JENIS-SUMBER DAN DAMPAK LIMBAH CAIR
Disajikan oleh:
Prof.Dr.Ir. Tri Widjaja, M.Eng.
DEPARTEMEN TEKNIK KIMIA, FTI-ITS
SEMESTER GENAP 2020/2021
Limbah Cair
Limbah cair adalah semua limbah yang berbentuk
cairan atau berada dalam fase cair (air seni atau
urine, air pencucian alat-alat).
Limbah cair merupakan sisa buangan hasil suatu
proses yang sudah tidak dipergunakan lagi, baik
berupa sisa industri, rumah tangga, peternakan,
pertanian, dan sebagainya. Komponen utama limbah
cair adalah air (99%) sedangkan komponen lainnya
bahan padat yang bergantung asal buangan
tersebut. (Rustama et. al, 1998).
2
Pentingnya Air
• Bagian fluida yang sngat penting bagi kehidupan
• Bagian dari fungsi ekosistem yang tak tergantikan
• Dibutuhkan untuk domestik, pertanian dan industri
• Berfungsi sebagai pembersih alami
• Setiap orang membutuhkan min 2,5 m3 air / hari.
Ketersediaan Air
• Bumi mengandung 325 juta kubik mil air,
yang meliputi 71% permukaan bumi.
• 97% air di bumi berupa air asin
• 2.5% berupa air tawar
• 0.77% tersedia untuk pemenuhan
kebutuhan manusia
Ketersediaan Air
Pemanfaatan Air
Sumber-sumber Air Segar
• Atmosphere (sebagai hujan)
• Danau dan sungai
• Glaciers, bongkahan salju
• Air tanah
• Air bawah tanah (aquifers)
SIKLUS AIR
SIKLUS AIR
• Siklus air merupakan proses dimana molekul air (H2O)
bergerak dalam suatu siklus : dari permukaan bumi
(lautan atau tanah ke atmosphere dan kembali lagi)
• Kuantitas air di bumi tidak pernah berubah,
• Siklus Air melalui perubahan wujud : Cair, Uap dan
merupakan proses yang dapat mempertahankan kualitas
air
• Kualitasnya dapat mengalami perubahan. Keberadaan Air
tidak pernah murni (sebagai H2O), selalu mengandung
impurities (kotoran) yang masuk ke lingkungan air.
Pencemaran (Pollusi)
Adanya suatu zat dalam lingkungan yang karena komposisi/
jumlahnya menyebabkan terganggunya proses alami,
lingkungan yang tidak diinginkan dan gangguan kesehatan
Pencemaran Air
•
Perubahan Kualitas Air secara Fisik, Kimia dan biologis yang
berakibat pada kehidupan mikroorganisme (misal degradasi)
•
Adanya bahan-bahan pengotor di badan air (danau, sungai,
aquifer, dll)
•
Pencemaran air adalah masuknya atau dimasukkannya
mahluk hidup, zat, energi dan atau komponen lain ke dalam
air oleh kegiatan manusia sehingga kualitas air turun sampai
ke tingkat tertentu yang menyebabkan air tidak berfungsi lagi
sesuai dengan peruntukkannya. (PP No.82 2001)
Sumber Pencemaran Air
• Sumber langsung:
Pencemaran yang
berasal dari sumber
tertentu
contoh : Pabrik, Sistim
buangan, sumur minyak
Sumber tak langsung :
Pencemaran yang berasal
dari penyebaran sumber yang
tidak jelas
contoh : Air dari jalan, dari
kegiatan pertanian
Sumber Pencemaran Air
PENCEMARAN AIR TANAH
Jenis & Karakteristik limbah cair
➢Karakteristik secara fisika
➢Karakteristik secara kimia
➢Karakteristik secara biologi
Karakteristik Fisika limbah cair
• Perubahan yang ditimbulkan parameter fisika dalam air
limbah yaitu:
❖ padatan
❖ kekeruhan
❖ bau
❖ temperatur
❖ daya hantar listrik dan warna
Padatan terdiri dari bahan padat organik maupun
anorganik yang larut, mengendap maupun suspensi.
Sifat Fisik Air Limbah
@ Kekeruhan
Dengan mengetahui penyebab kekeruhan akan
sangat
membantu
dalam
memilih
teknik
pengendapan yang akan diterapkan
Partikel 10 mikron dapat dihiloangkan dengan
penyaringan dan pengendapan
Partikel 1 mikron memerlukan sistem pemisahan
yang lebih tinggi, seperti dengan membran,
saringan mikro dan lainnya.
@ Warna
- Alamiah, seperti : tannin atau warna coklat pada tetes
-- Sintetis, seperti : bahan warna tekstil yang
umumnya menggunakan logam berat sebagai
bahan dasar, contoh chrome yellow
Tidak semua jenis bahan warna dapat diolah
dengan koagulan & flokulan konvensional, seperti:
tawas
Warna coklat pada limbah industri kinina, dapat
direduksi dengan khlorin dan kurang efektip bila
menggunakan FeSO4, FeCl3 apalagi tawas.
Keberadaan Padatan dalam
Cairan [settleable, in solution or in suspension]
Settleable
Solid
Total
Dissolved
Solid
Suspended
Solid
• Padatan yg bisa diendapkan dalam kondisi tenang dalam jangka
waktu antara 0,5-1jam, diukur dengan ImHoff Cone dalam fraksi
volume padatan yang mengendap dalam volume cairan total
• Filtrate dari cairan yang lewat kertas saring,
terdiri dari: small ions, macro molecules
dan very small colloids
• Bhn padatan yang didapatkan dg menyaring cairan limbah
menggunakan kertas saring 0,45 mm dan mengukur dry weight dari
material yang terkumpul dalam mg padatan/liter cairan.
• Disebut MLSS bila sample berasal dr activated sludge bioreactor,
sbg konsentrasi biomassa & inert di bioreaktor
Komposisi buangan limbah domestik
Karakteristik kimia limbah cair
Ada 7 macam yang terpenting:
• Keasaman : keasaman limbah cair dipengaruhi oleh
adanya bahan buangan yang bersifat asam atau basa.
Agar limbah tidak berbahaya, diupayakan untuk memiliki
pH netral.
• Logam berat beracun : Cadmium dari industri tekstil,
merkuri dari pabrik cat, raksa dari industri perhiasan dan
jenis logam berat yang lainnya.
• Nitrogen : umumnya terdapat sebagai bahan organic
dan diubah menjadi ammonia oleh bakteri sehingga
menghasilkan bau busuk dan bisa menyebabkan
permukaan air menjadi pekat sehingga tidak bisa
ditembus cahaya matahari.
Karakteristik kimia limbah cair
• Fenol : salah satu bahan organic yang berasal dari
industri tekstil, kertas, minyak dan batubara.
• BOD : kebutuhan oksigen yang dibutuhkan untuk
menguraikan senyawa organic yang ada di dalam air.
• COD : kebutuhan oksigen yang diperlukan mikroba
untuk menghancurkan bahan organik
Biological Oxygen Demand (BOD)
→Jumlah Oksigen yang dibutuhkan oleh
Bakteri untuk menguraikan zat Organik
dalam Limbah
Cara menentukan BOD
→Sampel air limbah dimasukkan bakteri
dan nutrien kemudian di inkubasi pada
suhu 200C selama 5 hari. Setelah
inkubasi perubahan oksigen terlarut
dalam sample diukur dalam mg O2 perliter
air limbah
Reaksi :
Bahan Organik+ O2 Bakteri
CO2 + waste product + NH3 + Energy + Bakteri Baru
Kurva BOD Dalam Air Limbah
Chemical Oxygen Demand(COD)
Jumlah Oksigen yang dibutuhkan untuk
mengoksidasi bahan-bahan organik
yang terdapat dalam limbah(Oksidasi
Kimiawi)
Cara menentukan COD
→Dengan mencampurkan air limbah
secara perlahan dengan potasium
dikromat selama 2-3 jam dan diukur
perubahan konsentrasi dikromat
Reaksi :
Bahan Organik+ Cr2O7-2
CO2 + H2O + Cr2 O4-2
Karakteristik biologis limbah cair
o Informasi secara biologis diperlukan untuk
menentukan kualitas air sehingga dapat
berguna untuk diminum, berenang, dll.
o Pada beberapa limbah cair,
mikroorganisme sangat berguna untuk
pretreatment (pengolahan tahap awal).
Klasifikasi miroorganisme yang
umum digunakan pada pengolahan
limbah cair:
–Bacteria
–Algae
–Fungi
–Protozoa
Sistem pengolahan limbah cair
“umumnya”
(1) Pengolahan primer (termasuk pengolahan
pendahuluan).
-
Proses pengolahan secara fisik & kimia
-
Pengoloahan fisik untuk pemisahan parameter
tersuspensi
-
Pengolahan secara kimia untuk pemisahan
parameter pencemar yang terlarut
-
Mengolah parameter pencemar inorganik
-
Pengolahan sekunder.
- Tahap ini digunakan untuk mereduksi parameter
pencemar organik
- Menggunakan aktivitas mikroorganisme
-
Pengolahan tersier atau pengolahan untuk
mereduksi parameter khusus , seperti mereduksi
warna, virus pada limbah rumah
sakit, dll
Primer
Sekunder
Tersier
Screen Grit removal Settling tank Aeration tank Settling tank Khlorinasi
Sludge
Aerator
Sludge
digester
Sludge drying bed
Primer
Sekunder
Tersier
Screen Grit removal Settling tank Aeration tank Settling tank Khlorinasi
Sludge
Aerator
Sludge
digester
Sludge drying bed
Macam-macam Pencemaran Air
• Bahan Patogen (penyakit)
• Bahan Kimia Organik
• Bahan Kimia Anorganik
• Kekurangan Oksigen/Eutrofikasi
• Endapan (Sedimentasi)
• Polusi Panas
Bahan-bahan Patogen
• Bakteri, virus,
parasit
• Buangan Manusia
& Hewan
• Demam
• Tiphoid/tifus,
• Kolera,
• Disentri
Bahan Kimia Organik
• Pestisida
• Minyak, oli, Bensin,
• Detergen
• Buangan Pabrik,
aliran dari
sawah/ladang
• Kanker
• Kerusakan sistim syaraf
Sumur Minyak Bor & Air Tanah
POPs
Persistent Organic Pollutants (POPs) adalah
bahan-bahan kimia yang tetap berada di
lingkungan, bioakumulasi melalui jaring-jaring
makanan, dan beresiko menyebabkan pengaruh
terhadap kesehatan manusia dan lingkungan.
The "dirty dozen" meliputi: PCBs, aldrin,
chlordane, DDT, dieldrin, endrin, heptachlor,
hexachlorbenzene, mirex, polychlorinated
dibenzo-p-dioxins, polychlorinated dibenzofurans,
and toxaphene.
Pollutan-pollutan Organik
contoh = Dioxin, PCB, DDT (Chlorinated)
Dioxin: stabil; lambat didegradasi
Penyebab:
kanker lemahnya sistem pertahanan
tubuh
Pollutan-pollutan Organik - PCBs
PCB = tidak mudah terbakar; tidak larut dalam air; bertitik
didih rendah; memiliki konduksi listrik rendah sehingga
digunakan sebagai transformers dan kapasitors.
• Terakumulasi dalam lemak hewan → biomagnifikasi
• penyebab:
•kanker
•pemecahan hormonal dan reproduksi
•menurunkan kemampuan cognitif (dopamine)
Pollutan-pollutan Organik - DDT
DDT adalah insektisida; stabil dan lambat di degradasi.
Paul Muller memenangkan penghargaan Nobel di tahun
1948 untuk pengembangan DDT.
Keuntungan = mengontrol penyebaran malaria;
melindungi hasil pertanian
Pollutan-pollutan Organik -DDT
Pollutan-pollutan Organik - DDT
Permasalahan dengan DDT:
DDT tidak dapat dimetabolisme dengan cepat oleh
hewan; sehingga akan terdeposit dan tersimpan
dalam jaringan lemak hewan → biomagnifikasi
Kesehatan Manusia
•penurunan fungsi mental
•infertilitas bagi perempuan
•kanker
Bahan Kimia AnOrganik
• Asam, Basa, garam
• logam berat (mercuri, timbal,
cadmium, selenium &arsen)
• Effluent industrial, buangan
perternakan,
pembangkit listrik
• Kanker, merusak sistem
syaraf
Pollutan-pollutan Anorganik
• Timbal dari pembakaran, pipa-pipa, solder , bensin
Menyebabkan:
- miscarriages
- kehilangan kemampuan mendengar
- kehilangan kemampuan belajar
• Arsen dari penambangan atau pengeringan tanah gurun
Menyebabkan : - anemia
- kanker
- kematian
• Natrium Klorida : Salinasi
• Asam-asam: Senyawa-senyawa sulfur dan nitrogen dari
batubara.
penyebab: - perubahan pH yang berefek terhadap species
- leaches aluminum
Pollutan Mercuri
• Berbagai sumber
• Pertambangan Emas
• Pembakaran
batubara-pembangkit
listrik
• Udara → Air
• Biomagnifikasi
Keracunan Mercuri :
• merusak sistem syaraf
• keterbelakangan mental
• perkembangan yang lambat
• kerusakan ginjal/gagal ginjal
Pengertian Bioakumulasi & Biomagnifikasi
Bioakumulasi : penimbunan (akumulasi)
suatu substansi / senyawa dalam jaringan
makhluk hidup.
Biomagnifikasi : Peningkatan konsentrasi
substansi atau senyawa dalam jaringan
makhluk hidup, dengan semakin tingginya
ikatan trofik dalam jaringan makanan.
Pollutan-pollutan Inorganik
Mother and Son
Minamata Bay, Japan
Oksigen dalam air
• Faktor yang mempengaruhi
jumlah O2 dalam air :
– Suhu
– Kecepatan Aliran
– Kekasaran permukaan yang dilintasi aliran air
• Oxigen terlarut (DO) dalam air akan berkurang
selama proses dekomposisi limbah organik [air segar
dapat mengandung hingga10 ppm (bagian per
sejuta) dibandingkan 200,000 ppm pada udara]
Kebutuhan Oksigen dalam Limbah
DO (Dissoved Oxygen): Oksigen terlarut dalam air sebagai indikator
kualitas air. 6 ppm O2 atau lebih mendukung kehidupan dalam air.
BOD (Biochemical Oxygen Demand) : Jumlah oksigen terlarut yang
dibutuhkan oleh mikroorganisme dalam air. Air limbah domestik, Pulp &
Kertas, limbah makanan dapat menyebabkan penurunan kadar oksigen
(Oxygen sag), dimana hanya sedikit makhluk hidup dalam air yang
bertahan.
Eutrophikasi
Eutrophikasi adalah proses alam dimana air (danau, sungai
dll) menjadi terlampau kaya akan nutrien, umumnya nitrogen
dan phospor. Ini merupakan salah satu cara dimana badan
air (danau, sungai, dan laut) berubah bentuk dari kondisi
kekurangan nutrien (oligotrophik), melalui fase yang sedikit
kaya nutrien (mesotrophik) hingga menjadi kondisi yang kaya
nutrien (eutrophik).
Aktivitas manusia kadangkala memperbesar laju perubahan,
karena aktivitas seperti perternakan, perhutanan, pembuatan
jalan, industri dan pengolahan limbah dapat menyebabkan
nutrien masuk ke dalam sumber-sumber air. Peningkatan
nutrien dengan cara tersebut seringkali menyebabkan
peledakan populasi alga dan tanaman air lainnya.
Pengaruh Ecosistem: Eutrophikasi
• Penyebab: Nitrat,
phosphat, &
ammonia
• Berasal dari: limbah
perternakan,
perkotaan, pertanian
(pupuk)
• Menyebabkan:
Kekurangan oksigen,
shg membunuh ikan
Eutrophikasi
BOD dan Eutrophikasi – perubahan cepat yang terjadi pada badan
air karena adanya peningkatan produktifitas biologi. (Danau dan sungai
oligotrophik memiliki air yang jernih dan produktifitas biologi yang
rendah).
Proses
eutrophikasi
Sedimentasi
• Definisi: pemenuhan badan air oleh partikelpartikel tanah, umumnya pasir dan kerikil.
• Pengaruh-pengaruh sedimentasi:
▪ Hilangnya tempat bersembunyi-tinggal bagi
ikan-ikan kecil
▪ Organisme-organisme air yang melekat
terlepas dari bebatuan dan pasir.
▪ berkurangnya cahaya masuk
▪ Kondisi anaerobik
Dampak
sedimentasi
Sedimen dari proses erosi dan terbawa aliran hujan:
•
mengisi danau
•
menghalangi jalur kapal
•
menyumbat turbin hydroelektrik
•
mempermahal proses purifikasi
Polusi Panas (thermal pollution)
Polusi Thermal = meningkatkan suhu
• menyebabkan
– thermal shock
– parasit dan penyakit
– berpeluang menjadi pollutan beracun
– Perubahan ecology
Dampak pencemaran air
• Dampak terhadap kehidupan biota air.
• Dampak terhadap kualitas air tanah.
• Dampak terhadap kesehatan.
• Dampak terhadap estetika lingkungan.
a. Dampak terhadap kehidupan biota air
• Dengan banyaknya zat pencemar yang ada di dalam air
limbah, maka akan menyebabkan menurunnya kadar
oksigen yang terlarut di dalam air limbah tersebut. →
– menyebabkan kehidupan terganggu (kematian, kurang
perkembangannya)
– kerusakan pada tanaman/tumbuhan air.
• Akibat matinya bakteri-bakteri, maka proses penjernihan
air secara alamiah juga terhambat. → air limbah menjadi
sulit teruraikan.
• Panas dari limbah industri juga membawa dampak pada
kematian organisme, apabila air limbah tersebut tidak
didinginkan terlebih dahulu.
b. Dampak terhadap kualitas air tanah
• Kualitas air tanah menurun (logam berat, bau, dll)
• Suatu survei sumur dangkal di Jakarta
menunjukkan bahwa pencemaran air tanah oleh
tinja telah terjadi dalam skala yang luas.
c. Dampak terhadap kesehatan
Pengaruh langsung terhadap kesehatan, umpamanya,
tergantung sekali pada kualitas air mengingat air yang
terkontaminasi dalam hal ini berfungsi sebagai media
penyalur ataupun penyebar penyakit.
Peran air sebagai pembawa penyakit menular bermacammacam, antara lain:
• air sebagai media untuk hidup mikroba patogen;
• air sebagai sarang insekta penyebar penyakit;
• jumlah air bersih yang tersedia tak cukup, sehingga
manusia bersangkutan tak dapat membersihkan dirinya;
• air sebagai media untuk hidup vektor penyebar penyakit.
Beberapa penyakit Bawaan Air dan Agennya
Agen
Penyakit
Virus:
•Rotavirus
•Virus Hepatitis A
•Virus Poliomyelitis
•Diare pada anak
•Hepatitis A
•Polio (myelitis anterior acuta)
Bakteri:
•Vibrio cholerae Escherichia coli
•Enteropatogenik Salmonella typhi
•Salmonella paratyphi
•Shigella dysenteriae
•Cholera Diare/Dysenterie
•Typhus abdominalis
•Paratyphus
•Dysenterie
Protozoa:
•Entamuba histolytica
•Balantidia coli
•Giarda lamblia
•Dysentrie amoeba
•Balantidiasis
•Giardiasis
Metazoa:
•Ascaris lumbricoides
•Clonorchis sinensis
•Diphyllobothrium latum
•Taenia saginata/solium
•Schistosoma
•Ascariasis
•Clonorchiasis
•Diphylobothriasis
•Taeniasis
•Schistosomiasis
d. Dampak terhadap estetika lingkungan
• Dengan semakin banyaknya zat organik yang
dibuang oleh perusahaan yang memproduksi
bahan organik seperti tapioka, maka setiap hari
akan dihasilkan air limbah yang berupa bahanbahan organik yang semakin besar → proses
pembusukan zat organik yang berada di
dalamnya → bau menyengat.
• Tumpukan limbah yang memerlukan tempat
yang luas.
d. Dampak terhadap estetika lingkungan
Peraturan Pemerintah Nomor 82 tahun 2001,
• Pengelolaan kualitas air adalah upaya
pemeliharaan air sehingga tercapai kualitas air
yang diinginkan sesuai peruntukannya untuk
menjamin agar kualitas air tetap dalam kondisi
alamiahnya;
• Pengendalian pencemaran air adalah upaya
pencegahan dan penanggulangan pencemaran
air serta pemulihan kualitas air untuk menjamin
kualitas sesuai dengan baku mutu air;
KlASIFIKASI AIR BAKU (PP 82/2001)
Air Kelas Satu : air yang peruntukkannya dapat digunakan untuk
air baku air minum, dan atau peruntukkan lain yang
mempersyaratkan mutu air yang sama dengan kegunaan
tersebut.
Air Kelas Dua : air yang peruntukannya dapat digunakan untuk
prasarana / sarana rekreasi air, pembudidayaan ikan air
tawar, peternakan, air untuk mengairi pertanaman, dan atau
peruntukan lain yang mempersyaratkan mutu air yang sama
dengan kegunaan tersebut.
Air Kelas Tiga : air yang peruntukannya dapat digunakan untuk
pembudidayaan ikan air tawar, peternakan, air untuk
mengairi pertanaman, dan atau peruntukan lain yang
mempersyaratkan mutu air yang sama dengan kegunaan
tersebut.
Air Kelas Empat : air yang peruntukannya dapat digunakan untuk
peternakan, air untuk mengairi pertanaman, dan atau
peruntukan lain yang mempersyaratkan mutu air yang sama
dengan kegunaan tersebut.
Konsep Pengendalian Pencemaran Air
▪ Mengurangi atau menghilangkan sumber-sumber pollusi
▪ contoh: pengurangan jumlah penggunaan pupuk
▪ Me-recycle buangan
▪ Mencari bahan-bahan pengganti yang ramah lingkungan
▪ contoh: penggantian phosphat pada deterjen laundry
▪ Mengembalikan secara alami, fungsi air sebagai
pembersih,
• contoh, menghidupkan kembali wetlands
• Meniru fungsi air sebagai pembersih alami dalam cara
mengontrol:
▪ Plant saluran pengolahan buangan
▪ Septik tank
TERIMA KASIH
Kuliah Minggu ke-2 PLI-A
DASAR-DASAR PENGOLAHAN LIMBAH CAIR
Oleh
Prof.Dr.Ir. Tri Widjaja, M.Eng.
DEPARTEMEN TEKNIK KIMIA FTIRS–ITS
TAHUNTemplates
2020
Powerpoint
Page 1
Komponen Air Limbah dan Analisanya
Karakteristik Fisik
Karakteristik Kimia
Karakteristik Biologis
Powerpoint Templates
Page 2
Karakteristik Fisik
Karakteristik Fisik
Kandungan Padat
(kekeruhan)
Warna
Suhu
Bau
Powerpoint Templates
Page 3
Kekeruhan (turbidity)
• Ukuran dari partikel merupakan penyebab kekeruhan di dalam air
• Dengan mengetahui ukuran dari partikel maka memudahkan dalam
memilih teknik pengendapan
• Jika ukuran partikel >10 µm umumnya dihilangkan dengan filtrasi
dan pengendapan
• Jika ukuran partikel < 1 µm maka memerlukan teknik pemisahan
yang lebih tinggi seperti membran dan saringan mikro
• Padatan tersuspensi dari bahan organik akibat proses biologis
anaerobik dapat terurai menjadi gas-gas beracun seperti H2S
• Untuk mengontrol hal tersebut maka digunakan bak equalisasi yang
sesuai dengan sifat fisik tersebut
Powerpoint Templates
Page 4
Suspended dan Filterable Solid
Powerpoint Templates
Page 5
Warna
• Untuk menguraikan bahan warna tersebut,
dibutuhkan bahan flokukan & koagulan serta
kondisi pengolahan (pH) yang sesuai dengan
sifat kimia dari bahan warna tersebut.
Alamiah
Warna coklat pada
limbah cair industri
bumbu masak karena
terdapat bahan baku
molases
Sintetis
Warna pada limbah
cair industri tekstil
Sifat warna air
limbah
Powerpoint Templates
Page 6
Karakter kimia
• Dari data sifat kimia, dapat ditentukan
sistem pengolahan mengikuti sistem fisik
diikuti kimia (koagulasi & flokasi) ataukah
masih dibutuhkan proses lanjut dengan
biologis.
• Disamping itu, sifat kimia juga perlu
diketahui agar dalam pengolahan air
limbah tersebut tidak terbentuk dampak
sampingan yang bersifat toksik.
Powerpoint Templates
Page 7
Karakter kimia (Contoh kasus)
Limbah cair
Industri Pelapisan
(electroplatting)
Limbah CN-, dan Cr+6
Cr+6 harus direduksi
menjadi Cr+3 agar
mengendap sebagai
Cr(OH)3
CN- toksik pada suasana
asam
Mutlak harus ada
pemisahan pengolahan
kedua
parameter
tersebut
Powerpoint
Templates
Page 8
Karakter biologis
1.Jenis Mikroorganisme
2.Daur Populasi
3.Indikator Bakteri dan
Analisanya
“ Secara umum organisme merupakan sludge biologis atau Mixed
Liquor Volatile Suspended Solid (MLSS). MLSS mengandung
material inert dan kultur biologi. MLVSS bersifat volatile”
Powerpoint Templates
Page 9
Karakter biologis
• Pemeriksaan biologis air limbah untuk
mengetahui apakah terdapat bakteribakteri patogen dalam air limbah.
• Pembagian mikroorganisme sangat
bervariasi, sebab terdapat banyak skema
yang bisa digunakan, namun umumnya
klasifikasi mikroorganisme cenderung
dibagi dalam jenis binatang, tumbuhan
dan protista.
Powerpoint Templates
Page 10
Karakter biologis
• Tabel berikut merupakan klasifikasi
mikroorganisme yang ada di dalam air
limbah
Powerpoint Templates
Page 11
Powerpoint Templates
Page 12
Contoh Kharakteristik Limbah
Domestik
Dari sewerage
• Total Suspended Solids (TSS)200-300
• (BOD5)
200-250
• (COD)
50-450
• Total Nitrogen as N
25-60
• Total Phosphorus as P
5-10
• Oil and Grease
80-120
Powerpoint Templates
mg/L
mg/L
mg/L
mg/L
mg/L
mg/L
Page 13
DASAR PENGOLAHAN LIMBAH CAIR
• Tujuan utama dari proses pengolahan air
limbah adalah mereduksi BOD, padatan
tersuspensi dan terlarut, organisme
patogen, zat beracun, dan senyawa
nonbiodegradable.
• Pengolahan air limbah dapat
diklasifikasikan menjadi :
– Pre-treatment (pengolahan awal)
– Primary treatment (pengolahan primer)
– Secondary treatment (pengolahan sekunder)
– Tertiery treatment (pengolahan tersier)
– Pembuangan sludge
Powerpoint Templates
Page 14
PROSES PENGOLAHAN AIR LIMBAH SECARA UMUM
PreTreatment
Primary
Treatment
Tertiery
treatment
Secondary
treatment
Powerpoint Templates
Sumber :
Wastewater Treatment, Donald W. Sundstrom, Herbert E. Klei
Page 15
Your Topic Goes Here
Screening
and grinding
Sedimentation
Carbon
adsorption
• Your Subtopics Go Here
equalization
Aerated
lagoon
membran
UF
Oil
separation
Stabillization
basin
Ion
Exchanger
Trickling
filter
Anaerobik
lagoon
Powerpoint Templates
Page 16
PRELIMINARY
TREATMENT
SCREENING
PENGUKURAN ALIRAN
EKUALISASI
GRIT CHAMBER
PRA AERASI
CLARIFIER KE SATU
LUMPUR
PENGOLAHAN PRIMER
PENGOLAHAN SKUNDER
PENGOLAHAN LANJUTAN
RAW WASTEWATER
DESINFEKSI
MANAGEMEN
LUMPUR
LUMPUR AKTIP
SARINGAN TETES
CLARIFIER
KEDUA
LUMPUR
SOLID REMOVAL
NITROGEN REMOVAL
PHOSPHORUS REMOVAL
Powerpoint Templates
DESINFEKSI
MANAGEMEN
LUMPUR
DESINFEKSI
MANAGEMEN
LUMPUR
Page 17
PROSES PENGOLAHAN BERDASARKAN UKURAN PARTIKEL
Ukuran partikel, mm
1.E-05
1.E-04
1.E-03
1.E-02
1.E-01
1.E+00
Org. & anorg. Suspensi koloid
terlarut
1.E+01
1.E+02
1.E+03
1.E+04
1.E+05
Padatan tersuspensi dan
padatan mengambang
Presipitasi
Screening
Transfer gas
Sedimentasi/flotasi
Ion exchange
Reverse osmosis
Filtrasi/microstraining
Elektrodialisis
Adsopsi
Koagulasi kimiawi
Powerpoint Templates
Biological oxidation
Page 18
PRE-TREATMENT
Screening
Pemisahan padatan dengan liquid
BAR RACK:
• memisah
suspended
solid yang terapung
•Solid material dibawa ke
landfill
• proses
pembersihan
secara
mekanikal
ataupun manual
Grit Chamber
• memisahkan padatan
seperti pasir, slit dan
kerikil
•Mencegah abrasi dari
pompa
dan mechanical
Powerpoint
Templates
lainnya
Beberapa peralatan
screening yang biasa
digunakan :
• Bar Screen
• Drum Screen
• Micro strainer
• Traveling
Water
Screen
• Communitor
Page 19
PRE-TREATMENT
Equalization
Mengendalikan fluktuasi aliran dan karakter air limbah di
industri agar proses pengolahan lebih lanjut lebih mudah
dilakukan untuk memenuhi baku mutu yang disyaratkan.
Fungsi :
1. Meredam beban kejut akibat fluktuasi beban organis yang dapat
mengganggu proses biologis aerobik ,
2. Mengendalikan pH air limbah,misalnya dengan pencampuran limbah
asam dan limbah basa yang ada.
3. Mengurangi fluktuasi debit aliran sehingga beban hidrolis yang tinggi
yang dapat mengganggu proses pengolahan,secara merata diatur
pengaliranya, menuju proses lebih lanjut atau dicampurkan ke
sistem riool kota, dan
4. Mencegah terjadinya konsentrasi bahan beracun yang tinggi
memasuki unit pengolahan biologis aerobik dan mematikan
mikroorganisme yang ada.
Powerpoint Templates
Page 20
CONTOH PERHITUNGAN
VOLUME TANGKI
EKUALISASI
Slope = debit
Powerpoint Templates
Ʃ gal x
time gal/min gal
10-3
8
50 3000
3,0
9
92 5520
8,5
10
230 13800
22,3
11
310 18600
40,9
12
270 16200
57,1
1
140 8400
65,5
2
90 5400
70,9
3
110 6600
77,5
4
80 4800
82,3
5
150 9000
91,3
6
230 13800 105,1
7
305 18300 123,4
8
380 22800 146,2
9
200 12000 158,2
10
80 4800 163,0
11
60 3600 166,6
12
70 4200 170,8
1
55 3300 174,1
2
40 2400 176,5
3
70 4200 180,7
4
75 4500 185,2
5
45 2700 187,9
6
55 3300 191,2
7
35 2100 193,3
Page 21
PRE-TREATMENT
Oil separation
Pemisahan dari lapisan minyak/oli
Alasan pemisahan minyak
➢ Densitas atau berat spesifik maupun dimensi sangat bervariasi
➢ sangat mengganggu dalamproses pengolahan air limbah lanjut, baik pada
proses kimia maupun proses biologis
➢ menghambat proses flokulasi & koagulasi
➢ menghambat masuknya oksigen ke air pada proses biologis
➢ sebelum proses koagulasi & flokulasi atau proses biologis, minyak dan
lemak (grease and oil) harus dipisah pada unit oil separator.
Parameter pemilihan separator minyak
• distribusi ukuran droplet minyak,
• kecepatan droplet,
• konsentrasi minyak dalam air, dan
• kondisi minyak,teremulsi atau tidak.
Powerpoint Templates
Page 22
API SEPARATOR
Oil skimmer
Vertical diffusion
Oilbaffle
skimmer
Oil
skimmer
Scraper
Oil
skimmer
Oil retention baffle Sludge pit
Coagulated plate interceptor
Cross Flow Interceptor
weir (dapat
diatur)
Corrugated
plate pack
Oil removal
outlet
inle
t
saluran
oil
oil
Inlet
weir
. .lapisan
. .. .
. . ...
oil .
.
masuk
•
Short
circuit
baffle
Slotted
distribution
plateDesludge
slu
dg
e
outlet
Corrugated
plate pack
slud
ge
sedim
en
weir, dapat
diatur
Penangka
p
sedimen
Powerpoint Templates
• . .
. o
.
• .
.
.
•
•
keluaran
.
sludge pit
sludg
e
Page 23
PRIMARY TREATMENT
Primary
treatment
physical
chemical
Netralisasi
Koagulasi
dan
Flokulasi
Sedimentasi
Powerpoint Templates
Flotasi
Page 24
PHYSICAL PRIMARY TREATMENT
merupakan tahapan yang berfungsi untuk menyisihkan polutan
yang berupa padatan (solids) yang bersifat settleable solids
maupun floatable solids dengan cara sedimentasi maupun
flotasi
1. Sedimentasi
2. Flotasi
Padatan akan mengendap
dibawah secara gravitasi.
Padatan akan
mengambang diatas
Powerpoint Templates
Page 25
SEDIMENTASI
Ada tiga klasifikasi pengendapan menurut jenis partikel yang diendapkan :
1. Pengendapan Partikel Diskrit
Partikel tidak mengalami perubahan bentuk, ukuran, maupun berat
jenisnya selama proses pengendapan
2. Pengendapan Partikel Flokulen
Partikel mengalami perubahan bentuk, ukuran dan massa jenisnya,
sehingga kecepatan pengendapannya bertambah besar
3. Pengendapan di ruang lumpur
Pengendapan
atau pemampatan
Powerpoint
Templateslumpur terjadi baik untuk partikel
Page 26
diskrit maupun untuk partikel flokulen
CHEMICAL PRIMARY TREATMENT
merupakan tahapan yang berfungsi untuk menyisihkan polutan
yang berupa padatan (solids) yang bersifat settleable solids
maupun floatable solids dengan cara penambahan bahan
kimia
1. NETRALISASI
Netralisasi dapat dilakukan :
a. Pencampuran limbah yang Asam dan Basa
b. Netralisasi Limbah Asam dengan Batu Kapur (aliran lewat
packed bed)
c. Mencampur Limbah Asam dengan CaO, Na2CO3,NaOH,dll
d. Limbah Basa dengan H2SO4,H3PO4, atau produk CO2
hasil proses reaksi.
Powerpoint Templates
Page 27
2. KOAGULASI DAN FLOKULASI
Sifat Koloid di air : hydrophobic atau hydrophilic.
Hydrophobic mudah dikoagulasi. (mis. tanah liat)
Hydrophilic sulit dikoagulasi, partikel diselimuti air
(mis. Protein)
Koagulasi: Hadirnya ion berlawanan mendestabilkan koloid.
Flokulasi : penggabungan koloid-koloid destabil, dan
Gabungan koloid berukuran lebih besar;
• mengendap (sedimentasi), bila ρ > ρ air
• mengapung (floatasi) , bila ρ < ρ air
Powerpoint Templates
Page 28
KOAGULASI
Pengadukan yang cepat
diperlukan untuk
mendistribusikan koagulan
secara uniform dalam cairan
Diusahakan dengan :
• motor driven impellers yang dipasang
pada kolam kecil dimana koagulan
dimasukkan ke dalamnya
• Memberikan koagulan pada beberapa
titik turbulensi
• in-line static mixers
• Menggunakan baffled chambers atau
channels atau hydraulic jumps
Powerpoint Templates
Page 29
Mekanisme Flokulasi
Flocculants Characteristics
Ø Bisa berupa kation,anion atau non
ion.
Ø Berupa bubuk,larutan atau latex.
Ø Dengan berat molekul yang sangat
besar
Ø Dapat membawa sistem koloid
dengan beragam muatan ditiap
rantai polymer.
Powerpoint Templates
Wastewater Treatment Course, 27 – 30 Juni 2006
Page 30
Ketiga unit koagulasi – flokulasi –
sedimentasi juga dapat dilakukan pada 1
unit, atau disebut Clarifier.
Seperti : Solid contact clarifier
Upflow clarifier, dll
Di plant, tahap koagulasi, flokulasi dan
sedimentasi dapat dilakukan terpisah
atau pada satu unit.
5.
5. Solid contact clarifier
Bahan kimia
motor
O O O O O
weir
O O O O
O
effluent
Inlet
Koagulan
influent
Scraper
scraper
sludge
Pengadukan
cepat
1 menit
(A)
Pengadukan
lambat
5 – 10 menit
(B)
Unit koagulasi-flokulasi-sedimentasi
Efflu
ent
Sedimenta
si 60 - 120
menit
Endapa
(C)
n
Powerpoint Templates
Page 31
ACTIVATED SLUDGE SYSTEM
Powerpoint Templates
Page 32
AERATED LAGOON
Aerated lagoon adalah bak dengan kedalaman 2,5 - 5 m,
dan luas permukaan beberapa ratus meter persegi serta
diaerasi secara mekanis atau difusi udara, sehingga
organik dalam air limbah dapat terurai.
Powerpoint Templates
Page 34
TRICKLING FILTER
Trickling filter adalah reaktor biologi berbentuk packed bed
dengan media filter berupa batu atau plastik, media packing
akan ditumbuhi lapisan slime / mikrobiologi aerobik.
SkemaTrickling Filter
Bentuk Media Trickling Filter
Powerpoint Templates
Page 35
Flowsheet for Physical-Chemical Treatment
Wastewater
Bar Racks
Chemical
Flocculation+
sedimentation
Sludge
Packed bed
filtration
Carbon
regeneration
Carbon
Chlorination
Powerpoint Templates
adsorption
Treated
effluent
Page 36
Pengolahan Tersier
• Beberapa operasi tersier secara umum
bertujuan untuk penyisihan senyawasenyawa tertentu seperti fosfor, nitrogen dan
zat-zat pewarna.
• Fosfor dengan cara koagulasi dengan suatu
bahan kimia
• Nitrogen dengan proses stripping ammonia
dengan udara
• Warna dengan adsorbsi dalam activated
karbon
Powerpoint Templates
Page 37
Desinfektan
• Pada akhir proses, efluent air limbah
akan diberi suatu agen desinfektan untuk
membunuh semua organisme
(khususnya yang patogen) sebelum
dibuang ke badan air
• Klorinasi : pemberian klorida pada
effluent limbah sebagai zat penghancur
dinding sel mikroba (lisis) sehingga
mikroba tersebut mati.
• Ozonisasi : pemberian ozon (O3) dalam
efluent sebagai oksidator kuat yang akan
mengoksidasi mikroba sehingga mati
Powerpoint Templates
Page 38
SLUDGE DISPOSAL
OPERASI
TUJUAN
DEWATERING
Vacuum nitration, Centrifugation,
Sand beds
Mengurangi Volume Lumpur
Dan Membentuk Lumpur Padat
DRYING AND OXIDATION
Inseneration, Heat drying, Wet
Air Oxidation
Mengringkan dan Mengoksidasi
Lumpur
ULTIMATE DISPOSAL
Landfill, Spreading on Soil, Lagoons,
Ocean
Memanfaatkan atau Mengolah
Lumpur Padat
Powerpoint Templates
Page 39
BLOCK DIAGRAM SLUDGE DISPOSAL
SLUDGE
GRAVIMETRY
THICKENER
ANAEROBIC
DIGESTER
VACUUM
FILTRATION
LANDFILL OR
INCINERATION
CHEMICALS
Powerpoint Templates
Page 40
Pembuangan Sludge
• Proses pengolahan limbah menghasilkan
lumpur dari padatan tersuspensi dalam
jumlah yang signifikan.
• Pemilihan urutan pengolahan lumpur
tergantung pada sifat lumpur, faktor
lingkungan, dan pemilihan pembuangan
akhir.
• Pembuangan sludge dapat dilakukan
dengan beberapa cara, antara lain :
– Landfill
– Incenerasi
Powerpoint Templates
Page 41
Powerpoint Templates
Page 42
Kuliah Gasal 2020/2021
Pengolahan Limbah Industri - A
TK 141282
PENGOLAHAN LIMBAH BIOLOGIS
PARAMETER CONTROL DAN MONITORING TOOLS
(SVI, F/M RATIO, MCRT, RAS, WAS, OUR, SOUR)
OLEH:
Prof.Dr.Ir. Tri Widjaja, M.Eng.
Departemen Teknik Kimia FTI-ITS
Primer
Sekunder
Tersier
Screen Grit removal Settling tank Aeration tank Settling tank Khlorinasi
Sludge
digester
Sludge drying bed
PRINSIP DASAR PENGOLAHAN BIOLOGIS
Proses Biologis
memanfaatkan pertumbuhan
mikroorganisme untuk mereduksi kandungan
bahan organik dalam air limbah
Dengan “diambilnya” unsur C dari bahan
organik, menyebabkan bahan organik tersebut
akan terurai & tingkat polutan tereduksi
Peran dari laju pertumbuhan
mikroorganisme sebagai kunci
keberhasilan
PENGOLAHAN BIOLOGIS
PENGOLAHAN BIOLOGIS
Tujuan : untuk mereduksi kandungan bahan organik
dalam air limbah dengan menggunakan
“jasa” mikroorganisme (biologis)
Mikroorganisme pengurai membutuhkan :
- oksigen >> diperoleh dari oksigen terlarut
- karbon sebagai sumber energi >>> diperoleh
dari bahan organik terlarut
- N dan P sebagai nutrient
PRINSIP DASAR PENGOLAHAN BIOLOGIS
Dilakukan dengan
menumbuhkan
mikroorganisme dalam bak
aerasi yang akan bertugas
untuk menguraikan bahan
organik
Bahan organik diolah adalah :
Carboneous organic
(C,H,O)
Nitrogenous organic
(C, H, O & N)
Secondary Treatment
Pengolahan Biologis
Proses penguraian dibutuhkan :
Proses biologis akan
menghasilkan mikrobiologi
berupa activated sludge
Jumlah oksigen (aerobik)
Nutrient N dan P
Unsur karbon (C)
Pertumbuhan mikroorganisme sesuai dengan konsentrasi
bahan organik yang akan diuraikan, memenuhi mekanisme:
- laju pertumbuhan mikroorganisme
- laju peruraian substrat (bahan organik)
•Reaksi proses biologis aerobik meliputi:
cell
Organik + O2 +N +P
Cell + O2
cell
cell baru + CO2 + H2O + SMP
CO2+ H2O +P + N +sisa cell +SMP
COD Balance
Mikroorganisme ( = cell) selalu tumbuh (growth ≈
ukuran bertambah besar) hingga maksimum.
Pada keadaan pertumbuhan maksimum (full growth),
cell membentuk cell baru serta ada cell yang mati
Selama pertumbuhan (masa hidupnya) , bagian cell
yang rusak aka “diperbaiki”, struktural komponen
diperbaharui dsb.
Ada molekul dalam dirinya yang rusak (catabolisme)
dan sebagian lainnya terbentuk lagi (anabolisme).
Catabolisme dan Anabolisme selalu dalam keadaan
setimbang
atau
yang
dinamakan
sebagai
Metabolisme
❖
Untuk menjaga kesetimbangan, cell dibutuhkan
energi,
seperti
(reproduksi),
energi
energi
untuk
untuk
pembelahan
cell
memperbaiki
dan
mengganti molekul yang rusak.
❖
Energi diperoleh dari :
- dari struktur kimia dari makanan yang dikonversi
menjadi energi
- radiant energi dari matahari yang dikonversi
menjadi energi kimia oleh cell yang mempunyai
kemampuan untuk proses photosyntesa
RESPIRASI
FOO
D
Reproduksi
ENZYMES
Respirasi
Energi
N
GE
Y
X
Cell
maintenance
O
Reaksi biokimia (respirasi) sangat tergantung
pada enzym (protein molecules) yang bertindak
sebagai katalis
❖
RESPIRASI AEROBIK
❖ RESPIRASI ANAEROBIK
CO2
n
ge
xy
O
H 2O
O2
New Cell
O2
cell
Organic
O2
O2
O2
New
energy
AEROBIK
Proses respirasi bahan organik oleh mikroorganisme, dimana aerobik
organisme hanya dapat hidup dengan baik apabila ada oksigen bebas
atau oksigen terlarut karena proses rspirasi membutuhkan oksigen
bebas
Lipids (fat and grease), carbohydrat (sugar and starch), dan protein
merupakan sumber energi (carbon)
Bahan organik akan terurai (respirasi) menjadi CO2, H2O dan energi
Su
l
H2S
fa
t
SO4
organic
CO2
Energy
SO4
CH4
cell
SO4
New cell
SO4
ANAEROBIK
Anaerobik respirasi terjadi walau tidak ada oksigen bebas atau
oksigen terlarut.
