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BIOMATH Department of Applied Mathematics, Biometrics and Process Control EMSEL, Kyung Hee or

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1 BIOMATH Department of Applied Mathematics, Biometrics and Process Control EMSEL, Kyung Hee or EMSEL Environmental Management Systems Engineering Lab. Optimizing Biological Nutrient Removal Processes 유창규 경희대학교 환경응용화학대학 환경관리시스템공학연구실 (EMSEL) 환경관리공단 (2007 년 3 월 9 일 )

2 ChangKyoo Yoo - 2 Presentation Review of Theory –Nitrification –Denitrification –Phosphorus Removal Optimization for nutrient removal –Nitrification Optimization –Denitrification Optimization Systematic optimization protocol for N and P removal –Case study 1 : SBR, Belgium –Case study 2 : Haaren, carrousel, Netherlands Problems and Troubleshooting (?)

3 ChangKyoo Yoo - 3 References 1.Jeanette A. Brown, P.E., DEE, (Executive Director SWPCA, CSWEA)- Optimizing Biological Nitrogen Removal Processes, USA 2.Dean Pond, Black & Veatch (WWTP Operators School)- Biological Wastewater Treatment Operators School, USA 3.Kim J.K, University of Wisconsin Madison, Biological Nutrient Removal Theories and Design, USA 4.Tao Jiang, BIOMATH, Belgium, UNESCO-IHE, Calibrating a Side-stream Membrane Bioreactor using ASM1, Belgium 5. Henze, DTU, Activated Sludge Model 1,2,2d,3, Denmark 6.Peter A. Vanrolleghem, Laval Univ., Optimal but robust N and P removal in SBRs, Canada

4 ChangKyoo Yoo - 4 Advanced Treatment Systems What are the effects of N and P in receiving waters?

5 ChangKyoo Yoo - 5 What are the effects of N and P in receiving waters? Increases aquatic growth (algae) Increases DO depletion Causes NH 4 toxicity Causes pH changes

6 ChangKyoo Yoo - 6 Nitrogen Removal Purpose –Reduce effluent N (ammonia and nitrates) –Biological or chemical –Reduce nutrient load on stream –Reduce algae growth –Reduce oxygen depletion

7 ChangKyoo Yoo - 7 Why is it necessary to treat the forms of nitrogen? Improve receiving stream quality Increase chlorination efficiency Minimize pH changes in plant Increase suitability for reuse Prevent NH 4 toxicity Protect groundwater from nitrate contamination

8 ChangKyoo Yoo - 8 What are the forms of nitrogen found in wastewater? TKN = 40% Organic + 60% Free Ammonia Typical concentrations: Ammonia-N = mg/L Organic N = 10 – 35 mg/L No nitrites or nitrates Forms of nitrogen: Organic N Ammonia Nitrite Nitrate TKN Total N

9 ChangKyoo Yoo - 9 Why is it sometimes necessary to remove P from municipal WWTPs? Reduce phosphorus, which is a key limiting nutrient in the environment Improve receiving water quality by: –Reducing aquatic plant growth and DO depletion –Preventing aquatic organism kill Reduce taste and odor problems in downstream drinking water supplies

10 ChangKyoo Yoo - 10 Advanced Treatment Systems Identify and explain the objectives of the following advanced treatment systems: –Further removal of organics –Further removal of suspended solids –Nutrient removal (N and P) –Removal of dissolved solids

11 ChangKyoo Yoo - 11 Advanced Treatment Systems How is N removed or altered by conventional secondary (biological) treatment?

12 ChangKyoo Yoo - 12 Nitrification

13 ChangKyoo Yoo - 13 Nitrification Oxidation of ammonia nitrogen to nitrite nitrogen by nitrosamonas group: –NH O 2 2H + + NO 2 - Oxidation of nitrite nitrogen by nitrobacter group: –NO O 2 NO 3 -

14 ChangKyoo Yoo - 14 Nitrification NH 4 +  Nitrosomonas  NO 2 - NO 2 -  Nitrobacter  NO 3 - Notes: –Aerobic process –Control by SRT (4 + days) –Uses oxygen  1 mg of NH 4 + uses 4.6 mg O 2 –Depletes alkalinity  1 mg NH 4 + consumes 7.14 mg alkalinity –Low oxygen and temperature = difficult to operate

