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 Primary Physical process Secondary Biological Tertiary Combination

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

In general, costs increase with increasing degree of treatment 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 Primary Physical process Secondary Biological Tertiary Combination

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 ? 1000 100 10000 100000 1 10 0.1 Wastewater COD (mg/L) Hydraulic Retention Time (days) Aerobic treatment Anaerobic digestion High Rate Anaerobic Treatment Low Rate Anaerobic

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 Avoided Facultative (Stabilisation) Anaerobic/ Aerobic Natural Aerated Aerobic Mechanical Aerobic (Maturation, Oxidation)

1. Anaerobic Ponds Characteristics: High organic load; Deep (3-6m); Biomass formation small (5-15% of C in feed)

Anaerobic Pond Design & Operation Parameter Unit Typical values Loading (volumetric) kg BOD5/m3/d 0.1-0.3 Temperature °C 25-35 Mean HRT days 6-25 Influent COD mg/L 1000-6000 Effluent COD 200-1000 Operational Considerations: BOD removal 60-80% Scum formation to contain odour emissions Monitor pH (should be 6.4 - 7.8)

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 over-loading 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 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 over-loading 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 Fast growing, robust bacteria Slow growing, pH sensitive archaea

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 SLUDGE DIGESTER Biogas Treated effluent Wastewater SLUDGE DIGESTER Mixing mechanically or often by biogas recirculation

1. Upflow Anaerobic Sludge Blanket (UASB) Treated effluent Biogas From Pre-acidification Tank Gas collector Granular biomass Gas collection below water level to reduce turbulence at overflow Uniform flow distribution essential Sludge blanket

2. Hybrid Reactor Biogas Treated effluent Packed bed From Pre-acidification Tank Granular biomass Uniform flow distribution essential Packed bed (plastic material) for biofilm growth Sludge blanket

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 high-rate 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  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 Acidif. Tank Mix Tank Sludge blanket Methanogenesis Acidogenesis Biogas Biomass retention as granules Recycle and mix tank reduce pH control dosing CSTR-type tank usually not heated

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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