Tetapi oksigen tetap dibutuhkan untuk respirasi, dan diperoleh
dari oksigen dalam compound (seperti NO3, SO4) sehingga
sebagai hasil akhir akan terbentuk gas-gas H2S, CO2, N2 dan CH4
serta energi
Pada pengolahan air limbah dengan proses anaerobik, kondisi
operasi diatur agar gas methane (CH4) terbentuk maksimal
AEROBIC
O2 : oksigen terlarut
ORGANIK
O
o2
o2
2
AEROBIC
FAKULTATIP
ANAEROBIC
O2 dari SO4, NOx, dll
SO4
SO4
FAKULTATIP
ANAEROBIC
ORGANIK
C, H, O DAN N
OKSIGEN
(UDARA)
BIOLOGIS
(AEROBIK)
N DAN P
(NUTRIENT)
CELL
1 O2 + 1 C
ENZYM
CO2 + H2O + ENERGI
BOD5
BOD5 = BOD yg diamati setelah hari ke-5
Untuk “mengambil” 1 mole C , biologis membutuhkan
atau menghabiskan 1 mole O2
Atau : untuk 12 mg C dari bahan organik dibutuhkan
32 mg O2
Banyaknya oksigen (O2) yang dihabiskan oleh
mikroorganisme merupakan gambaran tingginya
kandungan bahan organik
Dianalisa sebagai BOD5
Biological Oxygen Demand
MEKANISME PROSES BIOLOGIS
Cell baru
O2
Organik terlarut
Absorbsi
Nutrient
N, P
CELL
CO2
H2O
Adsorpsi
Koloid dan Suspended Solid
Catatan
Pada proses anaerobik yang keluar dari cell
berupa CH4 , CO2
(untuk bakteri2 methan)
berupa CO2, H2, asam2 organik
(untuk bakteri2 acetogenik)
Mekanisme Biodegradasi
cell = mikroorganismenya
SMP:
Soluble Microbial Product
outputnya
Biologis membutuhkan oksigen untuk
melakukan beberapa reaksi biokimia
Reaksi oksidasi bahan organik
(CH2O)n + nO2 ⎯⎯→
enzym n CO2 + nH2O + energi
Sintesa cell
(CH2O)n + NH3 + O2 ⎯⎯→ komponen cell + CO2
enzym
+ H 2O + energi
Oksidasi cell
enzym CO + H O + NH +
Komponen cell + O2 ⎯⎯→
2
2
3
energi
Metode Aerasi
AERASI ALAMI
AERASI DIFUSI
AERASI MEKANIK
To increase
contact area
bubbles
Mekanisme Difusi O2 & Tahanan Aerasi
1. Tahanan pada iquid film antara
gas-liquid interface dan bulk liquid
O2
2. Tahanan pada bulk liquid karena
pengadukan yang buruk
1
3. Tahanan pada liquid-film pada
solid interface
2
3
4
5
sel
4. Tahanan pada bulk-solid
5. Tahanan akibat reaksi kimia di
sel
Bahan organik mengandung nitrogen diolah
Bakteri:
NH3
O2
⎯⎯⎯⎯→ NO2
Nitrosomonas
-
O2
⎯⎯⎯⎯→
NO
Nitrobacter
3
Reaksi Denitrifikasi oleh bakteri
❖ Reduksi nitrat ( reaksi energi):
6 NO3- + 2 CH3OH
6 NO2 - + 2 CO2 + 4 H2O
❖ Reduksi nitrat ( reaksi energi):
6 NO2- + 3 CH3OH
3 N2 + 3 CO2 + 3 H2O + 6 OH-
❖ Reduksi nitrat overall (reaksi energi) :
6 NO3- + 5 CH3OH
5 CO2 + 3 N2 + 7 H2O + 6 OH-
METODE PENGUKURAN
KANDUNGAN BAHAN ORGANIK
BOD
Metode
pengukuran
bahan organik
COD
TOC
TOD
22
BOD
• BOD Merupakan O2 yang dibutuhkan mikroorganisme
untuk mendegradasi bahan buangan organik di dalam
air.
• BOD air limbah berhubungan dengan jumlah oksigen
yang harus disuplai ke dalam air limbah oleh aerator
selama secondary treatment atau dengan aerasi alami
oleh air yang diterima.
• BOD Biasanya digunakan sebagai indikator (tidak
langsung) bahan organik
23
BOD
• Sample air limbah diberi bakteri dan nutrien (bila
diperlukan) dan diinkubasi selama 5 hari pada 20°C,
dimana bisa juga diinkubasikan dengan suhu dan
waktu yang berbeda (tergantung kebutuhan). Setelah
inkubasi, perubahan konsentrasi Dissolved Oxygen
(DO) pada sample diukur, dan BOD5 dinyatakan dalam
mg O2/liter air limbah.
• BOD5 = BOD yang diukur setelah 5 hari (mg
O2/liter air limbah).
24
Organisme yang dimasukkan pada air limbah semula menggunakan bahan organik secara
cepat untuk pembentukan sel dan metabolisme, hal ini menyebabkan kenaikan BOD secara
cepat dalam waktu 1-3 hari & setelah bahan organik sebagian besar terasimilasi dari larutan,
kenaikan oksigen naik secara perlahan.
Reaksi karbon umumnya berhenti setelah 20-30 hari dan bahan organik menjadi stabil.
Selama 5 hari periode tes BOD, 60-70% jumlah akhir karbon yang dibutuhkan, BODL,
Gambar 1-5
25
• Selama 5 hari periode tes BOD, 60-70% jumlah akhir karbon
yang dibutuhkan, BODL, dicatat.
• Nitrogen pada bahan organik sebagian besar berasal dari urea
dan protein, dan berubah menjadi NH3 selama dekomposisi
bahan karbon.
• Selama sekitar 12 hari, nitrifying bacteria mulai mengoksidasi
ammonia dan menggunakan BOD seperti reaksi:
NH3 + 3/2 O2 Nitrosomonas H2O + HNO2
HNO2 + ½ O2 Nitrosobacter HNO3
• Tahap kedua oksigen yang dibutuhkan memiliki hasil akhir
BODLN setelah sekitar 50 hari.
26
• Berdasarkan persamaan:
• Maka,jumlah oksigen yang terpakai :
BODt = BODL (1 – e-kt)
= BODL (1 – 10-Kt)
dimana :
Keterangan:
BODL : jumlah awal BOD pada sample
BODt : jumlah BOD yang dikonsumsi pada sample setelah t hari
k
: rata2 konstanta reaksi (hari-1)
27
• Laju reaksi konstan dikendalikan oleh temperatur
dan diekspresikan sebagai :
kT = k20°C . θ(T-20°C)
dimana θ = 1.047 – 1.135
• θ yang biasanya digunakan adalah 1.056 untuk
suhu 20–30°C
28
COD
Merupakan O2 equivalen yg dibutuhkan untuk
reaksi oksidasi terhadap bahan polutan organik dalam
air.
COD biasanya > BOD, karena banyak senyawa
yang lebih mudah dioksidasi secara kimia daripada
dioksidasi secara biologi.
Bahan organik + Cr2O72- → Δ
CO2 +H2O + Cr2O42-
COD
Merupakan O2 equivalen yang dibutuhkan untuk
reaksi oksidasi terhadap bahan polutan bahan buangan
dalam air.
COD biasanya > BOD, karena banyak senyawa
yang lebih mudah dioksidasi secara kimia daripada
dioksidasi secara biologi.
Bahan organik + Cr2O72- → CO2 +H2O + Cr2O42Δ
Tes COD dilakukan dengan merefluks sample air limbah
dengan potassium dichromate selama 2-3 jam dan
mengukur perubahan konsentrasi dichromate.
30
TOD DAN TOC
• TOD : jumlah oksigen total yang dibutuhkan
untuk mengkonversi (menguraikan) carbon,
nitrogen dan mineral lain
• TOC: jumlah organik carbon yang terikat dalam
suatu senyawa organik, metode pengukurannya
sama dengan TOD, hanya pada TOC, jumlah CO2
nya dihitung dan diekspresikan sebagai mg
karbon/liter air limbah.
31
Menentukan TOD dan TOC
Tentukan TOD dan TOC pada larutan mengandung
2 gr/liter glukosa
Untuk
menentukan
bahan/zat
yang
mudah
teroksidasi seperti glukosa, TOC dan TOD bisa
dihitung berdasarkan stoikiometrireaksi oksidasi:
C6H12O6 + 6O2 🡪 6CO2 + 6H2O
(180,1) (32) (44)
(18)
32
Menentukan TOD dan TOC
TOD / TOC = 2.66 mg O2/mg C
33
Faktor yang mempengaruhi laju pertumbuhan
mikroorganisme (MO) atau sering disebut juga
sebagai faktor “growth pressure”
antara lain meliputi
❖ BOD5 (tipe dan jumlah makanan/unsur karbon)
❖ F/M ratio (food to MO ratio)
Terkait dengan
❖ DO (Dissolved Oxygen)
MLSS, MLVSS,
sludge age, RAS,
biomass X, dll
❖ Nutrient (N & P)
❖ Hydraulics (retention time)
❖ pH, temperatur
O2 dari udara
Q
Qe
DAF
ReaktorKeaerasi
Qr
Qw
Notasi dalam proses biologis :
sludge
- Biomass, MLVSS, X
Q = laju alir air limbah masuk
- Substrat , BOD, COD = S
Qr = laju alir RAS atau R
- HRT, waktu tinggat hidraulik, t , θ
Qw = laju alir sludge yang dibuang
- Umur sludge , tC, θC
Qe = laju alir effluent
- F/M atau U atau SLR
- Sludge Volume index, SVI
- RAS (return activated sludge, sludge yg direcycle)
36
PROTOZOA
• Ditemukan di semua jenis permukaan air dan
tanah.
• Ukuran bervariasi.
• Memetabolisme colloidal waste, bakteri, sesama
protozoa atau organisme yang lebih kecil.
• Dalam memproses activated sludge, Protozoa
yang digunakan adalah kelas Ciliata dan Suctoria.
37
ROTIFIERS
• Multiseluler, hidup pada kondisi aerobik.
• Memakan bakteri
• Membutuhkan kondisi lingkungan dengan oksigen
terlarut yang tinggi dan level bahan organik yang
rendah.
• Keberadaannya menjadi indikator “air telah
bersih (dipurifikasi tingkat tinggi).
38
JENIS – JENIS
PENGOLAHAN
LIMBAH SECARA
BIOLOGIS
JENIS PENGOLAHAN LIMBAH BIOLOGIS
PROSES AEROBIK (SUSPENDED GROWTH)
Merupakan jenis pengolahan limbah
secara biologis menggunakan peran
mikroorganisme tersuspensi untuk
mendegradasi limbah ORGANIK &
NITROGEN.
SUSPENDED GROWTH (ACTIVATED SLUDGE)
• Merupakan kolam ber-aerasi dan
berpengaduk
• Terjadi dekomposisi material
organik oleh mikroorganisme
yang diinokulasikan sehingga
dapat mengendap
• Bakteri dalam “activated sludge”
diresirkulasi secara kontinu ke
kolam aerasi utk meningkatkan
rate dekomposisi organik.
SUSPENDED GROWTH (ACTIVATED SLUDGE)
• Memiliki waktu tinggal 4 – 8 jam
• Jumlah udara yang dibutuhkan oleh mikroorganisme tidak konstan
• Konsentrasi MLSS antara 2000-3500 mg/L
• F/M rasio antara 0.2 - 0.5
• Sludge age / MCRT antara 5-15 hari.
SUSPENDED GROWTH (ACTIVATED SLUDGE)
KONTAK STABILISASI
• Pada contact tank, terjadi proses adsorpsi
• Pada stabilization tank, terjadi proses oksidasi
• Memiliki waktu tinggal yang kecil, sehingga volume contact tank
tidak perlu terlalu besar
• Waktu tinggal pada contact basin 0.5-2 jam, sedangkan pada
stabilization basin 4-8 jam
• MLSS concentration 1200-2000 mg/L pada contact chamber dan
4000-6000 mg/L pada stabilization chamber
• Contact stabilization memiliki F/M Rasio tertinggi hingga 0.6
SUSPENDED GROWTH (ACTIVATED SLUDGE)
EXTENDED AERATION
Waktu tinggal antara 16-24 jam
- Range MLSS antara 3000-5000 mg/L
- F/M rasio terendah untuk activated sludge process
biasanya 0.05- 0.2
- Padatan berada dalam bentuk abu inert
- Biasanya memiliki pretreatment namun tidak primary clarifiers
- RAS dikembalikan ke headworks dan lumpur limbah dikirim ke
digester aerobik.
SUSPENDED GROWTH (ACTIVATED SLUDGE)
Contoh Proses Extended Aeration
SUSPENDED GROWTH (LAGOON)
PROSES AEROBIK (ATTACHED GROWTH)
• Pengolahan limbah biologis menggunakan
peran mikroorganisme menempel pada
suatu media untuk mendegradasi limbah.
ATTHACED GROWTH AEROBIK (TRICKLING FILTER)
Trickling filter adalah reaktor biologi berbentuk packed bed
dari batu atau plastik sebagai media filternya dimana proses
kimia-biologis berlangsung, media packing akan ditumbuhiyang
menempel sbg lapisan slime/mikrobiologi aerobik.
PROSES AEROBIK (TRICKLING FILTER)
PROSES AEROBIK (ROTATING BIOLOGICAL
CONTACTOR)
•RBC dibuat dari lempengan2 plastik yang
dipasang pada sumbu berputar.
•40% volume alat ini dibenamkan dalam tangki air
limbah.
•Dipermukaan lempengan akan tumbuh lapisan
mikroba setebal 1–4 mm.
PROSES AEROBIK (ROTATING BIOLOGICAL
CONTACTOR)
PROSES AEROBIK (ROTATING BIOLOGICAL
CONTACTOR)
PARAMETER BIOLOGIS
Parameter penting yang mempengaruhi proses biologis:
• Rasio F/M ( Food to Microorganism )
• Sludge Age (MCRT) Mean Cell Residence Time (umur mikroba/sel di dalam sludge)
• SVI (Sludge Volume Index)
• RAS (Return Waste Sludge)
• WAS (Waste Activated Sludge)
• OUR (Oxygen Uptake Rate)
• Temperatur
• pH
Food : Mass Ratio
Salah satu dari parameter kontrol utama pada
lumpur aktif adalah Food:Mass Ratio atau Sludge
loading Rate. Dapat dihitung dengan persamaan
berikut:
Besar F:M ratio yang optimum berkisar antara
0,2-0,6 kg BOD/kg MLSS (sludge yang terbentuk
mudah mengendap/good settling)
Food : Mass Ratio
❖ Pada grafik tersebut
jika F/M ratio antara
0.2-0.6 maka zone
settling velocity (ZSV)
akan mudah
mengendap, dengan
sludge age sekitar 3-14
hari.
❖ Efisiensi dari
penurunan BOD
removal sangat kecil
pada range tersebut,
biasanya diatas 95%
dalam sistem yang
konvensional
Food : Mass Ratio
❖ Apabila F:M ratio terlalu rendah maka
dapat menimbulkan tumbuhnya filamen
bakteri atau kondisi bulking.
Pengendapan di tangki sedimentasi
terganggu/sulit.
❖ Jika F:M Ratio terlalu tinggi maka dapat
menyebabkan kenaikan kebutuhan
oksigen dan menaikan clarifier loading.
Food : Mass Ratio deficiency
Type 0041
Pertumbuhan Mikroba vs F:M ratio
Sludge Age (MCRT)
Sludge age atau Mean Cell Residence
Time (MCRT) adalah waktu tinggal
rata-rata mikroorganisme di dalam sistem
aerasi. Dapat dihitung dengan persamaan
berikut:
WAS and final effluent
Sludge Age (MCRT)
V, X, S
Q0, XE
QW, XW
Sludge age (MCRT) = (V*X)/(QW*XW+Q0*XE)
Keterangan:
V = Volume tanki aerasi
Q0 = rate effluent
X = konsentrasi biomass dalam tangki aerasi X = konsentrasi biomasss dalam effluent
E
Qw = rate waste activated sludge (WAS)
Xw = konsentrasi biomass dalam aliran (WAS)
Sludge Age (MCRT)
❖ Sludge age biasanya antara 3-14 hari
untuk menghasilkan biological floc.
❖ Jika MCRT < 3 hari maka biomassnya
kurang cukup tebal, sehingga terbentuk
“bulking sludge” dan straggler flok.
❖ Jika MCRT > 14 hari maka flok partikel
yang terbentuk akan terlalu kecil (Pin
Flok).
Pertumbuhan Mikroba vs MCRT
Sludge age vs efisiensi BOD removal
• Dibawah sludge age minimum, biomass dipindahkan lebih cepat di
tangki aerasi daripada digantikan oleh pertumbuhan sel baru. Proses
ini dimaksudkan sebagai Washout
• Ada juga sludge age maximum atau critical. Diatas age ini, semua
peningkatan performa diabaikan
• Ada periode antara washout dan critical sludge age dimana aktivitas
biomass mungkin naik atau turun secara teratur
Sludge Volume Index (SVI)
Berat endapan per satuan volume (mg/l) larutan setelah 30 menit
proses pengendapan, digunakan untuk mengetahui karakteristik
pengendapan sludge dan sangat berguna dalam proses kontrol
pengendapan.
Dimana: V = Volume dari settled solids setelah 30 menit
V0 = Initial volume dari sludged tested (liters)
X = Konsentrasi MLSS dari lumpur sebelum tes
(gm/liter)
Sludge Volume Index (SVI)
Keterangan :
SVI = Sludge Volume Index.
SV = Sludge Volume.
SS = Suspended Solid
Sludge Volume Index (SVI)
Settleable solid merupakan partikel
padat yang yang akan mengendap
setelah satu jam karena pengaruh
gaya
gravitasi
bumi.
Biasanya
pengukuran dilakukan menggunakan
“Imhoff Cone” dan data yang
dihasilkan berupa volume padatan
(ml) per liter larutan limbah.Untuk
mengetahui
total
solid
yang
mengendap, salah satu cara yang
dapat
dilakukan
adalah
dengan
penyaringan menggunakan membran
yang memiliki ukuran lubang sampai
0.45
micron.
Untuk
kemudian
diperoleh data berat kering endapan
limbah (mg/l).
Solids concentration vs SVI
Solids concentration vs SVI
● Dari gambar di atas untuk typical sludge. Di bawah poin (a) SVI relatif
tidak tergantung pada konsentrasi solid
● Di atas poin (a) dan di bawah poin (b) SVI sangat tergantung pada
konsentrasi solid karena kegagalan sludge untuk menggumpal menjadi
coarse open lattice
● Di atas poin (b) SVI turun ke parallel kurva maximum attainable
● SVI bisa digunakan sebagai alat operasional untuk in-plant control dari laju
recycle solid. Jika konsentrasi solid meninggalkan settler diasumsikan
1/SVI, maka solids balance disekitar reaktor (mengabaikan sintesa solid
di dalam reaktor) bisa dipetakan
• Sebagai contoh: Untuk menjaga konsentrasi solid pada 3.000 mg/L &
ketika SVI 75 ml/gm, maka laju recycle solid diperoleh 29%.
Pertumbuhan Mikroba vs SVI
Hubungan F/M ratio, DO dan SVI
Oksigen harus terdifusi kedalam flok agar dapat berkontak dengan mikroorganisme
dalam flok tersebut. Tingkat kedalaman penetrasi oksigen kedalam flok tergantung
pada bulk konentrasi oksigen di sekitar liquid dan laju utilitas oksigen pada flok. Laju
utilitas oksigen sebanding dengan dengan beban organik (F/M). Oleh karena itu,
apabila beban organik meningkat, DO yang dibutuhkan untuk membentuk flok secara
total aerobik juga meningkat.
RAS / WAS
Return
Activated Sludge
Waste
Activated Sludge
RASWAS
: Settled
: Sebagian
Sludge yang
lagidikembalikan
dibuang untuk
ke tangki
supaya
mengimbangi
activated pertambahan
solid cukup untuk
MLSS
mengatasi
di tangki
BOD
yang datang
RAS dan WAS
Jika rate WAS terlalu tinggi maka akan terjadi
straggler flok. Hal ini karena banyak sludge
yang masih terlalu “muda”
Jika rate WAS terlalu rendah maka
akan terjadi pin flok. Hal ini karena
banyak sludge yang terlalu “tua”
DISSOLVED OXYGEN (DO)
❑
Kebutuhan oksigen yang
diperlukan
untuk
respirasi
mikroorganisme dan oksidasi
lainnya.
❑ Secara
teoritis
dapat
ditentukan
dengan
mengetahui BOD air limbah
dan jumlah mikroorganisme
(MLSS) yang dibuang dari
sistem lumpur aktif setiap
hari.
❑ Suplai oksigen harus dapat
mencukupi kebutuhan BOD
limbah.
Akibat defisiensi DO
Filamentous
bulking
Kebutuhan
oksigen
untuk
oksidasi
substrat
=
Oksigen
untuk
sintesa
dan
respirasi
energi
+
Oksigen
untuk
respirasi
+
endogenous
Kebutuhan
oksigen =
( So – S) Qo + a
untuk
respirasi
Oksigen
untuk
oksidasi
biomass
+
Oksigen
untuk
oksidasi
inert
Laju
produksi
biomass
Laju
- b produksi
inert
SOUR (Specific Oxygen Uptake Rate)
SOUR adalah parameter yang menggambarkan
kecepatan mikroorganisme dalam mengkonsumsi
oksigen terlarut (DO) pada periode waktu tertentu
•
•
•
Cara mengukur SOUR :
Menetapkan periode waktu (t1 dan t2) untuk
pengukuran DO pada sampel
Mengukur DO pada waktu t1 dan t2
Mengukur MLVSS dalam sampel
Rumus perhitungan SOUR :
consumption
MLVSS mg/L
SOUR (Specific Oxygen Uptake Rate)
Contoh perhitungan SOUR :
Suatu sampel didapat dari tes parameter pada sebuah proses
pengolahan limbah.Oksigen level pada sampel di monitor setiap
interval 30 detik. Setelah 10 menit, DO level dibawah 1 mg/L.
Penngurangan oksigen dari tiap pembacaan rata-rata 0.40 mg/L.
MLVSS telah dianalisa dan tercatat sebesar 3500 mg/L. Hitung
SOUR?
Pertama, DO level tercatat tiap 30 detik, maka rata-rata harus dikalikan
2 untuk mendapatkan konsumsi oksigen.
MLVSS mg/L
MLVSS mg/L
SOUR (Specific Oxygen Uptake Rate)
>20 High
Hal ini mengindikasikan sludge tidak cukup
untuk mendegradasi BOD yang tersedia
(terjadi pada fase Logarithmic growth,
Flagellates, dispersed flock, settling slow).
12 – 20 Normal Untuk interval ini biasanya menghasilkan
BOD removal yang bagus dan lumpur
yang mudah mengendap dalam clarifier
(Declining growth, Ciliates, Flocks forming,
Settling normal).
<12 Low
Hal ini mengindikasikan bahwa terlalu
banyak sludge BOD sedikit atau terdapat
racun dalam sistem (Endogenous Respiration,
Rotifers, Pin Flock, dan Settling Fast,).
SOUR (Specific Oxygen Uptake Rate)
SOUR tertinggi
saat fase log
SOUR rendah saat
fase endogenous
Temperature
Pertumbuhan
mikroorganisme sebagai
reaksi metabolisme
Sebagai
reaksi
kimia,
kenaikan temp. 10oC akan
menaikkan
laju
reaksi
hingga 2 x nya.
Pada temp. terlalu tinggi, sistem enzym akan pecah
mikroorganisme mati, atau terjadi Denaturasi ,
Mikroorganisme : 35o – 40o C >> mesophilic
: 55o – 65o C>> thermophilic
pH
pH berpengaruh pada laju
pertumbuhan
mikroorganisme
Umumnya
pertumbuhan ideal
pada pH : 6.0 - 8.0
Perubahan pH sangat berpengaruh pada laju
removal dari subsrat (organic compound)
Beberapa mikroorganisme
dapat hidup pada pH < 6
seperti Fungi.
SAVE THE WORLD !!!
Terima kasih
Model Reaktor Biologis
Pengolahan Limbah Industri
TK 141282
Departemen TEKNIK KIMIA, FTI-ITS
Kuliah Gasal 2017/2018
MODEL REAKTOR BIOLOGIS
1. REAKTOR BATCH
Reaksi mengikuti model kinetika MONOD, maka
MAT. BALANCE substrat:
dS
koXS
−
=
dt Y (Km + S )
(1)
MAT. BALANCE Biomassa :
dX
koXS
=
- kd X
dt
(Km + S )
(2)
Utk penyederhanaan, dimana respirasi
endogenous diabaikan, maka pers (1) dan (2)
dapat diintegrasikan
0
0
0
0
0
0
0
0
S
X
+
Y
S
X
+
Y(S
−
S)
k0
t
(
X
+
Y
S
)
0
ln
ln S = ln S + Y ( S − S ) 0 +
−
0
X YKm
X
YKm
(3)
X
0
YKm
YS
ln
ln X = ko t + ln X0 −
0
0
0
X + YS X YS 0 + X0 − X
(4)
2. Well-mixed Reactors tanpa recycle
Well- mixed reactor adalah tergolong reaktor alir,
shg aliran keluar reaktor mempunyai komposisi
yg sama dg fluida dlm reaktor ( Gambar dibawah )
Q0 S0,X0
Q0 , S, X
V,S.X
Mat. balance sekitar reaktor:
dS
0 S0 - Q0 S + r V
V
=
Q
utk.M.B Substrat: dt
s
(5)
dX
V
= Q0 X0 - Q0 X + rx V
dt
(6)
utk. biomassa :
Bila rate reaksi mengikuti model kinetika Monod :
rs = -
k 0 SX
Y ( Km + S )
k 0 SX
rx = - kd X
( Km + S )
(7)
(8)
Substitusi ekspresi rate, maka M.B. menjadi
dS
ko SXV
0
0
0
V
=Q S -Q Sdt
Y (Km + S )
(9)
dX
ko SXV
0
0
0
V
=Q X -Q X- kd X V
dt
(Km + S )
( 10 )
Pers (9) dan (10) utk unsteady-state well-mixed reactor
Bila reaktor beroperasi pd kondisi steady-state
( konstan S0, X0, dan Q0 ), maka M.B menjadi:
ko SXV
=0
Y (Km + S )
( 11 )
ko SXV
0
0
0
Q X -Q X +
- kd X V = 0
(Km + S )
( 12 )
Q0S0 - Q0 S -
Bila konsentrasi sel dlm aliran feed diabaikan, maka
balance biomassa menjadi:
ko SXV
0
-Q X +
- kd X V = 0
(Km + S )
( 13 )
Bila reaktor beroperasi di phase pertumbuhan
logaritmik dimana respirasi endogenous
dianggap kecil, maka balance biomassa
disederhanakan sbb :
ko SXV
0
-Q X +
=0
(Km + S )
( 14 )
Bila M.B ditulis dlm bentuk residence
(Θ = V/Q0 ), maka :
ko SXθ
=0
Y(Km + S )
( 15 )
ko SXθ
X0 − X +
- kd X θ = 0
(Km + S )
( 16 )
S0 − S +
Bila tidak ada mikroorganisme yg masuk bersama feed, maka
kondisi ini mengakibatkan rate aliran kritis dimana
mikroorganisme terbawa aliran keluar (washed out) dari
reaktor lebih cepat d.p yg terbentuk karena reaksi.
Pada kondisi washed out, konsentrasi mikroorganisme reaktor
turun sampai nol dan tidak ada konversi substrat.
Utk X0 = 0 di pers ( 16 ), maka
1
θ=
koS/(Km + S) − kd
( 17 )
Residence time kritis utk washed out terjadi bila S = S0
θW =
1
koS 0 /(Km + S0 ) − kd
( 18 )
Bila residence time <=
Km + S 0
θW =
koS 0
θW
terjadi wash out
( 19 )
Utk reaktor yg beroperasi stabil pada residence
time > θW
maka Pers. (5-17) dapat diatur menjadi :
S=
(1 + kd θ )Km
k 0 θ - 1 - kd θ
( 20 )
3. WELL-MIXED REACTOR DENGAN RECYCLE
Tujuan utama recycle adalah untuk meningkatkan
konsentrasi biomass dlm reaktor.
Penambahan dari suatu aliran recycle juga akan
mengencerkan substrate yg masuk dan menurunkan
residence time element-2 dlm fluida di reaktor.
Qo+Qr
Qo,So,Xo
Se=S
S,X
Reactor
V,S,X
Qr,S,Xr
Qe,Xe
Clarifier
Qw,Xr
Diagram dari Well-mixed reactor with recycle
Material balance sekitar reaktor :
Accumulation = Feed input + recycle input –
output + Formation by reaction
dS
koSXV
V = Qo So + Qr S − (Qo + Qr)S −
dt
Y(Km + S)
( 21 )
dX
koSXV
V
= QoXo + Qr Xr − (Qo + Qr)X +
- kdXV
dt
(Km + S)
( 22 )
R=Qr/Qo
Residence time based on fresh feed : θ = V/Qo
Material balance menjadi :
dS 1
koSXV
( 23 )
= ( So - S) dt
Y(Km + S)
dX 1
koSX
= [ Xo + RXr − (1 + R) X ] +
− kd X ( 24 )
dt
Km + S
Residence time based on fresh feed adalah
konstan utk flow rate feed masuk.
True residence time adalah V/(Qo+Qr), berubah
dengan berubahnya recycle ratio dari proses.
Untuk suatu steady-state reactor, material
balance sbb :
koSX
( So - S) =0
Y(Km + S)
koSX
Xo + RXr − (1 + R ) X +
− kd X = 0
Km + S
( 25 )
( 26 )
Bila biomassa masuk bersama fresh feed
diabaikan, maka material balance utk
padatan biologis adalah:
koSX
RXr − (1 + R) X +
− kd X = 0
Km + S
( 27 )
Apabila reaktor beroperasi pada phase
pertumbuhan biomassa , dimana phase
endogenous respiration tdk begitu berpengaruh,
maka material balance biomassa sbb :
koSX
RXr − (1 + R) X +
=0
Km + S
( 28 )
Biasanya reaktor biologis menghasilkan padatan
biologis yg berlebih, shg harus dibuang dari
sistem.
Besaran biomass berlebih yg terbentuk sama dg
net growth biomassa dlm reaktor
ko S X V
Net biomassa =
− kdXV
Km + S
( 29 )
Excess biomass ini biasanya dibuang dari
separator yang menghasilkan padatan pekat utk
dikembalikan ke sistem sbg recycle.
Dua Parameter operasi yg secara luas digunakan
dlm design dan operasi sistem pengolahan
biologis adalah :
- Process loading Factor ( U ):
yaitu massa dari substrate yg dikonsumsi
selama periode tertentu oleh bioamassa dlm
reaktor atau Substrate removal velocity atau
food to microorganism ratio.
ko S X V
Y ( Km + S )
koS
U=
=
XV
Y ( Km + S )
- Solid residence time
( 30 )
m
adalah ratio jumlah padatan dalam sistem terhadap
jumlahan rate sintesa biomas dan padatan masuk pada
fresh feed.
Bila padatan diasumsikan sangat banyak di reaktor, dan
reaktor adalah well-mixed, maka true mean residence time
utk conventional activated sludge process menjadi:
XV
m =
koXSV
XoQo +
( Km + S )
( 31)
Pada kondisi steady –state, pembagi di pers. ( 31) sama
dg rate dimana biomas dibuang dari sistem oleh
outflow dan endogenous respiration.
XV
m =
XoQo + QeXe + kdXV
( 32 )
Dimana : Qw = flow rate dari waste dari recycle line
Qe = overflow rate dari clarifier
Xe = konsentrasi biomas di overflow dari clarifier
Biasanya mean cell-residence time belum dipakai secara
luas karena harga kd seringnya tidak diketahui untuk
aliran limbah masuk. Oleh karenanya, sebagai gantinya
sludge age sering dipakai, dimana menyatakan ratio
biomas di reaktor terhadap net pembentukan biomas, yg
dinyatakan sbb :
XV
Km + S
c =
=
koSXV
− kdXV ko S - kd (Km + S)
Km + S
( 33 )
Process loading factor berhubungan dg sludge
age melalui parameter kinetika Y dan kd, sbb :
1
= YU − kd
c
( 34 )
Pers. (30) dan (34) menghasilkan :
Y U Km Km + Km koc
S=
=
ko - Y U ko c - kd c - 1
( 35 )
Pada kondisi steady-state, net rate
pembentukan biomas sama dg rate dimana
biomas keluar dari sistem.
Bila biomas dibuang dari pembuangan recycle
line dan dari overflow clarifier, maka sludge age
adalah :
XV
c =
QwXr + QeXe
( 36 )
Untuk menggambarkan effek variabel proses dlm
performance reaktor, 2 model separator ideal sering
digunakan.
1. Separator yang memberikan harga konstan dari
ratio output konsentrasi terhadap konsentrasi
padatan input
Xr
=
= kons tan
X
Type ini dapat dicontohkan sbg tangki
sedimentasi.
2. Separator yang diasumsikan memberikan
konsentrasi biomas konstan pada aliran
recycle , spt . Xr = konstan.
Bila ratio konsentrasi melalui separator diasumsi
konstan, maka biomassa Pers ( 28 ) menjadi :
R ( − 1) − 1 + Km + S
koS
Km(1 + R - R)
S=
ko - (1 + R - R)
= 0 atau
( 37 )
Konsentrasi biomassa steady-state dpt dihitung
dari Pers ( 25 ) :
(So - S)(Km + S ) Y
X=
ko S
( 38 )
Pada kondisi wash-out, konsentrasi biomas jatuh
sampai 0 dan tdk ada konversi substrate yg
terjadi dlm reaktor.
Critical fresh-feed residence time utk wash-out
didpt dg mensubstitusi S= So dlm pers, ( 37 ) :
(Km + So)(1 + R + R)
w =
So ko
( 39 )
Tdk ada konversi substrate utk fresh feed residence
time ≤ w
TANGKI AERASI YANG DISUSUN SERIE
S*, X*
Qo + Q r
Qe
Qo, So
1
n
N
Xo
Se
SN, XN
Qr, Xr, Sr
Qw, Xr, Sr
Neraca massa substrat, misal untuk tangki aerasi ke –
n:
Substrat dan biomass masuk = Sn-1 ; Xn-1
Substrat dan biomass keluar = Sn ; Xn
Akumulasi = Substrat masuk - Substrat keluar – yang terurai
Untuk waktu tinggal dengan basis feed masuk awal, t = Vn / Qo
0=
1+R (S
n-1 – Sn ) -
tn
Ko Sn Xn
……………( 40 )
Y( Km + Sn )
Neraca massa untuk biomass untuk reaktor ke-n :
0=
1+R (X
n-1 – Xn ) -
tn
Ko Sn Xn
Km + Sn
- Kd Xn
……………( 41 )
Atau :
( Sn-1 – Sn ) -
Ko Sn Xn tn
Y( Km + Sn ) ( 1 + R )
=0
……( 42 )
( Xn-1 – Xn ) -
Ko Sn Xn tn
Y( Km + Sn ) ( 1 + R )
Kd Xn tn
=0
(1+R)
……( 43 )
Konsentrasi substrat dan biomass masuk ke reaktor 1 :
S* =
So + R Sr
1+R
X* =
Xo + R Xr
1+R
……( 44 )
Contoh : Tangki aerasi bersifat teraduk
sempurna
( well mixed reactor )
a.
b.
a.
Air Limbah masuk ke unir IPAL tipe activated sludge dengan
kadar BOD5 300 mg/liter. Sebagian sludge dikembalikan ke
tangki aerasi dari separator dengan laju aliran 0,013 m3/detik (
0,3 mgd) dengan kadar biomass 6000 mg/liter. Apabila
konversi reduksi substrat diingini 95%, maka tentukan volume
reaktor pada :
Aerator teraduk sempurna
Bila digunakan 3 unit reaktor teraduk secara serie
Solution :
Untuk tangki aerasi yang teraduk ( teraerasi ) sempurna.
Dari neraca massa untuk substrat :
( So – Sn ) -
k oS X t
Y( Km + S )
=0
Neraca biomass dengan Xo = 0
R Xr - ( 1 + R ) X +
k oS X t
=0
Km + S
R = 0,3/2 = 0,15 ; Xr = 6000 mg/liter ; So = 300 mg/liter ;
S = 0,05(300) = 15 mg/liter
Atau
{ 300 – 15 } -
0,4 (15) X t
0,6 (75+15)
=0
……( a )
Dan
0,4 (15) X t
0,15 (6000) – (1+0,15) X +
75+15
Dari ( a) dan ( b ) :
X = 930 mg/l
t = 2,76 jam
= 0 ……( b )
V = t. Qo = 2,76 x 0,088 x 3600
= 874 m3 ( 2,3 x 105 gal )
b.
Reaktor Seri
Reaktor ke 1
( So – S1 ) -
( Xo – X1 ) -
Ko S1 X1 t1
=0
Y( Km + S1 ) ( 1 + R )
Ko S1 X1 t1
=0
Y( Km + S1 ) ( 1 + R )
Reaktor ke 2
( S1 – S2 ) -
( X1 – X2 ) -
Ko S2 X2 t2
=0
Y( Km + S2 ) ( 1 + R )
Ko S2 X2 t2
=0
Y( Km + S2 ) ( 1 + R )
Reaktor ke 3
( S2 – S3 ) -
( X2 – X3 ) -
Ko S3 X3 t3
=0
Y( Km + S3 ) ( 1 + R )
Ko S3 X3 t3
=0
Y( Km + S3 ) ( 1 + R )
Konsentrasi masuk ke reaktor ke -1 :
S* =
X* =
So + R Sr
=
300 + 0,15 (15)
1+R
1 + 0,15
Xo + R Xr
0 + 0,15 (6000)
1+R
=
1 + 0,15
Semua reaktor dengan volume yang sama, hingga t1 = t2 = t3
Dan diingini S3 = 15 mg/l.
Di sini ada 6 variabel yang belum diketahui, yaitu S1, S2,
X1, X2, X3, dan t
Dapat diselesaikan dengan trial and error, yaitu asumsi
harga t dan dari 6 persamaan di atas, di cari harga S3
sampai ≈ 15, maka diperoleh :
t = t1 = t2 = t3 = 0,42 jam
Ttotal = 1,26 jam
V1 = V2 = V3 = 132 m3
Vtotal = 396 m3 ( 105.000 galon )
Misal pada tipe Oxidation Ditch. Pola aliran dari
Mammoth rotor hingga ke mammoth rotor
berikutnya mengikuti pola aliran plug (plug flow),
yaitu pada semua titik searah arah aliran
mempunyai laju aliran yang sama.
L
Qo
So
Xo
Q o + Qr
Si, Xi
RAS
Qr, Xr, Sr
S
X
S + dS
X + dX
dL
Misal : diambil satu bagian tipis dari unit tangki
aerasi, dengan panjang dL
L = panjang reaktor
Se
Xe
Neraca massa substrat pada unit aerator dengan panjang dL
Akumulasi = masuk – keluar – yang hilang karena terurai
…...(45)
……………….....(46)
Dengan cara yang sama untuk biomass :
……(47)
dV = A dL
Dimana A = luas penampang reaktor searah aliran
Konsentrasi substrat dan biomass masuk ke sistem
QoSo + QrSr = (Qo+Qr)Si dan QoXo + QrXr = (Qo + Qr)Si
Diketahui bahwa R=Qr/Qo maka :
Pada L=0
Dan
Pers (46) dan (47) merupakan persamaan diferensial non
liniar sehingga sulit untuk diselesaikan
secara analitik, karena
setiap persamaan mengandung dua variabel tidak diketahui, yaitu
S dan X. Sehingga harus menggunakan teknologi komputasi.
Secara analitik dapat diselesaikan bila salah satu variabel
tersebut diasumsi konstan, misal biomass X.
Disini, biomass yang terbentuk relative tidak besar
dibanding biomass masuk, sehingga konsentrasi biomass
sepanjang reaktor dianggap konstan, yaitu Xa
……(49)
Dimana : A.L = V
Perubahan walaupun kecil dari biomass sepanjang
reaktor, dapat diestimasi dari :
……(50)
Substitusi harga (Si – Se) dari pers (51) ke (52)
……(51)
Lawrence dan McCarty (1969) mengasumsi bahwa :
1. Konsentrasi
mikroorganisme
dalam
influent
reaktor diperkirakan sama dengan pada effluent
reaktor. Asumsi ini valid hanya apabila tc/t>5
2. Laju reduksi substrat rs adalah :
Contoh : Aerasi Aliran Plug (Plug-Flow Aerator)
Data seperti pada contoh sebelumnnya, tetapi
digunakan tangki aerasi tipe aliran plug
Solution :
Dari neraca massa untuk substrat :
Neraca biomass :
Pada V=0
Substitusi data-data ini ke neraca massa :
Karena jumlah biomass yang terbentuk dalam reaktor relative besar
terhadap biomass yang masuk, persamaan dapat diselesaikan secara
numerik. V volume reaktor atau A dL maka S dan X dapat dihitung
X = 930 mg/l
V = 288 m3 (76.000 gal)
t = 0,91 jam
Secondary Treatment Proses Biologis
(Anaerob)
Prof.Dr.Ir. Tri Widjaja, M.Eng.
DEPARTEMEN TEKNIK KIMIA, FTI-ITS
Anaerobic Digestion
Merupakan
salah
satu
proses
pengolahan sludge dari clarifier tanpa
mengalirkan
menguraikan
udara
(oksigen)
senyawa
untuk
organik.
Berdasarkan bakteri yang digunakan,
proses anaerobik ini
terbagi menjadi
dua, yaitu :
1. Mesophilic Digestion
Merupakan proses dimana sludge disimpan dalam tangki yang
cukup besar selama beberapa minggu (15-30 hari) pada kisaran
suhu 20oC-40oC. Pada proses ini digunakan bakteri mesophilic
2
yang dapat bertahan pada suhu tersebut.
Anaerobic Digestion
2. Thermophilic Digestion.
Merupakan proses dimana sludge difermentasikan dalam tangki
dengan kisaran suhu mencapai 70oC menggunakan bakteri
thermophilic dengan variasi waktu tergantung dari jumlah feed
dan temperature yang digunakan. Proses ini lebih cepat
dibandingkan mesophilic digestion,biasanya waktu yang
diperlukan hanya sekitar 2 minggu (14 hari).Namun
demikian,Proses ini lebih mahal karena memerlukan lebih banyak
energi untuk proses pemanasan guna mengaktifkan bakteri yang
digunakan.
3
4
Tahapan proses pengolahan limbah
secara anaerobik
Biodegradable Particulates
1
1
Proteins and Carbohydrates
Lipids
1
1
Volatile Acids
(propionic, butyric)
1
Amino acids and
simple sugars
Long chain fatty acids
2
2
2
Acetic acid
3
H2 and CO2
4
5
CH4 and CO2
Protein
Bakteria
Hidrolytic
Polimer Kompleks
Karbohidrat
Asam-asam amino,
gula
Bakteria
Acidogenic
Proses
Hidrolysis
Asam-asam lemak,
alkohol
Produk Intermediate:
Propionat, Butirat,
Valerat
Asetat
Aceticlastic
Methanogens
Lipids
Proses
Acidogenesis
H2 , CO2
Metana
CO2
Proses
Methanogenesis
Hydrogenophilic
Methanogens
6
Proses Anaerobik
Senyawa Organik Kompleks
Bakteri Anaerobik
(Acidogenesi)
Asam-asam Organik, Alkohol, CO2 + H2
Bakteri Pembentuk Metan
(Methanogenesis)
CH4 + CO2 + H2O
7
Mekanisme Fermentasi Anaerobik
Fermentasi anaerobic dibagi 4 grup microorganisme (MO):
- Mo Hydrolitic mengurai senyawa polymer (polysakarida dan
protein menjadi monomer).
Penguraian ini dihasilkan tanpa penurunan COD.
- Mo Acidogenic, dimana monomer dikonersi menjadi volatile
fatty acid (VFA) dengan sejumlah kecil H2.
VFA (acetat, propionat, dan butirat dengan sejumlah kecil
valerat). Proses acidifikasi reduksi COD adalah minimal.
- Dan dalam hal terbentuk %H2 reduksi COD yang terjadi
jarang melebihi 10%. Seluruh asam yang lebih tinggi dari
asam acetat dikonversikan menjadi acetat dan H2 oleh
mikroorganisme acetogenic.
8
Konversi asam propionat (acetogenic mo):
C3H6O2
+ 2H2O
→ C2H4O2 + CO2 + 3H2O
➢ Rekasi ini, reduksi COD terjadi pada pembentukan H2 sangat
rendah.
➢ Reaksi terjadi hanya jika konsentrasi H2 sangat rendah.
➢ Asam asetat dan H2 dikonversi menjadi CH4 oleh
microorganisme methanogen
Dilanjutkan konversi Asam asetat (methanogenic mo):
C2H4O2
→ CO2 + CH4
CH3COO- + H2O → CH4 + HCO3-
Hidrogen :
HCO3-
+
4H2 → CH4 + 2H2O + OH-
9
• Spesifik activity proses pengolahan limbah industri
dapat larut mendekati 1 kg COD utilized/(kg
biomass.d).