15 ChangKyoo Yoo - 15 Un-aerated Bioreactor (Anoxic Zone) RAS WAS Primary Effluent Nitrate Recycle AnoxicAerobic

16 ChangKyoo Yoo - 16 Characteristics of an Un-aerated Bioreactor Anoxic Microorganisms –Facultative heterotrophic-use carbon for the formation of new biomass –Use nitrate/nitrite instead of oxygen –Oxygen is preferred

17 ChangKyoo Yoo - 17 Nitrifier Minimum Aerobic SRT Varies with Temperature. Nitrification No Nitrification

18 ChangKyoo Yoo - 18 Effective Nitrification Achieved by: Effective nitrification –Adequate Aerobic SRT Temperature –Sufficient Oxygen Transfer Capacity Maintain a DO of 2 mg/l at peak loadings –pH > 6.5, preferably >7 Accomplished by sufficient alkalinity (Effluent concentration at least 50 mg/l –No inhibitory materials

19 ChangKyoo Yoo - 19 Nitrification Optimization Summary Test nitrification rate occasionally Select appropriate SRT Keep DO at 2 mg/l Keep pH about neutral (optimal 7.5 to 8.5) Provide sufficient alkalinity

20 ChangKyoo Yoo - 20 Denitrification

21 ChangKyoo Yoo - 21 Denitrification Using methanol as carbon source: 6 NO CH 3 OH N CO H 2 O + 6 OH - Using an endogenous carbon source: C 5 H 7 NO NO N CO H 2 O OH -

22 ChangKyoo Yoo - 22 Denitrification NO 3 -  denitrifiers (facultative bacteria)  N 2 gas + CO 2 gas Notes: –Anoxic process –Control by volume and oxic MLSS recycle to anoxic zone –N used as O 2 source = 1 mg NO 3 - yields 2.85 mg O 2 equivalent –Adds alkalinity  1 mg NO 3 - restores 3.57 mg alkalinity –High BOD and NO 3 - load and low temperature = difficult to operate

23 ChangKyoo Yoo - 23 Denitrification with Supplemental Carbon Methanol or other carbon source RAS WAS Aerobic Anoxic Primary Effluent Nitrate Recycle Aerobic

24 ChangKyoo Yoo - 24 Denitrification is Controlled by Mixed Liquor Recirculation. % Denit = R/(R+Q) * 100

25 ChangKyoo Yoo - 25 Effective Denitrification Size based on anoxic SRT –Typically 1 to 2 days depending on temperature Effective Denitrification –Sufficient Anoxic Volume (Anoxic SRT) –Sufficient Carbon –Sufficient mixed liquor recirculation

26 ChangKyoo Yoo - 26 External Carbon Methanol Stoichiometry –2.5 (NO3-N) (NO2-N) (DO) –Or, approximately 3 mg CH 3 OH/mg NO 3 -N –Requires 1 to 3 day SRT in secondary anoxic zone depending on temperature Other carbon sources technically feasible but generally more expensive.

27 ChangKyoo Yoo - 27 Denitrification Optimization Summary Minimize DO in anoxic zone (< 0.2 mg/l) Have 2Q to 4 Q recycle capabilities Provide sufficient carbon (readily biodegradable COD) Maximize use of secondary anoxic zones

28 ChangKyoo Yoo - 28 Phosphorus

29 ChangKyoo Yoo - 29 Phosphorus Removal Purpose –Reduce effluent P –Biological or chemical method –Reduce nutrient load on stream –Reduce algae growth –Reduce oxygen depletion Application / Mechanism –Biological –Chemical

30 ChangKyoo Yoo - 30 Phosphorus Removing Mechanism Facultative bacteria Energy Acetate plus fermentation products Substrate Acinetobacter spp. Anaerobic (Phosphorus removing bacteria, slow grower) Aerobic PHB Energy New biomass + Poly-P

31 ChangKyoo Yoo - 31 Phosphorus Removal Biological Continued … Final Clarifier RAS WAS Effl Q P Release Anaerobic Zone Aerobic Zone P Luxury Uptake P Removal

32 ChangKyoo Yoo - 32 Phosphorus Removal Chemical Continued … Final Clarifier RAS WAS Effl Q Aerobic Zone Chemical Coagulant P Removal Primary Clarifier Chemical Coagulant

