Pengolahan Kimia


Penyisihan unsur pencemar dengan cara
penambahan chemical agent/bahan kimia
sehingga terjadi reaksi kimia, contoh :
koagulasi dan presipitasi
Prinsip dasar: perubahan bentuk
terlarut/tersuspensi menjadi bentuk yang
terendapkan (kecuali desinfeksi) sehingga
lumpur yang terendapkan termasuk kategori
B3 (perlu treatment khusus)

Kelebihan pengolahan secara kimia:



Efisiensi tinggi (dapat mencapai angka yang
diinginkan)
Waktu dentensi relatif singkat sehingga volume
reaktor/unit pengolahan relatif lebih kecil
Kekurangan



Ada penambahan zat aditif sehingga
meningkatkan konsentrasi Total Dissolved Solid
(TDS). Penyisihan TDS relatif sulit dan mahal:
membran atau destilasi
Meningkatkan beban pengolahan
Biaya bahan kimia cukup mahal = biaya untuk
energi
Softening
Benno Rahardyan
Faculty of Civil and Environmental Engineering - ITB
Private Water System Resources
Hard water
What is "Hard Water"?



Perhaps you have on occassion noticed mineral deposits on your
cooking dishes, or rings of insoluble soap scum in your bathtub.
These are not signs of poor housekeeping, but are rather signs
of hard water from the municipal water supply.
Hard water is water that contains cations with
a charge of +2, especially Ca2+ and Mg2+.
These ions do not pose any health threat, but they can engage
in reactions that leave insoluble mineral deposits. These
deposits can make hard water unsuitable for many uses, and so
a variety of means have been developed to "soften" hard water;
i.e.,remove the calcium and magnesium ions.
water hardness
Hard water is water contaminated with compounds of
calcium and magnesium. Dissolved iron, manganese,
and strontium compounds can also contribute to the
"total hardness" of the water, which is usually
expressed as ppm CaCO3.
Water with a hardness over 80 ppm CaCO3 is often
treated with water softeners , since hard water
produces scale in hot water pipes and boilers and
lowers the effectiveness of detergents.
Table 1. Hardness Classification
---------------------------------------------------------Concentration of
hardness minerals
milligrams per
in grains per
liter (mg/l) or
gallon (gpg)
Level of Hardness parts per million (ppm)
---------------------------------------------------------below 1.0
soft
less than 17
1.0 to 3.5
slightly hard
17 to 60
3.5 to 7.0
moderately hard
61 to 120
7.0 to 10.5
hard
121 to 180
above 10.5
very hard
more than 180
----------------------------------------------------------
Problems with Hard Water

Mineral deposits are formed by ionic reactions resulting in
the formation of an insoluble precipitate. For example,
when hard water is heated, Ca2+ ions react with
bicarbonate (HCO3-) ions to form insoluble calcium
carbonate (CaCO3), as shown in Equation 1.




This precipitate, known as scale, coats the vessels in which the
water is heated,
reduce the efficiency of heat transfer
serious effect for industrial-sized water boilers
scale can accumulate on the inside of appliances, such as
dishwashers, and pipes.
Softening
Precipitation
Neutralization
CO2+Ca(OH)2  CaCO3(s) + H2O
Ca+2 Precipitation at pH 10
Ca+2 +2HCO3-+Ca(OH)2  2CaCO3(s) + 2H2O
Mg+2 Precipitation at pH 11
Mg+2 +2HCO3-+Ca(OH)2  2MgCO3 + CaCO3(s) + 2H2O
Mg+2 + CO3= +Ca(OH)2  Mg(OH)2((s) + CaCO3(s)
Ionic Balance: addnon non hardness ionic (Na+) :
Mg+2 +NaOH  Mg(OH)2((s) + Na +
Ca+2 +Na2CO3  CaCO3(s) + 2Na+
Precipitation




For large-scale municipal operations, a process known as the
"lime-soda process" is used to remove Ca2+ and Mg2+ from the
water supply.
The water is treated with a combination of slaked lime,
Ca(OH)2, and soda ash, Na2CO3. Calcium precipitates as
CaCO3, and magnesium precipitates as Mg(OH)2. These solids
can be collected, thus removing the scale-forming cations from
the water supply.
To see this process in more detail, let us consider the reaction
for the precipitation of Mg(OH)2.
Consultation of the solubility guidelines in the experiment
reveals that the Ca(OH)2 of slaked lime is moderately soluble in
water. Hence, it can dissociate in water to give one Ca2+ ion
and two OH- ions for each unit of Ca(OH)2 that dissolves.