• Ada 2 kelas methanogen yang akan mengkonversi
acetat menjadi methan, yaitu methanotorhix dan
methanosarcina.
• Methanotorhix mempunyai spesific activity rendah
sehingga akan menguasai sistem tinggi.
• Methanosarcina akan mempunyai spesific activity
lebih tinggi 3-5 kali methanotorhix jika adanya trace
nutrient. Trace nutrient itu adalah besi, cobalt, nikel,
molybdenum, selenium, kalsium, magnesium &
microgram perliter level vitamin B12.
• Pada suhu dan tekanan standart 0,454 kg COD
atau penurunan BOD pada proses ini akan
menghasilkan 0,16 m3 methan.
10
Faktor Yang Mempengaruhi Proses Operasi
• Process anaerobik berfungsi efektif pada 2 range suhu, range
mesophilic 85 - 100F (29 - 38C) dan range termophilic 120 135F (49 - 57C).
• Walaupun laju reaksi lebih besar pada range termophilic akan
tetapi perlu pertimbangan membutuhkan biaya lebih besar.
• Organisme methan bekerja pada range pH 6,6 – 7,6 dengan
pH optimum 7.
• Bila laju pembentukan asam melampaui laju pemecahannya
menjadi methan, proses akan menjadi tidak seimbang dimana
pH akan turun, produksi gas berkurang dan kandungan CO2
pada gas naik.
11
Umur sludge terhadap lb SS/lb BOD
Jumlah produksi sel selama fermentasi akan tergantung pada12
kekuatan limbah, karakter limbah dan retention sel
•Proses ANAEROBIK SENSITIF terhadap pH
•Untuk proses peruraian yang baik, pH antara 6.5 – 7.5
•Bila pH turun maka proses menjadi tidak seimbang
•Volatie acid yang terbentuk berlebihan akan merusak
kapasitas buffer dari alkalinitas di dalam sludge yang
berakibatkan pH dan produksi gas menjadi turun
•Selama sludge memiliki alkalinitas tinggi, peningkatan
produksi asam akan memberi sedikit efek pH sehingga
pengukuran volatile acid dapat digunakan sebagai
parameter contoh yang lebih baik
13
➢Konsentrasi normal dari volatile acid adalah 250 -1000
mg/l, bila > 2000 mg/l maka akan terjadi masalah
➢Kapur biasa digunakan untuk memulihkan degredasi bila
produksi acid berlebih, tetapi perlu diingat akan terjadi
kalsium karbonat
➢Na bicarbonat dapat digunakan sebagai alternatif,
biasanya alkalinitas bicarbonat pada range 2500-5000 mg/l
untuk penyediaan bufer.
➢ Bila alkalinitas tidak cukup, dapat menambahkan
alkalinitas dari limbah masuk atau dapat dikontrol dg
mengurangi rate feed
14
Alkalinitas dibutuhkan untuk menetralisir VFA
dan H2CO3 yang berlebihyang dihasilkan dari
tekanan CO2 yang tinggi dalam reactor
Asam inorganic dengan konsentrasi rendah dapat
diberikan sebagai perangsang (stimulan), pada
konsentrasi tinggi menyebabkan toksik.
Pada beberapa kasus, adaptasi akan menaikkan tingkat
toleransi mikroorganisme
Keberadaan antagonis ion dapat mereduksi inhibitory.
Penambahan 300 mg/l Kalium dapat mereduksi 80 %
pengaruh inhibitory 7000 mg/l. Inhibitory akan dapat
dieleminasi secara lengkap bila ditambahkan 130 mg/l
Kalsium. Tetapi kalsium tanpa Kalium tidak ada artinya.
15
KEBUTUHAN ALKALINITAS
16
Produk samping “Anaerobic Digestion”
Dari pengolahan sludge secara an aerobic ini dihasilkan produk
samping sbb :
1. Biogas
Merupakan campuran gas yang terdiri dari metana dan
karbondioksida dengan sedikit kandungan gas hydrogen
2. Produk Fertilizer
Merupakan hasil samping olahan sludge yang dapat digunakan
sebagai pupuk.Kualitas dari produk ini tergantung dari komposisi
sludgeJika sludge mengandung sedikit bahan organik sintetik atau
logam berat beracun, maka perlu sedikit treatment untuk kemudian
dihasilkan fertilizer.
17
Anaerobic Digestion:
Bagaimana prinsip kerjanya??
Green Power
Anaerobic Environment
CH4
High BOD
Waste
Organics
→ Acids
Acids →
CH4
Cogeneration
Hot Water
High Nutrient
Low Odor
Waste
Cogeneration atau gabungan panas dan daya (CHP) adalah penggunaan mesin
panas atau pembangkit listrik untuk menghasilkan listrik dan panas yang
bermanfaat pada saat yang sama.
19
Pembangkit Listrik biogas (methane) dari sludge
limbah air buangan
20
Sistem operasi “Anaerobic Digestion”
Beberapa sistem yang digunakan dalam operasi anaerobik, yaitu :
1. Sistem Batch.
Merupakan proses yang paling sederhana dengan menambahkan
feed (sludge) selama periode waktu tertentu.
2. Sistem Continuous.
Merupakan proses yang sering digunakan dimana feed (sludge)
dimasukkan dan dikeluarkan dari reaktor secara terus menerus
(continuous).
21
Proses Anaerobik
• Keuntungan Anerobik:
– Produksi Bahan Padat Rendah;
– Kebutuhan Nutrient Rendah;
– Produk Akhir Metan Punyai Nilai Sebagai Energi ;
Beban Organik Tinggi Memungkinkan.
• Kerugian Anerobik:
– Pertumbuhan Mikroba Rendah;
– Menghasilkan Bau;
– Kebutuhan Bufer Tinggi Untuk Kontrol pH; dan
– Efsiensi Removal Rendah Dengan Limbah Encer.
22
Proses Anaerobik
Beban Untuk Anaerobik
5 – 10 kg COD/m3.hari,dengan COD > 5000 mg/l;
Beban Untuk Aerobik
= < 1 kg COD/m3.hari
Tujuan Anaerobik :
⚫Stabilisasi Bahan
⚫Energi Rendah
Menggunakan Proses Untuk Mereduksi BOD
(>80 %)
⚫Digunakan dengan BOD > 10.000 mg/l
⚫Keuntungan :
-Produksi Gas (Energi)
-Reduksi Total Padatan (VS destruction)
23
BOD/COD ratio karakteristik efluent:
24
Konversi Polutant Organik Menjadi Biogas
Oleh Microorganisme Anaerobik
25
COD Balance
26
Aerobic Treatment
27
Pengolahan An/Aerobik
• Anaerobik: COD Dalam Air Limbah Sebagian
Besar Diubah Menjadi Methan, Sebagian Kecil
COD Diubah Menjadisludge.
• Aerobik: COD Dalam Air Limbah sebagian besar
diubah menjadi sludge, waste product, Yang
Mempunayai Biaya Tinggi Untuk Mengolahnya.
28
Konfigurasi BioReaktor
Proses Kontak Anaerobik
• Kapasitas Organic Loading: 2-5 kgCOD/
m3.hari
• HRT 0,5 hari
• SRT 4 hari
• Removal 91-95 %
• Design Internal Mixing
• Biaya Operasi Rendah
29
Upflow Packed Bed
(Anaerobic Filter)
• Menggunakan Media (Padatan Rock, Plastik)
• COD: 375 – 12000 mg/l
• HRT (Detention Time) = 4 -36 jam
30
Anaerobic Contact Process
Digunakan untuk menyediakan dan mensirkulasikan
bibit organisme
❖ Retention Time= 6-12 jam
❖
❖
Beroperasi optimum pada suhu 32oc
31
Anaerobic Filter
32
Anaerobic Contact Process
33
Fluidized Bed Reactor
Air limbah dipompa
menuju sand bed yang
terdapat mikrobanya
✓ Effluent direcycle
kembali menuju fluidized
bed
✓ Efisiensi removal sekitar
80%
✓
34
Fluidized Bed Anaerobic
Reactor
35
upward-flow anaerobic sludge
blanket (UASB) reactor
• Granules = Acetate Utilising Methanogen
(Methanothrix dan Methanosarcina)
• Partikel Aktif = 1 -2 g COD/gVSS.hari
• Organic Loading = 4 – 15 kg COD/m3.hari
36
upward-flow anaerobic sludge
blanket (UASB) reactor
• Air Limbah Mengalir Keatas Melalui Sludge
Bed Anaerobic Dimana mikroorganisme Dalam
sludge Kontak Dengan Substrat Air Limbah.
• Sludge Bed Tersusun Dari Mikroorganisme
Secara Alami Membentuk granules (pellets)
Diameter 0.5 to 2 mm, Mempunyai Kecepatan
Sedimentasi Tinggi, Tahan “wash-out” Dari
System Bahkan Pada Beban hydraulik Tinggi
37
Lanjutan
• Hasil Proses Degradasi Anaerobik Adalah
Untuk Produksi gas (e.g. biogas
Mengandung CH4 dan CO2)
• Gerakan Keatas Gelembung Gas,
Menyebabkan Turbulensi hydraulik,
Memberikan Mixing Pada Reactor Tanpa
Bantuan Proses Mekanik.
38
Lanjutan
• Bagian Atas Reaktor, Air Dipisahkan Dari
Padatan Sludge Dan Gas (Pemisahan
Tiga Fase).
• Pemisahan Tiga Fase Berupa “gas cap”
Dengan Settler Dipasang Diatasnya.
Dibawah Bukaan Gas Cap, Baffle
Digunakan Untuk Deflect Gas.
39
40
41
Animasi UASB
42
An expanded granular sludge bed
(EGSB)
• Reaktor Expanded Granular Sludge Bed
(EGSB) Adalah Variasi Dari Konsep UASB
• Kenaikan Flux Membuat Expansi
(fluidization) Granular Sludge Bed,
Meningkatkan Kontak Air Limbah-sludge
Juga Meningkatkan segregasi Partikel
Suspensi inactive Ukuran Kecil Dari
sludge bed.
43
Lanjutan
• Kenaikan Kecepatan Alir Juga Dilakukan
Dengan Mengatur Ketinggian Reaktor,
Atau Dengan Sirkulasi Effluent (Atau
Keduanya).
• Design EGSB Sesuai Untuk Air Limbah
Beban Rendah (Kurang Dari 1 - 2 g COD
Terlarut/l)
44
An expanded granular sludge bed
(EGSB)
• Kapasitas Organic Loading Tinggi ( 15-35
kg COD/m3.hari)
• Granular Biomass Mudah Mengendap
•
•
Produksi Methane Sebagai Energy
Biaya Operasi Murah
45
46
47
48
49
50
Pengolahan Limbah Industri klas A
Nitrogen Removal
Departemen Teknik Kimia FTIRS ITS 2022
Pendahuluan
Limbah Cair & Nitrogen
NH4+
N
NO3-
N
NO2-
N
Pendahuluan
Limbah Cair & Nitrogen
LIMBAH
N2 Terlarut
N2 dalam Protein
NH4+ & NH3
NO2- & NO3-
Pendahuluan
Limbah Cair & Nitrogen
Siklus Nitrogen
Udara sekitar kita mengandung 79% Nitrogen,
dimana Nitrogen ini diubah menjadi protein oleh
tumbuhan melalui proses fiksasi biologis yang
mana nantinya tumbuhan tumbuhan ini
dikonsumsi oleh hewan maupun manusia dan
disimpan dalam bentuk protein dan dibuang dalam
proses ekskresi pada feses dan urine dalam bentuk
Ammonia dan NH3 , selain dari proses ekskresi
manusia dan hewan sumber kandungan nitrogen
pada limbah biasanya bersumber dari sampah
dapur dan buangan industri. Ion Ammonia
nantinya akan dikonsumsi oleh bakteri/organisme,
dan kembali menjadi gas N2 setelah melewati
proses denitrifikasi bakteri.
Pendahuluan
Dissolved
Oxygen
Depletion
Methemoglobinemia
Masalah yang
timbul dari
polusi
Nitrogen
Eutrophication
Toxicity
Proses Transformasi N2
▪ Proses Amonifikasi
+ Definisi : Proses degradasi senyawa organik berikatan N sehingga terjadi pembebasan amonia,
sehingga organisme pengurai akan menguraikan senyawa-senyawa organik berprotein, menghasilkan amonia.
+ Media : Sedimen, Air, Tanah, dan proses biologi
+ Peran pada lingkungan : Degradasi senyawa organik kompleks bernitrogen seperti protein,
menghasilkan senyawa karbon organik sebagai penyedia energi dan berfungsi sebagai substrat untuk sintesis
senyawa organik dan sebagian amonium sebagai nutrient bagi pertumbuhan sel bakteri yang baru.
+ 3 cara Proses Amonifikasi terjadi :
- Dari senyawa ekstraselular yang mengandung senyawa nitrogen organik, secara kimia atau
biokimiawi (misal: urea).
- Dari sel-sel bakteri selama respirasi endogen.
- Dari sel-sel yang mati dan lisis.
+ Faktor yang memengaruhi proses :
- pH limbah
- Temperature
- Waktu penyiimpanan limbah
Proses Transformasi N2
▪Proses Nitrifikasi
+ Definisi : Pada proses nitrifikasi terdapat dua tahap proses, yang dilakukan oleh dua tipe
bakteri kemotrofi (bakteri yang memperoleh energi dari reaksi eksotermis nitrifikasi).
Bakteri Nitrosomonas
Proses Transformasi N2
▪Proses Nitrifikasi
Bakteri Nitrobacter
Proses Transformasi N2
▪Proses Nitrifikasi
+ Faktor lingkungan yang mempengaruhi laju proses nitrifikasi, yaitu:
- Reaksi bersifat aerobik, sehingga apabila konsentrasi O2 turun dibawah 2 mg/l, laju
reaksi menjadi turun drastis.
- pH optimum reaksi antara 8 dan 9, dan pH dibawah 6 akan menghentikan reaksi.
- Organisme nitrifikasi cenderung menempel pada sedimen atau permukaan zat
padat.
- Laju pertumbuhan organisme nitrifikasi lebih rendah dari pertumbuhan dekomposer
heterofilik, sehingga jika konsentrasi zat organik mudah urai tinggi, bakteri
heterotrop akan menghambat pertumbuhan nitrifier dan proses nitrifikasi
terhambat.
- Suhu optimal antara 20 – 25 0C.
Proses Transformasi N2
▪Proses Nitrifikasi
Proses Transformasi N2
▪Proses Denitrifikasi
+ Definisi : Denitrifikasi adalah proses reduksi nitrat menjadi gas nitrogen (N2)
secara biologis pada kondisi anoxic (tanpa oksigen). Denitrifikasi dilakukan oleh
bakteri heterotrofik (pseudomonas).
Pada denitrifikasi nitrat diubah menjadi nitrit yang kemudian diubah menjadi gas
nitrogen, seperti reaksi dibawah ini :
6NO3- + 2CH3OH → 6NO2- + 2CO2 + 4H2O
6NO2- + 3CH3OH → 3N2 + 3CO2 + 3H2O + 6OH-
Proses Transformasi N2
▪Proses Denitrifikasi
Bakteri Pseudomonas
Proses Transformasi N2
▪Secara keseluruhan proses nitrifikasi-denitrifikasi tersebut dapat ditulis
sebagai berikut :
Proses Nitrogen Removal
▪ Proses Bardenpho
▪ Proses A2/O
▪ Proses UCT (University of Cape Town)
▪ Proses VIP (nama untuk Virginia Initiative Plant di Norfolk, Virginia)
▪ Proses SBR (Sequencing Batch Reactor)
▪ Proses Phostrip
▪ Proses Anoxic / anaerobic
Proses Bardenpho
Proses Bardenpho untuk removal nitrogen
dimodifikasi (Phoredox Modification) untuk
mengkombinasi removal nitrogen dan
phosphorus. Proses terdiri dari 5 tingkat
zone anaerobic, anoxic, dan aerobic dan
dilengkapi dengan internal resirkulasi.
Proses A2/O
Modifikasi dari proses A/O untuk removal nitrogen adalah proses A2/O. Diagram
proses terdiri dari anaerobic, anoxic, dan aerobic dengan resirkulasi sludge dan
resirkulasi internal. Proses mempunyai stabilitas dan menghasilkan effluent
dengan kualitas tinggi.
Proses UCT
▪ Modifikasi proses UCT untuk removal nitrogen dengan menggunakan reaktor
anaerobic, anoxic, dan anaerobic secara bergantian baik lumpur aktif dan
kandungan dari tangki aerasi diresirkulasi kedalam zone anoxic, dan kandungan
dari zone anoxic kemudian diresirkulasi kedalam zone anaerobic.
Proses VIP
▪ Proses VIP pada dasarnya hampir sama dengan proses A2O dan UCT kecuali
untuk metode resirkulasi sludge. Diagram proses diberikan pada gambar. Pada
proses terdapat konfigurasi satu tingkatan reaktor yang di pergunakan oleh
paling tidak dua sel campuran pada kondisi seri untuk setiap zone dari reaktor
biologic.
Proses SBR (Sequencing Batch Reactor)
▪ SBR dapat dipergunakan untuk
menghilangkan nitrogen dan phsphorus.
Hilangnya phosphorus dan penggunaan
sejumlah BOD terjadi selama pengisian dan
pengadukan pada operasi anaerobik.
Pengambilan phosphorus, oksidasi BOD,
dan nitrifikasi terjadi pada kondisi siklus
aerobik. Denitrifikasi terjadi selama siklus
pengadukan anoxic dan siklus
pengendapan
Proses Phostrip
▪ Proses Phostrip mempunyai tangki yang berfungsi sebagai tangki stripping dimana sebagian dari
return sludge mengalami diversifikasi. Effluent primer dapat juga ditambahkan pada return sludge
unutk memberikan sumber karbon yang diperlukan untuk pelepasan phosphorus. Nitrogen
dihilangkan secara denitrifikasi pada kondisi anaerobik, dan phosphorus dilepas ke dalam liquid.
Padatan biologis dipisahkan dan dikembalikan ke dalam proses. Supernatan yang kaya akan
phosphorus dikoagulasi untuk memisahkan phosphorus.
Proses Anoxic/Anaerobic
▪ Reaktor anoxic/anaerobic/aerobic dilengkapi dengan resirkulasi dari tangki
pengendapan menuju tangki anoxic. Dari hasil study yang dilakukan
menunjukan bahwa sejumlah total nitrogen (TN) dan total phosphorus (TP)
secara significant dapat dihilangkan dengan menggunakan fasilitas BNR. Return
sludge yang tinggi mampu meningkatkan resirkulasi nitrogen dan menurunkan
kadar nitrogen melalui denitrifikasi dalam reaktor anoxic. Pengaruh keberadaan
konsentrasi karbon biogredable yang cukup tinggi di return sludge
mempengaruhi pelepasan total phosphorus di reaktor anoxic, anaerobic dan
pengambilan phosphorus di reaktor aerobic
TERIMAKASIH
KARAKTERISTIK ACTIVATED SLUDGE
Kuliah PLI
Semester Gasal 2020/2021
ACTIVATED SLUDGE
- Pengolahan limbah secara biologis yang
terjadi dengan reduksi kandungan bahan
organik menggunakan “jasa” mikroorganisme
- Mikroorganisme menjadi kunci keberhasilan
proses pengolahan, baik dari segi jenis,
kesehatan M.O dan laju pertumbuhannya
KARATERISTIK ACTIVATED SLUDGE
1. Warna Sludge
2. Zone Settling Velocity
3. Volume of Settled Sludge (Sludge Volume Index/SVI)
4. Sludge Age (Umur Sludge)
5. F/M Ratio
6. MLSS/MLVSS
WARNA SLUDGE YANG SEHAT
Berwarna kehitaman hingga coklat dan berbau khas tanah
Supernatant berwarna jernih dengan tanpa sedikitpun
partikel floc
WARNA SLUDGE YANG BAIK
ZONE SETTLING VELOCITY (ZSV)
Representasi laju maksimum sedimentasi
Nilai ZSV berbanding lurus dengan kualitas
activated sludge
Ditunjukkan oleh slope dari bagian linear kurva
sedimentasi
KURVA SEDIMENTASI
TABEL KARATERISTIK SLUDGE BERDASARKAN NILAI ZSV
Tipe Sludge
ZSV pada 3.5 g/L (m/h)
Well settling
>3
Light
2-3
Bulking
<1.2
SLUDGE VOLUME INDEX
(SVI)
SVI menunjukkan
kemampuan sludge
untuk settling
Tipe Sludge
Well Settling
Light (encer)
Bulking
SVI ( mL/g )
<80
80-150
>150
Settled Sludge VolumeΤSample Volume setelah 30 menit, mLΤL 1,000 mg
SVI, mLΤg =
x
Suspended Solids Concentration, mgΤL
gram
Supernatan
Sludge
SLUDGE AGE
Nama lain: Sludge (atau Solids) Retention Time (SRT),
Mean Cell Residence Time (MCRT)
Ukuran lama sludge berada di bawahaerasi
Suspended Solids di Tangki Aerasi
Sludge Age =
Rate Suspended Solids yang keluar dari Tangki Aerasi
MLSS
mg
L x V (L)
Rate Suspended Solids yang keluar dari Tangki Aerasi [Xw
mg
L
x
Qw
L
Jam ]
MATERIAL BALANCE DI ACTIVATED SLUDGE
F/M RATIO
Rasio makanan (BOD) yang masuk ke sludge dan
mikroorganisme (MLVSS) di tangki aerasi
Setiap jenis proses activated sludge memiliki range nilai
F/M tertentu
MLSS/MLVSS
MLSS (Mixed Liqiuor Suspended Solids) mengandung sebagian
besar mikroorganisme yang bertugas untuk mengolah limbah
MLVSS (Mixed Liquor Volatile Suspended Solids) jumlah dari
organik dan volatile solid tersuspensi
Sludge yang sehat memiliki nilai MLSS antara 1000-4000 mg/L
FILAMENTOUS BULKING AND
FOAMING
MIKROORGANISME BERFILAMEN
Bentuk panjang dan tipis
Membantu pembentukan flok (dalam
konsentrasi kecil)
Mengganggu pengendapan di secondary
clarifier (dalam konsentrasi besar)
Penyebab Filamentous Bulking dan Foaming
KARAKTERISTIK FILAMENTOUS BULKING
SVI tinggi
Gumpalan besar namun tidak dapat mengendap
Bila terdapat supernatant, biasanya jernih
Sifat settling dipengaruhi jenis mikroorganisme
filamen
ACTIVATED SLUDGE
Struktur Floc yang Baik
Struktur Floc yang Buruk
Jumlah dan
panjang filamen
dihitung
menggunakan
mikroskop electron
oleh Palm.
HUBUNGAN SVI DENGAN PANJANG
FILAMEN
Palm, J.C.; Jenkins, D.; and Parker, D.S.
1980. Relationship between organic
loading, dissolved oxygen concentration
and sludge settleability in the completelymixed activated sludge process. Journal
of the Water Pollution Control Federation.
52(10):2484-2506.
PENYEBAB TUMBUHNYA BAKTERI BERFILAMEN
F/M ratio rendah (< 0,2-0,3)
DO rendah (< 2mg/L)
Defisiensi nutrisi (N dan P)
SVI tinggi (>150mL/g)
Kandungan sulfide, karbohidrat, dan asam lemak tinggi
JENIS FILAMEN
(MODEL CHIESA AND IRVINE)
• Fast Growing zoogleal (floc forming) bacteria
• Resisten terhadap kurangnya makanan
• Aktivitas metabolism berkurang pada DO rendah
• Slow growing starvation resistant filament
• Nilai afinitas substrat tinggi
• Nilai Ks rendah
• Fast growing starvation susceptible filament
• Afinitas untuk DO tinggi
• Resisten tehadap nilai DO yang rendah
JENIS FILAMEN
Tipe Filamen
Kondisi yang Berkaitan
Yang umum:
Thiothrix II
Busuk; kandungan nutrisi rendah (N)
Thiothrix I
Busuk; Busuk; kandungan nutrisi rendah (N)
Nostocoida limicola II
Busuk
Tipe 0914
Busuk
H. Hydrossis
Oksigen terlarut rendah
Nostocoida limicola III
Busuk, kandungan nutrisi rendah (P)
Tipe 1851
Organik Loading rendah (F/M rendah)
Tipe 1701
Oksigen terlarut rendah
Tipe 021N
Busuk; kandungan nutrisi rendah (N)
JENIS FILAMEN
Tipe Filamen
Kondisi yang Berkaitan
Kurang Umum:
Tipe 0092
Busuk
Tipe 0411
Busuk
Tipe 0675
Organik Loading rendah (F/M rendah)
Sphaerotilus natans
Oksigen terlarut rendah
Tipe 0041
Organik Loading rendah (F/M rendah)
Tipe 0581
Busuk
Tipe 0803
Organik Loading rendah (F/M rendah)
Tipe 0211
Busuk
PENYELESAIAN JANGKA PENDEK
Membantu settling (pengendapan) pada secondary clarifier dengan:
• Polymer
• Lime
• Ferric Chloride
Menambah Toxic Agents (untuk membunuh bakteri penyebab Bulking)
Zat Oksidan (klorin/hipoklorit, peroksida)
2 – 10 lb Cl2 /hari/1000 lb MLSS
pH shock (penambahan zat asam)
Mengatur debit RAS (Sludge juggling)
PENYELESAIAN JANGKA PANJANG
Menghilangkan penyebabnya
Mengatur DO, F/M ratio, septisitas, nutrisi
Low F/M Problems : Bisa dilakukan dengan meningkatkan rasio F/M (M yang
dinaikkan), misal menggunakan selektor. Selektor adalah bak pencampuran antara
RAS dengan aliran limbah masuk (sebelum masuk bak aerasi). Kegunaan selektor
adalah untuk menciptakan suatu kondisi dimana menumbuhkan bakteri pembentuk
tetapi mencegah pertumbuhan filamen.
Nutrient Deficiency: Diukur kadar TIN (total Inorganic Nitrogen, minimal 1 mg/L)
dan kadar ortho-phosporus (minimal 0,5-1 mg/L). Penambahan nutrien ini
disesuaikan dengan kadar BOD yang ada dalam air limbah.
PENYELESAIAN JANGKA PANJANG
Low Dissolved Oxygen Problem: Pada bak aerasi, kadar DO yang diberikan ke limbah
dinaikkan konsentrasinya. Bisa juga dengan menurunkan rasio F/M, baik dengan
meningkatkan MLSS atau dengan meningkatkan RAS.
Wastewater Septicity and Organic Acids: Aliran limbah yang masuk (pra-aerasi) bisa
diberi bahan kimia oksidator (misal chlorine) atau presipitat kimia (ferric chloride). Bila
kadar septisitas tidak bisa diturunkan, maka model bak aerasi bisa diubah (bisa stepaeration atau mixed-aeration) untuk meminimalisir kontak antara biomasa dengan
bahan septisitas.
FILAMENTOUS FOAMING
FILAMENTOUS FOAMING
Masalah
Penyebab
Aksi Korektif
Foam tebal berminyak berwarna
gelap menutupi permukaan
aeration basin dan terbawa
hingga clarifier
Organisme berfilamen
(Nocardia, M. parvicella)
Meningkatkan laju WAS (tidak lebih dari 10%
per hari) untuk mengurangi MCRT.
Pengendalian filament normal dengan klorin
atau peroksida harus menyertakan treatment
(di semprotan air) dan penghilangan buih di
permukaan. Periksa MLVSS dan F/M ratio
untuk optimasi parameter proses.
Foam berbusa berwarna coklat
gelap (hampir hitam) dengan
bau busuk atau asam. Mixed
liquor juga berwarna coklat
gelap ke hitam
a) Kondisi anaerob di
aeration basin
b) Limbah mengandung
pewarna atau tinta
a) Periksa tingkat DO di basin, dan tingkatkan
aerasi/pencampuran. Mengurangi organic
loading jika dimungkinkan.
b) Periksa ulang strategi pre-treatment
Foam berwarna coklat muda
dalam jumlah rendah
Ini merupakan tanda dari
proses yang berjalan
dengan baik.
FILAMENTOUS FOAMING
Masalah
Penyebab
Foam putih, kaku,
a) Shock akibat start up atau BOD
mengepul atau berbuih
tinggi sehingga F/M menjadi tinggi
yang melingkupi sebagian
dan MCRT rendah
besar atau seluruh
b) Wasting yang berlebihan atau
aeration basin
hydraulic washout
c) Limbah beracun atau temperature
shock
d) RAS terlalu rendah
e) Lemak dairy, deterjen atau bahan
foaming lain berlebih
Aksi Korektif
a) Meningkatkan RAS atau menurunkan WAS.
Pertahankan DO level (1-3 mg/L)
b) Mengurangi wasting dan mengatur RAS hingga
kondisi normal. Mengalihkan aliran yang
berlebih ke collection basin untuk treatment
selanjutnya. Menambah hydraulic equalization
basin.
c) Membentuk kembali organisme activated
sludge. Melakukan bioaugmentasi.
Mengembalikan suhu normal atau mengatur
kondisi MCRT.
d) Mengatur ulang laju RAS
e) Pre-treatment dengan anti-foam atau DAF.
Menghilangkan minyak. Mempertimbangkan
bioaugmentasi untuk mendegradasi limbah
secara agresiif
FILAMENTOUS FOAMING
Masalah
Penyebab
Aksi Korektif
Foam mengkilat, tipis, coklat
gelap di sebagian besar
permukaan aeration basin
Aeration basin menuju ke kondisi
under loaded (F/M rendah)
karena kurang sludge wasting
Meningkatkan WAS hingga
proses kembali ke parameter
kontrol normal dan hanya sedikit
foam coklat muda yang tersisa.
Cek MLVSS, F/M dan MCRT untuk
dioptimalkan.
Foam tebal berminyak berwarna
coklat gelap melingkupi hamper
seluruh permukaan aeration
basin
Aeration basin secara kritis under Meningkatkan WAS hingga
loaded (terlalu banyak solid)
kelebihan solid terbuang dari
sistem dan mencapai
kesetimbangan. Cek MLVSS, F/M
dan MCRT untuk dioptimalkan.
PENANGANAN FILAMENTOUS FOAMING
Metode Non-spesifik
Metode Spesifik
PENANGANAN SECARA NON-SPESIFIK
▪Pengaturan operasional ( menurunkan MCRT)
▪Penambahan Struktur ( Penggunaan Selector)
▪Pengaturan konsentrasi DO pada pre-oxidation reactor
▪Pengukuran non spesifik – aplikasi steam
▪Skimming system
▪Penggunaan Water Sprays
▪Pump Inlet system
PENANGANAN SECARA NON-SPESIFIK
•
PENGATURAN OPERASIONAL (MENURUNKAN MCRT)
Penurunan pada MCRT (Mean Cell Residence Time) adalah salah satu
metode paling efektif untuk menekan pertumbuhan mikroorganisme
filamentous (M.Parvicella).
Pengaturan operasional pada MCRT berbeda-beda, tergantung pada
jenis mikroorganisme yang ditangani.
M.Parvicella → Penurunan MCRT 8-10 hari
Nocardia → Penurunan MCRT <3hari
PENANGANAN SECARA NON-SPESIFIK
•
PENAMBAHAN STRUKTUR (PENGGUNAAN SELEKTOR)
Selektor adalah tangki berpengaduk dimana RAS dan limbah masuk (Influent)
dicampur sebelum diteruskan ke tangki aerasi. Kegunaan selektor adalah untuk
menciptakan suatu kondisi dimana menumbuhkan bakteri pembentuk tetapi
mencegah pertumbuhan filamen.
Mekanisme selektor adalah dengan menseleksi organisme pembentuk flok dengan
mikroorganisme penyebab foam.
Selektor dibagi atas 3:
• Selektor Anoksi
• Selektor Aerobic
• Selektor Anaerobic
PENANGANAN SECARA NON-SPESIFIK
• Selektor Anoksi
Pada keadaan tanpa oksigen (O2) dimana nitrat sebagai
pengganti oksigen dalam akseptor elektron.
• Selektor Aerob
Memanfaatkan Oksigen sebagai akseptor elektron.
• Selektor Anaerob
Kondisi dimana tidak adanya zat kimia yang terlarut maupun
berikatan dengan unsur O .
PENANGANAN SECARA NON-SPESIFIK
•
PENGATURAN KONSENTRASI DO
Menurunnya konsentrasi DO dapat memicu tumbuhnya bakteri
penyebab foaming maupun bulking.
Penambahan sejumlah konstrasi DO dapat dilakukan dengan
melakukan injeksi pada bagian tangki Aerasi dengan
menggunakan Blower.
PENANGANAN SECARA NON-SPESIFIK
•
PENGATURAN KONSENTRASI DO
PENANGANAN SECARA NON-SPESIFIK
•
PENGUKURAN NON-SPESIFIK (APLIKASI STEAM)
Hoyle et al., 2006
PENANGANAN SECARA NON-SPESIFIK
•
SKIMMING SYSTEM
Sistem Skimming merupakan salah satu bagian dari proses Activated
Sludge sehingga lapisan foam yang terbentuk di bagian atas tangki
aerasi dapat dipisahkan.
Semua benda terapung dapat dipisahkan dengan sistem skimming.
Pemisahan dengan sistem skimming hanya efektif apabila foam yang
terbentuk memiliki ketebalan tidak lebih dari 3cm.
PENANGANAN SECARA NON-SPESIFIK
•
SKIMMING SYSTEM
PENANGANAN SECARA NON-SPESIFIK
•
PENGGUNAAN WATER SPRAYS (SURFACE OVERFLOWS)
Menyemprotkan sejumlah air ke dalam tangki aerasi dari bagian
bawah hingga overflow dan dilakukan skimming untuk membuang foam
yang terbentuk di bagian atas liquid.
PENANGANAN SECARA NON-SPESIFIK
•
PUMP INLET SYSTEM
Pompa digunakan untuk memindahkan sejumlah material yang
terdapat di permukaan liquid dan meneruskannya ke tangki
pembuangan.
Desain dari pompa harus terintegrasi dengan peralatan pengendalian
pompa, dimana pompa harus dapat memindahkan benda terapung
(floating material), dan mencegah terbentuknya vortex.
PENANGANAN SECARA NON-SPESIFIK
•
PUMP INLET SYSTEM
PENANGANAN SECARA SPESIFIK
•
PENGGUNAAN BAHAN KIMIA ( CHEMICAL METHODS)
Penggunaan bahan kimia pada pengendalian foaming bertujuan untuk
membunuh sejumlah mikroorganisme penyebab foaming.
Penggunaan dosis bahan kimia haruslah tepat dimana dosis bahan
kimia tersebut hanya dapat membunuh mikroorganisme penyebab
foaming tanpa membunuh mikroorganisme penyebab flok.
Penggunaan bahan kimia tidak ada yang mencapai efektivitas 100%.
PENANGANAN SECARA SPESIFIK
•
JENIS BAHAN KIMIA YANG SERING DIGUNAKAN
PENANGANAN SECARA SPESIFIK
•
KEKURANGAN METODE KIMIAWI
• Naiknya kelarutan COD karena chlorination treatment
• Meningkatnya yield Trihalomethanes (THM) ketika raw water bereaksi
dengan Chlorine
• Tingginya Chlorine dapat mempengaruhi kinerja bakteri autotrophic
dan heterotrophic
• Penggunaan dosis kimia seperti NaClO dan (SO4)3Al2 tidak efektif
untuk pengendalian jangka pendek.
• Tidak Efektif untuk pengendalian Nocardia
• Susah dalam menentukan dosis bahan kimia
PENANGANAN SECARA SPESIFIK
•
SECARA MIKROBIOLOGIS
Dilakukan dengan cara menurunkan jumlah sel mycolata dengan
fase litik. Bakteriofag berkembang biak di dalam sel bakteri dan
menyebabkan lisis (hancurnya dinding sel bakteri).
Sangat efektif karena hanya merusak mikroorganisme penyebab
foaming tanpa mempengaruhi kinerja mikroorganisme pembentuk
flok.
Masih dalam skala laboratorium, untuk skala industri masih belum
dapat diaplikasikan.
PENANGANAN SECARA SPESIFIK
•
SECARA MIKROBIOLOGIS
Kuliah PLI Semester Gasal 2020/2021
OUTLINE
I
Pendahuluan
II
Waste Stabilization Ponds
III
Constructed Wetlands
2/3 Negara
Berkembang
1/3
negara
maju
Negara2 maju memiliki TIDAK COCOK UNTUK
NEGARA BERKEMBANG
pengolahan air limbah
dengan teknologi canggih
dan otomatis
Anaerobic sludge digesters
Bailonggang Wastewater Treatment Plant
Shanghai Cina
Wastewater Stabilization
Ponds (WSP)
LOW
operation
Costs
Simple
Constructions
Environmentally
Friendly
Wastewater Stabilization
Ponds (WSP) atau kolam
stabilisasi air limbah
merupakan kolam dangkal
yang menggunakan proses
fisis dan biologis untuk
mengurangi kandungan
bahan pencemar yang
terdapat pada air limbah.
TERDAPAT PROSES
Pengendapan partikel padat
Penguraian zat organik
Pengurangan nutrien (P dan N)
Pengurangan organisme patogenik seperti
bakteri, telur cacing,dll
Prinsip dasar WSP
1. Menyeimbangkan dan menjaga fluktuasi beban organik dan beban hidrolis
limbah air,
2. Mengendapkan partikel padatan dari air limbah di kolam pertama,
3. Memanfaatkan proses fotosintesis yang dilakukan oleh algae sebagai sumber
utama oksigen,
4. Proses penguraian zat organik secara biologis yang dilakukan oleh
mikroorganisme (baik secara aerobik maupun anaerobik), dan
5. Pengurangan organisme patogenik melalui beberapa proses interaktif antara
alga dan bakteria.
Anaerobic
Ponds
Penggolongan WSP
Facultative
Ponds
Maturation
Ponds
Kolam anaerobik (anaerobic ponds)
Partikel padat (material organik) yang dapat terurai secara biologis dapat
mengendap dan diuraikan melalui proses anaerobik oleh bakteri. Kolam anaerobik
tidak memiliki oksigen terlarut dan algae. Kolam ini mempunyai kedalaman 2 - 5
meter dengan masa tinggal hidrolis (hydraulic retention time) antara 1 - 20 hari.
Kolam fakultatif (facultative ponds)
Kolam fakultatif biasanya mempunyai kedalaman berkisar 1 - 2 meter dengan proses
penguraian secara aerobik dibagian atas dan penguraian secara anaerobik di lapisan
bawahnya. Jenis kolam ini mempunyai masa tinggal hidrolis antara 5 sampai 30 hari.
Penggunaan kolam fakultatif bertujuan untuk menyeimbangkan input oksigen dari proses
fotosintesis alga dengan pemakaian oksigen yang digunakan untuk penguraian zat organik.
Kolam pematangan (maturation ponds)
Kolam pematangan adalah kolam dangkal dengan kedalaman hanya 1 - 1,5 meter.
Tujuan : agar keseluruhan kolam dapat ditumbuhi oleh alga sehingga oksigen yang dihasilkan
selama proses fotosintesis dapat dipergunakan untuk proses penguraian secara aerobik. Kolam
ini digunakan untuk memperbaiki kualitas air yang dihasilkan oleh pengolahan di kolam
fakultatif dan untuk mengurangi jumlah organisme patogenik.
1
Series
OR
Parallel
1
2
2
Typical Ponds System
#1
X
#3
Influent
Effluent
X
X
#2
SERIES OPERATION
Typical Ponds System
#1
Influent
#3
Effluent
X
X
#2
PARALLEL OPERATION
Hubungan Zona pada Ponds
SUN
O2
Dike
Zone
1 Aerobic
(Dissolved Oxygen Present)
O2
2 Facultative Zone
Zona adaptasi (Perubahan), tetap berdekomposisi
3 Anaerobic Zone (No Dissolved Oxygen Present)
Sludge
Influent
Sludge
Algae and Aerobic
Bacteria
Influent
Facultative
Bacteria
Anaerobic Bacteria
Effluent
Faktor Yang Mempengaruhi Proses Pengolahan
“Pengaruh Angin”
Meningkatkan kandungan oksigen
Membantu proses pencampuran
Harus dikontrol
Dengan meminimalisasi akumulasi
baik di dalam, permukaan dan
sekitar kolam
Faktor Yang Mempengaruhi Proses Pengolahan
SUN
“Pengaruh Sinar Matahari”
Membantu proses fotosintesis
Desinfeksi
Harus dikontrol
Dengan meminimalisasi akumulasi
baik di dalam, permukaan dan
sekitar kolam
Faktor Yang Mempengaruhi Proses Pengolahan
“Pengaruh Suhu”
Mempengaruhi aktivitas bakteri
Pertumbuhan alga
Penyerapan oksigen terlarut
Aerobic
Facultative
Anaerobic
O2
Faktor Yang Mempengaruhi Proses Pengolahan
SUN
“Pengaruh Fluktuasi Harian”
Suhu
Dissolved Oxygen
pH
Aerobic
Facultative
Anaerobic
O2
PHOTOSYNTHESIS
CO2
Sumber O2 : absorbsi dari atmosfer
Fungsi O2 : Mencegah bau
efisiensi treatment
Kelebihan dari WSP meliputi :
• Kesederhanaan dalam desain dan konstruksi
• Menghasilkan biological sludge rendah
• Modal, operasi dan biaya pemeliharaan yang rendah
• Efisiensi pengolahan air limbah tinggi jika dirancang dengan baik
• Kuat dan relatif dapat diandalkan
Kekurangan dari WSP meliputi :
• Kebutuhan lahan besar untuk kolam
• Akumulasi Sludge akan lebih tinggi di iklim dingin karena berkurangnya mikroba
• Nyamuk dan serangga lainnya dapat berkembang biak jika vegetasi tidak dikendalikan
• Jika tidak dirancang dengan baik dapat menyebabkan masalah bau
• Sulit untuk mengontrol atau memprediksi kadar amonia dalam limbah
Wetland merupakan suatu rawa buatan yang di buat untuk
mengolah air limbah domestik, untuk aliran air hujan dan
mengolah lindi (leachate) atau sebagai tempat hidup
habitat liar lainnya, selain dapat juga digunakan untuk
reklamasi lahan penambangan atau gangguan lingkungan
lainnya.
Dongtan Wetland Park, dikembangkan oleh Shanghai Dongtan
International Wetland Co, Ltd. Sebuah taman ekologi lahan
basah dengan konservasi alam, penelitian ilmiah, ilmu
pemasyarakatan, pariwisata ekologi dan resort rekreasi.
(Foto: shcm.gov.cn)
O2 pada udara
Proses Pengolahan sistem wetland
Proses-proses yang terjadi dalam
sistem pengolahan air limbah dengan
memanfaatkan
tanaman
air
:
Respirasi melalui
daun batang,akar
dan rizhoma
tumbuhan air
(dilepaskan pada area
perakaran(rizosphere)
Proses Fisika
• Sedimentasi
• Filtrasi
Proses Fisika &
Kimia
Proses
Biokimiawi
• Mekanisme
removal adsorpsi
• Presipitasi fosfor
• Logam berat
• Penurunan
bahan organik
• Nitrifikasi
• Denitrifikasi
• Dekomposisi
anaerobik
• Penyerapan
tumbuhan air
Manfaat Wetland yaitu :
1.)Organic Carbon (BOD) Removal
2.)Nitrogen Removal
3.)Phosphorus Removal
4.)Trace Metals Removal
5.)Removal of Toxic Organic Compounds
Kelemahan :
Akibat bahan kontaminan yang tertinggal di area, wetland tersebut
harus terus dimonitoring untuk memastikan kondisi kestabilan dan
kondisi lingkungannya. Jika bahan konsentrasi pencemaran
meningkat, efek toxic dari bahan organik maupun anorganik dari
limbah dapat menghambat pertumbuhan tanaman tersebut.