33 ChangKyoo Yoo - 33 Effective Phosphorus Removal Size based on SRT –Typically 7 to 10 days depending on temperature Effective Denitrification –Sufficient Anaerobic Volume (Anaerobic SRT) –Sufficient influent carbon –Competition between denitrification and phosphorous removal bacteria Sensitive to influent carbon Unstable process

34 ChangKyoo Yoo - 34 질소 / 인 제거를 위한 생물학적 처리방법의 연구내용

35 ChangKyoo Yoo - 35 Guidelines for Biological Nutrient Removal (BNR) Process Selection Nitrogen Removal v Four Stage Bardenpho Process v Modified Ludzack-Ettinger (MLE) Process Phosphorus Removal Only v A/O Process Nitrogen and Phosphorus Removal v Five Stage Bardenpho (Phoredox) Process v University of Cape Town (UCT) Process v Modified UCT Process v Virginia Initiate Process (VIP)

36 ChangKyoo Yoo - 36 Schematic Process Configuration for Optional Operations AnaerobicAerobic Secondary clarifier Effluent Influent Modified UCT process UCT process Phoredox process Sludge recycle, s Mixed liquor recycle, a Mixed liquor recycle, r Anoxic

37 ChangKyoo Yoo - 37 Process Selection Based on TKN/COD ratio (Initial Screening) Nitrogen Removal v TKN/COD < 0.09: Bardenpho process v TKN/COD > 0.10: MLE process Nitrogen and/or Phosphorus Removal v TKN/COD < 0.07 ~ 0.08: A/O, A2/O, Phoredox process (modified Bardenpho) v TKN/COD < 0.12 ~ 0.14: UCT process v TKN/COD < 0.11: Modified UCT process

38 ChangKyoo Yoo - 38 Systematic optimization protocol for N and P removal

39 ChangKyoo Yoo - 39 Introduction WWTP are complex systems Complex models can help in: –Understanding the processes –Plant design –Plant optimisation –Plant control In practice –Which model to choose? –How to calibrate the model? –How to optimize the process need for calibration and optimization protocol

40 ChangKyoo Yoo - 40 Why Model-based Optimization ? Solving Problems for wwtp systems Optimized System System under study Experimenting Virtual Reality Reality Model of the System Modelling Solution for the System Simulate Implement

41 ChangKyoo Yoo - 41 The systematic optimisation protocol  Systematize and standardize the model-based optimisation using mechanistic models (ASM2d for N- & P- removal)  Objective oriented & iterative protocol  A grid of scenarios (full-factorial design) built on the basis of the degrees of freedom and the constraints of the SBR system  Selection and calibration of a suitable model to describe the biological processes  Simulation and evaluation of a multitude of scenarios  Selection of the best scenario  Implementation & final evaluation 2. Framework of the optimization 3. Model selection and calibration 4. Scenario analysis 5. Evaluation of the results of scenarios 6. Implementation of the best scenario 7. Measurement campaign Target reached? END Yes No 1. Objective(s)

42 ChangKyoo Yoo - 42 Necessary information for model calibration Model based w.w. characterization Flowrates COD fractionation (S S, S F, X S, X I, S I ) N, P fractions, TSS Biomass characterization Kinetic, stoichiometric parameters Active biomass fractions Biomass composition Plant Information Mass balance, Operating parameters (SRT, HRT, control) Aeration & Hydraulics

43 ChangKyoo Yoo - 43 Biomath calibration protocol Stage II – Plant survey/data analysis Design data –Plant layout/configuration, volumes, pumps, aerators,... Operational data –Flow rates, sludge recycle/waste, control strategies,... Measured data –Influent/effluent characterisation (COD,TKN,PO 4,NO 3,...) –On-line measurements (DO,T,pH,...) –TSS (RAS and effluent), sludge age/production,... Mass balances –Flow rate, sludge (including N & P) –Important for data quality check (e.g. sludge age)

44 ChangKyoo Yoo Cycle time (min) Anaerobi c + filling Draw Settling AerobicAnoxic Aerobic Influent Effluent Concentratio n NO 3 - PO 4 3- Case study (I) - SBR  Developing a robust biological system – Detect the major sources of process disturbances as soon as possible – Useful to keep the sludge as stable as possible – Volume (80 L), SRT (10 d) and HRT (12 h), 6 hour cycle mode – Six on-line measurements (DO, ORP, pH, conductivity, temperature, weight)

