Biological Treatment
Processes
Outline
Overview
3.1 Criteria for Successful Biological
Treatment
3.2 Principles of Biological Reactions
3.3Wastewater Treatment Ponds
3.4 Anaerobic Treatment Processes
Wastewater Treatment
• Physical
process
Primary
Secondary
• Combination
• Biological
Tertiary
2.1 Overview of Treatment
Processes
Preliminary & Primary Treatment
Physical / chemical processes to prepare
wastewater for biological treatment
Removal of solids mainly
Usually cheaper/ easier than secondary processes
Examples:
a. equalisation (flow and load),
b. neutralisation,
c. settling of solids,
d. flotation of oil and grease,
e. filtration etc
2.1 Overview of Treatment Processes
Secondary Treatment
Biological removal of biodegradable, mostly
soluble organic compounds (carbon removal)
Aerobically
• activated sludge plants,
• aerated ponds
• trickling filters etc.
Anaerobically
• non-aerated ponds,
• high rate anaerobic (biogas) plants
Tertiary Treatment
Removal of specific pollutants with physical,
chemical and/or biological methods
Examples:
a. adsorption of organics by activated carbon
b. precipitation or flocculation of phosphate etc.
c. biological nitrogen removal
d. disinfection
In general, costs increase with increasing degree
of treatment
Wastewater Treatment
• Physical
process
Primary
Secondary
• Combination
• Biological
Tertiary
Outline
Overview
3.1 Criteria for Successful Biological
Treatment
3.2 Principles of Biological Reactions
3.3Wastewater Treatment Ponds
3.4 Anaerobic Treatment Processes
3.1 Criteria for Successful Biological Treatment
Produce biological catalyst (biomass)
• source of energy
• source of cellular components (C, H, N, O, P, S etc.)
Maintain biomass
• adequate environment (T, pH, toxics)
• adequate retention time (rate of treatment)
Separation of biomass
• grow suitable types of organisms ie. floc forming
bacteria
Outline
Overview
3.1 Criteria for Successful Biological
Treatment
3.2 Principles of Biological Reactions
3.3 Wastewater Treatment Ponds
3.4 Anaerobic Treatment Processes
3.2 Principles of Biological Reactions
A. Three Important Biological Reactions
Aerobic
CHO + O2  biomass + CO2 + H2O
≈ 50 % ≈ 50 %
respiratory metabolism
Anaerobic
CHO  biomass + CO2 + CH4 + H20
10 - 20
% 80 - 90 %
fermentative metabolism
Photosynthesis
CO2 + H2O  biomass + O2
energy supplied externally (light)
B. Aerobic or Anaerobic ?
Hydraulic Retention Time (days)
100
Anaerobic
digestion
10
Aerobic
treatment
Low Rate Anaerobic
Treatment
1
High Rate Anaerobic
Treatment
0.1
100
1000
10000
Wastewater COD (mg/L)
100000
3.2 Principles of Biological Reactions
C. Nutrient Requirements
"Major" elements: C, H, O, N
"Minor" elements:
•
•
•
•
P  DNA/RNA, phospholipids, ATP
S  for proteins, amino acids
K  in RNA, coenzymes
Mg  in RNA, coenzymes, as cation
Trace elements
• Often essential: Ca, Mn, Fe, Co, Cu, Zn
• Rarely essential: B, Na, Al, Si, Cl, V, Cr, Ni, As, Se, Mo, Sn, I
Outline
Overview
3.1 Criteria for Successful Biological
Treatment
3.2 Principles of Biological Reactions
3.3 Wastewater Treatment Ponds
3.4 Anaerobic Treatment Processes
3.4 Wastewater Treatment Ponds
Applied mostly in rural industries and small communities
Main benefits are low construction and operating cost
Classification based on biological activity, form of aeration
and influent composition
POND TYPE
BIOLOGICAL
ACTIVITY
TYPE OF
AERATION
Anaerobic
Anaerobic
Avoided
Facultative
(Stabilisation)
Anaerobic/ Aerobic
Natural
Aerated
Aerobic
Mechanical
Aerobic
(Maturation,
Oxidation)
Aerobic
Natural
1. Anaerobic Ponds
Characteristics:
High organic load;
Deep (3-6m);
Biomass formation small (5-15% of C in feed)
Anaerobic Pond Design & Operation
Parameter
Loading (volumetric)
Temperature
Mean HRT
Unit
kg BOD5/m3/d
°C
Influent COD
days
mg/L
Effluent COD
mg/L
Operational Considerations:
• BOD removal 60-80%
• Scum formation to contain odour emissions
• Monitor pH (should be 6.4 - 7.8)
Typical values
0.1-0.3
25-35
6-25
1000-6000
200-1000
2. Facultative Ponds
Characteristics:
• “two zone” environment, depth 1.5 - 4 m; large
• microbial diversity; medium organic load; odour free
Facultative Pond Design & Operation
Design: Area Loading Rate
• 40 - 140 kg BOD5/ha/d T>15oC
• 20 - 40 kg BOD5/ha/d T<15oC
• HRT 5 - 30 days
Operational Considerations:
• Maintain aerobic conditions. Beware of overloading causing the pond to turn anaerobic odour problems
3. Aerated Ponds
Characteristics:
• Mode is determined by the mixing intensity
• Completely mixed: P/V = 2.3 - 4 W/m3
• Facultative: P/V ≈ 0.8 W/m3
Aerated Pond Design & Operation
Design:
• HRT 0.5 - 3 days
• Aeration capacity ≈ 2*BOD load
• Aerators: 1 - 1.5 kg O2/kWh
• ΔBOD: 50 - 70%
Operational Considerations:
• Can be very efficient for soluble BOD/ COD removal
but solids concentrations too high for discharge
(irrigation ok).