• BOD removal
Proses penghilangan BOD dengan cara pengendapan dan penyaringan particulat BOD,
kemudian partikulat BOD dikonversi menjadi soluble BOD dengan cara hidrolisis
Soluble BOD didegradasi dengan cara penambahan pertumbuhan mikroba (biofilm pada
akar, batang, partikel kerikil,dll)
• Suspended Solid removal
Penghilangan suspended solid terjadi dengan cara pengendapan dan filtrasi dengan jarak
beberapa meter mendekati inlet
• Nitrogen removal
Nitrifikasi/denitrifikasi didalam biofilm
Plant Uptake (Penyerapan tanaman)
Volatilisasi ommonia pada pH > 8,5
• Phosphorus removal
Plant uptake
Penyerapan tanah (adsorption)
Presipitasi dengan Ca, Al dan Fe
• Pathogen removal
Sedimentasi/filtrasi
Predasi oleh protozoa
Absorbsi
Dari kondisi lingkungan seperti
( sinar UV, pH, dan suhu)
• Heavy metal Removal
Presipitasi dan adsorpsi
Plant uptake
• Trace Organic removal
Adsorpsi oleh partikel organik dan tanah liat/clay
• Pathogen removal
Sedimentasi/filtrasi
Predasi oleh protozoa
Absorbsi
Dari kondisi lingkungan ( sinar UV, pH, dan suhu)
Fungsi ekologi :
1.)Tempat makan dan habitat kehidupan liar
2.)Peningkatan kualitas air
3.)Perlindungan terhadap banjir
4.)Kontrol abrasi garis pantai
5.)Untuk rekreasi
Wetland sebagai Habitat :
Burung
Mamalia
Reptiles dan Amfibi
Ikan
Invertebrata
Faktor yang Mempengaruhi Sistem Wetland
Tanaman/Vegetasi
Media Tumbuh(Substrat)
Temperatur
Mikroorganisme
Kelompok macrophytes
Helophytes
Pleustophytes
Pleustophytes
Hydrophytes
Tanaman/ Vegetasi
Tanaman/ vegetasi dalam sistem Wetland mengambil peranan penting karena memiliki
beberapa fungsi sebagai berikut (Wood, 1990):
• Menyediakan kebutuhan oksigen bagi akar dan daerah perakaran dengan proses
fotosintesa, yang digunakan untuk pertanaman biologis bagi mikroorganisme yang
berada di zona akar. Dalam hal ini, tanaman memiliki kemampuan
memompa
udara melalui sistem akar.
• Menjadi komponen penting dalam proses transformasi nutrien yang berlangsung
secara fisik dan kimia mendukung proses pengendapan terhadap partikel tersuspensi.
• Proses kematian pada akar disertai pelepasan bahan organik yang mendukung proses
denitrifikasi.
• Sebagai media tumbuh mikroorganisme.
• Mendukung proses filtrasi bahan solid.
Tanaman/Vegetasi
Tanaman
yang
sering
untuk Wetland di Indonesia :
1.Anturium Merah /Kuning
2.Alamanda Kuning /Ungu
3.Akar Wangi
4.Bambu
5.Dahlia
6.Dracenia Merah /Hijau
7.Keladi
8.Lili
digunakan
9.Bintang Air dan tanaman air lainnya
(Dirjen Cipta Karya, Dep. Pekerjaan Umum, 2010).
Media Tumbuh (Substrat)
Media tumbuh dalam dapat dikelompokkan menjadi tiga, yaitu (Metcalf and Edy, 1991) :
a. Medium sand, media dengan struktur halus karena komposisi butiran lebih sedikit dari
pasir, berdiameter antara 0,04-0,11 mm dan lolos ayakan 2-20.
b. Coarse sand, media dengan struktur komposisi tanah berupa butiran besar dengan
kandungan kerikil kurang dari 15% dan pasir lebih dari 85%. Struktur media ini antara
medium sand dan gravel dan lolos ayakan 2-20.
c. Gravelly sand, media ini kombinasi antara pasir-kerikil dengan prosentase pasir 85%
dan kerikil 15%. Tanah mengandung lebih dari 70% pasir, porositas kurang dari 40%.
Mikroorganisme
Jenis mikroorganisme yang diharapkan berkembang adalah
heterotropik aerobic. Hal ini dikarenakan penguraian bahan organik dalam
tanah basah/ rawa buatan berlangsung secara aerobik dan anaerobik .
Aktifitas mikroorganisme dalam wetland dapat disamakan dengan aktifitas
mikroorganisme dalam pengolahan konvensional (lumpur aktif) dan Trickling
filter. Tumbuhan menyediakan media penyangga bagi bakteri pengurai zat
organik yang tumbuh melekat. Tumbuhan juga berfungsi menyediakan
komponen Lingkungan perairan yang dapat meningkatkan efisiensi
pengolahan (Yohanna, 2007).
Temperatur
Temperatur yang sesuai untuk Konstruksi wetland adalah 200C-300C (Wood, 1990).
Bagian-bagian pada Wetland
Udara
Tanaman air /alang-alang
Pipa
distribusi
Water
Lapisan cekungan/kolam
Zona akar
Sand filter
Pompa
Gravel + shells
Drainage
floor
Titik sampling
Tipe Desain Konstruksi Wetland
Sub Surface Flow Systems
Umumnya di Eropa
Surface Flow Systems
Lebih banyak di US/
Amerika Utara
FWS ( Free water Surface) berisi tanah sebagai tempat hidup tanaman
yang hidup pada air tergenang (emerge plant) dengan kedalaman 0,1-0,6
m. Pada sistem ini limbah cair melewati permukaan tanah. Pengolahan
limbah terjadi ketika air limbah melewati akar tanaman, kemudian air
limbah akan diserap oleh akar tanaman dengan bantuan bakteri .
Aliran air diatas
permukaan tanah
Lapisan impermeable
FWS
LAY-OUT
Sub-surface Flow System (SSF)
Proses : Filtrasi dan absorbsi oleh mikroorganisme
Media : Pasir dan kerikil dengan diameter
bervariasi antara 3-32 mm
Aliran Effluent
melalui dalam
tanah
Sub-surface Flow System (SSF)
Inlet zone (small rocks)
Gravel bed (fine gravel)
Outlet zone (small rocks)
Horizontal-SSF
Persamaan FWS & SSF
Menyerap nutrient
Menyerap nutrient
Logam berat terakumulasi
pada jaringan tanaman
Logam berat terakumulasi
pada jaringan tanaman
Habitat untuk hewan liar
Habitat untuk hewan liar
Memiliki Estetika
Memiliki Estetika
Batang : mechanical filter +
attachment pada biofilm
Sistem akar = mechanical
filter + attachment pada
biofilm
FWS( Free Water Surface)
SSF(Sub-Surface Flow System)
Membatasi pertumbuhan
algae
Sistem akar dapat
mempertahankan
konduktivitas hidrolik
Mengurangi velositas air
sehingga kecepatan
pengendapan meningkat
Transfer oksigen (aktif & Pasif)
dengan akar tanaman
mengangkut oksigen ke zona
akar, memungkinkan untuk
bertahan hidup pada kondisi
anaerob.
FWS umumnya aerobik pada
bagian lapisan atas, dan
anaerobik pada proses
sedimentasi
HSSF paling sering anaerobik,
sedangkan VF paling sering
aerobik
Aturan kebutuhan area pada jenis wetland
Type
Design area requirement
(m2/PE)
Type 1: FWS
5-10
Type 2: HSSF
3-5
Type 3: VSSF
2-3
Type 4: Hybrids (horizontal – vertical, or H-V)
2.5 – 3
(but note: better nitrogen removal
(denitrification) than VSSF)
Important:
1 PE = 1 Person Equivalent = 60 gBOD/cap/d
= 120 L/cap/d (in the Netherlands)
Design parameter
FWS
(free water
surface)
HSSF
(horizontal subsurface flow)
VSSF (vertical
sub-surface flow)
Data is for which wastewater type
Mixed domestic wastewater
Detention time (days)
5 - 14
2-7
N/A
8
7.5
4-6
Water or substrate depth (m)
0.1 – 0.5
0.1 – 1.0
N/A
Hydraulic loading rate (mm/d)
7 - 60
2 - 30
40 - 80
Area requirement (ha/m³/day)
0.002 – 0.014
0.001 – 0.007
N/A
Aspect ratio – length/width
2:1 to 10:1
0.25:1 to 5:1
N/A
Mosquito control
Required
Not required
Not required
3-5
3-5
N/A
Max. BOD loading rate (g/m2/day)
Harvest frequency (years)
greywater
Masalah pada konstruksi Wetland
Nyamuk
Solusi
•
•
•
•
•
Pre treatment untuk mengurangi organik loading rate
Pengeringan bed/wetland dapat membasmi larva
Mengurangi kedalaman air, meningkatkan kecepatan aliran
Menambahkan bahan biologi,seperti L Bacillus thuringiensis, Gambusia
Membuka area unplanted water untuk mendukung pertumbuhan
predator (ikan, invertebrata)
Proses yang terjadi filtrasi, absorbsi oleh mikroorganisme, dan absorbsi oleh akar-akar tanaman
terhadap tanah dan bahan organik
Tipe pengaliran air limbah pada umumnya secara horizontal, karena jenis ini memiliki efisiensi
pengolahan terhadap suspended solid dan bakteri lebih tinggi dibandingkan tipe yang lain. Hal
ini disebabkan karena daya filtrasinya lebih baik. Penurunan BOD nya juga lebih baik karena
kapasitas transfer oksigen lebih besar
Kelebihan pada konstruksi Wetland
•
•
Lokasi Fleksibel
Operasi dan perawatan simpel
Kekurangan pada konstruksi Wetland
•
•
•
Masalah Start-Up
Kebutuhan Ruang
Desain Wetland dan variabel kinerja
Sludge Disposal
Kuliah PLI A
Semester Gasal 2017-2018
SLUDGE DISPOSAL
• Kebanyakan proses pengolahan diterapkan pada pencemaran air
dalam industrI yang mengkontrol yield sludge dari proses
pemisahan padatan-cairan (sedimentasi, flotasi, dll).
• Padatan ini biasanya mengalami serangkaian tahap pengolahan
yang diantaranya sludge disposal dan melibatkan thickening,
dewatering, dan pembuangan akhir.
• Sifat reologi diperlukan untuk merancang pemompaan dan pipa
sistem transportasi dari sludges. Viskositas dari lumpur
bergantung pada sumber, konsentrasi, suhu, dan laju transfer
nya.
KARAKTERISTIK SLUDGE
Kuantitas dan karakteristik sludge air limbah bergantung pada limbah itu sendiri,
tipe plant air limbah.
Nilai-nilai tipikal untuk volume dan berat sludge yang dihasilkan oleh proses
pengolahan diberikan dalam Tabel berikut:
Volume (m3 sludge
per 1000 m3 water)
Mass (kg solid per
m3 water)
Primary Settling
3
0.144
Trickling Filter
0.7
0.054
Activated Sludge
19
0.216
Treatment Process
• Bila jumlah lumpur aktif sangat besar, maka volume besar harus ditangani.
Fasilitas pembuangan lumpur biasanya merupakan
40 sampai 60% dari biaya pembangunan pabrik
pengolahan air limbah.
KARAKTERISTIK SLUDGE
• Konsentrasi tipikal padatan tersuspensi lumpur limbah domestik pada Tabel:
Source of Sludge
WEIGHT % OF SOLIDS
Fresh
Thickened
Digested
Primary
2.5-6
8-10
5-12
Trickling Filter
4-8
7-10
-
Activated
0.5-1.5
2.5-3
2-3
Primary and
3-5
5-10
6-8
Activated
• Konsentrasi solid dalam lumpur primer umumnya 3 sampai 4 kali lebih besar
daripada di lumpur aktif. Meskipun konsentrasi biasanya diukur sebagai berat per
satuan volume (misalnya, mg/l), biasanya dinyatakan % berat padatan.
• Konsentrasi lumpur biasanya diukur sebagai berat per satuan volume (misalnya,
mg/l), umumnya dinyatakan sebagai % berat padatan. Dengan demikian,
konsentrasi diukur dari 10000 mg/l akan dianggap sebagai 1% berat.
• Viskositas lumpur aktif adalah sekitar 6 cp dan lumpur primer sekitar 25 cp.
• Konsentrasi lumpur dapat dikurangi dengan penambahan koagulan, tetapi dg
pertimbangan ekonomi dipilih pembuangan akhir ke tanah atau insinerasi.
KARAKTERISTIK SLUDGE
Komposisi kimia tipikal dari lumpur limbah domestik diberikan dalam Tabel:
Component
Organic matter
Total ash
Protein
Grease and fats
Cellulose
Nitrogen
Phosphorus (P2O5)
Potash (K2O)
Primary
60-80
20-40
20-30
6-35
5-15
2-4
1-3
0-1
% COMPONENT
Activated
60-75
25-40
30-40
5-12
5-15
2-6
2-7
0-2
Digested
45-60
40-55
15-20
3-20
5-15
1.5-6
1.5-4
0-2
Komposisi kimia dari lumpur berkaitan dalam memilih metode pembuangan akhir
dan dalam pengevaluasian kesesuaian untuk digunakan produk sampingan, seperti
pupuk.
Sludge Treatment
Processes
Concentration
• Sludge secara umum perlu dikonsentrasikan (menjadi
lebih kental) dengan memanfaatkan gaya gravitasi
ataupun proses flokulasi.
• Terjadinya pengentalan akan membuat volume dari
sludge menjadi lebih kecil sehingga lebih mudah
untuk ditransportasikan menggunakan pipa.
Stabilization
• Proses stabilisasi mengubah sludge menjadi bentuk yang
lebih ramah dalam hal bau, kemampuan penguraian
maupun kandungan organisme patogen.
• Terdapat 2 macam proses stabilisasi yaitu
1. Anaerobic digestion
2. Aerobic digestion.
•
Proses stabilisasi ini membentuk ulang sludge secara biologi
menjadi cairan, padatan yang terlarut dan gas.
➢Anaerobic digestion menghasilkan gas metana yang dapat digunakan
sebagai sumber energy
➢Aerobic digestion akan memberikan paling sedikit masalah operasional
Berikut ini adalah contoh unit anaerobic disgestion
berdasarkan jumlah stage;
1. Anaerobic single-stage disgestion, dan
2. Anaerobic two-stage disgestion.
1. Anaerobic single stage
2. Anaerobic two stage
Conditioning
• Conditioning adalah proses awal dari sebuah sludge yang bertujuan untuk
menghilangkan air dalam proses pengentalan ataupun dewatering.
• Proses ini dilakukan dengan 2 cara, yaitu:
1.
Coagulants
2.
Heat Treatment
Koagulan:
menggunakan
coagulants
bertujuan
untuk
inorganic
coagulants
menggabungkan
maupun
organic
partikel-partikel
kecil
menjadi lebih besar
Heat treatment: dengan memanaskan sludge under pressure untuk
membunuh sel patogen, sludge yang telah dipanasi telah steril dan tak
berbau serta dapat disaring tanpa memerlukan zat kimia
Dewatering
• Proses dewatering dilakukan untuk mengurangi kandungan air
dari sludge sampai level dimana sludge dapat diproses sebagai
padatan daripada cairan.
• Sebagai semi-padatan, sludge menjadi bentuk yang lebih simpel
khususnya dalam hal area, spreading, incineration, heat drying
ataupun trasportasi.
• Pada
umumnya,
vacuum
filtration
dan
centrifugation
(merupakan proses mekanik) lebih banyak digunakan pada
proses dewatering sludge
• Berikut adalah contoh unit dewatering:
1. Belt Filter Press
2. Filter Press
3. Centrifugal Dewatering
Belt Filter Press
Filter Press
Centrifugal Dewatering
Metode Sludge Disposal
❖ Thermal Process
❑ Heat Drying
❑ Incineration
❑ Wet Air Oxidation
Heat Drying
• Proses ini digunakan untuk mempersiapkan sludge yang akan dijual
sebagai pupuk atau untuk proses insinerasi.
• Kandungan air dalam sludge yang telah dikeringkan pada umumnya kurang
dari 10 %.
• Apabila dried sludge diinsinerasi, maka untuk energi drying dapat disuplai
dari proses insinerasi.
• Apabila sludge dijadikan pupuk, maka energi untuk mengevaporasi air
didapat dari bahan bakar.
• Metode untuk untuk heat drying yang biasa digunakan adalah flash dries
dan rotary drum
Metode Heat Drying
❑ Flash Driers
❑ Rotary Drum
• Pada metode ini, sludge
• Rotary drum merupakan sebuah silinder
partikel terdispersi dalam
yang berputar dengan kecepatan
aliran gas panas, dan
disalurkan melalui suatu
saluran, dimana kebanyakan
drying terjadi.
• Cyclone merupakan alat
untuk memisahkan dried
solid dengan uap
rendah
• Sludge memasuki bagian ujung atas
dan disalurkan dengan gravitasi
berlawanan, ke aliran gas panas.
• Meskipun sludge drying terjadi dibawah
suhu 500°C pada kedua proses di atas,
gas keluaran harus dipanaskan dengan
suhu sekitar 650-750°C untuk mematikan
bau yang tidak sedap
Insinerasi
o Proses ini membakar material organic yang ada dalam sludge
dan memproduksi inert ash.
o Bagian yang mudah terbakar dari sludge dibawah 75%, dan abu
yang tersisa untuk pembuangan sludge.
o Energi dibutuhkan untuk memanaskan sludge, menguapkan air,
dan memanaskan aliran uap, yang didapat dari pembakaran
sludge solids dimana dapat mencukupi suplai energi.
o Sludge primer dan sekunder yang melebihi kadar 30% solid
memiliki nilai bahan bakar yang cukup menjadi sustainable.
o Digested sludge memiliki nilai bahan bakar rendah dan biasanya
dibutuhkan dalam bahan bakar tambahan.
Metode Insinerasi
❑ Multiple Hearth Incinerators
• Metode ini mengandung larutan pasir yang terfluidisasi
oleh aliran bagian atas udara.
❑ Fluidized Bed Incinerators
• Metode pembakaran yang
paling umum untuk sludge
• Sebagai metode untuk
regenerasi adsorben activated
carbon
• Cara kerja nya dapat dilihat
pada gambar berikut
• Sludge dioksidasi dengan pasir yang terfluidisasi dan
abu yang dibawa oleh exhaust gas ke dalam
scrubber.
• Suhu proses ini biasanya djaga pada range 700 -800°C
• Bahan bakar tambahan dibutuhkan untuk pemanasan
awal dari pasir pada keadaan mula mula dan untuk
mencukupi reaksi jika fuel valve pada sludge rendah
• Alat ini mengoperasikan 20 – 30% udara berlebih dan
energi nya lebih efisien dibandingkan dengan multiple
hearth incinerators.
Cross Section of a
Typical Multiple
Hearth Incinerator
Cross Section of a
Fluid bed reactor
Thermal Drying
Keuntungan
o Pengurangan sludge yang signifikan
dalam sludge volume
Kerugian
o Lebih ekonomis
• Keterbatasan produksi liquid
effluent
o Produk akhir bebas organisme
• Pelepasan dari gas ke
atmosfer
pathogen
o Produk akhir mengandung karakteristik
tanah bebas endapan kotoran
o Produk akhir cocok untuk
pembuangan sludge di tempat
pembuangan akhir, dan inserenasi
• Mudah berbau dan
menimbulkan kebisingan
Wet Air Oxidation
• Pada mulanya, wet air oxidation dikembangkan di Norwegia
untuk pengolahan limbah industri kertas.
• Lalu, diadopsi untuk pengolahan lumpur limbah di Amerika
pada tahun 1960an. .
• Wet oxidation bisa bekerja dengan baik apabila limbah cair
bersifat encer dan mudah dibakar, sedangkan racun/unsur
yang tidak bisa dibakar diolah dengen pengolahan biologis.
Wet Air Oxidation
• Proses wet air oxidation bergantung pada mudah tidaknya sebuah
unsur untuk larut atau adanya partikel organik yang ada dalam cairan
yang akan dioksidasi pada suhu kisaran 100 ◦C-374 ◦C (titik kritis air).
• Dengan suhu 374 ◦C, unsur air akan hilang, walaupun dalam tekanan
tinggi.
• Adanya oksigen yang mudah larut dalam larutan air di suhu tinggi
memudahkan proses oksidasi.
• Proses ini sangat efisien untuk penghancuran bahan organik dari
limbah dengan rentang konsentrasi kepadatan 1% -20%, sehingga
bahan organik cukup untuk meningkatkan suhu internal reaktor tanpa
adanya pasokan energi eksternal.
• Batas maksimal konsentrasi kepadatan bahan organik
adalah 200 g / L (20%), agar panas dari surplus tidak
membuat suhu reaktor internal melebihi suhu maksimal.
• Apabila suhunya melebihi suhu maksimal, maka akan
terevaporasi seluruhnya.
• Limbah lumpur organik ketika diproses dengan wet air
oxidation bisa dengan mudah dioksidasi, ataupun
sebaliknya. Unsur utama limbah lumpur organik adalah
protein, lipid, gula, dan fiber dengan total prosentase
mecapai 60% dari unsur organik.
Proses Wet Air Oxidation
Low Pressure
Oxidation
Intermediate
Pressure
Oxidation
High Pressure
Oxidation
• Variabel control wet air oxidation
Tekanan
Temperatur
Konsentrasi
Solid
Oksigen
supply
Wet Air Oxidation
• Tujuan utama dari low-pressure wet air oxidation adalah untuk mengurangi volume
lumpur agar mudah untuk dewatering. Sedangkan oksidasi dengan tekanan
menengah dan tekanan tinggi digunakan untuk mengurangi volume lumpur
melalui oksidasi bahan organik yang mudah menguap menjadi CO2 dan air.
• Walapun cukup efisien, wet air oxidation masih memiliki beberapa kekurangan dan
jika digunakan dalam skala industri membutuhkan pengoperasian yang efisien dan
juga pemeliharaan. Masalah utama wet air oxidation dalam penggunaan skala
industri adalah:
Menghasilkan bau yang tidak
enak
Korosi pada heat exchanger
dan rekator
Membutuhkan energi yang
cukup banyak untuk memulai
proses oksidasi.
Limbah cair mengandung COD
dengan kadar yang tinggi
Terdapat kandungan logam yang
tinggi pada abu sisa.
Proses Wet Air Oxidation
Proses Wet Air Oxidation
• Proses
diatas
menunjukkan
wet
air
oxidation
(WAO),
menggunakan reaktor vertikal. Limbah lumpur dipompa menuju
reaktor WAO, melewati HE untuk menaikkan suhunya.
• Reaktor limbah cair WAO dialirkan ke phase splitter, mengalirkan
lumpur untuk dewatering. Sedangkan, larutan cair dialirkan
kembali ke heat exchanger agar panasnya dapat ditransmisikan
ke lumpur yang masuk.
• Limbah gas dibuang ke udara setelah diolah dengan electrostatic
precipitator dan disaring untuk memisahkan partikel padat dan
zat yang berbau.
• WAO memerlukan suplai oksigen yang cukup, menggunakan
udara biasa ataupun oksigen murni spy prosesnya dapat berjalan.
• Beberapa studi tentang pengolahan limbah yang 20%-nya adalah zat padat
menunjukkan bahwa biaya yang dibutuhkan untuk proses wet air oxidation lebih
tinggi daripada proses yang menggunakan insinerator (pembakaran) karena
tidak terlalu membutuhkan energi eksternal, walaupun biaya operasionalnya
lebih rendah.
• Wet air oxidation dapat mengolah hampir semua jenis lumpur organik yang
dihasilkan oleh rumah tangga maupun air limbah pabrik.
• Zat padat yang dihasilkan bersifat steril, tidak membusuk, mudah mengendap
dan mekanisme dewatering-nya mudah.
• Zat padat dengan konsentrasi nitrogen rendah dan kandungan logam yang
cukup tinggi tidak cocok untuk penggunaan lahan berkelanjutan.
• Gas yang dihasilkan dari wet air oxidation merupakan campuran dari nitrogen,
oksigen, carbon dioksida, dan hidrokarbon. Bau busuk yang dihasilkan tergantung
dari tingkat oksidasi yang dicapai di dalam reaktor.
Tabel Range Operasi Wet Air Oxidation
ULTIMATE DISPOSAL
• Sludge sisa dan abu dari proses pengolahan lumpur dapat dibuang di darat,
di air, atau di udara.
• Metode pembuangan akhir menentukan proses pengolahan yang
dibutuhkan untuk mempersiapkan sludge.
• Idealnya, pembuangan akhir tidak boleh mencemari lingkungan, dan harus
ekonomis.
• Banyak produk pembakaran yang berbahaya bagi lingkungan. Dengan
demikian, alternatif utama untuk pembuangan lumpur utama adalah tanah
dan air.
• Pembuangan ke udara hanya solusi parsial karena sebagian lumpur
mengandung jumlah yang cukup dari tidak mudah terbakar.
ULTIMATE DISPOSAL
Ocean Disposal
• Sludge diangkut ke lokasi pembuangan dengan tongkang atau pipa.
-Pipa -> sludge dimasukkan pada bagian bawah laut dan diffuser sering
digunakan untuk mencampur sludge dengan air laut.
-Jika pembuangan lumpur ke laut dipraktekkan, sludge harus
distabilkan dan tempatnya harus memiliki arus yang kuat untuk
memberikan pengenceran yang cepat.
• Dalam beberapa situasi, pembuangan lepas pantai lumpur
menyebabkan kerusakan yang signifikan terhadap lingkungan
kelautan.
• Ocean disposal limbah mungkin dibatasi atau dilarang di masa depan karena efek
yang berpotensi merugikan pada ekologi laut, →Saat ini sudah dilarang
ULTIMATE DISPOSAL
Ocean Disposal
ULTIMATE DISPOSAL
Land Disposal
o Treated sludge dibuang di tanah dalam bentuk dikeringkan, telah di-dewater dan
cair -> Sludge yang telah dikeringkan dan abu insinerator sering dibuang di TPA
sanitari dimana limbah ditutupi berkala dengan lapisan tanah.
o Investasi tanah bersih harus dipilih untuk menghindari drainase atau pencucian
polutan ke sungai atau persediaan air tanah.
o Lekatan sludge yang telah di-dewater biasanya dibuang sebagai TPA, oleh
pembakaran atau dengan menyebarkan di darat.
o Tempat sampah yg terletak di tempat terbuka, hanya memuaskan untuk lumpur
yang stabil atau abu yang tidak menciptakan kondisi gangguan.
o Ketika dibajak ke dalam lahan pertanian berpasir, digested sludge mengkondisikan
tanah dan meningkatkan retensi kelembaban. Beberapa lumpur telah memberikan
pertumbuhan tanaman dan hasil yang mirip dengan pupuk anorganik.
o Beberapa kota telah menjual lumpur kering sebagai pupuk atau kondisioner tanah.
Karena tingkat nutrisi yang rendah relatif terhadap pupuk anorganik, pasar untuk
bahan ini masih terbatas.
ULTIMATE DISPOSAL
Land Disposal
o Laguna -> metode sederhana untuk pembuangan lumpur dalam bentuk bubur cair
o -Dapat digunakan sebagai cadangan untuk drying beds dengan menghapus lumpur
berkala setelah pengeringan dan pengeringan.
o -Sebagai tempat pembuangan akhir, ditinggalkan ketika diisi dengan padatan.
o Lumpur dalam bentuk bubur cair dapat diterapkan untuk tanah irigasi semprot atau
teknik alur.
o Kelebihan cairan dari laguna dikembalikan ke pabrik untuk pengobatan. Laguna mungkin
dipraktekkan di daerah di mana lahan yang luas yang tersedia dan di mana lumpur tidak
akan membuat gangguan lingkungan.
o Dengan menyebarkan sludge di darat sebagai lumpur cair, dewatering tidak diperlukan
dan air irigasi disediakan.
o Pembuangan lumpur stabil pada lahan pertanian meningkat dalam penerimaan
meskipun ada keprihatinan serius tentang bahaya kesehatan, seperti organisme patogen
dan tanah dapat menjadi anaerobik atau mungkin tidak menyerap nutrisi. Misalnya,
kelebihan nitrogen mungkin teroksidasi menjadi nitrat dan bermigrasi ke persediaan air
tanah. Ketika ditangani dengan baik, bagaimanapun banyak sludge yang
merepresentasikan sumber daya potensial yang berharga untuk aplikasi pertanian.
ULTIMATE DISPOSAL
Land Disposal
DAFTAR PUSTAKA
• Eckenfelder, W. W.: Water Industrial Pollution Control,
CBI, Boston, Mass. 2000.
PENGOAHAN LIMBAH INDUSTRI
BULKING DAN FOAMING
Oleh:
Prof.Dr.Ir. Tri Widjaja, M.Eng.
DEPARTEMEN TEKNIK KIMIA
FAKULTAS BISNIS & MANAJEMEN TEKNOLOGI (FBMT)
Activated sludge
Pada Tahap ini mikroorganisme
memproses dg merubah bahan
organik dari non-settleable
solids menjadi settleable solids.
Merupakan kolam ber-aerasi
dan berpengaduk, yang
memungkinkan dekomposisi
material organik oleh
mikroorganisme yang
diinokulasikan sehingga dapat
mengendap. Bakteri dalam
“activated sludge”
diresirkulasi secara kontinu ke
kolam aerasi utk
meningkatkan rate
dekomposisi organik.
2
KARAKTERISTIK SLUDGE
Sludge yang buruk
Sludge yang baik
•
•
•
•
Settleability sludge baik
(mudah mengendap dan tidak
mengambang lagi ±1 jam)
Memiliki karakter flok yang
bagus :
1. Round & compact
2. Berukuran sekitar 5 mm
3. Berwarna cokelat cerah
4. Rapat karena bridging
yang bagus oleh bakteri
berfilamen
5. Memiliki kandungan
exopolisakarida (EPS)
cukup sekitar 10-20%
FM Ratio cukup (0.05 – 2)
Toksisitas rendah
•
Settleability sludge mudah (Sulit
mengendap dan tmengambang lagi ±1 jam)
• Memiliki karakter flok yang bagus :
1. Tidak berbentuk dan renggang (dispersed
growth)
2. Berukuran terlalu kecil (pin dan straggler
flok)
3. Berwarna gelap (pin dan straggler flok)
4. Renggang karena bridging yang jelek oleh
bakteri berfilamen terlalu banyak ataupun
terlalu sedikit (filamentous bulking dan
foaming, dispersed growth,dll)
5. Memiliki kandungan exopolisakarida (EPS)
terlalu tinggi (zoogleal bulking dan slimy
sludge)
• F/M Ratio terlalu tinggi ataupun terlalu
rendah (foaming dan bulking)
• Toksisitas tinggi (toxicity)`
Sludge yang baik
Sludge yang
buruk
3
Microbiology Problem dan Penyebabnya
Permasalahan Pin-Floc dan Dispersed Growth
Permasalahan Nitrifikasi dan Denitrifikasi
Kekurangan Nutrien dan Polisakarida Bulking dan Foaming
Zoogloea Bulking dan Foaming
Filamentous Bulking
Filamentous Foaming
Terbentuknya Foam dan Scum
Permasalahan Pin-Floc dan Dispersed Growth
Pin Floc merupakan flok yang berukuran sangat kecil
(diameter < 50 mm), tipis, dan lemah.
Penyebab :
Jumlah bakteri berfilamen dalam lumpur aktif sedikit, rasio F/M sangat rendah,
umur lumpur terlalu lama, dan toksisitas berkepanjangan
Akibat :
Flok kehilangan strukturnya, kemampuan solid untuk mengendap menjadi rendah
dan effluent tetap keruh
Solution:
-Redesigning secondary settling tanks agar lebih sesuai pada reflocculation process
-Semua struktur dengan high turbulence, sebaiknya dihindari
-Keep F/M ratio in the range 0.05 to 0.2
Permasalahan Pin-Floc dan Dispersed Growth
Spesies pembentuk flok bisa tumbuh dalam bentuk yang tersebar dan non-settleable
jika tingkat pertumbuhan terlalu cepat. Hal ini dikenal sebagai dispersed growth.
Penyebab :
Beban organik yang tinggi (kondisi F/M yang tinggi)
Akibat:
Tidak ada pembentukan flok dan pengendapan biomassa tidak terjadi, menghasilkan
limbah yang sangat keruh.
Solusi:
-Redesigning secondary settling tanks agar lebih sesuai pada reflocculation process
-Semua struktur dengan high turbulence, sebaiknya dihindari
-Keep F/M ratio in the range 0.05 to 0.2
Permasalahan Nitrifikasi dan Denitrifikasi
Dalam pengolahan activated sludge, bakteri biasanya bernafas menggunakan
nitrat pada tempat dimana bebas oksigen dengan diiringi pelepasan gas
nitrogen sebagai by product.
Gas nitrogen sedikit larut dalam air serta dapat membentuk gelembunggelembung gas kecil dalam activated sludge yang dapat mengakibatkan
terbentuknya lapisan sludge terapung dalam clarifier akhir.
Akibat Nitrifikasi:
Indikasi denitrifikasi
-rendahnya nilai pH (pH ≤ 6)
-alkalinitas air limbah yang rendah
-ditentukan dengan metode pengendapan
jar test selama beberapa jam pada sludge.
-pin floc dan kekeruhan yang tinggi
pada effluent
-Bila sludge meningkat dalam kurun waktu 2
jam atau kurang, maka masalah denitrifikasi
mungkin terjadi.
Permasalahan Nitrifikasi dan Denitrifikasi
Solusi:
1. Mereduksi umur sludge
2. Mereduksi proses aerasi
3. Meningkatkan laju RAS
4. Meningkatkan konsentrasi DO pada
clarifier akhir
Kekurangan Nutrien dan Polisakarida Bulking dan Foaming
Akibat Produksi polisakarida berlebih:
1. berkurangnya nutrient dalam air limbah (termasuk berkurangnya oksigen dan
meningkatnya F/M rasio)
2. dapat meningkatkan jumlah sludge yang terbentuk yang sulit terdegradasi
3. menyulitkan proses pengendapan yang dihubungkan terhadap permasalahan
slime bulking dan permasalahan dewatering sludge.
Activated sludge normal berisi 10%-20% polisakarida pada basis berat kering, dengan
kadar polisakarida tertinggi terjadi pada permulaan umur sludge.
Sludge dengan kadar polisakarida >20% akan bermasalah dalam hal proses
dewatering dan pengendapan.
Kekurangan Nutrien dan Polisakarida Bulking dan Foaming
Tanda-tanda dari kekurangan nutrien:
1. Terjadinya filamentous bulking
2. Activated sludge yang viskos
3. Terbentuknya foaming pada bak aerasi yang berisi polisakarida
yang memiliki properti permukaan aktif.
4. Mengecek/memastikan kadar amonia, nitrat, serta orthophosphate yang tersisa dalam effluent setiap saat.
Kadar amonia, nitrat, serta ortho-phosphate yang disarankan untuk
memastikan bahwa effluent memiliki kadar nutrien mencukupi
adalah sebesar 1-2 mg/L.
Zoogloea Bulking dan Foaming
Zooglea bulking terjadi pada kondisi F/M rasio yang tinggi pada
saat jumlah senyawa asam organik spesifik serta alkohol tinggi
dimana kadar oksigen rendah.
Dalam zooglea bulking, organisme yang berperan adalah
Zoogloea ramigera yang merupakan bakteri klasik pembentuk
flok. Dalam proses ini, zooglea berkembang biak pada activated
sludge yang mengakibatkan pengendapan sludge terhalangi.
Filamentous Bulking
Bulking merupakan problem berupa
lambatnya pengendapan dan tidak
kompaknya padatan di clarifier.
Penyebab :
Pertumbuhan yang berlebih dari
mikroorganisme berfilamen seperti Thiothrix
sp. Mikroorganisme dari kelompok ini
memiliki karakterikstik dimana koloninya
sulit untuk membentuk flok.
Akibat :
Lambatnya pengendapan solid di clarifier
dalam sistem lumpur aktif
Thiotrix sp
Penyebab Bulking
1. Konsentrasi DO
4 bakteri filamen tumbuh dengan DO rendah.
Pada sludge yang rendah hingga sedang: type 1701, S. natans and
Haliscomenobacter hydrossis
Pada sludge age tinggi: Microthrix parvicella
Semua MO tsb merespon dengan baik peningkatan DO.
2. Defisiensi Nutrien
Type 021N, Thiothrix spp., type 0041 and type 0675 tumbuh dengan
defisiensi nitrogen dan atau fosfor . Biological slime sering berakumulasi
dengan pertumbuhan MO tsb. Penambahan amonia atau asam fosfat ke
tangki aerasi biasanya butuh kontrol thd MO tsb.
3. pH Rendah
Jamur dapat tumbuh di bawah kondisi pH rendah. Kondisi ini sering
ditemukan saat pH influent rendah. Tetapi, hal ini dapat diamati dalam
nitrifying systems or ASS dimana alkalinitas alami influent rendah.
Eliminasi pH influent yang rendah atau penambahan alkalinitas thd sistem
dibutuhkan untuk kontrol fungi.
13
Penyebab Bulking
4. Sulfida
Thiothrix spp., type 021N, Beggiatoa spp. and type 0914 dapat mengoksidasi
sulfida menjadi elemen sulfur dan memisahkan sulfur ke dalam sel. Sulfur dalam
sel dapat diamati dengan mikroskop. Sumber sulfida harus dieliminasi atau
sulfida “diikat” secara kimia.
5. Readily metabolized food
Beberapa MO tumbuh dengan baik pada materi soluble yang mudah hancur.
Contoh: gula dan materi karbon rantai pendek. MO ini adalah S. natans, type
021N, Thiothrix spp., H. hydrossis, Nostocoida limicola and type 1851.
Mengurangi sludge dan memasang selektor membantu mengontrol N. limicola
and type1851.
6. Slowly metabolized food
Types 0041, 0092, and 0675 and M. parvicella dapat tumbuh dengan lambat
pada makanan yang mudah hancur dan dalam sitem shg secara biologis
meremove nitrogen and fosfor. Pertumbuhannya disebabkan oleh penggunaan
complete mixing pada tangki aerasi. Kontrol: mengurangi sludge age, dengan
menggunakan plug flow, dan mempertahankan DO tetap seragam. Selektor
anoksi tidak dapat digunakan karena beberapa MO dapat melakukan
14
denitrifikasi.
Penyebab Bulking
7. Konfigurasi Reaktor
Pada sludge age rendah hingga sedang, selektor yang didesain dengan tepat
(aerobic, anoxic, and anaerobic) dapat mengontrol MO:
– Type 021N, Thiothrix spp., S. natans, type 1701, H. Hydrossis, and
Nocardia
8. Zona yang Tidak Teraerasi
Pada sludge age tinggi(e.g., nitrifying BNR plants), adanya zona yang tidak
teraerasi menyebabkan pertumbuhan MO berikut yang tidak terkontrol oleh
berbagai selektor:
– M. parvicella, type 0092, type 0041, and type 0675
9. Sifat substrate
MO yang dapat larut dan mudah terbiodegradasi vs partikulat dan MO
lambat terbiodegradasi lebih suka substrat terlarut yang mudah
terbiodegradasi
– S. natans, type 021N, Thiothrix spp., H. hydrossis, N. limicola, and type
1851
15
Filamentous Bulking
Sejumlah organisme berfilamen diidentifikasi pada activated sludge
pengolahan air industri. Selain bergantung pada kondisi operasi, satu atau lebih
organisme mendominasi proses.
16
Filamentous Bulking
Beberapa tipe Filamen pada industri pengolahan limbah :
17
Filamentous Bulking
Faktor yang mempengaruhi pertumbuhan Filamen :
1.
Komposisi air Limbah
air limbah yang terdiri dari glucoselike saccharides (glukosa, saccharin,
laktosa, maltosa dsb) akan meningkatkan pertumbuhan filamen, sedangkan
Loundry, tekstil, dan air limbah kimia yang kompleks menghambat
pertumbuhan filamen dalam sistem yang tercampur sempurna. Pada
umumnya, semakin mudah mendegrdasi substrat maka sistem ini semakin
rentan untuk terjadi filamentous bulking.
2.
Konsentrasi Oksigen Terlarut
oksigen terdifusi ke dalam flok untuk mencukupi kebutuhan oksigen
organisme yang ada di dalam flok. Laju penggunaan oksigen adalah
seimbang dengan organic loading (F/M). Dimana, jika F/M meningkat, maka
oksigen terlarut yang diperlukan juga meningkat. Hubungan ini dapat dilihat
pada grafik berikut :
18
Fig. 6.32 dan 6.33
19
Efek Pertumbuhan Filamentous Bakteri
Ideal, non bulking floc
High
Pinpoint Floc
20
Filament Floc
Ideal, Non-Bulking Activated Sludge Floc
Filamentous
backbone
• MO filamen dan pembentuk flok setimbang
• Flok besar dan kuat
• Filamen tidak berinterferensi
• Flok yang mengambang jelas
• SVI rendah
Filamentous Bulking Activated Sludge Floc
• MO filamen dominan
• Flok besar dan kuat
• Filaments berinterferensi dengan settling
• Flok yang mengambang jelas
• SVI tinggi
Pin Point floc
• MO filamen rendah
• Flok kecil dan lemah
• Flok yang mengambang
keruh
• SVI tinggi
21
Keterangan :
a) Pinpoint- floc
b) small, weak flocs
c) flocs contining filamentous organisms
d) flocs containing filamentous organism “network“ or “backbone."
Filamentous Bulking
Beberapa filamen yang dikenal dapat menyebabkan terjadinya
Activated Sludge Bulking atau Foaming antara lain:
Beberapa Kondisi yang Menyebabkan Tumbuhnya Filamen pada Sistem
Air Limbah Industri
Terjadi pada
Sistem air limbah
Perkotaan dan
Industri
Hanya terjadi
Pada sistem
Air limbah
Industri
Filamentous Foaming
3 Jenis organisme filamen yang dapat menyebabkan terjadinya
activated sludge foaming adalah Nocardia, Microthrix Parvicella,
dan type 1863.
Nocardial foaming sangat sering muncul dan terjadi pada proses
activated sludge dengan kisaran angka 40% yang diikuti Microthrix
Parvicella kemudian type 1863 sebagai filamen yang jarang sekali
menjadi penyebab pada proses terjadinya filamentous foaming.
Spesies Nocardia yang umumnya ditemukan dalam proses foaming
antara lain: N. amarae, N. caviae, N. brasiliensis, dan N. asteroides
Filamentous Foaming
Foaming dalam proses activated sludge dapat dipengaruhi oleh
Beberapa hal dengan beberapa efek fisik foaming yang terjadi.
Adapun deskripsi dan beberapa penyebab tersebut antara lain:
Foam Description
Penyebab
Foam tipis, berwarna putih hingga abu-abu
Residence time sel rendah (startup foam)
Foam bergelembung, berbusa, berwarna putih
Nonbiodegradable detergents
Foam berwarna abu-abu kelabu
Recycle dari proses lain (misal: digester anaerobik)
yang terlalu berlebihan
Lapisan sludge tebal pada clarifier akhir
Denitrifikasi
Foam tebal, seperti lumpur, dan keabu-abuan
Foam kekurangan nutrien, foam berisi material
polisakarida yang dilepaskan dari flok
Foam stabil, tebal, berwarna coklat
Foaming yang disebabkan oleh filamen seperti
Nocardia, Microthrix, dan type 1863
Konsentrasi Oksigen terlarut rendah
• F/M ratio tinggi membutuhkan konsentrasi DO
tinggi untuk pengobatan yang efektif dari beban
BOD.
• Konsentrasi DO rendah dan tinggi di F/M rasio
menimbulkan pertumbuhan filamen yang
berlebihan (bulking)
• Filamen mikro-organisme yang terkait dengan DO
rendah adalah S. natans, tipe 1701, M. parvicella
dan mungkin H. hydrossis.
28
Faktor-faktor berpengaruh
dalam tangki aerasi
• Anorganik N larut dan konsentrasi P 0,5 sampai 1 mg/l harus
dipertahankan dalam tangki aerasi.
• Beban BOD sangat larut maka konsentrasi minimum N dan P
harus ditingkatkan menjadi 1 sampai 3 mg / l.
• Beberapa tanaman menggunakan pupuk komersial yang
mengandung urea, amonia dan nitrat. Namun dalam
campuran nitrat dapat menyebabkan denitrifikasi
29
Busa pada Tangki Aerasi
• Uji toksisitas dalam cekungan aerasi akan selalu menghasilkan
beberapa penumpukan busa dalam baskom.
• Warna busa merupakan indikator umur lumpur dan kondisi.
• busa, kering putih menunjukkan lumpur muda.
• Busa cokelat kaya media dikaitkan dengan pengendapan lumpur
yang baik.
• Coklat gelap, busa berminyak dapat ditemukan di lumpur yang lebih
tua. Hal ini umum dalam digester aerobik, tetapi bukan pertanda
baik di cekungan aerasi. Hal ini juga dapat menunjukkan masalah
bakteri berfilamen
30
Busa pada Tangki Aerasi
31
Terbentuknya foam dan scum
Problem ini disebabkan oleh tidak
terurainya surfactan serta adanya
mikroorganisme Nocardia sp dan
kadang-kadang juga disebabkan oleh
adanya Microthhrix parvicella.
Solusi :
1.Menggunakan antifoam
2.Menghilangkan
busa
secara
mekanis sebelum masuk Clarifier
Foaming
32
Metode Pengendalian Proses
Jangka Pendek
Jangka Panjang
• Pengaturan Debit RAS
• Penambahan bahan kimia
• Klorinasi
• Kekurangan nutrisi
• Rendahnya konsentrasi O2
terlarut
• Konfigurasi Tangki Aerasi
Meningkatnya
Metode Jangka Pendek
Untuk WAS ditutup aja supaya gak kebuang banyak
Pengaturan debit RAS (return activated sludge)
Meningkatkan debit RAS untuk mencegah solid wash out
ke saluran efluen.
2. Penambahan bahan kimia
Bertujuan untuk meningkatkan laju pengendapan solid.
Bahan yang ditambahkan berupa kaogulan/polimer
3. Desinfaksi
Pemberian desinfektan (klorin) untuk membasmi
mikroorganisme berfilamen.
1.
Metode Jangka Panjang
1.