45 ChangKyoo Yoo - 45 Introduction  Both N & P removal successfully demonstrated at lab-scale and full-scale SBR installations.  SBR offers more flexibility in operation (compared to continuous systems) –a key aspect in process optimisation.  A myriad of operating strategies to optimise nutrient removal performance in SBRs.  Usually developed at lab- or pilot-scale & only comparison of a few operating scenarios  Increasingly, mathematical models (e.g. ASM1 for N-removal and ASM2d for N- & P- removal) are used to search for the optimal operating scenario

46 ChangKyoo Yoo - 46 ASM2d extended with hydrolysis of organic nitrogen module of ASM1 model was selected and calibrated for the SBR system. Model selection and calibration

47 ChangKyoo Yoo - 47 Scenario analysis  Construction of grids of scenarios  Choose a range and interval for the degrees of freedoms: S O -sp: [0.2, 0.4, 0.6, 0.8, 1,2] V step-feed : [0, 5, 10] T ANB : [60, 70, 80] T AER : [130, 140, 150] Intermittent aeration frequency:[1, 2, 4, 8] Full-factorial design of degrees of freedoms:  total 648 scenarios  Each scenario simulated for 30 days (3 X SRT)

48 ChangKyoo Yoo - 48 Scenario analysis  Formulation of grids of scenarios: Configuration of intermittent aeration frequencies & step-feed of influent (  ) IAF1 IAF2 IAF4 IAF8 reference Scenario analysis

49 ChangKyoo Yoo - 49 Evaluation of the scenarios  Effluent quality Effluent quality of 648 scenarios were analysed, general conclusions:  Increasing T ANB improves P-removal but decreases N- removal  Increasing T AER slightly improves the nitrification but negative effect on denitrification.  The S O -sp is the most critical/dictates the overall behaviour of the system.  The step-feed has a positive effect on the denitrification.  Increasing the intermittent aeration frequency (IAF) increases N & P removal

50 ChangKyoo Yoo - 50 Evaluation of the advanced Nutrient Removal  Simulation of N and P trends in the best scenario (BSC)  Off-line effluent results of N and P in SBR  Effective for N-removal but ? for P-removal and need for settling  Integrated monitoring of microbial community dynamics

51 ChangKyoo Yoo - 51 Conclusions & perspectives  A systematic protocol for model-based optimisation of SBRs is developed and successfully evaluated at a lab-scale SBR to achieve an optimal but robust N & P removal.  Oxygen set-point found as the most important parameter determining the overall behaviour of the system  Step-feed of influent is positive on denitrification and hence reduces the negative NO 3 -N effect on P-removal  Frequent intermittent aeration during react phase is positive for overall N & P-removal  The effluent quality & the robustness criteria conflict –a compromise is needed for the selection of best scenario.  The systematic protocol is made flexible and objective oriented which can be used for different activated sludge systems.

52 ChangKyoo Yoo - 52 Case study (II) - Haaren, Netherlands

53 ChangKyoo Yoo - 53 Haaren wwtp, Netherlands Plant survey – PE – Qin = m 3 /d, Q RAS = m 3 /d –1 anaerobic selector (1.400m 3 ) –2 caroussels ( m 3 in total) –4 clarifiers (2.460 m 3 in total) –Oxygen control (day: mg/L, night: intermittent) Measurement campaign data –On-line DO, NO 3 –Effluent orthophosphate and TSS –Influent data: total/filtered (0.45  m) COD,VFA,BOD 5, total/filtered TKN, NH4-N, total/filtered PO 4 and TSS Data analysis was performed (quality check) Characterisation steps ommited due to lack of data Target - Decrease effluent nutrient concentrations (N&P) of a carrousel type WWTP

54 ChangKyoo Yoo - 54 Modelling Haaren WWTP in a simulator The plant layout in simulation & modelling software

55 ChangKyoo Yoo - 55 Better! Validation of the calibrated ASM2d model

56 ChangKyoo Yoo - 56 Scenario Selection Internal Recycle control => Submerged propellers needed? RAS flow control => No information about clarifier? Aeration control => Optimization of Intermittent Aeration (mixing problems) => DO level control ( ) – Settling Problem! Selector(s) DO controller Sludge Recycle Internal Recycle Point Settler Influent (flux based average) Effluent Wastage X X X

