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Biolog ical Treatment Processes. 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. Wastewater Treatment. 2.1 Overview of Treatment Processes.

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outline
Outline
  • Overview
  • 3.1 Criteria for Successful Biological Treatment
  • 3.2 Principles of Biological Reactions
  • 3.3Wastewater Treatment Ponds
  • 3.4 Anaerobic Treatment Processes
2 1 overview of treatment processes
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 processes1
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
slide6

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
outline1
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
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
outline2
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
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

100

Anaerobic digestion

Aerobic treatment

Low Rate Anaerobic

Treatment

10

Hydraulic Retention Time (days)

1

High Rate Anaerobic

Treatment

0.1

100

1000

10000

100000

Wastewater COD (mg/L)

B. Aerobic or Anaerobic ?
3 2 principles of biological reactions1
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
outline3
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
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
1 anaerobic ponds
1. Anaerobic Ponds

Characteristics:

  • High organic load;
  • Deep (3-6m);
  • Biomass formation small (5-15% of C in feed)
anaerobic pond design operation
Anaerobic Pond Design & Operation

Operational Considerations:

  • BOD removal 60-80%
  • Scum formation to contain odour emissions
  • Monitor pH (should be 6.4 - 7.8)
2 facultative ponds
2. Facultative Ponds

Characteristics:

  • “two zone” environment, depth 1.5 - 4 m; large
  • microbial diversity; medium organic load; odour free
facultative pond design operation
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 over-loading causing the pond to turn anaerobic - odour problems
3 aerated ponds
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
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
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
Aerobic Pond Design & Operation
  • Design: 40 - 120 kg BOD5/ha/d
  • Operational Considerations:
    • Maintain aerobic conditions. Beware of over-loading causing the pond to turn anaerobic.
outline4
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
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

Slow growing, pH

sensitive archaea

Fast growing,

robust bacteria

Anaerobic Process Principles

Pathways of organics in anaerobic treatment

process types
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

A. Single Stage Process
1 upflow anaerobic sludge blanket uasb

Gas collection

below water

level to reduce

turbulence at

overflow

Biogas

Treated effluent

Gas collector

Uniform flow

distribution

essential

Sludge blanket

From

Pre-acidification

Tank

Granular biomass

1. Upflow Anaerobic Sludge Blanket (UASB)
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

2. Hybrid Reactor
b two stage reactor performance
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
anaerobic reactor design
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 design1
Anaerobic Reactor Design

2. Methanogenic (2nd stage) reactor

    • Volume-based organic loading rate (OLR)

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

Recycle and mix tank reduce pH

control dosing

Biogas

CSTR-type tank usually not heated

Methanogenesis

Sludge

blanket

Acidif. Tank

Mix Tank

Acidogenesis

Biomass

retention as

granules

High Rate Anaerobic Treatment
  • Typical process flowsheet using Upflow Anaerobic Sludge Blanket (UASB) reactor
anaerobic reactor design2
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)
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