▪
▪
▪
▪
▪
Kurangnya Nutrien
Rasio makronutrien BOD:N:P = 100:5:1
Rasio BOD:N:P < 100:5:1 menyebabkan defisiensi nutrien
di dalam proses
Analisa mikrobiologi → Adanya mikroba berfilamen tipe
021N (Thiothrix spp., S. natans, H. hydrossis, dan N.
limicola III)
Rasio 100:5:1 digunakan sebagai acuan karena
penambahan nutrien juga tidak boleh berlebihan agar
tidak terjadi toksifikasi pada sistem.
Dipengaruhi oleh temperatur dan umur lumpur
Metode Jangka Panjang
2.
Rendahnya konsentrasi O2 terlarut
▪
Secara umum rasio F/M = 0,05-0,6 sedangkan DO = ±2
mg/L. Jika konsentrasi DO rendah maka memicu
pertumbuhan mikroorganisme berfilamen.
▪
Kebutuhan O2 di dalam tangki meningkat seiring dengan
rasio F/M. Jika kita tidak dapat meningkatkan konsentrasi
DO, maka kita harus menurunkan rasio F/M dengan cara
meningkatkan konsentrasi MLSS (memperbesar faktor M –
mikroorganisme)
Metode Jangka Panjang
2.
Rendahnya konsentrasi O2 terlarut
▪
Akan tetapi, menurunkan F/M dapat berakibat pada
peningkatan sludge age yang berakibat pada
meningkatnya kebutuhan oksigen untuk respirasi
endogenous.
▪
Apabila masalah seperti ini ditemukan, maka cara yang
dapat dilakukan untuk mengatasi bulking adalah tetap
mengoperasikan pada DO rendah dan melakukan
klorinasi untuk membasi organisme filamentous.
Metode Jangka Panjang
3.
Konfigurasi Tangki Aerasi
▪
Sistem biasanya dioperasikan secara kontinyu dan
teraduk secara sempurna (completely mixed).
Sistem ini memiliki karakteristik pengendapan yang lebih
rendah dibandingkan sistem yang dioperasikan secara
intermiten.
Selain dioperasikan secara completely mixed biasanya
ditambahkan selector.
▪
▪
Metode Jangka Panjang
3.
Konfigurasi Tangki Aerasi
▪
Pemasangan selector pada tangki aerasi sebenarnya
melakukan pembagian zona pada tangki dan melakukan
modifikasi konsentrasi oksigen pada zona-zona tersebut.
▪
Konfigurasi tangki aerasi dengan selector memungkinkan
mikroorganisme pembentuk flok untuk tumbuh dan
mencegah pertumbuhan mikroorganisme penyebab
bulking.
Metode Jangka Panjang
Contoh Selector pada Tangki Aerasi
The Bio-selector Effluent flows to the aeration basins.
42
State of Michigan
Department of Environmental Quality
ACTIVATED SLUDGE
PROCESS CONTROL
TRAINING MANUAL FOR WASTEWATER
TREATMENT PLANT OPERATORS
Water Resources Division
Rick Snyder, Governor
Dan Wyant, Director
www.michigan.gov/deq
800-662-9278 Environmental Assistance Center
4/17
This Activated Sludge Training Manual was prepared by the Michigan
Department of Environmental Quality, Operator Training staff. It is
intended to be used as an aid in the presentation of the MDEQ Activated
Sludge Process Control Training Course, along with numerous
handouts, class discussion, and an abundance of slides.
The manual is not intended to be an exhaustive reference, nor a design
manual. The student is encouraged to seek more detailed information from
manufacturer’s literature and the facility operation and maintenance
manual. References to specific equipment or manufacturers do not
indicate endorsement or preference by the State of Michigan. Although
there are many suppliers of related equipment, those that are most often
used to treat wastewater in Michigan are discussed in more detail than
those that are not as common.
The manual is generally organized to follow the topics discussed in the
training course, although some deviation is to be expected. The first
section of the manual (and the first day of the course) reviews the basic
principles of the activated sludge process. This is followed with information
more specific to operating the activated sludge process, including nutrient
removal, and troubleshooting.
It is hoped that participants in the Activated Sludge Process Control Course
will find this manual helpful as they follow class discussions and as they
review the information presented in class.
A.S.
BASICS
Activated Sludge Manual
Activated Sludge Manual
I. Background Stuff
Wastewater Sources and Quantities
Wastewater may be described as water that is used
Typical Water Use
to convey pollutants away from a source of
Residential
pollution. It originates in homes, businesses,
100 gal per day per person
schools, hospitals, prisons, and industries, and is
Offices
ultimately discharged back into the environment.
40-50 gal per day per person
Depending upon the collection system, wastewater
Hotels
may become diluted with groundwater or surface
400 - 500 gal per day per room
water as it passes from the source to the point of
Hospitals
treatment. Infiltration into sewage collection
200 gal per day per bed
systems may account for large increases in the
Schools
amount of wastewater that requires treatment.
200 - 300 gal per day per room
Although typical quantities of domestic wastewater
generation are somewhat predictable, industrial
contributions are more varied. While many industries treat wastewater on-site, it is not
unusual for a publicly owned wastewater treatment plant (POTW) in an industrialized city
to treat wastewater comprised of up to 40 % industrial wastewater.
Wastewater Characteristics
Solids
Components in wastewater may be generally classified in several different ways. One
might refer to the pollutants in wastewater as being either inorganic or organic.
Inorganic materials include sand, grit, minerals and metals, and are not biodegradable.
Organic materials can be thought of as those which contain carbon, originate from living
plants and animals, and are usable as a food source by living organisms. Obviously that
is an over-simplification, since organic substances may be synthesized commercially,
many compounds of which may not be biodegradable. Contributors of organic pollutants
include animal wastes, food processing, household wastes, and oil and grease.
Solids Composition of Wastewater
Total Solids
100 %
Settleable and
Suspended 60 %
Volatile
50 %
Fixed
10 %
Fixed
30 %
Dissolved
40 %
Volatile
20 %
Volatile
70 %
Total Solids
100 %
Fixed
20 %
Solids are present in nearly every wastewater, may
be very detrimental environmentally, and so are very
often regulated in discharges of wastewater. Solids
increase the amount of sedimentation in aquatic
systems, choking off plants and animals and limiting
the use of the receiving water. The term “solids”
actually includes several possible components. The
term “suspended solids” refers to particles which
may be visible, add turbidity, and may be filtered out.
“Dissolved solids” are those which pass through a
filter and are not seen. Only when the water is
evaporated from a sample is the amount of dissolved
material apparent. So “total solids” refers to the
amount of material that would be recovered if the
water was evaporated from a sample, including
particulates and dissolved materials.
1
Activated Sludge Manual
The term “settleable solids” refers to those particulates which will settle within a defined
period of time under quiescent conditions. Although no longer typically used for
wastewater discharge monitoring, the settleability test is often used in controlling
biological wastewater treatment plant operations, especially the activated sludge
process. Another solids term that is often used is “colloidal solids”. This refers to
particles which are so finely divided that they are microscopic in size and will not settle.
These may pass through a filter paper and give the water a hazy appearance.
Solids may be organic or inorganic. For example, table salt in water would be an
inorganic, dissolved solid. Pepper in water would be an organic, suspended solid. The
fraction of organic solids is often estimated by burning the material. Organic materials
will burn or “volatilize” at a temperature of 550°C, while inorganic materials will remain as
a residue and are referred to as “fixed”. The table on the right indicates typical solids
composition of domestic wastewater. Overall, the amount of solids donated to a
domestic wastewater is estimated at about 0.20 - 0.25 lbs/d/capita.
It is important to note that the fraction of organic solids
which are dissolved in wastewater may change. As the
table on the right indicates, when wastewater goes
from fresh to stale, particulate size is reduced, the
percentage of particulate organics drops and the
percentage of dissolved (soluble) organic solids
increases. Stale wastewater is more difficult to treat,
requiring longer detention times and reducing the
efficiency of a biological wastewater treatment process.
Thus, it is important to limit detention time of the
sewage in the collection system, equalization tanks,
primary clarifiers, etc.
Fresh
Stale
15 - 25 %
50 %
Colloidal
75 - 85 %
Suspended
50 %
Soluble
As Wastewater Becomes Stale,
More of the Suspended Organics
Become Dissolved
Biochemical Oxygen Demand
Another characteristic of wastewater that is closely
regulated is the “Biochemical Oxygen Demand” (BOD). As the term implies, many of
the components of wastewater cause an oxygen demand to occur on a wastewater
treatment system or on a receiving stream. This demand occurs as microorganisms,
mainly bacteria, feed on the pollutants in the wastewater. As bacteria metabolize the
pollutants they require oxygen, and dissolved oxygen is taken from the stream. As the
pollutant load on the stream increases, the amount of oxygen required to consume the
pollutants increases. Also as the term “Biochemical Oxygen Demand” implies, most of
the demand occurs as a result of biological (organic) pollutants, but some inorganic
pollutants, for the most part ammonia, can also contribute to the oxygen demand. As
ammonia is biologically oxidized to nitrate (nitrification), oxygen is used up. So Total
BOD is the sum of the carbonaceous oxygen demand (CBOD) and the nitrogenous
oxygen demand (NOD).
2
Activated Sludge Manual
In the laboratory, the BOD of a wastewater is
determined by diluting a portion of the wastewater
sample with nutrient-rich, pH buffered dilution water in
a 300 mL BOD bottle. The initial dissolved oxygen
(D.O.) concentration of the diluted sample is
determined and the bottle is incubated at 20°C for 5
days. The final D.O. in the bottle is determined and
the BOD of the sample is calculated based on the
oxygen depletion and the amount of sample dilution.
If only the CBOD of the wastewater is to be
determined, a nitrification inhibitor is added to the
BOD bottle during dilution.
Bio
Chemical
Oxygen
Demand
Carbonaceous BOD
Nitrogenous BOD
Total Biochemical
Oxygen Demand
Total BOD = CBOD + NOD
5
The environmental impact of BOD on a receiving
stream may be illustrated with the chart below. On
0.17 - 0.22 lbs/d/capita
the left hand side of the chart, clean stream
conditions are indicated by a relatively high D.O.
concentration, maybe in the range of 5 – 7 mg/L and little sediment.
Clean Stream
Recovery
Active
Decomposition
Degradation
Clean Stream
BOD
When a pollutant load is discharged into the
stream (Zone of Degradation) the BOD
concentration increases as the bacteria naturally
present in the stream find a ready source of food.
The bacteria become acclimated to the food
supply and the population quickly increases in the
Zone of Active Decomposition. In this zone
oxygen consumption peaks, and the D.O. of the
stream sags. Sediment increases as the pollutant
is converted to bacterial mass which accumulates
on the stream bottom.
As the pollutant is consumed the food supply for
the bacteria becomes limited. Oxygen transfer
D.O.
Sediment
Dissolved
from the
Oxygen
Variety
atmosphere
Oxygen Sag
Sediment
overtakes
oxygen consumption and the D.O. of the stream
begins to increase in the Zone of Recovery.
Eventually, as the BOD drops to minimal levels, the
stream is returned to Clean Stream conditions (with
some additional sediment).
3
Clean
Water
Recovery
Active
Decomposition
Degradation
Population
Clean
Water
Since the biology of a stream is related to the D.O.
available, the types of organism expected in each of
these zones will change. In a clean stream with high
D.O. one could expect to find organisms which can
not tolerate pollution. This would include a large
variety of organisms, but relatively low overall
population. In the zones of degradation and active
decomposition organisms which thrive at low D.O.
conditions have the advantage and their population
Activated Sludge Manual
increases greatly, but biological diversity is reduced as the less tolerant organisms
decline.
So a stream can purify itself as long as the pollutant load is not so large that the system
becomes stuck in the process of active decomposition. In considering the environmental
impact of a particular pollutant on a stream, though must be given to the numerous other
possible pollutant loads. Non-point source pollution such as agricultural run-off and
storm-water discharges, as well as other point-source discharges such as combined
sewer overflows add significant pollutant loads to streams. If the total oxygen demand
on the stream exceeds its capacity to recover, fish kills, objectionable odors, and very
limited water use will result.
Nutrients
Nitrogen and phosphorus are nutrients that are
required by every living organism, becoming a
component of every cell. Domestic wastes, animal
wastes, food processing wastes, and many industrial
wastes will contain these nutrients. If these are
discharged into a stream or lake they act as fertilizer,
increasing the growth rate of aquatic plants. As this
growth rate increases the lake may become choked
with weeds and the amount of sediment increases.
Over time, the lake begins to fill in with sediment.
Eutrophication is the term used to describe the aging
process that lakes undergo as they gradually fill in
with sediment, forming a bog or swamp. Careful
control of the nutrient load discharged into the
environment helps to slow that process. More
detailed information on the impacts of nutrients and
their treatment/removal will be included later.
Nutrients
Nitrogen
Phosphorus
Human Health Hazards and Toxins
Wastewater may contain an untold variety of components that may be hazardous to
humans. Domestic wastes always present the possibility of containing infectious
microorganisms, or pathogens. Wastewater
treatment plant workers exposed to these may
Human Health Hazards
contract any of several waterborne diseases. One of
Pathogens
the most critical aspects of wastewater treatment is to
Nitrate
prevent the discharge of these organisms into the
Toxic Materials
environment where others may also be at risk.
Materials Toxic to Biota
Metals
Ammonia
Pesticides,
Herbicides
Chlorine
Acids/Bases
Materials toxic to humans or aquatic organisms may
enter the wastewater collection system from
agricultural, industrial, or domestic sources. Metal
solutions, pesticides, herbicides, acids and bases,
and chlorine used to disinfect wastewater flows are
included in the list of potentially toxic materials.
Though the field of wastewater treatment has
progressed by leaps and bounds in the past one
hundred years, it must still be realized that the
4
Activated Sludge Manual
wastewater treatment plant operator has a very difficult task. Wastewater received by
the treatment facility is a complex mixture of largely unknown substances which must not
be released into the environment. It may include solids, oxygen demanding substances,
nutrients, pathogens, and toxins. The operator should always try to bear in mind the
importance of this position in the protection of natural resources and the protection of
public health.
II. Wastewater Treatment Processes
Typical Wastewater Treatment Plant Schematic
Wastewater treatment processes may be grouped into two general categories, the first
being physical/chemical. This category includes screening, sedimentation, filtration,
precipitation, and chemical destruct systems. The second category, biological, includes
processes which rely on living organisms to remove pollutants from the wastewater.
This includes processes such as waste stabilization lagoons, trickling filters, rotating
biological contactors, and activated sludge.
In most cases wastewater treatment is accomplished through the use of a combination
of physical/chemical and biological treatment processes. For instance, a typical
treatment plant might include preliminary treatment (physical) to remove large debris and
grit, primary treatment (physical) to remove settleable suspended solids, secondary
treatment (biological) to remove the remaining particulates and dissolved organic
material, chemical precipitation to remove nutrients, tertiary filtration (physical) to remove
remaining fine particulates, and chemical or ultraviolet light disinfection.
Preliminary Treatment
Preliminary treatment is intended to protect downstream
processes by removing large debris that might plug or jam
equipment. This often involves bar screens to remove large
particulates such as sticks, rocks, rags, etc. Coarse bar screens
are usually inclined in the flow with the bars spaced about 1½
inches apart. The screen may be manually cleaned in smaller
facilities or mechanically cleaned automatically in larger
facilities. Fine bar screens have found increased use in the past
several years. Often these screens are designed to remove
particles as small as ¼ inch, obviously removing a larger
amount of material from the wastewater flow, and providing
increased protection for downstream processes.
Grinding and shredding mechanisms such as comminuters
have been used for many years to reduce the size of large
debris, sending the shredded debris further into the treatment
process. Although more modern equipment may be more
efficient and less maintenance intensive than the older
comminuters, they still deposit the debris back into the
wastewater flow where it must be treated again later (possibly
an advantage that the fine screen has over grinders).
Screening or grinding is usually followed by grit removal. Grit
includes mainly the heavy inorganic materials such as sand,
gravel, etc. that are abrasive to pumps, accumulating in
5
Activated Sludge Manual
primary and secondary treatment processes, and adding unwanted inorganic content to
sludge.
There are several options for grit removal, but two general methods are gravity grit
separators and aerated grit separators. Gravity separators simply slow the flow velocity
to a point at which the grit will settle out of the flow, but the organic material will remain
suspended; this targeted velocity is generally taken as 1 foot per second. The simplest
of the gravity separators is a channel in which the velocity is controlled, with the grit
being removed manually. Another type of gravity separator is the detritor process, a
settling tank(often rectangular) with a revolving plow which directs the grit to a sump
were it is removed from the tank.
Aerated grit chambers operate along the same principle as
the gravity separators, except that rather than adjusting the
flow velocity through a channel, air is injected near the floor
of the separator. The air causes enough turbulence in the
tank to keep the organics in suspension while allowing the
grit to settle. Often air lift pumps are used to remove the
grit from the separator.
Aerated Grit Separator
Flow monitoring is typically included in the preliminary part
of the process. This may involve a closed pipe flow
measurement system, but probably more often flow is
directed through a Parshall flume where a flow level
detecting device (such as an ultrasonic sensor) relates
liquid level in the flume to the flow rate through the flume.
Flow
Measurement
Determine Loading on WWTP
Determine Loading on Stream
Discharge Permit Parameters
Budgeting
Flow measurement is important for many reasons,
budgetary, operationally, and compliance-wise. Sewer
use charges are usually based on flow, hydraulic and
organic loading rates on plant processes require flow data,
in-plant return flows and chemical feed rates are often
paced relative to the influent flow measurement, and
reporting plant flow information is almost always required
in discharge monitoring reports. It is obvious that the flow
measuring equipment must be periodically calibrated and
serviced. It is recommended that this maintenance be
performed by a qualified technician at least once per year.
Sewer Use Charges
While the preliminary part of the treatment process may
not be the most glamorous area of the plant in which to
work, it is important that this equipment be kept maintained in good working order. If the
operator observes large debris or grit accumulating in down-stream processes, the
preliminary treatment processes should be closely examined for areas that should be
better maintained or operated. If this situation is allowed to exist, the overall treatment
efficiency and operability of the plant will be reduced.
Pace Chemical Feed &
Process Return Flows
Primary Treatment
Most, but not all, activated sludge facilities include a primary treatment step ahead of the
secondary process. Primary clarification is a sedimentation process which is intended to
6
Activated Sludge Manual
remove heavy settleable organic material. This removes a portion of the organic load
before the secondary process, allowing the solids to be removed more economically as
primary sludge rather than more dilute and harder to dewater secondary biomass.
Primary clarifiers may be designed as either rectangular or circular tanks with a
minimum depth of about 10 feet. Tank dimensions vary according to the expected
hydraulic load, but generally allow for a detention time of approximately 2 hours and a
surface overflow rate of 400 – 600 gallons per day per square foot at average daily flow.
Whether rectangular or circular, provision is made for collecting and removing the settled
sludge, skimming and removing grease and other floating material, and discharging the
clarified primary effluent to the secondary process.
The graphs below show the expected removal efficiency for solids and for BOD in a
primary clarifier. Note that removal efficiency increases with time until a detention time
of about 2 hours; after that little additional removal occurs. As discussed previously,
removal rates for BOD may actually begin to decrease as degradation begins to occur in
the clarifier. Solids begin to be broken down into smaller particles, and eventually are
resolubilized, increasing the organic load on the secondary process and producing
organic acids and other byproducts which are difficult to treat. It is therefore important
that the primary treatment system be operated properly. Sludge must be removed from
the clarifier before decomposition begins, and hydraulic detention time must be allowed
to become excessive. Either situation will decrease performance of both the primary
and the secondary treatment systems.
Primary Sedimentation Efficiency
Primary Sedimentation Efficiency
100
100
Settleable Solids
Percent Removal
80
80
60
Suspended Solids
60
BOD
40
40
20
20
0
1
2
3
0
4
Time, Hrs.
1
2
Time, Hrs.
7
3
4
Activated Sludge Manual
Activated Sludge Basics
The Activated Sludge Process is a biological wastewater
treatment process. This means that treatment occurs as
pollutants are used as a food source by many different
types of microorganisms. It is a suspended growth
process, since the organisms are suspended in the
wastewater rather than attached to a media as in the
trickling filter or rotating biological contactor processes.
Since this is a biological process, understanding some basic biology will be needed.
Some terms that will be used in this discussion and definitions follow:
Anaerobic
Aerobic
Facultative
Organisms that need no D.O. or nitrate (NO3) oxygen
Organisms that must have D.O.
Organisms that can exist with or without D.O.
Heterotrophic
Autotrophic
Organisms which consume organics in the wastewater
Organisms which are able to use inorganic compounds as
a food source
The activated sludge process relies on the cultivation of a population of millions of
microorganisms of many different types, mostly aerobic and facultative heterotrophic
bacteria suspended in the wastewater, as it passes through a reactor (aeration tank).
Air
Primary
Effluent
Aeration
Tank
MLSS
Secondary
Clarifier
Secondary
Effluent
Return Activated
Sludge
Waste Activated
Sludge
This suspension, referred to as mixed liquor (or Mixed Liquor Suspended Solids,
MLSS), is supplied with oxygen and kept mixed by bubbling air through it. These are
naturally occurring organisms; there is no need to supply them from an external source.
As the organisms feed on the organic pollutants in the wastewater, the pollutants are
converted to more organisms (biomass) and some byproducts. The amount of biomass
produced is often estimated as about 0.7 pounds for each pound of BOD removed in the
secondary process. While an individual bacterium is not visible to the eye, they stick to
one another to form a biological mass which may be easily seen as a brown colored floc.
Following an adequate amount of treatment time the mixed liquor flows from the aeration
tank to a secondary clarifier where the biomass is allowed to settle out of the wastewater
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Activated Sludge Manual
and the effluent passes to the next treatment step. The settled biomass is returned to
the treatment process to provide organisms which will continue removing pollutants.
This returned biomass is referred to as Return Activated Sludge (RAS).
Since this is a living and growing process, it will continue to build biomass to the point of
having too much. The amount of biomass in the process is controlled by removing
(wasting) a portion of it each day. This excess biomass removed from the secondary
system is known as Waste Activated Sludge (WAS).
Bacterial Cells
It is helpful to understand a little about the bacterial cell
if we wish to know how it is able to remove pollutants.
The diagram at the right shows a typical cell. The inside
of the cell contains reproductive information, food
storage mechanisms, etc. Surrounding the cell is a
membrane which keeps the organism together, and
through which dissolved food may pass. The cell wall is
coated with a slime layer which is used to trap particles.
Wastewater
New Cells
Slime Layer
Oxygen
Cell
Food Membrane
Storage
Enzymes
(Absorption)
Soluble Organics
Adsorbed
Particle
NH3
CO2
H2O
The diagram at the left
shows a bacterial cell suspended in wastewater
containing both soluble and particulate organic
pollutants. Soluble organic pollutants pass through the
cell membrane (absorption) and are used as a direct
food source. Particulate organics cannot pass through
the membrane, but stick to the slime layer (adsorption).
The organism begins to produce enzymes which are
secreted through the membrane and solubilize the
particulate, allowing it to pass through the membrane
where it too is used as food. In this way the organism is
able to remove both soluble and particulate organics
from the wastewater.
Also indicated in the diagram, oxygen must be supplied
to the organisms as they metabolize the organics, and
new bacterial cells are produced. Byproducts to this metabolism include ammonia
(NH3), carbon dioxide (CO2), and water (H2O).
Three Steps of Biological Treatment
Biological wastewater treatment is often described as occurring in three steps. In step
one, Transfer, food from the wastewater is transferred to the cell. Adequate mixing and
detention time are needed to assure that the organism comes into contact with the food
source.
Step two, Conversion, occurs as the organism metabolizes the food supply, converting
it to new cells. In order for this to occur the food supply must be a usable type and in a
usable form. Some compounds are easily degraded by the bacteria, while others are
metabolized more slowly. Some pollutants may not be metabolized until the organisms
become acclimated to it, producing the right kinds of enzymes. A proper D.O.
environment must be present; aerobic organisms will not efficiently remove pollutants in
an anaerobic environment. The nutrient balance must be proper for conversion to take
9
Activated Sludge Manual
place. Like other life forms, the organism needs nitrogen and phosphorus, among other
minor nutrients, in order to metabolize food and build new cells. The ratio of carbon to
nitrogen to phosphorus is generally taken as 100:5:1.
In the third step of treatment, Flocculation and Separation, the microorganisms stick
together to form large particles that will settle out of the purified wastewater in the
secondary clarifier. Flocculation occurs when mixing allows the organisms to contact
one another, but does not cause conditions so turbulent that the flocculated material is
torn apart. Settleability and compaction of the floc particles depends on the density, size
and shape of the particles as well as the efficiency of the clarifier. Settleability is
affected by the abundance of filamentous bacteria, those that form strings as they
grow rather than forming floc. An excessive growth rate of these bacteria may cause a
bulking condition in which the mixed liquor does not compact well, taking up much more
volume in the clarifier. This condition may be caused by many factors, among which are
improper D.O. environment and nutrient imbalance, and may result in solids loss in the
clarifier effluent. More about bulking and control of filamentous organisms later.
III. Activated Sludge Control Factors and Calculations
Control Factors Overview
Proper operation of an activated sludge plant will require knowledge of biological and
physical factors that influence the efficiency of the process. These factors include:
• organic and hydraulic loading on the aeration tank
• dissolved oxygen in the aeration tank
• biosolids wasting rate
• return activated sludge rate
• clarifier loading
• solids settling and compaction characteristics
Organic Loading
Organic loading refers to the number of pounds per day
of BOD entering the process. In most activated sludge
plants this is based on the primary effluent, but in plants
without primary clarifiers it would be based on the plant
influent flow. Pounds per day of BOD loading may be
easily calculated using the Pounds Formula.
Multiplying the flow rate in million gallons per day by the
weight of a gallon of water (8.34 Lbs/Gallon) and times
the concentration in milligrams per liter of BOD in the
flow yields the number of pounds per day of BOD in that
flow.
Pounds Formula
Lbs = MG X 8.34 Lbs
D
Gal
D
X Conc, mg
L
Lbs = MG X 8.34 Lbs
D
D
Gal
X Parts
M Parts
Use to Determine the Pounds per Day of a
Material in a Given Flow at a Given
Concentration
note:
•Flow Must Be In Million Gallons/Day
•8.34 Lbs is the Weight of a Gallon of
Water
•Conc Must be in Part per Million
Terms
It may be advantageous to calculate the organic loading
as a five-day or seven-day moving average. This
helps to average out day to day loading fluctuations,
allowing more consistent control of the operation. A
seven day moving average would be calculated by averaging the pounds of BOD for a
particular day with values for the six days previous.
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Activated Sludge Manual
Quantity of Microorganisms
Determination of Mixed Liquor Suspended
Solids (MLSS) and Mixed Liquor Volatile
Suspended Solids (MLVSS)
The concentration of mixed liquor (MLSS) is
determined by suspended solids analysis of the
suspension in the aeration tank. Since this suspension
includes biological mass as well as inorganic material
present in the wastewater, the amount of biological
mass is estimated by determining the organic content
of the MLSS. Mixed Liquor Volatile Suspended Solids
(MLVSS) is determined by igniting a sample of the
dried MLSS in a muffle furnace at 550°C. The material
that burns at that temperature is considered to be
organic, and therefore estimates the biological mass.
The material that remains (non-volatile, or fixed)
estimates the inorganic fraction of the MLSS. So in
process control calculations where all of the solids must
be considered, MLSS is used in the calculation. In
calculations where just the active biological population
should be considered, MLVSS is used.
The quantity of microorganisms available for treatment is also calculated using the
pounds formula. Since the microorganisms are in the aeration tank, pounds of
microorganisms are calculated by multiplying the volume of the aeration tank(s) in
millions of gallons times the weight of a gallon of water (8.34 lb/gal) times the MLVSS
concentration in milligrams per liter.
Food to Microorganism Ratio
Food to Microorganism Ratio (F:M) is one of
the primary controls used in activated sludge
plants. This helps the operator to maintain a
balance between the quantity of food
available, with the quantity of
microorganisms in the aeration tanks. Since
the food available to the microorganisms is
represented by the BOD of the wastewater,
the F:M ratio is calculated by dividing the
number of pounds of BOD entering the
secondary treatment system by the number
of pounds of MLVSS in the aeration tanks.
While best treatment may not occur at the
same F:M ratio in different plants, the range
for Conventional activated sludge plants is
often given as 0.25 to 0.45. Activated sludge
plants that operate in the Extended
Aeration mode typically operate with F:M in
the 0.05 to 0.15 range.
F:M
F:M Calculations
Calculations
Problem A:
How many pounds of MLVSS should be
maintained in an aeration tank with a volume of
0.105 MG receiving primary effluent BOD of
630 lbs/d ? The desired F:M is 0.3.
F =M
F/M
= 630 lbs/d = 2100 lbs MLVSS
0.3
Problem B:
What will be the MLVSS concentration in mg/L ?
2100 lbs = Conc X 0.105 MG X 8.34 lbs/gal
2100 lbs
= 2398 mg/L
0.105 MG X 8.34 lbs/gal
Since the operator usually has no control
over the number of pounds of BOD entering the wastewater treatment plant, F:M is
adjusted by adjusting the number of pounds of MLVSS in the secondary system. If more
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Activated Sludge Manual
biomass is needed (raising MLVSS) the amount of biomass wasted must be reduced,
and if less biomass is needed (lowering MLVSS) the wasting rate must be increased
until the necessary pounds of biomass is achieved. A couple of things that must be
remembered regarding making operational changes: 1) biological systems react slowly
to these types of control changes; give the system time to adapt to a change before
making another adjustment, and 2) consistency is often the key to successful operation;
use a moving average to calculate pounds of BOD, and make few adjustments; only
when necessary.
F:M is most useful as an operational tool when the operator sets a target F:M, and
based on the average BOD loading calculates the pounds of MLVSS needed to achieve
that F:M. Divide the pounds of BOD by the target F:M to calculate the pounds of
MLVSS needed. The concentration of MLVSS in the aeration tank can then be
calculated by dividing the pounds of MLVSS by the aeration tank volume in million
gallons and by 8.34 lbs/gallon.
Cell Residence Time
Cell Residence Time (CRT), also known as Sludge Age (SA) or Solids Retention
Time (SRT), may be defined as the average length of time in days that an organism
remains in the secondary treatment system.
Growth Rate of Organisms
The graph on the right illustrates the growth
Lag
Log Declining Endogenous
phases in a biological system and how the
Growth
Growth
Growth
Growth
growth rate of microorganisms changes with
increased CRT. When a food supply is
introduced into a biological treatment plant that
Food
is in start-up, there is an abundance of food but
very few organisms. The organisms are said to
Conventional
be in Lag Phase as they begin to acclimate to
Treatment
the waste, producing the needed enzymes, and
Extended Air
the population begins to increase. Once the
organisms have acclimated, the growth rate
increases rapidly in the Log Growth Phase. At
this point the food supply is not a limiting factor
as BOD is converted to biological mass,
producing large amounts of sludge. In the
Sludge Production
Declining Growth Phase the population has
grown to the point that the available food supply
begins to limit the production of new cells and
Time
organisms begin to compete for food. As the
population ages (CRT 5 days or greater), larger
and more complex organisms which are able to compete for the remaining food are
more numerous, and predatory organisms begin to feed on smaller ones as a food chain
develops. In the Endogenous Phase the food supply has been depleted and as the
age of the population increases (CRT is now up to >15 days), the organism growth rate
continues to decline. Food which the organisms have stored is metabolized and the
organisms feed on one another in Endogenous Respiration. Although the
concentration of organisms is large, sludge production is lower.
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Activated Sludge Manual
Considering the objectives and the expense of
wastewater treatment, there are obvious advantages in
Conventional Activated Sludge
operating an activated sludge system in the extended
Aerator Detention Time 4 - 8 Hrs.
aeration mode. BOD will be almost completely gone,
F:M 0.25 - 0.45
producing a high quality effluent, and sludge production
CRT 4 - 6 Days
will be at its lowest. Nitrification (the oxidation of
ammonia) is almost certain. Because of the large
organism population and large aeration tanks used the
system is more resistant to upset, with day to day
operation being typically very consistent. These plants
Extended Aeration Activated Sludge
are usually designed without primary clarifiers, since
Aerator Detention Time 16 - 24 Hrs.
organic loading on the secondary system is fairly light.
F:M 0.05 - 0.15
Biosolids wasted from the secondary system is often
CRT 15 - 25 Days
digested and concentrated by means of aerobic
digestion, followed by disposal on agricultural land. The
popularity of the extended aeration mode is evidenced
by the large number of oxidation ditch and sequencing
batch reactor facilities (both are typically operated in the extended air mode) built in
Michigan in recent years.
But it is not practical to build an extended aeration process for many municipalities and
industries. Extended air plants must be capable of long detention times in the aeration
basin, usually in the range of about 24 hours. They must also be capable of maintaining
large microorganism populations; MLSS is often in the 5000 mg/L range. In situations
where the wastewater flow rate or the BOD loading is large, constructing an extended air
plant would be too expensive, requiring very large aeration tanks and secondary
clarifiers.
Most of the larger activated sludge plants in Michigan (> 5 MGD) operate in the
conventional mode. Primary clarifiers remove a large portion of the organic loading
before the wastewater is treated in the secondary system. Primary sludge and biomass
wasted from the secondary system may be thickened followed by anaerobic digestion,
lime stabilization, or incineration. Effluent quality is typically high; nitrification may or
may not occur, depending largely on organic and hydraulic loading.
Cell Residence Time
Cell Residence Time, CRT
Sludge Age, SA
Mean Cell Residence Time, MCRT
The Average Length of Time in Days
that an Organism Remains in
the Secondary Treatment System
CRT, days = Total MLVSS, lbs
Total MLVSS Wasted, lbs/d
Cell residence time is calculated by dividing the total
pounds of MLVSS in the aeration system by the number of
pounds of biomass wasted per day. Note the use of the
volatile portion of the biomass in the CRT formula
(MLVSS). Actually the formula will work with either total or
volatile solids as long as it is consistent on both the top
and bottom. But since we are again interested in the
biological portion of the solids, we will avoid confusion by
using volatile solids.
Example Problem:
CRT =
MLVSS = 6681 lbs
MLVSS Wasted = 835 lbs/d
Calculate the CRT.
6681 lbs
835 lbs/d
CRT = 8.0 Days
In plants with a number of secondary clarifiers, a
significant quantity of the total amount of biomass may be
held in the clarifiers. In this situation the operator may
include these solids in the calculation; the total quantity of
biomass in the aeration tank and secondary clarifiers
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Activated Sludge Manual
divided by the quantity of biomass wasted per day is the Mean Cell Residence Time
(MCRT). Sometimes in calculating the MCRT the quantity of solids lost in the effluent is
added to the quantity of biomass wasted per day. But for most facilities, calculation of
the CRT is appropriate. Like F:M, the most efficient CRT at which a plant will operate is
best determined by experience.
One tool which many activated sludge operators utilize is the microscope. A more
detailed discussion of the use of the microscope will be given later, but it will be helpful
at this point to discuss the use of indicator organisms in
controlling the process. As the CRT increases, the types
of organisms in the mixed liquor become larger and more
complex. While the operator is not able to see most
bacteria even with the use of a microscope, the larger,
more complex organisms are more easily identified.
Since the size and complexity of the organisms increase
as the CRT increases, the types of organisms
predominating in the Mixed Liquor give an indication as
to the age of the sludge.
At low CRT (< about 4 days) the simpler life forms are
present. This includes amoebas and flagellates. As
the sludge age increases (> about 4 days), more complex
organisms such as the free swimming ciliates and
stalked ciliates appear. And at high CRT, multi-celled
animals such as the rotifers and nematodes may be found. Again, a more detailed
discussion will come later, but at this point it should be recognized that as the CRT
increases, complexity and size of organisms increase, allowing the operator who is
skilled in the basic use of the microscope to determine the quality of the biomass in the
treatment plant.
Operational problems may often be linked to inappropriate CRT.
Young sludge (low CRT) may be related to an inadequate
microorganism population or an excessive BOD load (high F:M)
which causes a log growth situation. The cells become dispersed
rather than flocculated, settleability is poor, and the effluent
becomes turbid. In this condition oxygen is used up quickly due to
the high metabolism rate, and sludge production is high. One telltale sign of this condition is the production of huge amounts of a
billowing white foam.
At the other extreme, an old sludge (high CRT) may be related to
operating with an excessive microorganism population. All of the influent BOD has been
used up and the organisms are now in endogenous respiration
(low F:M). Oxygen use is lower as metabolism and sludge
production declines. Mixed liquor settles rapidly due to the
dense, compact floc that forms. Effluent may be generally
clear, with some straggler floc left behind. Plants operating
with an old sludge often experience slurp, a dense, greasy,
brown foam which accumulates on the aeration tanks and
sometimes on the secondary clarifiers. Although it is not
uncommon to see some slurp on plants operated in the
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Activated Sludge Manual
extended air mode, an excessive amount of slurp is not only unsightly, but may cause
walkways to become dangerously slippery. In cold temperatures slurp in aeration tanks
and clarifiers may freeze, causing operational difficulties and possibly mechanical failure.
Slurp is caused by the growth of a highly branched
filamentous organism (usually Nocardia). As the filaments
float to the surface they entrap other particles and air
bubbles to form a scum which is very resistant to just about
any effort to eliminate it. The best solution is usually to
reduce CRT, being careful not to remove too much
biomass, and with time the amount of slurp may diminish.
As a short term solution, it may be necessary to physically
remove the slurp by means of a vacuum truck or pump.
Wasting Rates
CRT was defined earlier in this discussion as the average length of time in days that an
organism remains in the secondary treatment system. The operator determines the
operating CRT for the facility and maintains it through wasting the appropriate amount of
excess biomass (Waste Activated Sludge, WAS) from the secondary system. In other
words, the amount of biomass (MLSS) in the secondary system is controlled and
maintained through solids wasting.
In nearly all activated sludge plants, wasting is accomplished by directing a portion of the
Return Sludge to the solids handing facility. Wasting Return Sludge rather than Mixed
Liquor minimizes the volume of water that must be processed by the sludge
thickening/dewatering equipment.
While it may be best to waste these solids continuously to achieve maximum system
stability, this is often not possible due to plumbing and time constraints. If intermittent
wasting is practiced, it is usually best to waste over as long a time period as practical,
and when the loading on the secondary system is at the low point of the day. Drastic
changes should not be made in wasting rates from one day to the next; allow the
biological system time to acclimate to a change before another change is made.
Consistency is a key element in successful activated sludge plant operation.
Many activated sludge plants were originally designed to waste secondary solids into the
primary clarifiers. The reasoning was that as the less dense biological solids co-settle
with the heavier primary solids the combined sludge density would be increased. The
problem which often results from this arrangement is that much of the biological solids
do not settle in the primary clarifier and end up back in the aeration tank, increasing CRT
and often causing operational problems. A more efficient operation will result if the WAS
is wasted directly to a solids handling process and not allowed to return to the treatment
system. It is crucial that adequate solids concentrating equipment and solids storage
capability are part of any plans for building or expanding an activated sludge plant.
Secondary sludge wasting is one of the most important controls available to the operator
because it controls the most important aspect of treatment, biomass population.
Unfortunately, control of wasting is limited in some facilities by poor design. Inability to
return sludge and waste at the same time, WAS piping being too large or too small, and
inadequate or non-existent WAS flow metering capability add to the level of difficulty in
operating some plants. Again, it is very important that system design include adequate
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Activated Sludge Manual
WAS control and metering equipment. A good WAS control situation is one that allows
the operator to set a totalizer which determines the maximum number of gallons wasted
in a particular day and also allows the operator to control and monitor the WAS flow rate.
In determining an appropriate WAS rate, we need to return to the CRT relationship:
CRT(days) = Lbs of MLVSS in aerators
Lbs/day WAS VSS
Determining a wasting rate first involves deciding upon a target CRT. For instance, the
CRT for a conventional activated sludge plant may be set at 8 days, or for an extended
air plant the CRT may be set at 20 days. Assuming that the plant has an established
biomass concentration based on F:M, the target CRT will be used to establish a wasting
rate that will provide a stable biomass population.
Since we already know the target CRT, the relationship may be rearranged to solve for
the number of pounds of WAS:
Lbs WAS VSS = Lbs of MLVSS in aerators
day
CRT (days)
While the above calculation gives the number of pounds of biomass that must be wasted
each day, the operator is usually interested in calculating the number of gallons of that
material that must be wasted per day. If the concentration of the WAS is known (same
as RAS VSS), this can be done by rearranging the pounds formula to solve for MGD:
WAS (MGD) =
lbs/day WAS VSS
.
RAS VSS (mg/L) x 8.34 lbs
gal
Keep in mind that the end result of this calculation is the flow in MGD, and since waste
flow rates are not large, the calculated MGD is typically a small number. Convert this to
gallons per day from MGD:
MGD x 1,000,000 = gallons per day
If wasting is to be done over a 24 hr. period:
.
WAS (gpm) = gallons/day
1440 minutes/day
If wasting is to be done over a shorter period of time:
WAS (gpm) =
gallons/day
.
min wasting to be done/day
Return Activated Sludge
Return Activated Sludge (RAS) refers to the biological solids (mixed liquor solids) that
settle in the secondary clarifier and are continuously returned back to the aeration tank.
There are two important reasons for returning these organisms. First, if they were not
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Activated Sludge Manual
continuously drawn from the clarifier it would quickly fill up with solids and they would be
lost in the effluent. Second, these organisms are the major component of the treatment
system. If they were not returned the biomass population could not be sustained and
the treatment system would quickly fail. RAS brings active, hungry microorganisms back
into the aeration tank where they can again feed on incoming wastes.
There are a couple of fairly common misconceptions regarding RAS. Some mistakenly
think that RAS returns food back into the aeration tank for the microorganisms in the
tank. Keep in mind that the food (BOD) should be completely gone at this point. The
object is to return microorganisms to sustain the biological population in the aeration
tank. Another misunderstanding is that RAS controls the MLSS concentration in the
system. RAS can only change the MLSS concentration on a short term basis, and only
when the solids in the system are not in balance. The operational parameter which
controls the biomass concentration in the system is WAS; if more biomass is needed
WAS is reduced, and if less biomass is needed WAS is increased.
Return sludge is drawn continually from the secondary clarifiers. Operators typically try
to control the RAS so that the sludge blanket in the clarifier is maintained between 1 and
3 feet deep. It is important not to allow the blanket to get too deep, so that a safe
distance is maintained between the top of the sludge blanket and the surface of the
water in the clarifier, assuring that solids will not be lost as the flow rate through the
clarifier fluctuates. It is usually desirable to maintain some solids in the clarifier; this
helps to provide a thicker return sludge and will minimize the number of gallons of
material that must be wasted.
The RAS rate in most plants is electronically controlled through metering and automatic
valves. In many plants the rate may be set to track with the influent flow rate; as the
influent flow rate increases the RAS rate increases. So a 40 % RAS rate means that if
the plant influent flow rate is 1 MGD, the RAS rate would be 0.4 MGD. Typical RAS
rates range from about 30 % to about 125 %. RAS controllers often provide both
percent of influent flow and MGD indicators. Tracking the RAS flow rate with the influent
flow rate helps to avoid solids build-up in the secondary clarifiers during high flow
periods and avoids pumping all of the solids from the clarifier during low flow periods.
While electronic flow meters are needed to determine RAS rates, a
simpler approach may be best for determining the depth of sludge in the
secondary clarifier and making RAS adjustments. Many operators use
a device known as a “sludge judge” for this purpose. This is a clear
plastic tube marked in 1 foot increments, about 1 inch in diameter, in
lengths that thread together to allow the operator to reach from the
clarifier bridge to the bottom of the clarifier. The bottom section of the
tube has a check valve which allows the tube to fill as it is lowered into
the clarifier, but seats when the operator draws the tube up. The
operator then is able to actually see the depth of sludge in the tank, and
the RAS rate increased or decreased according to the desired blanket
depth. An alternate approach to sludge blanket measurement is to use
one of the many electronic devices on the market for determining the
sludge/water interface. Regardless of the method of measurement, this
determination should be made at least once per shift on each clarifier,
with increased frequency if changes in the RAS rate are being made or if reduced
settleability has become a problem.
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Activated Sludge Manual
The RAS rate may be calculated using a mass balance approach around the secondary
clarifier.
Q + Rq
Influent
Flow, Q
MLSS
RAS Flow, Rq
RAS SS
Basically, this approach is centered on the thought that in a secondary clarifier the
pounds of solids drawn from the clarifier (RAS) must be equal to the pounds of material
entering the clarifier (MLSS).
Without detailing all of the algebra involved, using the pounds formula to express pounds
of material going into the clarifier, setting that equal to the pounds of material drawn from
the clarifier, and solving for the RAS flow rate (Rq) yields the formula:
Rq =
Q X MLSS .
RAS SS – MLSS
In this formula Rq is the calculated Return Sludge flow rate, Q is the influent flow rate,
RAS SS is the RAS suspended solids concentration in milligrams per liter, and MLSS is
the Mixed Liquor suspended solids concentration in milligrams per liter.