The OH- ions react with Mg2+ ions in the water to form
the insoluble precipitate. The Ca2+ ions are unaffected
by this reaction, and so we do not include them in the
net ionic reaction. They are removed by the separate
reaction with CO32- ions from the soda ash.
Ion-exchange



Ion-exchange devices consist of a bed of plastic
(polymer) beads covalently bound to anion groups,
such as -COO-.
The negative charge of these anions is balanced by
Na+ cations attached to them. When water
containing Ca2+ and Mg2+ is passed through the ion
exchanger, the Ca2+ and Mg2+ ions are more
attracted to the anion groups than the Na+ ions.
Hence, they replace the Na+ ions on the beads, and
so the Na+ ions (which do not form scale) go into
the water in their place.
The ion exchange process
 Calcium (Ca2+) and magnesium (Mg2+) ions that
cause water hardness can be removed fairly easily by
using an ion exchange procedure.
 Water softeners are cation exchange devices. Cations
refer to positively charged ions. Cation exchange
involves the replacement of the hardness ions with a
nonhardness ion.
 Water softeners usually use sodium (Na+) as the
exchange ion. Sodium ions are supplied from
dissolved sodium chloride salt, also called brine. In
the ion exchange process, sodium ions are used to
coat an exchange medium in the softener.
 The exchange medium can be natural "zeolites" or
synthetic resin beads that resemble wet sand.
The exchange medium can be
natural "zeolites" or synthetic resin
beads that resemble wet sand.

Softening Process
NaZeolite + Ca2+ --> CaZeolite + Na+
and
NaZeolite + Mg2+ --> MgZeolite + Na+

Recharging Process
NaCl + CaZeolite --> NaZeolite + CaCl
and
NaCl + MgZeolite --> NaZeolite + MgCl
Ion exchange softeners replace Ca++ and Mg++ with Na+ ions.
Zeolite medium is recharged with Na+ by NaCl brine when depleted.
Ion Exchange Water Softeners



Exchange sodium ions for calcium and magnesium
ions in water
May be dietary hazard - hypertension (adds 140
mg/l of sodium in “Hard” water)
Use potassium salt (KCl) for health reasons


many people with high blood pressure or other
health problems must restrict their intake of sodium.
Because water softened by this type of ion exchange
contains many sodium ions, people with limited
sodium intakes should avoid drinking water that
has been softened this way. Several new techniques
for softening water without introducing sodium ions
are beginning to appear on the market.
Types of water softening
equipment available
Water softeners are classified in five different categories:

Manual: There are several types of manual softeners. The
operator opens and closes valves to control the frequency, rate
and time length of backflushing or recharging.

Semi-automatic: The operator initiates only the recharging
cycle. A button is pushed when the softener needs recharging
and the unit will control and complete the recharging process.

Automatic: The automatic softener usually is equipped with a
timer that automatically initiates the recharging cycle and every
step in the process. The operator needs only to set the timer
and add salt when needed. It is the most popular type of
softener used.
Types of water softening
equipment available


Demand initiated regeneration (DIR): All operations are
initiated and performed automatically in response to the water
use demand for softened water. DIR systems generally have
two softening tanks and a brine tank. While one tank is
softening the other tank is recharging.
Off-site regeneration (generally rental units): A used
softening tank is physically replaced with a recharged tank.
Spent softening tanks are then recharged at a central location.
Ion Exchange Water
Softener with
Sensor- Controlled
Recharge
Softener Selection Considerations

Required grain capacity






Daily water use (household
population)
Water hardness
Desired regeneration schedule
Initial cost
Water conservation
Other (Iron removal, etc.)
Ion Exchange Water
Softener Capacity

Rated by grains of hardness treated between
regenerations
Example:
Water hardness = 200 mg/l
Softener Capacity = 2000 gr
Household Population = 4 persons
Calculate:
Water Use = 4 persons x 200 l/person-day = 800 l/day
Daily Hardness Treated = 800 l/day x 200 mg/l = 160 gr/day
Regeneration Interval = 2000 gr/ 160 gr/day = 12.5 days
Recommended Softener Sizes
Pump Capacity
(l/det)
Softener Capacity
(gr)
Water Hardness
(mg/l)
50
750
350
80
1000
500
120
1500
850
140
2000
1200
200
3000
1500
Ion Exchange Water Softener
Recharge Control Method
Water
Use
+
-
Initial Cost
-
-Time Clock
-Flow Meter
-Hardness
- Sensor
+
Water Softening


Permanent magnet water softeners
don’t work
Electrostatic and catalytic descalers may
“descale” water, but don’t soften it


Scale will not buildup on pipes, water heater
elements, bathtubs etc.
Sudsing action of soaps is not improved
Typical Programmable Water Softener
Controller
Reactions