57 ChangKyoo Yoo - 57 Intermittent aeration (BNR) MLE - Predenitrification air aerated unaerated IR RAS aerated Intermittent aeration unaerated RAS Aeration Time Cycle Time Denitrification Time Operating Parameters Aerated Fraction (AF) Cycle Time Ratio (CTR) AF = Aeration Time / Cycle Time CTR = Cycle Time / HRT

58 ChangKyoo Yoo - 58 AF = (Lin & Tsang, 1989, Ip et al., 1987) CTR = (Lin & Tsang, 1989, Nakanishi et al., 1990) Literature: Haaren: Intermittent aeration during day COD loss due to nitrification High NO3 flux carry over through selector (P removal) Aerated Fraction, AF = Cycle Time Ratio, CTR = Selection of the operating parameters Scenario tested

59 ChangKyoo Yoo - 59 Results (I) AF = 0.5 CTR

60 ChangKyoo Yoo - 60 Results (cont.) CTR = (Cycle time=110 min) Optimum => CTR : => AF : AF

61 ChangKyoo Yoo - 61 Conclusions and Perspectives N, P effluents can be optimized via aeration control Long term simulation is necessary for effluent stability and biomass composition Aeration cost can be minimized by adjusting the AF, CTR Other scenarios and alternatives

62 ChangKyoo Yoo - 62 Troubleshooting and Problem Solving

63 ChangKyoo Yoo - 63 Nitrification Inhibition Inhibition is defined as –Decrease in rate –Inability to convert NH3 to NO2, or NO2 to NO3 Indicators of Potential Inhibitors –Increase in effluent NH3-N concentration –Increase in NO2 concentration –Failure to nitrify at appropriate SRT –Decreased OUR –White foam –Increased effluent turbidity

64 ChangKyoo Yoo - 64 Probable Nitrification Inhibitors Metals –Cadmium –Lead –Zinc Organic Chemicals –Benzene –Cyanide –Thiourea –Surfactants Inhibition can be acute or chronic

65 ChangKyoo Yoo - 65 Potential Sources of Inhibition Industrial Discharges Haul-in with sludge or septic In-plant chemical spills Incinerator scrubber return

66 ChangKyoo Yoo - 66 Potential Solutions Confirm presence –Use simple nitrification test procedure with control Identify source –Can use test procedure for system-wide detective work Remove source or modify treatment strategy –Storage –Side stream options –Main stream options

67 ChangKyoo Yoo - 67 Solutions to Problems Foam on tanks:  Gray - brown - orange foam, viscous in nature - Nocardia type foam  Lon SRT ’ s, trapped surface, fluctuating SRT, fluctuating temperature Remove trapped surface, chlorinate foam selectively, chlorinate RAS  White foam - looks like soap  May have too low MCRT, not enough biomass in tank, excessive detergents

68 ChangKyoo Yoo - 68 Solutions to Problems High effluent ammonium, fluctuating effluent ammonium-N:  MCRT or DO may not be adequate in the aerobic zone to maintain nitrification.  Increase MCRT.  Evaluate Step Feed to increase MCRT without increasing MLSS to clarifier.

69 ChangKyoo Yoo - 69 Solutions to Problems Fluctuating chlorine demand: Partial nitrification of ammonium-N to nitrite-N without further conversion to nitrate-N. –Inadequate aeration to handle high flows, – inadequate biomass in system to handle diurnal peak nitrogen loads, or –inadequate biomass to handle spikes in influent TKN (e.g.: sudden septage discharges).

70 ChangKyoo Yoo - 70 Solutions to Problems Fluctuations in basin DO (with periods of low DO): Check if sufficient blowers are operating for peak loads Consider adding more blowers or upgrading to fine bubble diffusers. Excessive DO at certain times of the year or during low flow periods: –Look into ways of adjusting aeration based on time of day. e.g.: install timers or an automated DO Control system

71 ChangKyoo Yoo - 71 Solutions to Problems Gradual increase in secondary clarifier sludge blanket: Evaluate the trend in SVI. Is SVI too high for the clarifier solids loading? What is the blanket level? –If SVI is high because of filaments, are they low DO filaments? Where are these filaments growing? Is the anoxic zone behaving as a low DO zone? Is the aerobic zone suffering from low DOs. Can it be corrected? –Initiate RAS chlorination to reduce SVI. –Increase sludge wasting if MCRT can be reduced.