Note that the flow units for Rq will match the units used for influent flow Q; if Q is given in
MGD, Rq will be in MGD. If the operator needs to calculate Rq in terms of percent of
influent flow, just use 100 % for Q.
Rq, % = 100 % X MLSS
RAS - MLSS
Also note that total suspended solids rather than volatile solids are used in the RAS
formula for both RAS and MLSS. This makes sense, since we are concerned with the
total amount of sludge in the clarifier rather than just with the biological mass. Keep in
mind while using the RAS formula that while this calculation may provide a good starting
point or a check for the RAS rate, the most practical means of controlling the RAS rate is
by actual sludge blanket measurement.
RAS metering and control is another area in which the operator needs to be vigilant
when a new facility is being designed, or an existing plant expanded. The additional
capital costs associated with meters and valves is usually money well spent when the
operational difficulties existing at some facilities is considered.
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Activated Sludge Manual
Not Good
Better
Best
M1
Secondary
Clarifier
#1
RAS
Secondary
Clarifier
#1
Secondary
Clarifier
#2
RAS
Pump
What are the
Chances of
Controlling
Sludge blankets ?
P1
P2
V1
V2
Secondary
Clarifier
#1
Secondary
Clarifier
#2
Secondary
Clarifier
#2
V1
V2
M1
M2
RAS Wet Well
P1
P2
If we divide the meter reading in half
will we get the flow rate for each clarifier?
Consider RAS control in the schematics above. In the first example the RAS lines from
the two clarifiers are tied into a common pipe to the RAS pump. There is no provision
for controlling flow or metering flow from either of the clarifiers; sludge blankets would be
very difficult to control.
In the second diagram each clarifier has its own RAS flow control valve and pump. The
two pumps discharge into a common pipe where the flow is metered. This is a better
situation, allowing the operator to vary the amount of RAS from each clarifier, but since
the meter only measures the total flow from the two clarifiers, it will be very difficult to
balance the RAS flow. Even with equally sized pumps and with the control valves set
the same, the RAS pumped from the two clarifiers will not be the same, since the
hydraulic head pumped against will be different for each pump. Installing a meter on the
pipe from each clarifier rather than on the combined flow would provide better control,
but adjusting the flow from one clarifier would still affect the flow from the other.
The third diagram represents the best approach. RAS from each clarifier flows through
a control valve and flow meter to a wet well, where a set of pumps draws from the wet
well. This arrangement allows the operator to make adjustments to the RAS flow from
each clarifier as needed.
IV. Biomass Settleability
Earlier we discussed the three steps of biological wastewater treatment which includes
Transfer, Conversion, and Flocculation and Separation. Even though the first two steps
may take place effectively, without the ability to separate the biomass from the
wastewater the process will not function. The organisms must combine into properly
sized particles that have sufficient density to allow them to sink to the floor of the
secondary clarifier. The biomass must compact well enough that the sludge blanket
does not occupy an excessive amount of space in the clarifier or solids may be lost in
the effluent.
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Activated Sludge Manual
A simple procedure called the Settleometer Test is used to determine the settling
characteristics of mixed liquor. The test requires a settleometer, which is typically a
clear plastic cylinder with a capacity of
2 liters. Graduations on the cylinder range
from 100 to 1000 cubic centimeters (or
milliliters) of settled sludge per liter.
A sample of mixed liquor should be obtained
from the discharge end of the aeration tank,
being careful not to include scum in the
sampling container. Do not allow the sample
to set for more than a few minutes before the
settling test is performed. Determine the
MLSS concentration in milligrams per liter on
a portion of this sample.
Mix the sample well, and fill the settleometer
to the 1000 graduation. Immediately start a
timer and at the end of 30 minutes record the
settled sludge volume in the settleometer.
It is a good idea to occasionally record the
settled sludge volume every 5 minutes while
the solids are settling and prepare a graph of
settled sludge volume versus minutes. This
allows the operator to see whether the solids
are settling too quickly or slowly. Solids that
settle too quickly may be an indication of an
old sludge that will probably leave straggler floc in the effluent, while solids that settle too
slowly or do not compact well may be washed out of the clarifier during times of high
hydraulic load.
Just Right
Too Slow
20
Too Fast
Activated Sludge Manual
It is also a good practice to allow the sample to set in the settleometer for an additional
30 to 60 minutes after the settling test. Watch for tiny bubbles which form in the settled
sludge. These nitrogen bubbles form as nitrate is reduced to nitrogen gas
(denitrification) under anoxic conditions. As the bubbles rise they attach themselves to
floc particles and float them to the surface. A small amount of denitrification occurring in
the secondary clarifier will cause a scum to form on the surface, while a large amount of
denitrification may float a significant portion of the biomass to the top of the clarifier. The
settleometer test may give the operator the first warning that this may become a
problem.
Two main factors determine the settled sludge volume in the settleometer at the end of
the 30 minutes. The first, solids compaction indicates how much volume the biomass
will occupy. But the operator must recognize the influence of the second factor, MLSS
concentration, in settled sludge volume. As long as the MLSS doesn’t change,
settleometer test results can be compared from one day to the next. But as the MLSS
increases, the settled sludge volume in the settleometer will increase. Since we use the
settleometer test mainly to indicate how well the mixed liquor compacts, we must
account for the concentration of the biomass in the settleometer. This allows the
operator to track changes in sludge quality even though the MLSS concentration
changes.
Sludge Volume Index (SVI)
SVI is used by operators to determine and compare mixed liquor settleability. It
mathematically relates settled sludge volume in the settleometer to MLSS concentration.
The definition for SVI is: The volume in milliliters occupied by one gram of
activated sludge which has settled for 30 minutes. Note that SVI relates sludge
volume in milliliters to MLSS concentration in grams per liter. A simple formula for SVI
is:
SVI = mls Settled in 30 min
MLSS Conc, grams/L
or
SVI =
mls Settled
.
MLSS, mg/l / 1000
Consider an example where a settleometer is filled to the
mark with mixed liquor which has a concentration of
2400 mg/L, and after 30 minutes the settled sludge volume
is 260 ml. The SVI is calculated as follows:
SVI =
mls Settled
. =
MLSS, mg/L / 1000
260 ml
. =
2400 mg/L / 1000
260 = 108
2.4
The SVI of 108 indicates that each gram of settled sludge will occupy a volume of
108 milliliters. The SVI is typically given without units. Keep in mind that as the SVI
increases, the sludge is less compact, occupying more volume (the sludge blanket in the
clarifier increases). A perfect situation is considered to be that in which 1 gram of sludge
will occupy a volume of 100 ml (SVI = 100/1.0 = 1.0). SVI in a range of 80 to 120
indicates good settleability.
21
Activated Sludge Manual
Sludge Density Index (SDI)
SDI is another way to express sludge compaction, makes use of the same information
as SVI, but expresses it as sludge density (weight per volume rather than volume per
weight). The definition for SDI is: The grams of activated sludge which occupies a
volume of 100 ml after 30 minutes of settling. The formula for SDI is:
SDI =
grams/L of MLSS
.
mls settled in 30 min / 100
or
MLSS, mg/L / 1000
.
mls settled in 30 min / 100
Consider the example given above where MLSS is 2400 mg/L and after 30 minutes of
settling the sludge occupies a volume of 260 ml. The SDI is calculated as follows:
SDI = 2400 mg/l / 1000 = 2.4 = 0.92
260 ml / 100
2.6
The SDI of 0.92 indicates that each 0.92 grams of sludge will occupy a volume of
100 ml. As the SDI increases compaction increases, and the volume that the sludge
blanket occupies decreases. Like SVI, a perfect situation is taken as one where 1 gram
of sludge occupies a volume of 100 ml (SDI = 1.0 / 100/100 = 1.0). The range of good
settleability when using SDI is 0.8 to 1.2; again the SDI is typically given without units.
SVI / SDI Relationship
Operators do not typically use both SVI and SDI, since both are indicators
of sludge compaction, but usually use the one that they are most
comfortable with. For those not wishing to try to remember both
formulas there is a simple conversion between the two: divide the one
that you have into 100 and you get the other.
SDI = 100 / SVI
SVI = 100 / SDI
For instance, if the SVI is 133, the SDI is 100 / 133 = 0.75. If the SDI is 0.6, the SVI is
100 / 0.6 = 167.
Relationship of SDI to RAS Concentration
The settleometer test is used to approximate conditions in the secondary clarifier. This
means that the concentration of the settled sludge in the settleometer should be
approximately the same as the concentration of the settled sludge in the clarifier, which
is the Return Sludge.
Consider the mathematical definition of SDI:
SDI =
grams/L of MLSS
.
mls settled in 30 min / 100
An SDI of 1.0 means that 1 gram of sludge occupies a volume of 100 ml:
SDI 1.0 =
1 gram / Liter
100ml / 100
So the concentration of the settled sludge in the settleometer (and the RAS from the
clarifier) would be 1 gram / 100 ml, or 10 grams per liter. Since water (and most sludge)
22
Activated Sludge Manual
weighs 1 gram per milliliter, we can express the relationship as 1 gram of sludge per 100
grams of water, which is the same as a 1 % solution by weight:
SDI = 1.0 = 1 gram solids
100 ml water
=
1 gram solids . = 1 % RAS Concentration
100 grams water
Or we can express the concentration of the settled sludge (RAS) in terms of mg/L:
SDI = 1.0 = 1 gram solids = 1000 mg solids = 10,000 mg solids = 10,000 mg/L RAS
100 ml water
100 ml water
1000 ml water
So if the SDI is 1.0, the RAS concentration would be expected to be 1%, or 10,000 mg/L.
300
Sludge Volume Index
Relationship of F:M to Settleability
The graph at the right illustrates the
impact that F:M has on mixed liquor
settleability. Starting on the right hand
side of the graph and moving to the
left, one can see that as the F:M
drops from a starting point of 1.2, the
SVI increases drastically and then
drops again to within a controllable
area at an F:M range of 0.25 – 0.45.
If the F:M continues to decrease, the
SVI again rises sharply and then
drops to another controllable area at
F:M less than 0.20.
200
Operationally, this indicates that there
100
are three areas where settleability
should be good, defined by F:M range
Extended
as High Rate, Conventional, and
Conventional
High Rate
Air
Extended Air. While the High Rate
mode of operation is not practical for
0
0.20 0.40 0.60
0.80 1.00 1.20
most, many Activated Sludge facilities
F:M Ratio
operate in the
Conventional mode and many operate
in the Extended Air mode. The graph
also indicates the potential consequences of allowing conditions to wander too far from
these F:M ranges.
23
AERATION
Activated Sludge Manual
V. Aeration Requirements and Equipment
Aeration Requirements
Aeration of the contents of the activated sludge reactor accomplishes two important
requirements. Mixing must occur in order to provide contact between the biomass and
the incoming pollutants; assuring that the entire contents of the aeration tank are kept in
suspension. Dead zones in the tank may allow settling to occur and mixed liquor will
accumulate on the floor of the tank. As this settled material begins to decompose an
area of low dissolved oxygen is created, forming conditions conducive to the growth of
filamentous bacteria. These filaments bridge between floc particles, reducing the
density of the mixed liquor and causing settling problems in the secondary clarifier. In
facilities where aeration equipment does not provide adequate mixing, supplemental
mixing may be required.
Aeration must also provide oxygen to the huge population of aerobic and facultative
bacteria and other organisms in the mixed liquor. Operators typically control the
aeration rate to assure a concentration of 2 – 3 mg/L of dissolved oxygen (D.O.) at the
discharge end of the aeration tank. Higher D.O. concentrations waste power, while low
D.O. (<1 mg/L) may encourage the growth of filamentous bacteria.
The amount of air that must be supplied to the aeration tank to achieve the required D.O.
concentration depends on several factors. As BOD (biochemical oxygen demand)
loading increases the organisms will require more oxygen to metabolize the waste and
more air must be supplied to keep the D.O. concentration within the desired range.
Likewise, as the number of pounds of biomass in the system increases the air supply
must be increased; each organism will use the amount of oxygen needed to sustain
itself. Treatment objectives such as Nitrification and Denitrification are also factors
which determine how much air must be supplied. While it takes 1.0 to 1.5 pounds of
oxygen to degrade 1 pound of BOD, it takes 4.5 pounds of oxygen to convert 1 pound of
ammonia to nitrate (nitrification).
The Oxygen Transfer Efficiency of the aeration equipment plays a large role in
determining how much oxygen is supplied to the organisms with each cubic foot of air
delivered to the aeration tank. Not all of the oxygen supplied to the aeration tank is
dissolved into the water; most if it remains in gaseous form, bubbles to the surface, and
is lost to the atmosphere.
The Standard Oxygen Transfer Efficiency (SOTE) for various aeration equipment ranges
from about 10% up to about 40% in clean water and at 15 feet of submergence (diffused
aeration). Oxygen Transfer may also be given as Standard Oxygen Transfer Rate
(SOTR), given in units of pounds of oxygen transferred per horsepower hour.
The Actual Oxygen Transfer Efficiency (AOTE) or Actual Oxygen Transfer Rate (AOTR)
in wastewater will be considerably less than the SOTE or the SOTR. Oxygen transfer is
affected by many factors, including the type of equipment utilized (and how well it is
maintained), air temperature, chemical characteristics of the water, and the rate at which
the organisms use the oxygen (oxygen uptake rate). For instance, while the SOTR for
a particular aeration device may range as high as 6.5 pounds of oxygen per horsepower
hour, the AOTR may be expected to be in the range of 2.5 pounds of oxygen per
horsepower hour.
1
Activated Sludge Manual
Aeration Equipment – Mechanical Aeration
Air may be supplied to the aeration tank using
either mechanical aerators or a diffused
aeration system. Mechanical aerators splash
the mixed liquor into the air, causing oxygen to
dissolve into the water. There are many types of
mechanical aerators, including vertical and
horizontal designs. Vertical aerators may pump
mixed liquor from near the bottom of the tank
and discharge it against a deflector, or may
function as large impellers, spinning partially submerged in the mixed liquor near the
surface of the aeration tank. Adjustment of the depth of
submergence changes the amount of aeration and
mixing that occurs.
Horizontal mechanical aerators, or rotors, are commonly
seen in oxidation ditch facilities in which a long
horizontal shaft is suspended just above the surface of
the aeration tank.
Metal brushes or
plastic discs
attached to the shaft
rotate partially submerged in the mixed liquor,
providing aeration and imparting velocity to the
mixed liquor which keeps the biomass in
suspension. Again, the aeration rate may be
changed by adjusting the depth of liquid in the tank,
thus raising or lowering the submergence of the rotor.
Actual Oxygen Transfer Rates for mechanical aerators range from about 1.8 to
2.5 pounds of oxygen per horsepower hour.
Aeration Equipment – Diffused Aeration
The most commonly used method of aeration in conventional activated sludge plants is
the diffused aeration system. In this system a blower (compressor) is used to supply air
at low pressure into a piping arrangement with air
diffusers submerged near the floor of the aeration
tank. The diffusers break the air flow into small
bubbles from which oxygen is transferred to the liquid
as the bubbles rise to the surface. Increasing the
amount of time the bubble is in contact with the liquid
increases the oxygen transfer efficiency. Aeration
tanks are typically designed deep enough (often
about 15 – 18 feet) to maximize the travel time of the
bubble to the surface, but not so deep as to create so
much head (pressure) against the blower that it
operates beyond its range of maximum efficiency.
Aeration tanks are also sometimes configured to
cause a rolling motion of the liquid in the tank, again
to keep the air bubbles in contact with the mixed
liquor for as long a time as possible.
2
Activated Sludge Manual
Blowers may be classified as either positive displacement or centrifugal.
Centrifugal blowers are used in nearly all
medium to large activated sludge plants. These
operate as high rpm turbines, with air outputs of
20,000 - 150,000 CFM. The volume of air
pumped is variable within a range, adjusted by
controlling an Inlet Guide Vane (throttling valve
on the suction side of the blower).
Positive displacement blowers
are often referred to as rotary
lobe blowers. As the rotating
lobes rotate, a fixed volume of air
is displaced each time the lobes
come together. These blowers
operate at lower rpm than
centrifugal blowers and generally
produce less than 20,000 CFM of air. Unlike
centrifugal blowers, the air output of positive
displacement blowers cannot be varied by use of throttling valves. Air output may only
be changed by changing the speed at which the blower operates, for instance by
changing pulley size on blower or motor.
Blower Maintenance
Whether centrifugal or positive displacement, blowers are difficult and expensive to
repair, largely due to the fact that they operate at high speed and are machined to very
close tolerance. Wastewater treatment plant shops are seldom equipped to perform
major repairs on this type of equipment, and usually contract for this work to be done.
Heat, vibration, and dust are often causes of premature wear and blower failure.
Improperly maintained air filters which results in suction side air restrictions, or leaks
which allow dust to enter the blower are damaging to either type of blower. Lubrication
and other preventive maintenance must be done in accordance with the manufacturer’s
specifications.
Piping
Piping which connects the blower to the air diffusion
system at the aeration tank may be either a simple drop
pipe extending from the deck of the aeration tank to the
floor of the tank, or may be a swing type system. The
advantage of the swing system is that hinges in the piping
allow the operator to retrieve a bank of aerators out of the
tank using a crane on the deck of the tank when diffuser
maintenance is needed. Needless to say, this operation
requires an adequate crew and careful observance of all
safety considerations.
3
Activated Sludge Manual
Diffusion Equipment
Air diffusers have been designed in many shapes and sizes through the years; some
have been more successful than others. Air diffusers may be generally classified as
either coarse bubble or fine bubble.
Coarse bubble diffusers have been used for many years in
activated sludge plants. As pressurized air from the blower
flows from the air header through a small orifice in the
diffuser the air is broken up into small bubbles. The diffusers
are generally made of plastic or stainless steel and provide
good mixing and aeration with minimal head loss. These
diffusers are resistant to plugging and may operate for long
periods of time with minimal maintenance. Reported
Standard Oxygen Transfer Efficiency in clean water for coarse bubble diffusers varies
from 9% to 13% at 15 feet of submergence. Actual Oxygen Transfer Rates range from
about 1 to 2 pounds of oxygen per horsepower hour, depending on the type of diffuser
and the configuration of the aeration tank.
Fine bubble diffusers began gaining popularity in the 1970’s as energy costs increased
and discharge permit limits became more stringent. Given that power costs to operate
the aeration system in an activated sludge plant make up a very large part of the annual
budget for the facility, the need to maximize the efficiency of aeration systems is
obvious. Reported Standard Oxygen Transfer Efficiency varies widely depending on the
type of diffuser, ranging from 13% to 40% in clean water at 15 feet of submergence.
Again, it should be noted that actual transfer efficiency in wastewater will be lower than
in clean water, especially as the system ages. Actual Oxygen Transfer Rates vary from
about 1.3 to 2.5 pounds of oxygen per horsepower hour.
Early fine bubble diffusers included porous socks that were tied over coarse bubble
diffusers. These met with limited success, as it was not unusual for the operator to find
many of the socks floating in the aeration tank a short time after installation. Porous
plate diffusers were also developed which significantly improved oxygen transfer, but
often plugged and became maintenance intensive.
Currently there are several suppliers of fine bubble diffusers on the market, and many
activated sludge plants around Michigan have converted coarse bubble systems to fine
bubble systems. Modern fine bubble diffusers have reduced energy consumption in
activated sludge plants and are less susceptible to plugging.
Fine bubble diffusers are available in several configurations, including among others:
Ceramic Dome
Diffusers
Porous Flexible
Membrane Diffusers
4
Ceramic Disc
Diffusers
Activated Sludge Manual
Diffuser plugging is still a concern with fine bubble diffusion systems; fouling may occur
either from the air side of the diffuser or from the water side. Air side fouling may be
caused by dust and dirt that is drawn into the blower and deposited on the inside of the
diffuser, oil from the blower or piping, and pipe scale and rust.
Water side fouling may occur as a result of power failures, where pressure is lost on the
diffuser and solids begin to accumulate on and in the diffusion material. Poor air
distribution may allow contaminants to build up on diffusers where the air supply is
lower. Other causes of water side fouling include excessive amounts of oil and grease
in the wastewater, high organic load on the aeration tank, precipitation of inorganic
materials on the diffuser, and bioslime growth on the diffuser.
Facilities which utilize ceramic fine bubble systems are sometimes designed with a gas
cleaning system in which hydrogen chloride gas is injected into the diffusion system
periodically. This gas will form acidic conditions at the diffuser, killing biological slime
growths and dissolving inorganic precipitates. This may be performed by staff at the
facility or may be contracted to a firm that is more familiar with handling this type of
equipment. Although originally thought to be needed about every six months, many
facilities have found that cleaning is not required that frequently.
VI. Factors Affecting Biological Activity
Effect of Temperature on Activated Sludge
As is true of any biological system, the activity
and efficiency of the biomass in an activated
sludge facility is dependant to a fairly large
extent on the temperature of the wastewater. It
has been demonstrated that each 10 deg C
drop in water temperature in the aeration tank
reduces biomass activity by about 50%. This
means that as the water temperature drops,
organism growth rate slows down. BOD
removal will occur more slowly, and the system
will require a longer recovery after upset.
Wastewater temperature is usually more stable
in cities using ground water as the drinking
water supply. Influent sewage in these cities
usually stays pretty consistently in the 5055 degree F range. Facilities in cities that use
lake or river water as the drinking water source
may experience a wider range of wastewater
temperatures. Often the biggest changes in
wastewater temperature occur following a rapid snow melt and after rainfall.
As indicated in the graph above, biological activity increases to a maximum at a
temperature of about 100 degrees F. Increased temperature beyond that point would be
expected to result in a sudden die-off of the biomass.
5
Activated Sludge Manual
Effect of pH on Activated Sludge
As shown in the graph at the left, biological activity in an
activated sludge plant is best in a pH range of about 6.5
to 8.5. Growth may occur outside of that range, but at a
reduced rate, and may result in the filamentous bacteria,
especially at low pH values. Oxygen uptake is thought to
be optimum between pH 7.0 and 7.4. Generally pH
values below 7 are more detrimental than those above 7.
Sudden changes and frequent fluctuations in pH are
most damaging.
Although pH may be controlled at the wastewater
treatment plant by acid or base addition before the
aeration tanks, this is expensive, and not practical at
most municipal facilities and large industrial plants. The
best means of controlling influent pH is to control the source of acids and bases
discharged into the collection system.
Toxicity in Activated Sludge Facilities
A wide range of organic and inorganic compounds are known to be toxic to activated
sludge biomass. Many of the heavy metals such as cadmium, chrome, nickel, and lead
are toxic above about 1 mg/L. Silver, and arsenic, and mercury are toxic at
concentrations much less than 1 mg/L. Cyanide, herbicides, and pesticides are toxic to
this biological system as they are to any other.
Toxicity often first becomes apparent as a rise in D.O. concentration in the aeration tank.
As organisms become impaired or destroyed by the toxic material, the rate of oxygen
uptake decreases, so the D.O. concentration in the aeration tank increases. This may
be followed by deflocculation, where floc particles are no longer able to maintain their
structure and come apart to leave a very turbid effluent. Settleability in the secondary
clarifier is often impaired, and if the facility has been nitrifying a sudden increase in
effluent ammonia will usually occur as the nitrifying organisms are killed. Higher effluent
BOD may occur, as heterotrophic bacteria and other organisms are impaired or killed.
Organisms may acclimate to some extent to low concentrations of a toxic material, so
long as the concentration is fairly consistent. Activated sludge plants have been known
to remove significant amounts of heavy metals from wastewater flows after acclimation.
Sudden increases or long periods of time between dosages will produce toxic effects.
A problem which may result from the removal of toxic materials (especially metals) by
the biomass is that this material often concentrates in the sludge wasted from the
secondary system. This in turn causes problems for the facility that intends to dispose of
this sludge on agricultural land. The best solution to this problem is to control toxic
materials at their source, not allowing them into the collection system.
VI. Oxygen Uptake Rate (OUR)
The rate at which the biomass uses oxygen in the metabolism of pollutants in
wastewater is dependant on several factors. The amount and age of the biomass,
6
Activated Sludge Manual
water temperature, amount of food available, and the health of the biomass all affect
how quickly wastes are metabolized, and therefore how quickly oxygen is used.
Determination of the OUR may be used by the operator as a control tool, especially in
monitoring the system for toxic materials in the wastewater. A lower than normal OUR in
a system that has plenty of food and microorganisms is usually an indication that the
biomass has been impacted by toxicity; a higher than normal OUR may indicate an
organic overload. A high OUR that returns to normal within a short time may indicate a
shock or slug load.
While OUR must be determined on a fresh sample from the aeration tank, sample
collection point will be an important consideration. Samples should be collected at the
head of the tank where return sludge mixes with influent wastewater. OUR should be at
it’s highest at this point in the aeration tank and will give a good indication of metabolism
under high food conditions; probably the best sampling point to monitor for toxicity.
Samples collected near the discharge end of the aeration tank should indicate that
metabolism is nearly complete before passing to the secondary clarifier. A high OUR at
the discharge end of the aeration tank may be an indication of inadequate detention time
in the aeration tank or of an organic overload. It would be wise for the operator to collect
samples at both ends of the aeration tank and at the midpoint to have an overall
indication of how metabolism progresses through the tank.
The OUR test is simple and quickly performed with equipment already on hand in the
WWTP laboratory. Equipment needed includes a D.O. meter with BOD bottle probe and
a timer. OUR simply measures the amount of oxygen used up in a BOD bottle over a
10 minute time period, with the results reported as milligrams per liter of oxygen used
per hour.
The test can be modified to eliminate the variable due to the amount of biomass in the
sample by dividing the OUR by the grams per liter of MLSS. In this case the result is
reported as the Specific Oxygen Uptake Rate (SOUR), in units of milligrams oxygen
used per hour per gram MLSS. SOUR may also be referred to as the Respiration
Rate (RR).
7
Activated Sludge Manual
SPECIFIC OXYGEN UPTAKE RATE
(Respiration Rate)
1. Obtain a fresh sample of mixed liquor from the aeration tank.
2. Determine the concentration (Grams / Liter) of MLSS.
3. Saturate the sample with dissolved oxygen by shaking in a partially filled
container for at least 45 seconds (or use compressed air with an air
stone).
4. Completely fill a 300 ml BOD bottle and insert a calibrated D.O. probe.
Use stirrer on probe or a magnetic mixer and stir bar.
5. Wait for about a minute while the D.O. meter stabilizes.
6. Record mg/L D.O. decrease over a 10 minute interval.
7. Calculate SOUR, mg O2/hr/G
SOUR, mg O2/hr/gram = OUR, mg O2/L/hr
MLSS, grams/L
Example:
Determine the SOUR of a Mixed Liquor Sample Given the Following Data:
MLSS = 2500 mg/L Initial D.O. = 8.3 mg/L
Final D.O. = 2.4 mg/L
The oxygen depletion was recorded over 10 min.
8.3 mg/L - 2.4 mg/L X 60 min
10 min
hr
= SOUR = 14.16 mg O2/hr/G
2.5 G/L
8
Activated Sludge Manual
In some situations, it may be advantageous to determine the SOUR on a “Fed” sample;
in other words on a sample into which a source of food is added, typically a sugar
solution. If the SOUR is still low on a mixed liquor sample after the addition of the sugar
solution, the presence of a toxic or inhibitory substance is probably indicated.
The SOUR test can also be used to help determine whether a waste is biodegradable.
Substitute the supernatant from a settled mixed liquor sample with the wastewater in
question. If the SOUR is high, the waste is treatable in the biological process; if the
SOUR is low either the waste contributes little food, or a toxic effect may be indicated.
Again, the addition of sugar into the test would probably help to determine if the waste is
toxic.
An online respirometer, which monitors oxygen uptake continuously, may be helpful in
some facilities. Although expensive and fairly maintenance intensive, this
instrumentation may be warranted where influent loading characteristics may change
significantly and suddenly or where influent toxicity is a frequent concern.
9
MICROSCOPIC
EXAMINATION
Activated Sludge Manual
VI. Microscopic Examination of Activated Sludge
Microscopic examination of the MLSS can be a significant aid in the evaluation of the
activated sludge process. Although the heterotrophic and autotrophic bacteria which are
mainly responsible for purifying the wastewater are too small to be easily observed, the
presence of several other microorganisms within the sludge floc may give an indication
of treatment conditions and efficiency. The most important of these indicator
microorganisms are the protozoa and the rotifers. These higher life forms also play an
important role in clarifying the wastewater, consuming bacteria and small particulates,
and improving biomass settleability.
A predominance of ciliates and rotifers in the MLSS is a sign of good sludge quality.
Treatment under these conditions, with proper RAS, WAS and aeration rates, can be
expected to produce a high quality effluent. Inversely, a predominance of filamentous
organisms and a limited number of ciliates is characteristic of a poor quality activated
sludge. This condition is commonly associated with a sludge that settles poorly. The
sludge floc is usually light and fluffy because it has a low density. There are many other
organisms such as nematodes (worms) and waterborne insect larvae which may be
found; however, these typically do not significantly affect the quality of treatment.
The microorganisms which are important to the operator are the protozoa and rotifers.
As discussed previously, the protozoa eat the bacteria and help to provide a clear
effluent. Basically, the operator should be concerned with three groups of protozoa,
each of which have significance in the treatment of wastewater. These groups include
the following:
1) Amoeboids
2) Flagellates
3) Ciliates
Amoeboids
The cell membranes of Amoeboids are extremely flexible;
and the mobility of these organisms is created by the
movement of protoplasm within the cell. Food matter is
ingested by absorption through the cell membrane.
Amoeboids may predominate in the MLSS floc during startup periods of the activated sludge process or when the
process is recovering from an upset condition.
Flagellates
These organisms are characterized by the tail (Flagella)
which extends from their round or elliptical cell
configuration. Their mobility is created by a whipping
1
Activated Sludge Manual
motion of the tail, which allows them to move with somewhat of a corkscrew motion.
Flagellate predominance may be associated with a light-dispersed MLSS floc, a low
population of bacteria, and a high organic load (BOD). As a more dense sludge floc
develops, the flagellate predominance will decrease with an increase of bacteria. When
the flagellates no longer are able to successfully compete for the available food supply,
their population decreases to the point of insignificance.
Ciliates
These organisms are characterized by the rotating hair-like membrane (cilia) which
cover all or part of their cell membrane. Their mobility is created by the movement of the
cilia, and the cilia around the gullet are utilized for the intake of food. Ciliates may
predominate during the period of fair to good settling of the activated sludge.
They are considerably larger than flagellates and for the purposes of microscopic
examination may be classified into two basic groups, which are the free swimming and
the stalked ciliates.
Free Swimming Ciliates
Free swimming ciliates are usually apparent when there are a large
number of bacteria in the activated sludge. These organisms feed or
graze on the bacteria and clarify the effluent. Therefore, their
presence is generally indicative of a treatment process that is
approaching an optimum degree of treatment. A relative
predominance of flagellates indicates decreased treatment efficiency
and the MCRT of the system should be increased to maintain a
relative predominance of free swimmers, stalked ciliates and higher
forms of organisms such as rotifers.
Stalked Ciliates
These organisms are frequently present when the free
swimmers are unable to compete for the available food. A
relative predominance of these organisms along with rotifers will
indicate a stable and efficiently operating process.
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Activated Sludge Manual
Rotifers
Although large in comparison with most other activated sludge
organisms, rotifers are a group of the smallest multi-cellular
microorganisms. They are strictly aerobic, existing only where
dissolved oxygen is plentiful. Rotifers may attach to floc particles by use
of a forked tail or may swim freely in the wastewater, grazing on bacteria,
algae, protozoa, and small particulate matter in the waste. At their
anterior end they possess cilia which are rotated to gather food as well
as to propel the organism through the water. Rotifers are more abundant
at higher cell residence times and are an indication of a high level of
treatment.
MLSS Evaluation by Microscopic Examination
Observation of microorganism activity and predominance in the activated sludge can
provide guidance in making process control adjustments. The Worksheet for
Microscopic Examination for Activated Sludge can assist the operator in deciding
whether to increase or decrease the MCRT based on the relative predominance of the
organisms found in the MLSS. The decline of ciliates and rotifers is frequently indicative
of a poorly settling sludge. These observations may make it possible to detect a change
in organic loading or cell residence time before an upset occurs. These changes can be
correlated with observations of the settling characteristics of the MLSS in the 30-minute
settling test, and by calculation of the F/M. If the other tests confirm these observations,
adjustments to the MLSS should be made to alleviate the problem.
In summary, relative predominance of ciliates and rotifers are an indication of process
stability. This predominance is associated with the efficiency of treatment under various
loading conditions. An increase or decrease in the predominance of these organisms
may be indicative of process upset before there is a major effect on process
performance.
Selection of a Microscope
Features which should be considered the minimum when selecting a microscope to be
used for routine operational use include the following:
1) Built-in illumination or an external system which allows variations of light
intensity.
2) A condenser system.
3) A movable stage. Stage should be controlled by coaxial handle rather than a
manual push-pull.
4) 10X and 40X objectives.
5) 10X eyepiece.
Auxiliary equipment should include
6) Light blue filter (daylight type)
7) Slides
8) Coverslips
9) Several small dropping pipettes
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Activated Sludge Manual
Optical and Mechanical Features of
THE MICROSCOPE
4
Activated Sludge Manual
Use of the Microscope
Procedures for preparing slides and using the microscope include the following:
1) Select a clean cover slip and slide.
2) Use a pipette or a long tipped medicine
dropper to transfer one drop of the mixed
liquor sample onto the center of the glass
slide.
3) Carefully pick up the cover slip by two
corners. Do not touch the clean area.
4) Pull cover slip along glass slide towards drop
of sludge.
5) As soon as cover slip touches drop of sludge,
allow cover slip to fall onto glass slide.
6) Place slide on microscope stage.
7) Move stage up to within approximately 1/8
inch of objective. Look at glass slide through
the eyepiece of the microscope.
8) Use the coarse adjustment on the microscope to bring the sludge into the
field of focus.
9) Use fine adjustment to refine focus to suit your eyes.
10) Identify organisms in the sludge.
Procedures for Examination
When performing a microscopic examination of activated sludge, a sheet of paper
should be kept handy to sketch the types observed. In the event that unknown varieties
of microorganisms are made, the operator may identify these later. The objective of the
examination is to determine relative predominance of microorganisms. This may be
accomplished by the procedures outlined below and utilizing a worksheet as illustrated in
the attached.
Examination Procedures:
1) Record the date, time, temperature, and location of the sample on the
worksheet.
2) A minimum of three slides per sample should be examined.
3) Scan each slide and count the number of microorganisms in each group.
4) Provide a mark for each microorganism counted in the appropriate group
space on the worksheet.
5) After completing the examination of the three slides, total the number of
organisms counted in each group.
6) The three higher totals are interpreted as the predominating organisms.
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Activated Sludge Manual
WORKSHEET FOR
MICROSCOPIC EXAMINATION OF
ACTIVATED SLUDGE
Date:
Time:
AM
PM
BY:
Temp:
°C
Sample Location:
Microorganism
Group
Slide
No. 1
Slide
No. 2
Amoeboids
Flagellates
Free Swimming
Ciliates
Stalked
Ciliates
Rotifers
Worms
Relative Predominance:
1.
2.
3.
6
Slide
No. 3
TOTAL
Activated Sludge Manual
Good Settling
Rotifers
RELATIVE PREDOMINANCE
STRAGGLERS
PIN FLOC
Stalked
Ciliates
Rotifers
Stalked
Ciliates
Nematodes
Rotifers
Free Swimming
Ciliates
Nematodes
Free
Swimming
Ciliates
Stalked
Ciliates
Free
Swimming
Ciliates
Rotifers
Flagellates
Free
Swimming
Ciliates
Amoeboids
Flagellates
Amoeboids
Flagellates
Flagellates
Amoeboids
Amoeboids
RELATIVE NUMBER OF MICROORGANISMS VS. SLUDGE QUALITY
7
Stalked
Ciliates
Free Swimming
Ciliates
Flagellates
Amoeboids
CLARIFIERS
Activated Sludge Manual
VII. Clarifiers
Purpose and principle
Clarifiers may be referred to as sedimentation basins or settling tanks. All three terms
describe the function of clarifiers, to provide a quiescent area which will result in the
separation of settleable solids from the wastewater flow.
Preliminary
Treatment
Primary
Treatment
Secondary Treatment
Bar Screens
Grit Removal
Primary
Clarifier
Aeration
Tank
Secondary
Clarifier
Primary clarifiers may be used following preliminary treatment to separate the heavy
organic solids from wastewater before the flow continues to the secondary (activated
sludge) process. Design detention time is typically about 2 hours. Primary clarifiers
remove a large part (up to about 40%) of the solids and BOD load which would
otherwise have to be treated in the secondary process, allowing smaller secondary
process design and improved efficiency. Primary clarification is typically not used in
extended aeration activated sludge systems, since the secondary treatment system in
these facilities is much larger than in conventional activated sludge plants.
Solids that accumulate in the primary clarifiers are commonly referred to as primary
sludge or raw sludge. This sludge is usually not pumped from the clarifier continuously,
but is pumped on a periodic basis, typically once or twice per shift. Since there is little
dissolved oxygen in this part of the process and the sludge is unstable, primary sludge
must be frequently removed from the clarifier to prevent anaerobic decomposition.
Sludge that is not removed often enough may cause excessive odors and floating sludge
as gasses (carbon dioxide and methane) are released from the decomposing organics.
Floatable material such as oil and grease is skimmed from the surface of the clarifier. In
many facilities this is pumped directly to the solids handling process, while some plants
have a separate system for concentrating the waste before disposal.
Primary clarifier effluent will contain the particulates which are not readily settleable as
well as all of the dissolved solids in the influent wastewater. This flow then passes on to
the secondary portion of the treatment process.
Secondary clarifiers follow a secondary biological treatment process. In the activated
sludge process secondary clarifiers separate the biomass (mixed liquor) from the flow,
concentrating it in the bottom of the clarifier. These solids are then pumped from the
bottom of the clarifier back to the head of the aeration tank as return sludge.
Although secondary sludge is usually more stable than primary sludge, it is important to
keep the sludge blanket level under control. Secondary sludge is typically lighter than
primary sludge and may be carried out of the clarifier if the sludge blanket level gets too
1
Activated Sludge Manual
high. Secondary sludge is also susceptible to filamentous bulking and to floating sludge
due to denitrification.
The sludge blanket in the secondary clarifier should be maintained in the range of
1 to 2 feet thick. This will provide enough clear water on top of the sludge to avoid losing
solids from the clarifier. Keeping some sludge in the clarifier allows the solids to
concentrate so that fewer gallons of material must be wasted and return sludge flows
may be lower.
General Clarifier Considerations
Clarifiers, in general, are designed around Stoke’s Law. This mathematical model
relates the factors which determine the settling rate of particles in water.
VF = 2(p - po)g (d/2)2
9n
VF
(p - po)
n
g
d
Fall Velocity
Density difference between the particle and water
viscosity of water
gravitational acceleration constant
diameter of the particle
Stoke’s Law indicates that the vertical drop (settling rate) of particles in water is directly
related to the difference in density between the particles and water and the size of the
particle. The settling rate is inversely related to the viscosity of water. So as the density
and size of the particle increase the settling rate increases. As the density and viscosity
of water increase the settling rate decreases.
The temperature of the water in the clarifier is related to both the density, and to a less
extent, the viscosity of water. As the temperature of water drops its density increases
(until 4oC). This means that as water becomes colder, its density becomes closer to the
density of the particles that are trying to settle, and the settling rate decreases.
The efficiency of the clarifier in removing solids is also related to the physical aspects of
the clarifier. Generally, as surface area increases the more efficient the clarifier. Depth
must be adequate to prevent scouring solids from the sludge blanket into the effluent.
The size of the clarifier as it relates to the quantity of flow through it (hydraulic loading),
the size of the clarifier as it relates to the quantity of solids (solids loading), the shape of
the clarifier, influent/effluent design, the solids removal mechanism, and operation and
maintenance of the clarifier all affect efficiency.
Flow characteristics in the clarifier are a big factor in determining its ability to settle
solids. The flow must be dispersed as evenly as possible throughout the clarifier so that
solids are not carried out by areas of high flow velocity. Short circuiting in clarifiers
may be defined as a situation in which the flow in part of the clarifier is higher than in the
rest of the clarifier. This condition may be caused by several factors including:
• Missing or poor influent baffling
• Uneven effluent weirs
• Plant growths or accumulations of trash in the effluent weirs
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Activated Sludge Manual
•
Temperature stratification due to very cold or very warm influent
wastewater
Types of Clarifiers
While there are many differences among the various manufacturers of clarifiers, they
can be generally grouped into rectangular, circular, or inclined plate.
Inclined plate (Lamella) clarifiers have increased surface
area available for settling due to a series of plates (or in some
cases tubes) inclined in the clarifier. This type of clarifier
works well for many industrial applications, but has limited
use in biological treatment plants. Slime growths accumulate
on the plates causing short circuiting and plugging and
become very maintenance intensive.
Rectangular clarifiers are often used as primary clarifiers in
activated sludge plants, and are sometimes used as
secondary clarifiers.
Inlet
Inlet
Baffle
Effluent
Weir
Drive
Unit
Flight
Drive
Gear
Sludge
Trough
Sludge
Withdrawal
Pipe
Scum
Trough
Effluent
Launder
Idler
Sprockets
Drive
Chain
Rectangular Clarifier
Influent wastewater is dispersed throughout the tank by an inlet baffle. This baffle may
be a vertical plate, an inlet “T”, or a series of gate valves. As the flow passes through
the tank solids settle to the bottom and the clarified water flows over an effluent weir and
into the discharge launder.
Solids that settle to the bottom are scraped into a sludge hopper at the influent end of
the clarifier by a set of wooden or fiberglass flights. These flights are nearly as long as
the clarifier is wide and are attached to a steel or plastic chain driven by an electric
motor and sprocket. In most plants where rectangular clarifiers are used as primary
treatment the flights do not run continuously, but are turned on an hour or so before
sludge pumping begins and are shut off when pumping is completed.
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Activated Sludge Manual
As the flights are drawn along the surface of the
clarifier they move floating material toward a scum
removal trough. This is generally a manually
operated pipe which is slotted. As the operator turns
the slot into the water the surface is skimmed and
scum is discharged to solids handling or scum
handling processes.
Effluent weirs are usually of the “V notched” or “saw
toothed” design. This helps to keep flow velocity constant as the flow rate over the weir
increases. It also tends to clear particles which tend to become trapped behind the weir.
In a rectangular clarifier there may be one weir or several
weirs. In general, as the number of linear feet of weir
increases, the more efficient the clarifier will be. The
weirs may be straight, or may be serpentine (S-shaped).
Serpentine weirs increase the overall length of weir in the
clarifier.
Circular Clarifiers are used mainly as secondary clarifiers, but may be used as primary
clarifiers.
In the most common circular clarifier configuration, influent enters in the center of the
Scum Beach/Trough
Center Baffle
Drive Unit
Skimmer Arm
Scum
Baffle
Effluent
Weir
Effluent
Launder
Clarifier Influent
Sludge Sump
Sludge Plow
Circular Clarifier
tank into the center well. A circular baffle prevents the solids from being carried by the
flow directly to the effluent weir, directing them toward the floor of tank. Solids settle to
the floor of the tank where they are scraped to a sludge sump in the center.
In activated sludge plants, the sludge is pumped continuously (return activated sludge)
back to the influent end of the aeration tank. The sludge collection mechanism consists
4
Activated Sludge Manual
of a rotating frame with plows attached to it. The plows are set at an angle so that as the
sludge collector rotates the sludge is moved to the sludge sump. Adjustment of the
clearance between the plows and the floor of the tank may be necessary periodically to
prevent contact between the plows and the floor, and also to avoid leaving an excess of
solids in the tank.
The mechanism which drives the sludge collection system also drives a skimmer at the
surface of the clarifier. As the skimmer rotates, floating material is directed onto a scum
beach and into the scum trough. Just before the skimmer arm reaches the scum beach
an underwater lever is activated, allowing a flow of flushing water through the scum
trough. The scum is then directed to the solids handling or scum handling process.
Around the perimeter of the tank, clarified water
passes under a scum baffle and over the effluent
weir into the discharge launder. The launder and
weir may be attached directly to the outer wall of
the clarifier (1), may be attached to the inner wall
(2), or the launder and weirs may be suspended
in the clarifier (3). As in primary clarifiers, Vnotched weirs are generally used.
Scum
Baffle
Effluent
Weir
Effluent
Launder
Effluent
Circular Clarifier Outlet
1
2
3
A problem that is commonly experienced with
the design shown in diagram #2 above, is that
the launder may contribute to a current in the
tank which contributes to the loss of solids over
the effluent weir. To minimize this some newer
clarifier designs include a baffle which extends
down from the inside corner of the launder at an
angle. This directs the current back toward the
center and bottom of the clarifier.
The design shown in diagram number three may also be prone to solids loss, especially
between the outside edge of the launder and the clarifier wall. Careful consideration of
the placement of the launder (not too close to the clarifier wall) and proper baffling is
especially necessary when building this type of clarifier.