CO2+Ca(OH)2CaCO3+H20
Ca(HCO3)2+Ca(OH)2=2CaCO3+2H20
Mg(HCO3)2+Ca(OH)2=MgCO3+CaCO3+2H20
MgCO3+Ca(OH) 2=Mg(OH)2+2CaCO3
MgSO4+Ca(OH) 2=CaSO4 + Mg(OH)2




CaSO4+Na2CO3=CaCO3 + Na2SO4
CaCl2+Na2CO3=CaCO3 + 2NaCl
MgSO4+Ca(OH)2=CaSO4 + Mg(OH)2
MgCl2+Ca(OH)2= Mg(OH)2 + CaCl2





CO2+Ca(OH)2Ca+2+2OHCO2+2OH- HCO3HCO3-+OH- CO3-2+H20
Ca+2 +CO3-2  CaCO3
Mg+2 + 2OH- Mg(OH)2
Pretreatment and other
variations
Prior to softening some preliminary treatment may be
advisable if
1.
Raw water turbidities exceed 3,000 NTU at times
2.
Raw water has high concentration of free carbon
dioxide (more than 10 mg/l)
3.
The raw water is high in organic colloids of a type
that impedes crystallization of calcium carbonate
4.
Raw water quality is highly variable over short
periods of time
5.
Recalcining of sludge is to be practiced
Variation of process








Single or two stage recarbonation ater conventional
lime-soda treatment
Sludge recirculation
Excess lime treatment with split treatment or
recarbonation
Post-treatment with polyphophates
Coagulation with alum, activated silica, or polymers
The use of three-stage treatment
The substitution of cation exchangers for soda ash to
remove non carbonate hardness
The use of caustic soda instead of soda ash




CO2+2NaOHNa2CO3+H20
Ca(HCO3)2+2NaOH=CaCO3+Na2CO3+2H20
Mg(HCO3)2+4NaOH=Mg(OH)2+2Na2CO3+2H20
MgSO4+2NaOH=Mg(OH)2 + Na2SO4
Systems expressing hardness
of water




German degree = Ca and Mg equivalent
with 10 mg CaO/liter
French degree = Ca and Mg equivalent
with 10 mg CaCO3/liter
English degree = one grain (0.06480 g)
of CaCO3 per gallon (3.785 L)
USA = ppm (mg/L) CaCO3
Expressing hardness in
milliequivalent/liter
1 milli-equivalent per liter =
-
-
2.8 German degree
5.0 French degree
3.5 English degree
50 mg CaCO3/liter
< 2 meq/L  soft water
> 5 meq/L  hard water
Total hardness = amount of Ca and Mg non
carbonate hardness + carbonate hardness.
Carbonate hardness = Ca and Mg equivalent to
bicarbonate content
The difference between total hardness and
bicarbonate (also called carbonate) hardness
is the non carbonate hardness, which
corresponds with ions like Cl-, and SO4--
HCO3-
I
Total hardness
Ca+2
Mg+2
HCO3-
II
Ca+2
Total hardness
Mg+2
Solubility in water
Substance
mg/l
meq/l
mg CaCO3/l
Ca(OH)2
1280
34.9
1730
CaCO3
15
0.3
15
Ca3(PO4)2
Nearly insoluble
Mg(OH)2
8.4
0.29
14.5
MgCO3
110
2.62
131
Mg3(PO4)2
Nearly insoluble
Disadvantage of phosphate method
- Rather expensive (cost of sodium
orthophosphate)
- Treated water will contain some rest of
PO43- For drinking water it is not necessary
and even not reccomendable to remove
all hardness
Lime soda process
I CO2+Ca(OH)2CaCO3+H20
II Ca(HCO3)2+Ca(OH)2=2CaCO3+2H20
1
1
IIIa Mg(HCO3)2+Ca(OH)2=MgCO3+CaCO3+2H20
IIIb MgCO3+Ca(OH)2=Mg(OH)2+2CaCO3
III Mg(HCO3)2+2Ca(OH)2=Mg(OH)2+2CaCO3+2H20 2
IV CaCl2+Na2CO3=CaCO3 + 2NaCl
1
Va MgCl2+Ca(OH)2=CaCl2 + Mg(OH)2
Vb CaCl2+Na2CO3=CaCO3 + 2NaCl
V MgCl2+Ca(OH)2+Na2CO3=CaCO3 + Mg(OH)2 + 2NaCl 1 1
II, III carbonate hardness reactions
IV, V non carbonate hardness reaction
needed Ca(OH)2 in meq
needed Na2CO3 in meq
Lime soda process
I
IV
Ca+2
II
CO2
V
Lime needed : [CO2] +[HCO3-]+ [Mg+2]
Soda needed : [Ca+2] - [HCO3-] + [Mg+2]
HCO3-
CO2
II
I
V
Mg+2
HCO3-
Ca+2
Mg+2
Lime needed : [CO2] +[HCO3-]+ 2[Mg+2]
I
II
III
Sodium hydroxide-soda process
Advantages
- Dosage of NaOH solutions is very
simple
- By using NaOH the amount of sludge is
much less than with Ca(OH)2 as the
precipitating agent)
NaOH process
I CO2+2 NaOHNa2CO3+H20
II Ca(HCO3)2+ 2NaOH =CaCO3+Na2CO3+2H20
IIIa Mg(HCO3)2+ 2NaOH =MgCO3+Na2CO3+2H20
IIIb MgCO3+ 2NaOH =Mg(OH)2+Na2CO3
III Mg(HCO3)2+ 4NaOH =Mg(OH)2+2Na2CO3+2H20
IV CaCl2+Na2CO3=CaCO3 + 2NaCl
1
V MgCl2+ 2NaOH = NaCl + Mg(OH)2
1
II, III carbonate hardness reactions
IV, V non carbonate hardness reaction
1
1
2
needed Ca(OH)2 in meq
needed Na2CO3 in meq
NaOH process
I
HCO3-
CO2
I
II
V
NaOH needed : [CO2] +[HCO3-]+ [Mg+2]
Soda needed : ([Ca+2] - HCO3- ) – ([CO2] +[HCO3-])
[Ca+2] - [CO2] - 2[HCO3-]
IV
Ca+2
II
CO2
I, II
Mg+2
HCO3-
Ca+2
Mg+2
NaOH needed : [CO2] +[HCO3-]+ 2[Mg+2]
I
II
III




