2 Outline Overview 3.1 Criteria for Successful Biological Treatment 3.2 Principles of Biological Reactions3.3Wastewater Treatment Ponds3.4 Anaerobic Treatment Processes
3 Wastewater Treatment Primary Physical process Secondary Biological TertiaryCombination
4 2.1 Overview of Treatment Processes Preliminary & Primary TreatmentPhysical / chemical processes to prepare wastewater for biological treatmentRemoval of solids mainlyUsually cheaper/ easier than secondary processesExamples:a. equalisation (flow and load),b. neutralisation,c. settling of solids,d. flotation of oil and grease,e. filtration etc
6 In general, costs increase with increasing degree of treatment Tertiary TreatmentRemoval of specific pollutants with physical, chemical and/or biological methodsExamples:a. adsorption of organics by activated carbonb. precipitation or flocculation of phosphate etc.c. biological nitrogen removald. disinfectionIn general, costs increase with increasing degree of treatment
7 Wastewater Treatment Primary Physical process Secondary Biological TertiaryCombination
8 Outline Overview 3.1 Criteria for Successful Biological Treatment 3.2 Principles of Biological Reactions3.3Wastewater Treatment Ponds3.4 Anaerobic Treatment Processes
9 3.1 Criteria for Successful Biological Treatment Produce biological catalyst (biomass)source of energysource of cellular components (C, H, N, O, P, S etc.)Maintain biomassadequate environment (T, pH, toxics)adequate retention time (rate of treatment)Separation of biomassgrow suitable types of organisms ie. floc forming bacteria
10 Outline Overview 3.1 Criteria for Successful Biological Treatment 3.2 Principles of Biological Reactions3.3 Wastewater Treatment Ponds3.4 Anaerobic Treatment Processes
11 3.2 Principles of Biological Reactions A. Three Important Biological ReactionsAerobicCHO + O2 biomass + CO H2O≈ 50 % ≈ 50 %respiratory metabolismAnaerobicCHO biomass + CO2 + CH4 + H20% %fermentative metabolismPhotosynthesisCO2 + H2O biomass + O2energy supplied externally (light)
12 B. Aerobic or Anaerobic ?1000100100001000001100.1Wastewater COD (mg/L)Hydraulic Retention Time (days)Aerobic treatmentAnaerobic digestionHigh Rate AnaerobicTreatmentLow Rate Anaerobic
13 3.2 Principles of Biological Reactions C. Nutrient Requirements"Major" elements: C, H, O, N"Minor" elements:P DNA/RNA, phospholipids, ATPS for proteins, amino acidsK in RNA, coenzymesMg in RNA, coenzymes, as cationTrace elementsOften essential: Ca, Mn, Fe, Co, Cu, ZnRarely essential: B, Na, Al, Si, Cl, V, Cr, Ni, As, Se, Mo, Sn, I
14 Outline Overview 3.1 Criteria for Successful Biological Treatment 3.2 Principles of Biological Reactions3.3 Wastewater Treatment Ponds3.4 Anaerobic Treatment Processes
15 3.4 Wastewater Treatment Ponds Applied mostly in rural industries and small communitiesMain benefits are low construction and operating costClassification based on biological activity, form of aeration and influent compositionPOND TYPEBIOLOGICALACTIVITYTYPE OFAERATIONAnaerobicAvoidedFacultative (Stabilisation)Anaerobic/ AerobicNaturalAeratedAerobicMechanicalAerobic (Maturation, Oxidation)
16 1. Anaerobic Ponds Characteristics: High organic load; Deep (3-6m); Biomass formation small (5-15% of C in feed)
18 2. Facultative Ponds Characteristics: “two zone” environment, depth m; largemicrobial diversity; medium organic load; odour free
19 Facultative Pond Design & Operation Design: Area Loading Ratekg BOD5/ha/d T>15oCkg BOD5/ha/d T<15oCHRT daysOperational Considerations:Maintain aerobic conditions. Beware of over-loading causing the pond to turn anaerobic - odour problems
20 3. Aerated Ponds Characteristics: Mode is determined by the mixing intensityCompletely mixed: P/V = W/m3Facultative: P/V ≈ 0.8 W/m3
21 Aerated Pond Design & Operation HRT daysAeration capacity ≈ 2*BOD loadAerators: kg O2/kWhΔBOD: %Operational Considerations:Can be very efficient for soluble BOD/ COD removal but solids concentrations too high for discharge (irrigation ok).
22 4. Aerobic (Oxidation) Ponds Characteristics:Natural oxygenation (wind, photosynthesis); large surface area; shallow ( m); low organic loading.Suitable for treating effluent from anaerobic ponds
23 Aerobic Pond Design & Operation Design: kg BOD5/ha/dOperational Considerations:Maintain aerobic conditions. Beware of over-loading causing the pond to turn anaerobic.
24 Outline Overview 3.1 Criteria for Successful Biological Treatment 3.2 Principles of Biological Reactions3.3Wastewater Treatment Ponds3.4 Anaerobic Treatment Processes
25 3.4 Anaerobic Treatment Processes Treatment under exclusion of oxygenCarbon mainly converted to methane (CH4) and carbon dioxide (CO2)Used for high organic loadingsEfficient and economic COD/BOD removalLow rate systems use very long HRT eg. Anaerobic pondsHigh rate systems use low HRT but need biomass retention mechanism eg. UASBIncrease rate of biological action by increasing temperature.
26 Anaerobic Process Principles Pathways of organics in anaerobic treatmentFast growing,robust bacteriaSlow growing, pHsensitive archaea
27 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) processesShort HRT in first stage, no biomass retentionShort HRT but with biomass retention in second stage, usually pH controlledEg. UASB, Hybrid, fluidised bed reactors etc.
28 A. Single Stage Process SLUDGE DIGESTER Biogas Treated effluent WastewaterSLUDGE DIGESTERMixing mechanicallyor often by biogasrecirculation
31 B. Two-Stage Reactor Performance COD removal %BOD removal %Gas production m3/kg CODremovedMethane production m3/kg CODremovedMethane conc %Sludge production kg VSS/kgCODremoved
32 Two-stage high-rate hybrid reactor for abattoir & industrial wastewater
33 Anaerobic Reactor Design 1. Pre-acidification tankOften on the basis of an equalisation tank (also variable volume operation)Typical HRT hpH 5-6 if controlled, 4-5 if uncontrolledMixing usually only by inflow importance to minimise solids in influentCovered tank, gas vented and treated or incinerated (with biogas in boiler or flare)
34 Anaerobic Reactor Design 2. Methanogenic (2nd stage) reactorVolume-based organic loading rate (OLR)Cin biodegradable COD conc. in influent mg/LQ wastewater flow rate m3/dVR methanogenic bioreactor volume m3Typical HRT h, Solids RT daysUsually heated to operate at °C
35 High Rate Anaerobic Treatment Typical process flowsheet using Upflow Anaerobic Sludge Blanket (UASB) reactorAcidif. TankMix TankSludgeblanketMethanogenesisAcidogenesisBiogasBiomassretention asgranulesRecycle and mix tank reduce pHcontrol dosingCSTR-type tank usually not heated
36 Anaerobic Reactor Design OLR designs for various reactor types:UASB 6-12 kg COD/m3/dInternal Circulation kg COD/m3/dFluidised/expanded bed 12-20 kg COD/m3/dHybrid Reactor 6-12 kg COD/m3/dOLR varies with degradability, temp., pH…Hydraulic loading up to 24 m3/(m2reactor area d)Gas loading m3 gas /(m2reactor area d)