Clarifier Variations
Through the years many alternative clarifier designs have been used, some successfully
and some not so successfully. The following discussion lists a few of the more often
used alternative designs.
5
Activated Sludge Manual
One design which has seen some use in
Michigan is a cross between the
rectangular and circular clarifiers. It is a
square clarifier with a circular sludge
collection and skimming mechanism. This
design is commonly referred to as the
Squircular clarifier. The advantage that
this design might have over a rectangular
clarifier is the effluent weir on all four sides
of the square rather than on just one end.
It also has a surface area advantage over the circular clarifier.
One potential disadvantage of this design is that the sludge collection mechanism must
be designed to collect the solids in the corners and along the sides of the square tank.
The sludge collector has a section which flips out to plow the sludge in the tank corners,
and then retracts back up into position after it has passed the corner. This has been a
maintenance concern in some plants with these clarifiers.
Some clarifiers are designed as
Peripheral feed, rather than center feed.
The wastewater enters a baffled area
around the perimeter and then passes
through slots in the bottom of the baffle
into the bottom of the clarifier. The sludge
blanket in the clarifier is intended to filter
fine particles out of the wastewater flow as
it passes up through the blanket. The
picture at the right shows a peripheral feed
clarifier with radial discharge launders.
Radial Discharge
Peripheral Feed
Peripheral feed, center discharge
The peripheral feed, center discharge design shown above is used in several facilities in
Michigan, and seems to be very successful. In this particular design, the baffle extends
almost to the floor of the tank, with a continuous slot along the bottom. Wastewater
passes through the slot, up into the clarifier, and travels to the discharge weirs at the
center. Scum is collect between the baffle and the clarifier wall.
6
Activated Sludge Manual
There are also variations in the way that sludge is removed from the clarifier. Rather
than plowing the sludge to a sump in the center of the clarifier, in some designs the
sludge collection mechanism sucks the sludge into pipes which rotate along with
triangular shaped sludge plows. The operator can control the sludge flow through each
pipe by adjustment of the telescoping valves pictured above at the far right.
In a somewhat similar design, the clarifier pictured at the
left, draws the sludge into a rotating horizontal tube.
Sludge collection systems using the vacuum approach
may have the sludge collector connected directly to a
sludge pump, or may rely on hydraulic pressure to draw
the sludge into the collector.
Clarifier Operation
Good clarifier operation begins with routine inspection and maintenance. Drive
mechanisms need to be lubricated and maintained as required by the manufacturer. At
least a couple of times each shift clarifiers should be observed for proper operation. At
least once each shift sludge depths should be determined in each clarifier.
In rectangular clarifiers the operator should watch for smooth operation of the sludge
collection system. Erratic movement may indicate broken flights, worn chains or
sprockets, or large debris in the tank. Floating sludge may indicate worn or broken
components. Tripped breakers or sheared pins on the drive mechanism are also
indications of excessive wear, improper alignment, or large debris jamming the
equipment.
In circular clarifiers the operator should check to see that the skimmer/sludge collector
operates smoothly. Also check to make sure that the scum trough is not plugged.
Solids tend to collect in the center baffle, along the weirs, and in the launder of circular
clarifiers, requiring frequent cleaning. Algae and water weeds grow quickly in summer
months when the water is warm and there is plenty of sunlight. Cleaning can be
accomplished in several ways, but most often involves an operator with a broom. Care
must be taken to not get into a situation where safety is compromised. Hosing the slime
growths off is probably safer and faster but may not be as effective.
7
Activated Sludge Manual
Some facilities have installed hoses and nozzles along the weir and launder through
which they spray chlorinated water to control slime growths. This may be effective, but
care must be taken to avoid causing a chlorine residual in the effluent that exceeds the
limit in the discharge permit.
Some facilities have installed brushes on the
skimmer arms of circular clarifier. As the
skimmer arm rotates, the brushes clean the
weir and launder. This has proven to be
effective and certainly much easier than
manually cleaning the clarifier. Usually the
brushes are spring loaded, and can be
disengaged so that the operator can control
whether or not they contact the clarifier
surfaces, minimizing wear on the brushes.
These systems are commercially available, but
some facilities have designed and installed their
own cleaning brushes.
A few facilities have installed covers over the
clarifier weirs and launders. This blocks the
sunlight and limits plant growth. This is a fairly
simple and effective method, but it does limit
access when the need for cleaning or maintenance
arises.
Many plants have installed aluminum or
fiberglass covers on the secondary clarifiers.
These also help to minimize algae by
minimizing light. The covers allow easy
access to the clarifiers and provide a much
more comfortable working situation when
working on the clarifier in cold weather.
Clarifier Loading
Clarifiers, like most wastewater treatment processes, are designed to treat a given
amount of wastewater. Flows in excess of that design value will result in loss of
efficiency and solids in the clarifier effluent. Clarifier loading may be expressed in terms
of hydraulic or solids loading. Hydraulic loading refers to the number of gallons going
into the clarifier as it relates to the size of the clarifier.
8
Activated Sludge Manual
Detention time is an expression of hydraulic loading, indicating how long it will take the
water to pass from clarifier influent to effluent. This is typically expressed in hours, with
a design value usually between 2 and 3 hours.
DT, hrs =
Tank Volume, MG X 24
Flow into Tank, MGD
Be careful to make sure that the tank volume units and flow units match; if volume is
given in millions of gallons use units of million gallons per day for the flow. If the volume
is given in gallons, use gallons per day as the flow. The “24” in the equation converts
from units of “day” to units of “hours”.
Surface Overflow Rate (SOR), a very commonly used expression of hydraulic loading
relates the number of gallons per day of water overflowing the clarifier, to its surface
area in square feet. Design values for average flow conditions are usually in the range
of 400 to 800 gallons per day per square foot.
SOR, gpd/ft2 = Flow, gallons/day
Surface Area, ft2
Weir Overflow Rate (WOR) is also often used to express hydraulic loading. WOR
relates the number of gallons per day overflowing the clarifier weirs to the number of
linear feet of weir. The design value for WOR is typically in the range of 10,000 gallons
per day per foot of weir.
WOR, gal/d/ft = Flow, gallons/day
Length of Weir, ft
Solids Loading Rate (SLR) relates the number of pounds of solids per day entering the
clarifier to its surface area in square feet. The design value for SLR is usually about 25
to 30 pounds per day per square foot.
SLR, lbs/d/ft2 = Solids, lbs/day
Surface Area, ft2
By becoming familiar with these clarifier loading calculations, the operator may be able
to troubleshoot settling problems. If clarifier effluent suspended solids are excessive, a
settleability test on the solids will help to determine whether they should have settled in
the clarifier. If the solids settle in a settleometer but not in the clarifier, determination of
hydraulic and solids loading on the clarifier may help to explain the problem.
9
NITROGEN
Activated Sludge Manual
VIII. Nitrogen
The Nitrogen Cycle
Nitrogen may be a concern for environmental reasons as well as for public heath
reasons, depending on the form or compound that the nitrogen is in. For instance,
ammonia (NH3) is often limited in discharge permits due to its toxicity to aquatic
organisms, and because it causes an oxygen demand in water. Ammonia, nitrite (NO2-)
and nitrate (NO3-) are fertilizers, increasing the growth rate of weeds in rivers and lakes.
Nitrite and nitrate are limited in discharges to groundwater because of they interfere with
the respiration process in infants, causing a condition known as “blue baby syndrome” or
methemoglobinemia. Nitrite is a concern in the operation of wastewater treatment plants
in which chlorine is used as a disinfectant in that the nitrite causes a large chlorine
demand.
Nitrogen exists in many forms which transform continually in the environment and in
wastewater treatment plants. The Nitrogen Cycle diagram below illustrates these forms
and the various transformations that occur. Understanding the nitrogen cycle will be
important for wastewater treatment plant operators, especially when working with
facilities where nitrification and/or denitrification are required.
5
6
1
2
4
3
1
Activated Sludge Manual
Starting at the center-right of the diagram (1), atmospheric nitrogen makes up about
79% of the air that we breathe. Lightning oxidizes this nitrogen gas to nitrate which
becomes fertilizer for plants. Fertilizer manufacturers also convert nitrogen gas to
nitrate (2) and to ammonia (5). Nitrate and ammonia in water and soil are used by
plants as fertilizer, and some plants take up (fix) nitrogen directly from the air (3). As
plants take up nitrogen and as animals consume plants (4), nitrogen becomes an
essential component of every cell of every living plant and animal; it is one of the key
elements required for survival, being used to build proteins and other complex organic
molecules. When plants die, bacterial decomposition of these organic nitrogen
compounds releases nitrogen in the forms of ammonia gas (NH3) and ammonium
dissolved in water as an ion (NH4+) (5). As animals metabolize food, and when the
animal dies and decomposes, byproducts also decompose to release ammonia. From
the diagram this far, atmospheric nitrogen has become part of plants and animals to
form organic nitrogen compounds. When these compounds decompose ammonia is
released. It is obvious that the influent to every wastewater treatment plant treating
domestic wastes will contain nitrogen in two main forms, organic nitrogen and
ammonia/ammonium. The sum of organic nitrogen and ammonia nitrogen is analytically
termed Total Kjeldahl Nitrogen (TKN). This is an important term, since it is a
measurement of the nitrogen in wastewater that will be available to the biomass.
Continuing around the diagram in a clockwise direction from NH3, some types of bacteria
(autotrophic bacteria) are able to oxidize ammonia first to NO2 (6) and then to NO3 (2);
this is the process of nitrification. Also notice that this process can be reversed, with
NO3 being reduced back to NO2 (6) and then to N2 (1); this is the process of
denitrification. At this point the cycle has been completed; nitrogen gas originating in
the atmosphere has been returned to the atmosphere.
Nitrification
NH3
Aerobic
NO2
NO3
Autotrophic Bacteria
The nitrification process is important in wastewater treatment due to the concern for
ammonia toxicity and nitrogenous oxygen demand in effluents. It is not unusual for
National Pollutant Discharge Elimination System (NPDES) permits for facilities
discharging to surface water to limit the concentration of ammonia in the effluent.
Depending on several factors, the limit may be less than 1 mg/L. With a typical influent
ammonia concentration of 15 – 20 mg/L, the facility must be capable of nitrification,
converting nearly all of the ammonia to nitrate before discharge. Nitrate is not generally
regulated in surface water discharges since it is not considered toxic and exerts no
oxygen demand.
Keep in mind that as organic pollutants are broken down by heterotrophic bacteria in the
activated sludge biomass, nitrogen is released in the form of ammonia. At the same
time the biomass is growing and taking up nitrogen; although the biomass typically does
not remove enough nitrogen to meet discharge permit limits for ammonia, biomass
uptake does account for a significant amount of nitrogen removal.
2
Activated Sludge Manual
Nitrification is also a biological process, but involves autotrophic bacteria. These
bacteria are able to utilize inorganic compounds (like ammonia) as an energy source,
using carbon dioxide (or bicarbonate) as a carbon source to build cells.
Two types of autotrophic bacteria are involved in the nitrification process: Nitrosomonas
oxidize ammonia (released by heterotrophic bacteria) to nitrite, and then Nitrobacter
oxidize nitrite to nitrate.
NH3
NO2
Nitrosomonas
NO3
Nitrobacter
In facilities where nitrification is required to reduce the concentration of ammonia in the
effluent, the objective is to provide conditions in the activated sludge system which will
encourage the accumulation of these autotrophic bacteria. In some plants this occurs
almost automatically due to system design (extended aeration), while it may be more of
a challenge at others.
Several factors must be considered to trying to achieve nitrification in an activated
sludge plant:
1. As indicated in the equation, nitrification is an oxygen consuming process,
requiring about 4.6 milligrams of oxygen for each milligram of ammonia oxidized
to nitrate. Nearly twice as much oxygen is required to nitrify as to remove CBOD.
Dissolved oxygen must be maintained at a higher concentration than for just
CBOD removal. Although nitrification may begin at D.O. as low as 0.5 mg/L,
typically D.O. in the aeration tank should be increased to 3 – 5 mg/L to assure
efficient nitrification.
2. Nitrifying organisms do not compete well with heterotrophic bacteria and are
inhibited by soluble carbonaceous BOD. Detention time in the aeration tank
must be long enough (> 5 hours) to achieve nearly complete removal of CBOD
before the nitrification process can begin. Nitrification is best at low organic
loading and low F:M (< 0.25).
In-plant return flows, such as from solids handling processes are usually high in
soluble BOD and ammonia and can cause loss of nitrification or exceed
nitrification capability if not carefully controlled. It is important that these highstrength wastes are returned through the plant slowly, in low quantity, and at
times when loading on the system is at its lowest.
3. Nitrifiers grow more slowly than heterotrophic bacteria. Nitrification may begin at
a CRT of 4 or 5 days, but is best if over 8 days.
4. Temperature is an important factor in the nitrification process. As wastewater
temperature decreases, nitrifier growth rate decreases, resulting in reduced
nitrification. Below a wastewater temperature of 50 oF nitrification becomes very
difficult, requiring long detention time and high CRT. If the nitrification process
becomes upset in cold weather, recovery may be nearly impossible until
wastewater temperatures rise. Low temperature especially impacts the
nitrobacter; this may result in an accumulation of nitrite, causing a disinfection
problem if using chlorine.
3
Activated Sludge Manual
Higher MLSS concentrations may help to compensate for cold weather
nitrification problems, but this approach is limited by the amount of biomass that
can be maintained in the system. Oxygen transfer must be adequate to support
the quantity of biomass in the system, the secondary clarifier must be capable of
settling all of the biomass, and high CRT problems (slurp, straggler floc) may
result.
5. Autotrophic bacteria are pH sensitive, with best nitrification occurring between pH
8.0 – 8.5. The nitrification process consumes alkalinity according to the following
formula:
NH4HCO3 + O2
HNO3 + H2O + CO2
This indicates that ammonium bicarbonate (bicarbonate alkalinity) is being used
up, and that nitric acid, water, and carbon dioxide are being produced. Recall
that the alkalinity of a solution provides buffering capacity (resistance to change
in pH). In the nitrification process buffering capacity is being consumed, and
nitric acid is being produced. If enough alkalinity is consumed and enough nitric
acid is produced the pH of the solution will drop, inhibiting the autotrophic
bacteria and the nitrification process.
For every pound of NH3 Oxidized, 7 pounds of alkalinity are destroyed. And
there may be additional reactions occurring that consume alkalinity; chemicals
added for phosphorus removal also destroy alkalinity:
5.3 - 13.5 lbs Alkalinity per lb Fe Added
6.0 - 9.0 lbs Alkalinity per lb Al Added
Wastewater and sludge that is allowed to become septic in the collection system,
primary clarifiers, sludge thickeners, or other solids handling processes also
generate acid which consumes alkalinity. All of these factors should be
considered when troubleshooting nitrification problems.
Reduction of pH due to insufficient alkalinity is not typically a problem in activated
sludge plants treating domestic wastes, especially where groundwater is used as
the drinking water source. Domestic wastewater generally does not contain an
excessive amount of ammonia, and groundwater is typically fairly hard,
containing sufficient alkalinity to nitrify without a depression of the pH. This may
be more of a concern in situations where the influent ammonia concentration is
high and surface water or softened water is used as a drinking water supply.
In general, if the influent total alkalinity is at least 150 mg/L there is sufficient
alkalinity for nitrification of ammonia in domestic wastewater treatment. If total
alkalinity of the effluent is at least 50 mg/L pH depression should not occur. If
low alkalinity is found to be the cause of nitrification problems, sodium
bicarbonate, sodium hydroxide, or lime may be added to the aeration tank to
increase pH and add buffering capacity.
One reason for continued success of extended aeration activated sludge systems such
as sequencing batch reactors and oxidation ditches is that they are designed with long
detention time in the aerator (24 hours), high CRT (20 days), and low F:M (0.05 – 0.15).
These conditions nearly assure nitrification to very low levels of effluent ammonia, given
4
Activated Sludge Manual
sufficient alkalinity, high enough wastewater temperature, and careful operator attention
when returning flows from solids handling processes back into the wastewater stream.
Denitrification
Denitrification is the reduction of nitrate to nitrite, and then to nitrogen gas by
heterotrophic (BOD consuming) bacteria.
NO3
Anoxic Environment
NO2
Heterotrophic
Bacteria
N2
Heterotrophic
Bacteria
As indicated by the equation above, in an anoxic environment (no dissolved oxygen, but
nitrate is present) heterotrophic bacteria are able to use oxygen from nitrate and nitrite,
releasing nitrogen gas.
Nitrogen gas released during denitrification sometimes presents an operational problem
in wwtp’s that are nitrifying. As mixed liquor enters the secondary clarifier, the solids
settle to the bottom. If the solids remain in the clarifier too long the D.O. may begin to
drop, creating an anoxic condition. Nitrate becomes denitrified, releasing nitrogen gas in
the form of very small bubbles which rise to the surface of the clarifier, bringing some of
the settled biomass with it. This may result in globs of biomass floating on the surface,
or it may result in a substantial amount of the sludge mass rising to the surface. If this
problem is suspected the operator should watch for nitrogen gas bubbles when
performing a settleability test. If bubbles are observed or if the biomass floats to the top
of the settleometer within a 1 hour settling time, care must be taken to keep the sludge
blanket in the secondary clarifier at minimal levels. Return sludge rates should be
adjusted to assure that the biomass does not remain in the clarifier long enough for
denitrification to occur.
Denitrification is required where nitrate or nitrite in the discharge is a concern. In
situations in which the wastewater treatment plant discharges effluent into groundwater,
a groundwater discharge permit is required. This permit usually limits the Total
Inorganic Nitrogen (TIN) (NH3-N + NO2-N + NO3-N) in the discharge to 5 mg/L. The
facility must be capable of first nitrifying the ammonia to nitrate, and then denitrifying the
nitrate to nitrogen gas.
The diagram below illustrates a typical flow schematic for a WWTP that is designed for
denitrification. BOD is reduced, and ammonia is nitrified to nitrate as the wastewater
passes through the first aerobic (oxic) reactor. The mixed liquor then flows into an
anoxic (zero D.O.) environment and is supplied with a food source. As bacteria in the
Return Sludge
Aeration Tank
BOD Removal and
Nitrification
(Oxic)
Denitrification
(Anoxic)
5
Oxic
C
Clarifier
Activated Sludge Manual
mixed liquor metabolize the BOD in the absence of D.O., they utilize the oxygen from the
nitrate molecule, releasing nitrogen into the atmosphere. To prevent these nitrogen gas
bubbles from causing a rising sludge problem in the secondary clarifier, the mixed liquor
flows from the anoxic zone into another aerobic zone where the nitrogen as is stripped
from the mixed liquor by the air bubbling through it.
Denitrification is also required in nitrifying facilities which remove phosphorus
biologically. Nitrate is a source of oxygen and would interfere with the ability to attain the
anaerobic condition necessary in this process.
Other modifications of the activated sludge process that are usually designed to nitrify
and denitrify include the Sequencing Batch Reactor and the Concentric Ring Oxidation
Ditch.
Probably the most often encountered problem when trying to achieve denitrification is
that of controlling dissolved oxygen. When wastewater treatment plants are newly
constructed the organic load is usually lower than design and aeration capability greatly
exceeds demand. This results in an excess of D.O. in the aerobic stage that impairs the
ability to achieve anoxic conditions where necessary. The effluent contains low
ammonia but high nitrate, which does not meet the 5 mg/L TIN groundwater discharge
permit limit. Designers and operators must assure flexibility in facility design that will
allow very close control of organic loading (number and size of reactors) and D.O.
control (number and size of aeration blowers) where denitrification is required.
There are benefits to denitrification in addition to discharge permit compliance. Zero
D.O. conditions may help to control the growth of filamentous bacteria, denitrification
recovers oxygen by forcing the bacteria to utilize oxygen that was required in the
nitrification process, and denitrification recovers alkalinity that was consumed in the
nitrification process.
3.57 mg Alkalinity (OH-) Produced per mg NO3 Reduced
NO3 + ORG
N2 + CO2 + H2O + OH-
Nitrite and Disinfection Problems
Nitrite (NO2-) can cause disinfection problems in wastewater treatment plants that use
chlorine. This is due to the fact that nitrite is unstable, and is easily oxidized to nitrate
(NO3-) by the chlorine. Nitrite is sometimes referred to as a chlorine “sponge” because
of its affinity for chlorine. The chlorine reacts with nitrite instead of disinfecting, leaving
the plant with increased fecal coliform in the effluent.
WWTP’s that typically nitrify most often experience this during the months of February
and March. WWTP’s that do not typically nitrify may see this problem later in the Spring
and in early Winter.
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Activated Sludge Manual
When chlorine is added to water, hydrogen chloride and hypochlorous acid are formed
(equation 1). Hypochlorous acid ionizes to hydrogen ion and hypochlorite ion
(equation 2). The hypochlorite ion (OCl-) is a strong oxidizer and will react with nitrite to
form nitrate and chloride (equation 3).
(1)
Cl2 + H20
(2)
HOCl
(3)
OCl- + NO2-
HCl + HOCl
H+ + OClNO3- + Cl-
Since chloride has no disinfecting properties, disinfection will be impaired. The amount
of chlorine consumption by nitrite may be estimated as 5 milligrams of chlorine per
milligram of nitrite.
Cold Water NO2 Problem
At wastewater temperatures above 63oF (17oC) the conversion of ammonia to nitrite is
the slowest step in the nitrification process, with the conversion of nitrite to nitrate
occurring rapidly. This results in low concentrations of ammonia, low concentrations of
nitrite (< 1 – 2 mg/L NO2), and high concentrations of nitrate in the effluent.
NH3
NO3
NO2
When wastewater temperatures drop to between 54 and 57oF (12 and 14oC) the first
reaction step becomes the fastest. Ammonia is oxidized to nitrite, but the conversion of
nitrite to nitrate takes longer.
NO2
NH3
NO3
This causes an accumulation of nitrite (NO2 can reach 15 mg/L) in the effluent. With
each milligram of nitrite using up 5 milligrams of chlorine, it may be nearly impossible to
supply enough chlorine to disinfect the flow.
Warm Water NO2 Problem
Nitrite consumption of chlorine is not only a cold weather problem, but may also occur in
wastewater treatment plants that are nitrifying in warm water. In the process of
denitrification, nitrates are biologically reduced first to nitrite and then to nitrogen gas
which is released into the air. Denitrification occurs in an anoxic (low D.O.) environment
and requires that a food source be present for the heterotrophic bacteria. In the
absence of D.O. and with a food source present, the bacteria utilize nitrate oxygen as
they metabolize food, releasing nitrogen gas.
The reduction of nitrate to nitrite occurs quickly, while the reduction of nitrite to nitrogen
gas is a slower process.
NO3
NO2
N2
If a food source is present both steps occur rapidly enough that nitrite accumulation is
minimal. If a food source is not available during denitrification, the second step takes
7
Activated Sludge Manual
place so slowly that nitrites accumulate and chlorine intended for disinfection is
consumed in oxidizing the nitrite back to nitrate.
Facilities that do not intentionally denitrify must be careful to keep the entire system
aerobic, and to be sure that return sludge rates are sufficient to prevent denitrification of
the secondary sludge in the clarifier. Facilities that intentionally denitrify should try to
assure that the organisms have a sufficient supply of BOD in the denitrifying part of the
process.
Toxicity and the NO2 Problem
Nitrifiers are usually more affected by toxic wastes than other types of bacteria. Toxicity
may result in the over-all loss of nitrification, with increased ammonia in the effluent.
Some toxins affect the Nitrobacter more than the Nitrosomonas. This means that the
conversion of ammonia to nitrite may occur, while the conversion of nitrite to nitrate may
be inhibited, with a corresponding accumulation of nitrite in the effluent. Fuels such as
kerosene, jet fuel, gasoline, and diesel oil are some of the materials known to be toxic to
Nitrobacter.
Effect of Ammonia on the NO2 Problem
When chlorine is dissolved in water that does not contain ammonia, the chlorine reacts
as “free chlorine”. While this provides disinfection, free chlorine also reacts very quickly
with nitrite to oxidize it to nitrate. So in wastewater treatment plants that are very low in
effluent ammonia but there is an accumulation of nitrite, disinfection problems are very
likely to occur.
NH3 +
NO2 + NO3 + Cl = Disinfection Problems
If NH3 is present, free chlorine reacts with the ammonia to form chloramines.
NH3 + Cl
NH2Cl
While not as strong a disinfectant as free chlorine, chloramines (such as
monochloramine, NH2Cl) are more stable in water and do not react quickly with the
nitrite. This allows disinfection to continue even in the presence of nitrite.
8
PHOSPHORUS
Activated Sludge Manual
IX. Phosphorus
Importance of Controlling Phosphorus
Phosphorus (P) is regulated in wastewater discharged to surface water due to its
properties as a fertilizer. Like nitrogen, phosphorus is taken up by living organisms
approximately according to the 100C:5N:1P ratio. It is one of the essential nutrients
needed to build cells and sustain life. Plants do not grow where there is not an adequate
supply of P. This is true in agriculture where P is added to soil to encourage the growth
of crops, but is also true in aquatic systems where limiting plant growth is desired.
Plant growth in lakes is related to the state of
eutrophication of the lake. Lakes that are cold and
deep, with minimal plant growth and very low nutrient
concentration are classified as Oligotrophic (oligo =
few, trophic = nutrient). Lakes that are in a changing
state with regard to nutrient load are classified as
Mesotrophic (meso = middle), while lakes that are
shallow, warm, and receive high nutrient loading are
classified as Eutrophic (eu = well, eutrophication = well
nourished).
Controlling the eutrophication rate of lakes involves controlling plant growth rate. This
can be accomplished by controlling the nutrient load into the lake. While nitrogen and
phosphorus are both nutrients needed by plants, nitrogen is too available naturally to be
used as a practical control method. Phosphorus on the other hand, is only available to
lakes as minerals containing phosphorus dissolve, or as fertilizers are discharged to the
lake from point source discharges such as wastewater treatment plants or from nonpoint sources such as agricultural run-off. Controlling phosphorus loading into lakes is a
practical method of limiting the growth rate of plants and the rate of eutrophication.
Phosphorus limits in Michigan have been established for most surface water dischargers
at 1.0 mg/L. However, this permit limit is determined by the quantity of flow to be
discharged, the characteristics of the receiving water, and current nutrient loading. In
addition to a concentration limit, many facilities are also limited as to the number of
pounds of phosphorus that can be discharged over a period of time. As the population
grows and development continues, phosphorus limits have become more restrictive in
some areas. This is especially true in areas near lakes where rapid development has
occurred and there are many sources of phosphorus contributing to the environment.
Forms of Phosphorus in Wastewater
The influent concentration of Total P for most municipal wastewater treatment plants
ranges from about 2.5 mg/L up to about 6 mg/L, depending largely on the amount of
inflow / infiltration into the collection system and industrial contributions. Total P includes
three common forms of P: Organic-P, Poly (condensed) P, and Ortho-P. All three
forms would be expected to occur in municipal wastewater influent.
Organic-P includes P that is a part of organic compounds; food scraps and human and
animal wastes contribute this form of P to the waste stream. Organic-P compounds may
be soluble (dissolved) in the wastewater, but are often associated with particulate
material. Poly-P is in the form of a long chain, consisting of many linked PO4 molecules.
Poly-P is soluble, found in many detergents, and is often added to public drinking water
1
Activated Sludge Manual
supplies to sequester (tie up) the iron which would otherwise cause scaling and staining
problems. Ortho-P may be thought of as the PO4 molecule; often referred to as “simple
phosphates”. This form of phosphorus is soluble, and is common in many detergents
and cleaning agents, especially industrial cleaners. For instance, milk producers and
other food processing facilities often use phosphoric acid (H3PO4) solutions for cleaning,
and may discharge high concentrations of Ortho-P.
Phosphorus Removal – Sedimentation and Biomass Uptake
Since much of the Organic-P is associated with particulate material in raw wastewater,
some P removal will occur when settleable solids are removed in the primary clarifier.
Primary sedimentation may account for between 5 - 15% removal of the Total-P coming
into the WWTP.
Phosphorus will also be taken up by the developing biomass in a biological WWTP in
about the ratio of 100C:5N:1P. A facility with a primary clarifier followed by a trickling
filter may remove 20 - 30% of the total influent P. A facility with a primary clarifier
followed by activated sludge may remove 30 - 50% of the total influent P. While this is
fairly substantial P removal, it is obvious that even 50% removal will not meet a 1 mg/L P
permit limit given a typical influent P concentration of 5 mg/L. The WWTP with a P
permit limit of 1 mg/L must usually include a process designed specifically for P removal.
Phosphorus Removal – Chemical Precipitation
The most commonly used method of P
removal is by precipitation with a metal salt.
In this process a chemical is added which will
combine with the P to form a particle which
will settle. The chemical sludge is then
removed along with other solids which settle
in the clarifier.
The dosage point for the metal salt is
important, considering the various forms of P
in the influent. Metal salts most effectively
remove Ortho-P. Since Organic-P and Poly-P
are converted to Ortho-P as they pass
through biological wastewater treatment
systems, the most effective dosage point will
be after this conversion has taken place. In
the activated sludge process, the metal salt
solution is usually added near the discharge
end of the aeration tank or just before the
secondary clarifier.
Chemical
Chemical Phosphorus
Phosphorus
Removal
Removal
Total Phosphorus
Organic
Phosphorus
Metal Salt
Addition
Condensed (Poly)
Phosphates
Ortho
Phosphates
WAS
Ortho
Phosphates
RAS
Iron salt solutions have been used to effectively remove P for many years. Iron may
be added in its divalent state as Ferrous Chloride (FeCl2), or in its trivalent state as
Ferric Chloride (FeCl3). Both solutions are acidic, corrosive, and stain everything that
they contact with an orange color. Appropriate safety precautions must be observed.
FeCl2 is purchased from suppliers who obtain this as a byproduct of steel pickling (it is
sometimes referred to as “pickle liquor”), where hydrochloric acid solution is used to
2
Activated Sludge Manual
clean steel parts before further processing. This solution is usually less expensive than
other options, but care must be taken to assure that “tramp metals” which also dissolve
into the acid during the pickling operation are not excessive. The operator should also
make sure that the purchased solution has been filtered by the supplier to remove
particulates which would otherwise form sludge in storage tanks, and cause plugging
problems in solution feed pumps, pipes, and valves. Ferric Chloride (FeCl3) is typically a
cleaner product with a higher percentage of iron in solution, but is more expensive.
Book values for dosage are about 5.2 lbs FeCl3 per lb P.
Aluminum salts solutions are also often used for P removal in many facilities. The
.
most commonly used salt is aluminum sulfate (Al2(SO4)3 14H2O, referred to as alum.
While still an acidic solution, this is not as hazardous as the iron salt solutions and does
not stain. The main disadvantage with alum is that the dosage is about double that of
iron salt solutions, and the cost may be higher. Book values for dosage are about
9.6 lbs Alum per lb P.
Phosphorus Removal – Enhanced Biological Uptake
During the late 1960’s and early 1970’s it was discovered that some activated sludge
plants around the country removed more P than the 100C:5N:1P ratio indicated without
chemical addition. Theories as to why this occurred centered on type of aeration and
wastewater chemistry. Eventually it was found that in these plants the MLSS cycled
from an anaerobic to an aerobic environment. This fairly simple concept is the basis for
Enhanced Biological Phosphorus Removal in activated sludge plants.
Research has found that in situations where the MLSS cycles
through anaerobic followed by aerobic conditions a type of
bacteria (Acinetobacter) begins to accumulate in the
biomass that uses P as an energy storage mechanism.
In the anaerobic reactor fermentation occurs as heterotrophic
bacteria begin to break down organic material in the waste,
releasing volatile acids (acetic acid) into the solution. These
easily biodegraded organic acids become the food supply for
the Acinetobacter. As these bacteria consume the organic
acids they release P into the solution to produce energy
needed for metabolism. The Ortho-P concentration in this
reactor will be much higher than that of the influent P.
Biological P Removal
Phosphorus Storing Bacteria
Acinetobacter (Assin Eato Back Ter)
Anaerobic
Fermentation
Acetate Production
Selection of Acinetobacter
P Released to Produce Energy
Aerobic
Stored Food Consumed
Excess P Taken Up
Sludge Wasted
When the MLSS passes into the aerobic reactor the Acinetobacter consume stored food,
and incorporate an excess amount of phosphorus into the biomass (often termed
“Luxury Uptake”). Secondary sludge is wasted while the organisms are aerobic, thus
removing P from the system.
Many different designs have risen
through the years in an effort to
maximize the quantity of P removed
by the biomass. The A/O
(Anaerobic / Oxic) process was one
of the earliest of the biological
phosphorus removal processes.
Though the A/O process had been
3
Activated Sludge Manual
used successfully in the southern U.S., it was not known initially how the system would
respond in colder climates. A demonstration project was initiated in 1984 in Pontiac,
Michigan where one half of the East Boulevard Wastewater Treatment Plant was
retrofitted to the A/O process. This project evaluated the effect of cold wastewater
temperatures, the ability of the plant to nitrify while removing phosphorus biologically,
and studied the effect of returning anaerobic digester supernatant to the process. The
final report for the project published in 1991 indicated that the process was able to
effectively remove phosphorus to less than 1 mg/L consistently, even in low temperature
(40 – 50oF). It was also found that nitrification continued and that digester supernatant
recycle was not detrimental to the A/O process.
Since the demonstration project was begun at Pontiac, many municipal facilities have
been retrofitted to the A/O process in Michigan with good success, and some new
facilities have been designed as A/O plants. It should be noted that in nearly all cases
they are able to achieve permit limit concentrations for phosphorus almost always, but
for as yet unexplained reasons, there are times in nearly every facility when biological P
removal is not adequate and metal salts are added to remove the excess.
Other Bio-P removal processes have been used in
Michigan besides A/O; the City of Adrian was retrofitted to
the Phostrip process. A portion of the return sludge is
diverted to an anaerobic stripper tank, and the released P
is elutriated to a precipitation tank. Lime is then added to
precipitate the phosphorus. After operating the Phostrip
process for several years the City reverted back to
conventional activated sludge with chemical phosphorus
removal due to operational and mechanical difficulties.
Biological P Removal
Phostrip
Chemical and Biological P Removal
Adrian, MI
Aeration Tank
Stripper
Lime
Precip
Clarifier
Sludge
The A2/O (Anaerobic, Anoxic, Oxic) process is similar to
Not very Popular Due to Difficulties
the A/O
With Lime
process,
but an
added process in the aeration tank
recirculates nitrified mixed liquor into
an anoxic zone where denitrification
occurs. This minimizes the nitrate
concentration that would otherwise be
recirculated to the anaerobic zone,
causing a potential interference.
The Concentric Ring Oxidation Ditch is a modification of
the activated sludge process which also usually removes
phosphorus biologically. The biological mechanism is the
same as described earlier, but treatment is achieved using a
different tank configuration. Designed as three aeration tanks
in concentric rings, influent wastewater enters the first ring
which is operated in an anaerobic condition. Mixed liquor
then passes into the inner two rings, both typically being
aerobic. As the mixed liquor passes through the anaerobic
ring phosphorus release occurs, followed by phosphorus
uptake in the inner two rings. Some of the aerobic bio-solids
4
Concentric Ring Oxidation Ditch
Anaerobic
Aerobic
Aerobic
Activated Sludge Manual
are then wasted, removing phosphorus from the waste stream.
Another activated sludge modification which typically removes phosphorus biologically is
the Sequencing Batch Reactor (SBR). This fill-and-draw system begins with an
anaerobic Fill Phase during which phosphorus release occurs and is followed by an
aerobic React
Phase during which
phosphorus is
Ortho-P
taken up by the
biomass. During
the Settle Phase
the biomass settles
to the bottom, and clear water is drawn from the top of the
D.O.
reactor and discharged during the Decant Phase. Excess
biomass is usually wasted during either the Decant or Idle
Phase, which also removes phosphorus from the system.
Biological Phosphorus Removal Considerations
Regardless of facility configuration, there are some important considerations in trying to
determine whether a particular waste stream or activated sludge facility should be able
to remove phosphorus biologically.
•
•
•
•
•
•
Bio-P removal is most effective where there is an adequate influent BOD
concentration. Weak, diluted wastes may not supply enough oxygen demand to
easily achieve the anaerobic environment needed. The amount of BOD needed
is often expressed as a ratio of BOD to Phosphorus (BOD:P), with a ratio of
about 15-20:1 usually being preferable.
There must be an adequate amount of anaerobic detention time, without being
so long as to promote reduction of sulfate to sulfide (septicity). Optimum
anaerobic detention time depends to a large extent on the BOD:P ratio, but is
generally in the 1-3 hour range.
Aerobic detention time must be long enough to provide adequate BOD removal
as well as nitrification if necessary. A typical minimum would be 4-5 hours.
The facility must be capable of producing an effluent Low in Suspended Solids
(Below 20 mg/L). Mixed liquor solids lost in the secondary effluent will contain
much more phosphorus (about 8% by weight) than mixed liquor solids in a
conventional activated sludge plant (about 2% by weight). It is possible for a BioP removal facility to be out of compliance with the discharge permit limit on
phosphorus due to solids loss even though suspended solids are within the
permit limit.
Nitrification (nitrate) may interfere if provision is not made for denitrification.
Nitrate becomes an oxygen source for bacteria when D.O. is not available, so
cannot be allowed to enter the anaerobic zone.
Supernatant from solids handling processes must be carefully controlled to
avoid overloading the Bio-P removal system with phosphorus. Aerobic digesters
5
Activated Sludge Manual
would be expected to release P into the supernatant if allowed to become
anaerobic.
Benefits of Bio-P Removal
As stated earlier, many activated sludge facilities in Michigan are being retrofitted to take
advantages of the Bio-P removal process. These advantages include reduced need for
chemical feed resulting in reduced amount of chemical sludge produced, lower cost, the
process is safer, the possibility of tramp metal contamination of sludge is reduced, and
cycling the biomass through alternating anaerobic / aerobic environments helps to
inhibits the growth of filamentous bacteria.
Special Considerations of Bio-P Removal
While the benefits outweigh the drawbacks of Bio-P removal, the operator must keep in
mind that there are some special factors to consider with this process:
1. There will be periods of time when chemical addition will be needed to
supplement Bio-P removal; a back-up chemical feed system will be required.
2. The D.O. environment needed for P release opposes that needed for nitrification.
In facilities where effluent ammonia is regulated by permit the operator must be
aware of the conditions needed for each process to occur and current conditions
in the plant. In trying to resolve problems, keep in mind that nitrification can only
take place biologically, whereas phosphorus removal can be achieved chemically
if necessary.
3. Solids handling considerations and supernatant recycle are more critical.
4. Controlling the effluent solids concentration is more critical.
5. Laboratory and process control testing may in increased:
P in anaerobic and aerobic zones
D.O. in anaerobic and aerobic zones
6. When retrofitting a facility to the Bio-P removal process, determine whether the
changes in question will require consideration of patent rights currently in effect.
6
TROUBLE
SHOOTING
Activated Sludge Manual
IX. Troubleshooting
Troubleshooting Tools
Probably one of the most common temptations for the operator, when troubleshooting
activated sludge problems, is to overlook obvious sources and solutions in favor of the
strange and unusual. Although situations sometimes do arise that are difficult to
determine and explain, the first approach should usually be to start with the most basic
information about the problem. Look for in-plant operational or mechanical causes first,
and then only if those can be eliminated, expand the search to the collection system and
changes in influent wastewater characteristics.
Begin by characterizing the problem; excessive odor, mixed liquor settleability, high
effluent BOD, Suspended Solids, Ammonia, TIN, P. Try to assure that information that
is available is reliable. Occasionally, much effort is expended in tracking down a
supposed problem only to discover that metering or laboratory data was not accurate.
Don’t assume that the laboratory data is faulty, but be sure that it is accurate.
Review operational records, starting with several months before the problem began.
Watch for correlation of operational problems with control changes, operational functions
such as digester supernating, or changes in chemical feed rates or solutions used. Look
for trends in the data that indicate cyclic problems. These might be cycles that occur in
the plant or in a contributor to the collection system.
Establish a log in which operators record general conditions in the plant and make note
of conditions that seem unusual. Spend time learning what the plant looks, sounds, and
smells like when operating well; this will help to make earlier and easier troubleshooting
decisions when problems occur.
Make operational changes affecting the biomass gradually; biological systems need time
to acclimate to changing conditions. Avoid the urge to make several changes at the
same time.
The following troubleshooting outline may provide some direction in trying to resolve
some of the more common activated sludge problems.
1
Activated Sludge Manual
Activated Sludge Troubleshooting Outline
A. High Secondary Clarifier Sludge Blanket
Use the Settleometer Test and the microscope to help determine the cause of this
condition. Calculate SVI or SDI to establish whether the problem is caused by poor
compaction or just too much biomass in the system.
1. High SVI, Low SDI
Microscopic Examination of MLSS - floc size, shape, structure, indicator
organisms, filaments
Filamentous Bulking
a. Possible Causes:
• Low Dissolved Oxygen
• Low Organic Loading Rate
(F:M < 0.05) (High CRT)
• High Organic Loading Rate
• Nutrient Deficiency (N or P)
• 100 parts C to 5 parts N to 1 part P
• Septic Wastes / Sulfides
• Low pH (< pH 6.0)
• High Carbohydrate Load (Sugars, Syrup, etc.)
b. Possible Cures:
Long Term – Try to eliminate or control the cause of the problem
Short Term – Control settleability with Chlorine (Cl2)
• Add to Return Sludge Before Mixing With Wastewater
• Feed Chlorine as :
Solid - HTH
Liquid - Bleach
Gas
• Lbs Cl = 0.0000834 X SVI X F X W
F = RAS MGD
W = RAS TSS, mg/L
• Enough Cl2 must be dosed to kill the filaments; start with the amount
calculated by the formula above, increase if settleability does not
improve within one day. Discontinue Cl2 feed when settleability is
under control.
2. Normal SVI, SDI
a. Excessive amount of Biomass
• Recalculate CRT, Wasting Rate
• Inadequate Sludge Storage – make sure that maximum sludge
storage capacity is available before winter to assure adequate WAS
ability
• High SS, BOD in supernatant from sludge handling
• WAS solids being carried through Primary Clarifiers
2
Activated Sludge Manual
b. Inadequate Return Sludge Rate
RAS Pump Control Settings
Mechanical Condition of RAS Pumps, Meters, Valves
Clarifier Sludge Sump blocked with debris
Sludge collector mechanical problem
c. Excessive Hydraulic or Solids Load on Clarifier
Determine Surface Overflow Rate (SOR)
B. Scum and Foam
1. White Billowing Foam
A.S. Plant Start-Up
Low CRT, High F:M
Organic Overload or Shock Load
Loss of Biomass – Over-wasting
Recovering from Toxicity
2. Dark, Scummy Brown Foam, Slurp
Some is normal in Extended Air
Excessive CRT
MLSS in Primary Effluent
Excessive Oil and Grease
Short Term Control
Manually Remove
Long Term Control
Increase Wasting
Slurp is caused by filamentous bacteria which
have low specific gravity (they float) and trap air to
form foam. Although there are two types of
bacteria which may cause slurp, Microthrix
parvecella and Nocardia, of these Nocardia
seems to be the cause most often in Michigan. It
is easily identified by microscopic examination of
the foam as a highly branched, short filament
which stains gram positive.
C. High Effluent BOD
1. If Total BOD is reported, also analyze the effluent for CBOD to determine if
nitrification is causing the increased BOD. If this is the case and there is no
ammonia limit in the discharge permit, try to get the plant out of nitrification, or
call the DEQ district office and explain the problem.
3
Activated Sludge Manual
2. If effluent CBOD is high:
a. Determine whether the CBOD is dissolved or particulate. If it is particulate,
improve settling in the secondary clarifier or improve tertiary filtration.
b. Look for organic overload – High F:M, possibly shock loading of high strength
organic waste, from within plant or from collection system.
c. Determine if CBOD is coming from a process downstream from the aeration
tanks, such as from tertiary filters, effluent equalization, polishing pond, etc.
d. Assure adequate detention time in the aeration tanks, look for short circuiting.
e. Determine nutrient balance going into A.S. system. Don’t remove phosphorus
before aeration tanks.
f. Recalculate F:M and CRT to assure operation is in typical range.
D. Nitrogen
1. Excessive Effluent Ammonia
a. Assure that processes downstream from aeration tanks (such as polishing
ponds) are not contributing ammonia directly to the effluent.
b. Carefully control supernatant from sludge handling, digestion, and storage
units. This recycled flow typically contains very high ammonia concentration
and is very often the cause of ammonia violations of the discharge permit.
c. Recalculate CRT and F:M to make sure that they are in the nitrification range.
Avoid over-wasting or slug loading the biomass.
d. Analyze influent and effluent for total alkalinity. Influent should be at least
150 mg/L and effluent should be at least 50 mg/L. Add sodium bicarbonate
or other form of alkalinity to aeration tanks if needed.
e. Increase aerator detention time if possible by putting more aeration tanks on
line. Avoid taking aeration tanks out of service or other situations that would
result in a loss of biomass or reduced aerator detention time, especially in
cold weather.
f. Assure adequate D.O. (3-5 mg/L) at the discharge end of the aeration tank.
g. Check for toxicity entering the WWTP.