CaSO4+Na2CO3=CaCO3 + Na2SO4
CaCl2+Na2CO3=CaCO3 + 2NaCl
MgSO4+Ca(OH)2=CaSO4 + Mg(OH)2
MgCl2+Ca(OH)2= Mg(OH)2 + CaCl2
Diketahui air mengandung ion
 Cl=142 mg/l,
 HCO3-=183 mg/l
 Ca++ = 120 mg/l
 Mg++=36 mg/l
 CO2 terlarut = 66 mg/l
Harga bahan kimia
 Na2CO3 = Rp. 4500/kg
 NaOH = 4000/kg
 Ca(OH) = 2500/kg
1.
2.
3.
Hitung tingkat kesadahan yang dapat dicapai
dengan metode pengendapan yang paling murah.
Jawaban didasarkan atas perhitungan dan reaksireaksi
Hitunglah tingkat kesadahan yang dapat dicapai
dalam proses pelunakan menggunakan Ca(OH)2 jika
diketahui bahan proses ini memerlukan kelebihan
dosis Ca(OH)2 sebanyak 18,5 mg/l
Idem soal 2 menggunakan NaOH jika diperlukan
kelebihan dosis NaOH sebesar 16 mg/l





Cl-=142 mg/l =142/35.5 = 4 meq/l
HCO3-=183 mg/l = 183/61 = 3 meq/l
Ca2+ = 120 mg/l = 120x2/40=6 meq/l
Mg2+ = 36 mg/l = 36x2/24 = 3 meq/l
CO2 = 66 mg/l = 66x2/44 = 3 meq/l
Ca2+ Mg2+ > HCO3-  bukan hanya kesadahan
sementara
Kesadahan total = 9 meq/l
Kesadahan sementara = 3 meq/l
Kesadahan tetap = 6 meq/l
Total Ca(OH)2 yang dibutuhkan
333+18,5 = 350 mg/l
Naik 350/333 = 1,05
Ksp CaCO3 < Ca2+ + CO3=
pada suhu tertentu adalah 0,3 meq/l
=[Ca2+][CO3=]/[CaCO3]
dengan memperhatikan
Ca(OH)2 = Ca2+ + OH350
350
---- mmol/l ----- mmol/l
74
74
Dengan kenaikan 1,05 pada ion Ca
maka Ksp  0,3 x 1,05 = 0,315 meq/l
Mg(OH)2 = Mg2+ + OHKsp = 0,24 meq/l
Total 0,555 meq/l = 1,64 D CaCO3
1 D = 10 mg/L CaCO3 = 10/(40+12+48)*2
= 0,2 meq/L CaCO3
Ca(OH)2 = Ca2+ + 2OH18,5 mg
18,5 x 2
--------- = 0,5 meq/L
40+34
Tingkat kesadahan 0,5/0,2 = 2,5 D
NaOH = Na+ + OH16 mg
16/40 =
0,4 meq
Kesadahan 0,4/0,2 = 2 D