4. Aerobic (Oxidation) Ponds
Characteristics:
• Natural oxygenation (wind, photosynthesis);
large surface area; shallow (1 - 1.5m); low
organic loading.
• Suitable for treating effluent from anaerobic
ponds
Aerobic Pond Design & Operation
Design: 40 - 120 kg BOD5/ha/d
Operational Considerations:
• Maintain aerobic conditions. Beware of overloading causing the pond to turn anaerobic.
Outline
Overview
3.1 Criteria for Successful Biological
Treatment
3.2 Principles of Biological Reactions
3.3Wastewater Treatment Ponds
3.4 Anaerobic Treatment Processes
3.4 Anaerobic Treatment Processes
Treatment under exclusion of oxygen
Carbon mainly converted to methane (CH4) and carbon
dioxide (CO2)
Used for high organic loadings
Efficient and economic COD/BOD removal
Low rate systems use very long HRT eg. Anaerobic ponds
High rate systems use low HRT but need biomass
retention mechanism eg. UASB
Increase rate of biological action by increasing
temperature.
Anaerobic Process Principles
Pathways of organics in anaerobic treatment
Process types
A. Single-stage processes
• Long solids & hydraulic retention times (HRT)
• Eg. Anaerobic digesters (20-30 d HRT)
Anaerobic ponds (10-30 d HRT)
B. Two-stage (high rate) processes
• Short HRT in first stage, no biomass retention
• Short HRT but with biomass retention in second stage,
usually pH controlled
• Eg. UASB, Hybrid, fluidised bed reactors etc.
A. Single Stage Process
Biogas
SLUDGE
DIGESTER
Treated
effluent
Wastewater
Mixing mechanically
or often by biogas
recirculation
1. Upflow Anaerobic Sludge Blanket
(UASB)
Gas collection
below water
level to reduce
turbulence at
overflow
Uniform
flow
distribution
essential
Biogas
Treated effluent
Gas
collector
Sludge
blanket
From
Pre-acidification
Tank
Granular biomass
2. Hybrid Reactor
Packed bed
(plastic material)
for biofilm growth
Biogas
Treated effluent
Uniform
flow
distribution
essential
Sludge
blanket
From
Pre-acidification
Tank
Granular biomass
B. Two-Stage Reactor Performance
COD removal 60 - 95%
BOD removal 80 - 95%
Gas production 0.3-0.6 m3/kg CODremoved
Methane production 0.2-0.35 m3/kg CODremoved
Methane conc. 55 - 75%
Sludge production 0.05-0.1 kg VSS/kgCODremoved
Two-stage highrate hybrid
reactor for
abattoir &
industrial
wastewater
Anaerobic Reactor Design
1. Pre-acidification tank
• Often on the basis of an equalisation tank
(also variable volume operation)
• Typical HRT 12-24 h
• pH 5-6 if controlled, 4-5 if uncontrolled
• Mixing usually only by inflow 􀃎 importance to
minimise solids in influent
• Covered tank, gas vented and treated or
incinerated (with biogas in boiler or flare)
Anaerobic Reactor Design
2. Methanogenic (2nd stage) reactor
• Volume-based organic loading rate (OLR)
Cin . Q
OLR 
VR
Cin  biodegradable COD conc. in influent mg/L
Q  wastewater flow rate m3/d
VR  methanogenic bioreactor volume m3
Typical HRT 12-24 h, Solids RT 10-150 days
Usually heated to operate at 30 - 40°C
High Rate Anaerobic Treatment
Typical process flowsheet using Upflow Anaerobic
Sludge Blanket (UASB) reactor
CSTR-type
tank usually
not heated
Recycle and
mix tank
reduce pH
control dosing
Acidif. Tank
Mix Tank
Acidogenesis
Biogas
Methanogenesis
Sludge
blanket
Biomass
retention as
granules
Anaerobic Reactor Design
OLR designs for various reactor types:
•
•
•
•
UASB
 6-12 kg COD/m3/d
Internal Circulation
 15-25 kg COD/m3/d
Fluidised/expanded bed 12-20 kg COD/m3/d
Hybrid Reactor
 6-12 kg COD/m3/d
OLR varies with degradability, temp., pH…
Hydraulic loading up to 24 m3/(m2reactor area d)
Gas loading 70 - 200 m3 gas /(m2reactor area d)
Questions?
Documentation
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