2. Excessive Total Inorganic Nitrogen (TIN)
a. Analyze for ammonia, nitrite, and nitrate.
b. If ammonia is high, refer to the previous troubleshooting section on ammonia.
c. If nitrite and nitrate are high, encourage denitrification.
• Reduce excess D.O. in aerobic reactor
• Avoid excessive detention time in aerobic reactor
• Add carbon source (influent wastewater, methanol) to anoxic reactor
3. Denitrification causing floating solids in
secondary clarifier
a. Verify denitrification with settling test;
watch for formation of small nitrogen
gas bubbles in MLSS during test.
b. Maintain a lower sludge blanket in
secondary clarifier; increase return rate
to remove solids before denitrification
occurs.
c. In facility is capable of denitrification, assure purging of nitrogen gas before
the mixed liquor enters the secondary clarifier.
4
Activated Sludge Manual
4. Nitrite & Disinfection – refer to nitrogen section in manual.
E. Phosphorus
1. Chemical P Removal
a. Make sure that Ferric or Alum solution concentration (specific gravity and %
solution) are as specified and have not been unexpectedly diluted.
b. Check chemical feed rate at pump discharge; use calibration tube on system
or container of known volume and stop-watch.
c. Evaluate chemical feed point(s). Chemical should be fed after biological
system and before sedimentation. Adequate mixing is important.
d. Perform jar test to verify appropriate dosage.
e. Analyze for Total P as well as Dissolved P to determine whether effluent P is
soluble (dissolved) or is particulate.
• If it is soluble, it should have reacted with the chemical to form a
solid which settled in the secondary clarifier. Check chemical feed
as outline above.
• If it is particulate, it should have settled in the secondary clarifier.
Improve settling conditions in the clarifier. Use polymer if
necessary.
f. Filter a sample and determine Total P and Ortho P on the filtrate.
• If Ortho-P is high, it should have reacted with the chemical. Check
chemical feed.
• If Ortho-P is low in the filtrate, but Total P is high, the biomass is not
converting the P to the Ortho form. Either conditions in the aeration
tank are not appropriate for this conversion to occur, or the form of
P cannot be converted by the biomass.
o Look for an industry using a Phosphite (PO3) compound
such as a metal finisher with an electroless nickel plating
process.
o If Phosphite is verified, it must be controlled at the source.
2. Biological P Removal
a. Verify anaerobic followed by aerobic conditions. Oxidation / Reduction
Potential (ORP) may be helpful in assessing this. The ORP in the anaerobic
reactor should be less than about -200 mV, and in the aerobic reactor greater
than about +50mV.
b. Monitor Ortho-P in MLSS supernatant. A P release should be observed in
the anaerobic reactor with resulting high Ortho-P in the liquid, followed by P
uptake in the aerobic reactor resulting in low Ortho-P in the liquid.
c. High amounts of inflow / infiltration may inhibit P removal by diluting the
influent wastewater and adding D.O. to the anaerobic reactor.
d. Control RAS from the secondary clarifier to avoid an excessive sludge
blanket depth that may result in a P release.
e. Avoid an excessive RAS flow from the secondary clarifier, especially in plants
that are nitrifying. This dilutes the influent flow, and also adds nitrate to the
head of the plant that can inhibit the release of P in the anaerobic reactor.
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Activated Sludge Manual
F. Toxicity
1. While not always an indication of a problem, sometimes unusual odor, color
change, or foam in the influent wastewater accompany a toxicity problem.
2. Monitor aeration tank D.O. continually if possible. If BOD load, blower output,
and MLSS have remained fairly constant, a sudden increase in aeration tank
D.O. may be an indication of reduced biological activity due to toxicity.
3. Use the microscope to observe indicator organisms in the mixed liquor. Watch
for inactivity among normally active organisms, and for disrupted cells.
4. Monitor Respiration Rate of the biomass. This will be most helpful if there is
sufficient past data for comparison. If the respiration rate on a fed sample (sugar
added) is lower than normal, toxicity is a definite possibility.
5. Be aware of potential contributors of toxicity to the collection system. This may
or may not be due to an industrial discharge. Several municipal wastewater
treatment plants have experienced severe toxicity problems following the use of
root killers in the collection system by sewer cleaning companies.
6. Deflocculation of the biomass is often the result of toxicity. The effluent becomes
turbid and settleability is suddenly poor as the biological floc comes apart,
releasing small particulates into the wastewater.
7. White billowing foam resembling soap suds may occur on the aeration tank as
the biomass begins to recover from a toxic waste.
8. Remember that toxicity due to heavy metals as well as some organic compounds
may result in contamination of the sludge. Pumping this into an anaerobic or
aerobic digester may compound the problem in the plant by inhibiting the sludge
stabilization process and interfering with the ability to dewater the solids. It may
also result in inability to dispose of the sludge by land application onto
agricultural land.
9. While effluent BOD will probably increase following a toxicity problem, increased
effluent ammonia will usually be more immediately apparent.
10. The list of materials potentially toxic to activated sludge biomass would be too
long to list, but includes pesticides, herbicides, disinfectants, many heavy metals,
cyanide, degreasing agents, and high concentrations of ammonia or sulfide. If
there is some question as to the treatability of a waste, the oxygen uptake rate
test may be used to help make this determination.
6
MODIFICATIONS
Activated Sludge Manual
X. Modifications of the Activated Sludge Process
The activated sludge process has been modified in many ways through the years for
several purposes. Modifications have occurred as a result of trying to improve
treatment for specific types of wastes or to achieve more complete removal of a
certain component of the waste. In some situations economics may be the overriding
factor in the development of an activated sludge modification, and in others the ability
to treat high wastewater flow rates or high strength organic wastes may be the
objective. Some modifications have been very successful and are used extensively,
while others have not been used to a large extent.
A. Conventional
F:M
CRT
Aerator DT
0.25 – 0.45
5 – 8 days
5 hours
B. A/O and A2/O
The A/O process is an
activated sludge
modification that is
designed to provide
biological P removal.
The MLSS passes
through an anaerobic
zone is followed by an
aerobic zone.
AO Process (Anaerobic/Oxic)
A2/O (Anaerobic/Anoxic/Oxic)
1
The A2/O process is like
the A/O process with an
added internal
recirculation of MLSS
through an anoxic zone.
This provides
denitrification and
prevents NO3 from
interfering with biological
P removal.
Activated Sludge Manual
C. Contact stabilization
This process takes advantage of
the adsorptive capability of
activated sludge. Particulates
are adsorbed by the biomass in a
Stabilization
Contact Tank with a short
Tank
detention time (about 2 hrs).
Return sludge is directed to a
Stabilization Tank where the
organisms metabolize the adsorbed material. This process works well for
wastewaters high in particulate material, but is not very effective for high soluble /
low particulate wastes due to the short detention time in the Contact Tank. An
added benefit is the large reserve of MLSS in the Stabilization Tank that can be
utilized in case of wash-out.
Contact
Tank
D. Tapered aeration
Amount of Air Added is tapered
from Aeration Tank influent to
effluent. More air is added
where organic load and
biological activity is greatest.
Aeration
RAS
E. Step aeration
WAS
Return Sludge Enters Head of
Aeration Tank; Wastewater is
added at various points along
length of tank. Intended to
distribute the wastewater
throughout the Aeration Tank.
Avoids areas of low and high D.O.
in Aeration Tank.
F. Step feed
Return Sludge and wastewater
are added at various points along
length of tank Intended to avoid
high and low areas of loading
and D.O. Approximates
Complete Mix.
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Activated Sludge Manual
G. Complete Mix
Return Sludge and influent waste are
distributed throughout the aeration tank.
Mixed liquor exits the reactor around the
tank perimeter. Treatment is uniform in
the reactor. Shock Loads are
immediately diluted. Generally smaller
tanks are more completely mixed than
large ones May be more susceptible to
filamentous bulking.
H. Pure oxygen
High quality
Oxygen is
generated onsite to be used
rather than air
in the aeration
tank. Aeration
system is
covered to
minimize loss of
O2. Operated
at high F:M
Ratio. Designed to reduce area needed to treat large or high strength flows.
City of Detroit City of Wyandotte
City of Holland French Paper Company
I. Plug Flow
Wastewater and Return Sludge
enter head of Aeration Tank and
travel as a “Plug” through the
tank. Minimizes short-circuiting.
D.O. at head of tank may be low
due to localized introduction of
entire BOD load and increases
as ML travels to outlet.
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Activated Sludge Manual
I. Extended Aeration
F:M
0.05-0.15
Long Detention Time
CRT
20 – 25 Days
High MLSS
DT
24 Hrs.
High CRT
Low F:M
High Quality Effluent
Nitrification is very likely. Resists upset due to large biomass population and long
detention time.
J. Oxidation ditch
1. Racetrack Ditch
Usually operated as Extended
Aeration. Typically uses
Mechanical Aeration called “rotors”
that aerate and mix. D.O.
Approximates Plug Flow
Due to varying D.O. environment in
ditch. Mixing Approximates
Complete Mix (ML stays in ditch
through many revolutions).
2. Concentric Ring
Oxidation Ditch
Usually Extended
Aeration.
Nitrification,
Denitrification, and
biological P removal is
often attained. RAS and
influent enter first ring
which is operated in anaerobic condition. MLSS passes into inner two rings
which are aerobic.
K. Sequencing Batch Reactor
The SBR is an activated sludge
modification in which treatment occurs in
batches rather than continuous flow.
Treatment takes place in 5 phases in the
reactor; each phase is computer
controlled. The biomass stays in the
reactor with a portion being wasted each
treatment cycle. There are no secondary clarifiers, no RAS, and usually no
primary treatment. The SBR process is typically operated in the Extended Aeration
mode, with nitrification usually occurring. Because the D.O. environment changes
from anaerobic during the fill phase to aerobic during the react phase, the system
is capable of denitrification, as well as biological P removal.
4
Activated Sludge Manual
L. Bio-Lac
The Bio-Lac system was developed by the
Parkson Corporation. It is an extended
aeration system, often designed as a large
rectangular basin with sloping sides. Fine
bubble aeration tubes are suspended from
air headers which float on the surface of
the basin. This arrangement helps to
eliminate dead spots in the basin, ensuring
even air distribution. This process is being
used in some municipal wastewater
treatment plants in Michigan.
M. Membrane Bio-Reactor (MBR)
The MBR process is an activated sludge
process in which the secondary clarifiers are
replaced by filtration units which are
submerged in the mixed liquor. Offered by
several manufacturers, MBR plants are
currently in operation at Traverse City and
at Dundee. The filtration units are designed
either as tubes or hollow fibers (Zenon), or
as cassettes of flat plates (Kubota).
Advantages include high biomass
concentration in the reactor (>10,000 mg/L),
biomass settleability is not a concern, and
the ability to remove very fine particulates.
N. Aero-Mod Sequox
The Sequox system by Aero-Mod is
a flow through activated sludge
process designed for biological
nutrient removal. An anaerobic
selector ahead of the aeration
system provides filament control
and encourages biological P
removal. Alternating aerobic/anoxic
reactors following first stage
aeration provide denitrification.
This is a compact design, utilizing air
lifts for most of the pumping
requirements. One of these facilities
is in operation at Kingsley.
5
BENCH SHEET
ACTIVATED SLUDGE BENCHSHEET
SAMPLE
DATE
MLSS
#1
MLSS
#2
RAS #1
RAS #2
MLVSS
MLVSS
RAS
VSS
RAS VSS
a. mls Sample
b. Filter Paper + Dry, grams
c. Filter Paper, grams
d. Dry Solids, grams
(b – c)
e. Suspended Solids, mg/L
f. Percent Solids
g. Dish + Ash, grams
h. Dish, grams
i. Ash, grams
j. Volatile Solids, grams
(d – i)
k. Volatile Solids, mg/L
l. % Volatile Solids
D.O., mg/L
A.T. #1
A.T. #2
SETTLEABILITY
mLs
Time
Settled
Time
5 min
10 min
15 min
20 min
30 min
SVI
Sludge Blanket Depth, ft
Time
Clar #1
Clar #2
RAS Flow
% Influent
Clar #1
Clar #2
WAS
gallons
OUR
Activated Sludge Manual
Interpretation of Oxygen Uptake Test Results
Overview
Respiration rate test results are very dependent on environmental conditions and
concentration of microorganisms. The example values given here are very general. The
actual results that you get will be very specific to the biological system you are
monitoring.
Units:
Values for oxygen uptake rates (OUR) are given in mg DO/l/min. Values for specific
oxygen uptake rates (SOUR) are given in mg/hr/gram of MLSS.
UNFED Values:
An unfed OUR is defined as a sample of return activated sludge plus secondary effluent.
This mixture's concentration is designed to imitate the MLSS concentration in the
aeration tank. For MLSS concentration between 2500 and 3500 mg/L, the unfed OUR
will range between 0.3 and 0.7 mg/l/min.
Please pay attention to the MLSS concentration. The higher the MLSS concentration,
the higher the number of "bugs" that are breathing. This means a MLSS of 2,500 mg/L
may show a higher OUR value than a MLSS of 1,500 mg/L on the same sample. The
lower MLSS concentration would normally create a younger sludge which might result in
a higher rate of respiration per gram. The OUR test results would make the higher MLSS
concentration "look like" a younger sludge. This problem is solved when you calculate
the SOUR.
Low UNFED Values: < 0.3 mg/l/min
The lower the value the older the sludge. You should also see other indications of older
sludge in the plant, fast settling floc, pin floc, etc..
High UNFED Values: > 0.8 mg/l/min
The higher the value the faster growing , younger the sludge is. A high unfed OUR
indicates an under-oxidized sludge, slower settling, low compaction, etc..
FED Values:
A fed OUR is defined as a sample of return activated sludge plus primary effluent or raw
influent. This mixture's concentration is designed to imitate the MLSS concentration in
the aeration tank. For MLSS concentration between 2500 and 3500 mg/L, the fed OUR
will range between 2 to 5 times the unfed rate. Example: 0.6 to 3.5 mg/l/min. Most
aeration systems should be able to handle a range of 2.0 to 2.5 mg/l/min.
Note: these numbers are example for MLSS concentrations between 2500 and 3500
mg/L. If your system is running at 1500 to 2500 mg/L, your average values would be
different.
Low FED Values: < 0.6 mg/l/min
1
Activated Sludge Nutrient Addition 2
A low fed OUR could indicate a number of conditions: low BOD load and/or too high of a
MLSS concentration, food type cannot be used easily by the microorganisms or a toxic
waste that is inhibiting the growth of the bacteria.
High FED Values: > 3.5 mg/l/min
High fed OUR numbers indicates a high food load on the system. Question about
adequate air supply and mixing must be answered. A move to step-feed may be
required.
Specific Oxygen Uptake Rate
Converting your OUR values into SOUR values is a common practice. OUR is converted
into SOUR by dividing the OUR by the MLSS or MLVSS (MLVSS is preferred) and
multiplying by 1,000 mg/gram and multiplying by 60 min/hour. The resulting unit is mg
oxygen per hour per gram of MLSS (mg/hr/gr). This removes the variable of changing
MLSS concentrations. If respiration rate numbers are given in a common unit of "one
gram of MLSS", SOUR values can be compared from plant to plant.
For example, A fed sample OUR = 1.4 mg/L/min. The MLVSS of this sample is 2100
mg/L. The SOUR would be calculated by the following:
If this same example has the same OUR but a MLVSS of 3500 mg/L, the SOUR would
be:
In this example you can see that for the same value of OUR, the SOUR values are
changed significantly based on the MLVSS concentration. Based on the SOUR values,
the higher MLVSS actually has a smaller respiration rate per gram of active solids
(VSS).
2
Activated Sludge Nutrient Addition 3
Load Factor
Another calculation is sometimes used to interpret the fed and unfed numbers. Load
factor or load index is a ratio between the fed and unfed values (the fed number is
divided by the unfed number). This value indicates activity before and after feeding.
1. LF < 1.0 - inhibitory or toxic load
2. LF > 1.0 but < 2.0 - dilute or stabilized load
3. LF > 2.0 but < 5.0 - acceptable loading
4. LF > 5.0 - possible oxygen supply problems
3
NUTRIENT
ADDITION
Activated Sludge Manual
Nutrient Requirements for Activated Sludge
For every 100 lbs. (or mg/L) of organic carbon (BOD5) entering the aeration system, a
minimum of 5 lbs. (or mg/L) of nitrogen, 1 lb. (or mg/L) of phosphorus, and 0.5 lbs. (or
mg/L) of iron is required.
100 BOD5 : 5 N : 1P : 0.5 Fe
A nutrient deficiency of any of these essential requirements may cause excessive growth
of filamentous organisms resulting in poor settling of the activated sludge. These
filamentous growths can not be successfully controlled by chlorination, hydrogen
peroxide, or polymers. If the nutrient deficiency is severe, the ability of the
microorganisms to remove soluble organic matter will be impaired, resulting in treatment
failure.
The above ratio is useful for detecting nutrient deficient wastes and supplemental
feeding of nutrients. Major factors that should be considered when feeding nutrients
include:
1.
Both ammonia and nitrate are available as nitrogen sources, as well as organic
nitrogen (urea). However, if organically bound nitrogen is used and the waste
contains a carbon source that is easily metabolized (simple sugars and organic
acids), the nitrogen may not be available rapidly enough during the metabolism
of these wastes.
2.
If the organic loading varies, the nutrient supply should vary with the loading.
3.
Since each wastewater has its own particular nutrient demand, measurement of
effluent concentrations for orthophosphate, ammonia, and nitrate should be
performed. Concentrations of total inorganic nitrogen (NH3, NO2, and NO3) of at
least 0.2 to 0.3 mg/L and soluble Orthophosphate of 0.2 mg/L should be
maintained. Phosphorus measurements should be made on effluent samples
filtered through 0.45 μm filter paper.
Supplemental nutrient feed is calculated based on the number of pounds of BOD coming
into the secondary process, the amount of nutrients already present in the incoming
waste, and the properties of the commercial chemical that will be used to supply the
nutrient. Take a step-by-step approach to make the calculation more understandable.
Start out by determining how much (mg/L) of each nutrient is required to supply
the amount indicated by the 100 C : 5 N :1 P : 0.5 Fe ratio.
Next, determine how much (mg/L) nutrient is needed in addition to that already in
the waste.
Using the pounds formula, calculate how many pounds of each nutrient must be
added.
Based on the properties of the commercial chemical that will be used to supply
the nutrient, calculate the pounds of the chemical must be used.
If the commercial chemical is a liquid, determine the number of gallons of the
solution that must be fed each day.
1
Activated Sludge Manual
Example Calculation of Nutrient Addition
Average Flow
Secondary Influent BOD5
Secondary Influent TKN
Secondary Influent P
Secondary Influent Fe
4.5 MGD
150 mg/L
2.5 mg/L
1.0 mg/L
1.0 mg/L
Commercial Chemicals to be Used:
Anhydrous Ammonia (NH3) = 80 % NH3 by Weight
Phosphoric Acid (H3PO4)
Target Ratio by Weight
1.
= 50 % H3PO4 Solution by Weight
Specific Gravity 1.335
100 BOD : 5 N : 1 P : 0.5 Fe
Determine the mg/L of each nutrient required to satisfy the ratio:
N Required, mg/L
P Required, mg/L
Fe Required, mg/L
=
BOD, mg/L
Ratio BOD:N
=
150 mg/L
100/5
=
150
20
=
7.5 mg/L N
=
BOD, mg/L
Ratio BOD:P
=
150 mg/L
100/1
=
1.5 mg/L P
=
BOD, mg/L
Ratio BOD:Fe
=
150 mg/L
100 / 0.5
=
0.75 mg/L Fe
2
=
150
100
= 150
200
Activated Sludge Manual
2.
3.
4.
Determine how many additional mg/L of each nutrient is needed:
N Additional, mg/L
=
=
7.5 mg/L N - 2.5 mg/L N
5.0 mg/L N additional needed
P Additional, mg/L
=
=
1.5 mg/L P - 1.0 mg/L P
0.5 mg/L P additional needed
Fe Additional, mg/L
=
=
0.75 mg/L Fe - 1.0 mg/L Fe
- 0.25 mg/L Fe (have excess, none needed)
Determine how many additional pounds of each nutrient are needed:
N addition, lbs/day
= Flow, MGD X N Addition, mg/L X 8.34 lbs/gal
=
4.5 MGD X 5 mg/L X 8.34 lbs/gal
=
188 lbs / day additional N needed
P addition, lbs/day
= Flow, MGD X P Additiion, mg/L X 8.34 lbs/gal
=
4.5 mg/L X 0.5 mg/L X 8.34 lbs/gal
=
18.8 lbs/day additional P needed
Determine how many pounds per day of each commercial chemical will be
needed to supply the required pounds of nutrient:
Commercial Chemical, lbs/day =
Nutrient Addition, lbs/day
Decimal % of Nutrient X Decimal % of
Pure Commercial Chemical
A. Determine lbs of Anhydrous Ammonia to be Fed Per Day:
Decimal % of N in Anhydrous Ammonia (NH3) =
N = 14 X 1 = 14
H = 1X3 = 3
NH3 =
17
Atomic Wt. of N
Molecular Wt. of NH3
Decimal % of N in Anhydrous Ammonia = 14 / 17 = 0.8235
Lbs/day of NH3 Needed = 188 lbs/d N Needed = 228 lbs/day
0.8235
Lbs/day of 80% Anhydrous Needed = 228 lbs/day = 285 lbs/day
0.80
3
Activated Sludge Manual
B. Determine Pounds of Phosphoric Acid Solution (H3PO4) to be Fed Per Day:
Decimal % of P in H3PO4 =
H
P
O
H3PO4
Atomic Wt. of P
Molecular Wt. of H3PO4
= 1X3 = 3
= 31 X 1 = 31
= 16 X 4 = 64
=
98
Decimal % of P in H3PO4 = 31 / 98 = 0.3163
Lbs/day of H3PO4 Needed = 18.8 lbs/d P Needed = 59 lbs/day
0.3163
5.
Determine Gallons of 50 % Phosphoric Acid Solution Needed Per Day:
Lbs/day of 50% H3PO4 Solution Needed =
59 lbs/day H3PO4 Needed
0.50 X 1.335 X 8.34 lbs/gal
= 10.6 gallons/day needed
4
A.S. HISTORY
THE GENESIS AND EVOLUTION OF
ACTIVATED SLUDGE TECHNOLOGY
James E. Alleman, Professor
School of Civil Engineering
West Lafayette, IN 47907-1284
[alleman@ecn.ecn.purdue.edu]
[PH: 765-494-7705]
[FX: 765-496-1107]
INTRODUCTION
The design and operation of activated sludge systems represents one of the key topics
covered during any environmental engineer’s education. Our courses and textbooks
thoroughly examine the topic, shedding light and wisdom on a process whose finer
details have been mastered to a point likely more advanced than that of any other
wastewater treatment technology.
However, should you happen to ask any one of our student’s what they know about the
formative history of the concept, let alone the names and backgrounds of the responsible
engineers, you’ll quickly find that their corresponding knowledge ranges from bare to nil.
In large measure, the technical roots for activated sludge have largely been ignored
within our current texts and classes, such that most students likely share a common
misconception that activated sludge has forever been the preeminent option for
wastewater purification.
Most practicing engineers, though, belatedly develop an appreciation for the ‘art’ of their
applied technology, transcending their textbook’s wisdom about kinetics and
microbiology with a personal, human interest in the ‘who, what, when, and why’ of the
affiliated history. This article will consequently examine the yesteryear circumstances
under which activated sludge was devised, and the early developments which largely
shaped its subsequent use.
Admittedly, there have been many articles written about this singular topic. The
following chronology provides a summarial overview of these prior ‘review’
publications: Porter, 1917; Ardern, 1917; Porter, 1921; Martin, 1927; Clark, 1930;
Mohlman, 1938; Greeley, 1945; Sawyer, 1965; and Alleman, 1984.
Activated Sludge Manual
These works have progressively provided informative reviews and updates on the everexpanding history of activated sludge treatment. Porter’s two works alone, respectively
written in 1917 and 1921, are certainly indicative of the explosive interest which this
technology first drew. Barely six years after the idea had originally been published, the
associated literature had already grown in number to nearly 800 articles. The paper athand will, therefore, unavoidably retrace technical developments thoroughly documented
by these preceding activated sludge historians. This latest review will, however, attempt
to provide yet another slant on the topic, with new insights regarding the personal and
scientific motivations which catalyzed the original concept.
FLEDGLING CONCERNS ABOUT WASTEWATER MANAGEMENT:
Mid-to Late-19th Century
In order to understand the impact which activated sludge had on wastewater treatment
technology, one must first appreciate the relative infancy of the ‘sanitary’ field which
existed during the mid- to late-1800’s. Midway through the 19th century, barely a handful
of European cities had any sort of organized approach to handling their daily wastewater
problem. While the Industrial Revolution had produced a manifold range of technical
blessings, these new industries also added a considerable increase in the magnitude and
strength of local waste output, heavily befouling their already burdened environment.
Installing sewers would prove to be the first step in the sequential process of waste
management, but very few cities had yet made this effort. The necessary technology had
been established nearly two millennia earlier by Greek and Roman planners, but for most
industrialized 19th century towns the notion of intentionally conveying these wastes
streams beyond their municipal borders was still in its infancy…and effectively beyond
their technical capabilities. For this matter, the delivery of clean water itself had hardly
become a commonplace practice.
Lacking any means of collecting and removing these wastewaters, therefore, the
convenient solution was either one of direct discharge from chamber pot to street or, for
those more affluent homes, to rely on a central cesspit. However, John Snow’s
investigation in the 1850’s of yet another cholera outbreak caused by one such cesspit
failure (i.e., near London’s Broad Street area) provided a frightfully compelling
motivation to find suitable solutions for the public’s intertwined water/wastewater
problems.
Over the next several decades there was a consequent move to install pumping and
supply systems for delivering clean water, followed shortly thereafter by complementary
sewer networks with which this incoming water could then be removed to some distant
point of discharge. The science of sewer design, though, was anything but a mature
technology. As implied by the classic 2858 ‘Punch’ poem, "Slow But Sewer," there was
considerable argument about the notion of co-mingling rain waters to flush and dilute
these streams, but the problems created by inadequately sloped sewers eventually led to
widespread adoption of the combined designs.
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At this point, the solution of downstream ‘dilution’ had become civilization’s best
strategy for dealing with wastes. However, with these wastes now being funneled to
discrete outfalls, the idea evolved that these streams might actually be used for beneficial
gain as a convenient, and free, source of fertilizing nutrients. Punch’s poem briefly
captures this mood, and Victor Hugh’s classic, "Les Miserables," offered an even more
convincing argument for reusing what he aptly described as the ‘detritus of capital’:
"A great city is the most powerful of stercoraries. To employ the city to enrich the plain
would be a sure success. If our gold in filth, then our filth is gold….these fetid streams of
subterranean slime which the pavement hides from you, do you know what all this is? It
is the flowering meadow, it is the green grass, it is marjoram and thyme and sage, it is
game, it is cattle, it is the satisfied low of huge oxen at evening, it is perfumed hay, it is
golden corn, it is bread on your table, it is warm blood in your veins, it is health, it is joy,
it is life."
These newly devised waste conduits were subsequently recognized as a prime
commodity for entrepreneurial gain, and a cottage industry of wastewater alchemists
quickly emerged intent on extracting the nutrient essence of sewage for monetary gain.
The Native Guano Company eventually dominated this ‘manure’ market in England,
franchised and licensed to cities with the lofty expectation that they could transform their
foul wastes into a profitable ‘artificial fertilizer.’ In retrospect, therefore, these original
treatment plants were frankly not built for environmental or sanitary gain. Instead, the
prime goal for this company’s patented technology, known as the "ABC Process,’ was
considerably more focused on nutrient recovery (nitrogen and phosphorus).
In retrospect, though, the ABC Process started a sanitary revolution whose technical
prodigy would eventually lead us to full-fledged wastewater treatment facilities. This
original procedure, using alum, blood, and clay (i.e., "ABC") to optimistically promote a
sort of natural ‘coagulation,’ no doubt qualifies as the seminal prototype for physicalchemical sewage treatment. Undoubtedly, this scheme was a malodorous first step, but
the precedent had been established against which future engineers could measure their
success.
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EARLY BIOLOGICAL WASTEWATER TECHNOLOGY:
1870’s -> 1900’s
Biological treatment was unquestionably a primitive science in the late 1800’s, having
only recently been elucidated through progressive European (i.e. Mueller, Frankland,
Bailey-Denton, Dibdin) and American (i.e. Mills, Hazen, Drown and Sedgwick of the
Lawrence Experimental Station situated in Lawrence, Massachusetts) filtration research.
(Peters & Alleman, 1982) The basic derivatives of their work included intermittent filters,
contact beds and trickling filters.
Septic tanks were also popular during this era, at least until Cameron obtained a
restrictive patent in 1896 and began to enforce substantial royalty charges despite bitter
public criticism. Although the popularity of septic tanks subsequently faded, alternative
anaerobic systems were soon available, including both the classic Imhoff Tank and its
predecessor, the Travis ‘Colloider’ or ‘Hydrolytic’ Tank. (Peters & Alleman, 1982)
Imhoff also patented his unit, but the associated royalty charges were considerably lower.
PRELIMINARY ‘BLOWING-AIR’ RESEARCH:
1880’s -> 1910’s
Searching for an improvement in sewage treatment, and with an intuitive inclination that
aerobic conditions would avoid undesirable, malodorous anaerobic results, several
researchers began to explore blowing air into sewage tanks. Dr. Angus Smith’s work in
1882 is commonly referenced as the original study, followed by Dibdin and Dupre,
Hartland and Kaye-Parry, Drown, and Mason and Hine. (Martin, 1927; Pearse, 1938) For
the most part, these early pioneers felt that oxygen presence ‘per se’ would provide the
desired oxidation of wastewater contaminants. Experimental results, though, were
nominal at best. Although putrescence was typically delayed, the effort and expense of
aeration seemed to lack significant compensation in terms of improved treatment.
Somewhat greater success was obtained, however, in studies of artificially aerated
contact filter beds conducted both by Col. George Waring and the Lawrence
Experimental Station. (Martin, 1927; Pearse, 1938; Peters & Alleman, 1982) In hindsight,
it is evident that these latter fixed-film units were receptive to the stimulus of aeration
because of their existing biomass, whereas the earlier aeration tanks lacked a recycled
biological population.
Over the course of the next few years, the importance of a suspended precipitant for
enhanced biological treatment became more accepted. Studies conducted by Mather and
Platt in 1893 indicated that precipitated impurities which accumulated at the bottom of
aeration tanks provided a marked enhancement of available treatment. (Martin, 1927) In
his presentation to the Royal Commission in 1905, Adeney reinforced this belief that
collected humus matters would accelerate the treatment capacity. (Martin, 1927; Pearse,
1938) Fowler’s experiments on sewage aeration in 1897 also yielded a clear effluent with
rapid settling deposits of particulate matter. (martin, 1938) However, Fowler conversely
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Activated Sludge Manual
viewed the enhanced deposition as a failure since he personally believed that sewage
impurities were to be rendered soluble or gasified for optimal treatment.
By 1910, the merits of aerating sewage in the presence of biological humus or slime were
beginning to find widespread acknowledgement. In their classic full-scale New York
study, Black and Phelps decided to abandon coarse rock media in favor of closely spaced,
wooden laths in order to achieve a higher surface area for desired slime accumulation.
(Black & Phelps, 1914) In essence, their unit was an aerated version of the prior Travis
‘Colloider’ or ‘Hydrolytic’ Tank (which had also used wood laths, but in an anaerobic
contact chamber).
Clark and Gage also initiated similar laboratory studies at Lawrence in 1912, comparing
aerated treatment efficiencies of bottles inoculated with algal suspensions against that
obtained in packed slate beds. (Martin, 1927; Pearse, 1938) The slate bed concept should,
however, be attributed to Dibdin. (Dibdin, 1913) Having been unsuccessful at simple
aeration in 1884, Dibdin had successively studied intermittent filtration, contact beds and
serial contact beds before coming full circle to the notion of combining aeration with
biological treatment in a slate bed contactor.
INITIAL GENESIS OF THE ACTIVATED SLUDGE CONCEPT:
1912 -> 1914
Giving these latter studies at New York and Lawrence, it was, therefore, serendipitous
that the eminent Englishman, Sir Gilbert John Fowler, was called to the United States to
review the New York Harbor pollution problem. (Martin, 1927; Pearse, 1938; Ardern &
Lockett, 1914a) In conjunction with this trip, Fowler had an opportunity to witness firsthand Clark and Gage’s ongoing experiments on Lawrence in 1912. Fowler subsequently
credited this visit as the impetus for his "illuminating idea" regarding activated sludge,
referring to Lawrence as the "Mecca of sewage purification."
Although disappointed with his prior aeration experiments, Fowler quickly seized upon
the concept of employing a suspended biomass culture and initiated several related
experiments upon returning to Manchester, England. One year after the Lawrence tour,
Fowler and one of his graduate students, Mrs. Mumford, published their successful
results covering a suspended-culture aeration system inoculated with iron salts and an
special ‘M-7’ iron bacterium. (Fowler & Mumford, 1913) Their treatment scheme
sequentially employed a ‘blowing tank’ and clarifier. However, their system had two
shortcomings. First, since it did not have a means of recycling solids, the unit required
continuous inoculation with their mysterious M-7 organisms. Secondly, Fowler at this
point was laboring under the misconception that his special ‘iron’ bacteria played a major
role in the overall efficacy of the process. To some extent, this misunderstanding might
have been linked to the use of coagulating, iron-rich blood started fifty years earlier with
the ABC Process. This misunderstanding about the role of iron and iron-bacteria, though,
would then persist for more than a decade (Wolman, 1927).
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Activated Sludge Manual
At this point, 31 years had elapsed since Dr. Smith first examined the aeration of sewage.
However, the seemingly simplistic notion of accumulating a suspended biomass through
solids recycle was still unknown. Hence, the revelation by Fowler’s students, Ardern and
Lockett, in May 1914 that these humus solids should be saved rather than discarded
proved to be an unqualified "bombshell" (using Fowler’s description, provided during an
audience reply following presentation of Ardern and Lockett’s paper).
Ever sensitive to the fiscal realities of academic research, Ardern and Lockett
acknowledged their gracious appreciation for the monetary support which had been
provided by the ‘Worshipful Company of Grocers.’ (Ardern & Lockett, 1914b) In
retrospect, the fact that ‘grocers’ would have been interested in this sort of research topic
does seem rather odd. However, upon reading the audience comments following their
presentation, it is readily evident that they genuinely thought this system’s waste sludge
would yield a marketable product given its nutrient content. Here again, as with the prior
‘ABC’ process, they were optimistically interested in recovering nitrogen and phosphorus
which otherwise was in critically short supply as a raw fertilizer feedstock. Rather
ironically, though, their ever-present shortage of fixed nitrogen would shortly have an
even more dramatic impact relative to its necessity for manufacturing the munitions
which would be needed for World War 1. Germany’s acute awareness of this problem led
them (i.e., via Fritz Haber’s Nobel-winning research) to develop industrial processes for
synthesizing ammonia and nitrates…at which point the opportunity or need for
recovering nitrogen from sewage largely became a moot issue.
Using fill-and-draw cycling, these latter authors had provided the premier demonstration
and pronouncement of activated sludge treatment. Even with viewed in the context of our
contemporary operations, their initial experiments were remarkably advanced. Indeed,
their presentation addressed such topics such as energy conservation, sludge handling,
and the sensitivity of nitrifying organisms to temperature and pH, all of which are still
debated in our contemporary literature. Perhaps more importantly, the audience of Ardern
and Lockett’s presentation immediately recognized the monumental value of their
discovery.
LAB TO FIELD TRANSFORMATION OF THE ACTIVATED SLUDGE
PROCESS:
1914 -> 1920’s
Ardern and Lockett subsequently presented two further papers in 1914 (b) and 1915
which touched on a range of practical issues, including: performance capabilities during
continuous-flow and fill-and-draw operation, the detrimental impact of trade wastes,
aeration levels using plain tubes and porous tiles, required aeration intensities, and
biomass acclimation. The startling fact that it could reliably produce clear, non-odorous
effluents had extreme appeal for municipalities long frustrated with their aesthetically
unattractive options. Even as Fowler’s pioneering research continued, therefore, the
process was already being tested on full-scale basis. In fact, at the same 1914 meeting
that Ardern and Lockett presented their second paper, Melling (1914) announced that he
had successfully applied activated sludge treatment to an 80,000 gallon per day flow at
Salford, England.
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In quick succession, several full-scale English installations were placed into operation.
The following listing provides a chronological summary of these facilities: Salford, 1914,
Davyhulme, 1915; Worcester, 1916; Sheffield, 1916; 1917; Stamford, 1917; Tunstall,
1920; Sheffield, 1920; Davyhulme, 1921; and Bury, 1921. In the United States,
progression of the activated sludge process moved with similarly amazing speed. Edward
Bartow, a Professor at the University of Illinois, visited Fowler’s group in Manchester in
August of 1914 and subsequently began his own bench-and pilot-scale experiments along
the lines established by Fowler’s group. Within a period of several months, numerous
other American researchers initiated similar studies, including those by Hammond,
Hendrick, Hurd, Frank, Mohlman, Hatton, and Pearse. (Maring, 1927; Pearse, 1938;
Metcalf & Eddy, 1916;
Babbitt, 1926) Full-scale U.S. installations began to appear to 1916, and by 1927 there
were nearly ten full-scale systems spread throughout the country, including: San Marcos
(TX), 1916; Milwaukee (WI), 1916; Cleveland (OH), 1916; Houston (TX), 1917 & 1918
(2 each); Des Plaines (IL), 1922; Calumet (IN), 1922; Milwaukee (WI), 1925; and
Indianapolis (IN), 1927.
ENTREPRENEURIAL TRANSFORMATION OF ACTIVATED SLUDGE:
1913 -> 1940’s
Within less than a decade, this rudimental, bench-scale concept had been installed at
numerous multi-MGD facilities. Based on its rapid growth during these first few years, it
would seem that activated sludge would have become the preeminent wastewater
treatment process virtually overnight. However, despite this initial intensity, activated
sludge did not truly find widespread application for several decades.
The cause for this delay is quite simple; namely, patent litigation curtailed most of the
technical momentum. Whereas Ardern and Lockett presented their research findings in
May of 1914, another pair of wastewater entrepreneurs (i.e., Jones and Attwood, Ltd.)
had actually beaten them to the punch by nearly a full year, filing four separate patent
applications dealing with "Improvements in Apparatus for the Purification of Sewage or
other Impure Waters" (UK patents #19915 at 1913; #22952 at 1913; #729 at 1914; and
#19916 at 1914). (Jones &
Attwood, 1913ab; 1914; 1915) Of these four, none actually employed the term ‘activated
sludge’. No. 729 clearly included the basics of the process though, particularly because of
its specific reference to solid recycle. Furthermore, the reactor figures provided by this
latter patent bear a striking similarity to contemporary looped designs marketed by
several proprietors.
Jones and Attwood must also be credited with much of the preliminary work towards
establishing the practical application of activated sludge. Several of the original full-scale
facilities (e.g. Worcester and Stamford) were, in fact, solely constructed at their expense
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Activated Sludge Manual
and risk as a means to demonstrate its pragmatic merit. In fact, the Worcester system was
designed and installed under a performance-based contract based on effluent quality.
The patent situation for activated sludge became even more complex in 1915 when Leslie
Frank, a U.S. Public Health Officer, obtained an American Patent (#1,139,024) which
covered much the same material as the Jones and Attwood claims. (Frank, 1915) Frank,
however, dedicated his patent for "activated sludge" (the misspelling reflects Frank’s
terminology) to all U.S. citizens. Hence, at this point, there were two different patent
entities dealing with activated sludge. Aside from these legal claims, Flowler’s own
standing as the originator of activated sludge was also being disputed by Clark at
Lawrence. (Clark, 1915; Mohlman 1938; Greeley, 1945) However, despite this confusion
regarding the legal status and origination of activated sludge, the American engineering
community pushed ahead with its technical application.
In late 1914, Jones and Attwood, Ltd. Warned American engineers and cities that they
should use caution regarding patent infringements. (Hatton, 1916) And when American
engineers took credit for certain innovations which transgressed into their (i.e. Jones and
Attwood, Ltd) patented procedures (e.g., Clarence Hurd’s announcement of the spiralflow aeration pattern being used at Indianapolis), they were quickly rebuffed by the Jones
and Attwood group. (Hurd, 1929; Sandford, 1929) But as more and more plants were
built, municipal concerns about patent problems and complications diminished.
This mood quickly changed, though, with a suit filed by Activated Sludge, Ltd. (the
licensed patentee for Jones and Attwood, Ltd.) against Chicago in the late 1920’s.
(Anonymous, 1933) Additional suits against Milwaukee, Cleveland, Indianapolis, and
several smaller cities soon followed. Legal rulings on all of these cases took several
years, during which time the sanitary engineering profession seriously reassessed the
prospects for near-term activated sludge utilization. In 1933, District Judge Geiger ruled
that Milwaukee had, indeed, violated patents held by Activated Sludge, Ltd.
(Anonymous, 1934a,b) An appeal was submitted, but in October, 1934 the Supreme
Court declined to rule against this decision.
In reflecting upon this outcome, Bloodgood (1982) indicated a belief that the District
Judge ruled against Milwukee moreso because of their outspoken lawyer than the
involved legal details. Whatever the case, the infringement ruling immediately rippled
throughout the country. Several existing plants quickly shut down to avoid monetary
fines, including the original San Marcos, Texas facility. (Otts, 1982) Many others chose
to continue their use of the activated sludge process based on a royalty fee of 25 cents per
capita. Amongst the 203 plants, Kappe (1938) reported that 150 were licensed by
Activated Sludge, Ltd. (Kappe, 1938) As for the large number of communities planning
to install new activated sludge plants, most simply elected either to build an alternative
system (oftentimes a trickling filter) or to wait until the applicable patents expired (e.g.
Washington, D.C. was a prime example).
Milwaukee and Chicago appear to have suffered the largest losses with each being fined
just under one million dollars (Activated Sludge, Inc., 1946) In Milwaukee’s case, these
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monies were secured from the proceeds on a relatively new (i.e. since 1926) sludge
product, Milorganite, whose annual sales in 1934 were estimated at 3 million dollars. In
retrospect, Chicago probably wishes it had accepted the terms of an out-of-court
settlement offered by Activated Sludge, Ltd. (Activated Sludge, Inc., 1946) Rather than
paying for the imposed fine and several years of legal involvement, the case could have
been settled with a $90,000 settlement.
ACTIVATED SLUDGE SUPREMACY:
1950’s -> present
Once the business of building wastewater treatment plants hit its peak in the United
States following World War II, the activated sludge process quickly became the dominant
design approach for secondary systems…and this ‘supremacy’ remains in effect to this
day. Had it not been for the litigation stemming from its original British patents, this
transition from fixed film processes would probably have moved even faster. At this
point in time, though, activated sludge has proven itself to be durable technology in an
era where most engineering methods lapse into obsolescence only decades, if not years,
after their original development.
SUMMARY
Sixteen years ago Frank Schaumburg published this ’Figure’ as a stand-alone paper with
the Journal of the Water Pollution Control Federation (NOTE: Even today, it is still
considered to be one of the most succinct publications ever carried on the topic of
activated sludge!). Entitled, "65 Years of Efficiency Progress in Activated Sludge," the
late Professor Schaumburg’s goal was to visually (and probably sarcastically)
demonstrate - the fact that the performance levels achieved with activated sludge (i.e.,
BOD removal efficiencies) have changed extremely little over the decades in spite of
considerable research and publication on the topic! It worked well when first developed,
it works about the same today, and it should serve our needs for many more years.
Ironically, though, the problem of handling the resultant sludge, which Melling cited as
their "greatest bugabear" while commenting on the landmark paper in 1914, still remains
a distinct challenge!
ACKNOWLEDGMENTS
The figures included within this text were respectively scanned from the following
sources:
-Pg. 1, Punch, pg. 41,31 July 1858;
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Activated Sludge Manual
-Pg. 2, top, Reyburn, W. (1989). "Flushed with Pride," Pavilion Books Limited, London,
UK;
-Pg. 2, bottom, Punch, pg. 71, 14 August 1858;
-Pg. 3, Minutes of Evidence, Royal Commission on Metropolitan Sewage Discharge,
Vol. III,
From May 1884 to October 1884 (1885);
-Pg. 4, Ardern, E. and Lockett, W.T. (1914). "Experiments on the Oxidation of Sewage
Without the
Aid of Filters." Journal of the Society of Chemical Industry, 33, pg. 524, 30 May;
-Pg. 7, Schaumburg, F. and Marsh, B.E. (1980), "65 Years of Efficiency Progress in
Activated
Sludge," Journal of the Water Pollution Control Federation, 51, pg.1.
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