72 ChangKyoo Yoo - 72 Solutions to Problems SVI increases during and immediately after periods of high flows: Does infiltration increase anoxic zone DO? Infiltration may also bring in filamentous bacteria. Plan on a maintenance dose of RAS chlorination.

73 ChangKyoo Yoo - 73 Solutions to Problems Increase in effluent soluble organic-N - possible causes or Increase in effluent TKN with increase in ammonium-N: Reduction in SRT below that required for nitrification. Sudden increase in influent TKN - septage dose, etc. Sudden addition of inhibitor - industrial chemical, pesticide, preservative, etc..

74 ChangKyoo Yoo - 74 Solutions to Problems Large clumps of sludge floating to top of secondary clarifier: Denitrification in clarifier - Increase RAS flow rate to reduce time sludge spends in clarifier, increase activated sludge tank effluent DO. Too high of a blanket Check sludge scraper for proper operation.

75 ChangKyoo Yoo - 75 Summary Good troubleshooting and problem solving comes only with experience. Every plant is different. We never know what is coming into the plant that can harm the process. Even under perfect process control, we can still have problems. The more we learn, the more we need to learn.

76 ChangKyoo Yoo - 76 Any Question ? 유창규 경희대학교 환경응용화학대학 환경관리시스템공학연구실 (EMSEL) Tel)

77 ChangKyoo Yoo - 77 Aerobic condition Aerobic respiration O 2 present Electron acceptor: O 2 H O 2 O

78 ChangKyoo Yoo - 78 Anoxic Condition A

79 ChangKyoo Yoo - 79 Anaerobic Condition AN

80 ChangKyoo Yoo - 80 Aerated Bioreactor RAS (microorganisms) WAS O 2 + Pollutants + Microorganisms

81 ChangKyoo Yoo - 81 Retrofit of Existing Plants +Aeration basin size and configuration +Clarifier capacity +Aeration requirements +Type of aeration system +Sludge processing units +Operator skills Considerations

82 ChangKyoo Yoo - 82 Clarifier Modification nUsually than center-feed clarifiers because the flow is usually up through the sludge blanket. nSome phosphorus release typically occurs in the clarifier sludge blanket of a BPR plant but in a properly operated center-feed clarifier the entire sludge blanket plus the released phosphorus is drawn off the bottom of the clarifier and recycled to the anaerobic zone.

83 ChangKyoo Yoo - 83 Aeration Requirements and Type of Aeration System nThe aeration equipment is usually removed from any zone that will permanently become a part of the anaerobic zone. nThere is no need to add additional aeration equipment because the processes in the anaerobic zone reduce the oxygen transfer requirements by 10 to 20%. nThe primary concern should be the protection of the anaerobic zone from the recycle of too much dissolved oxygen.

84 ChangKyoo Yoo - 84 Sludge Processing Units nThe inclusion of BNR results in a 5 to 15% reduction in WAS while the inclusion of BPR will increase the WAS production slightly. nThe sludge processing units are of primary concern. nThe recycle of any soluble P changes the COD:P ratio entering the activated sludge process. nThe use of anaerobic digesters, gravity thickeners for waste activated sludge (WAS), and the recycle of the WAS for settling with the primary sludge in the primary clarifier are detrimental if not properly managed.

85 ChangKyoo Yoo - 85 Sludge Processing Units - continued Sludge dewatering nSeparate the thickening of primary sludge and WAS. nFlotation thickening is ideal. After thickening, the sludge may be further dewatered by belt press with the addition of polymers. nNote that some polymers inhibit nitrification. nAfter thickening or dewatering the sludge may be treated by: composting, digestion, landfill, incineration, heat treatment

86 ChangKyoo Yoo - 86 Sludge Processing Units - continued Composting nPrimary sludge can be dewatered to 22% solids and WAS to 16%. No phosphates will be released. Digestion (aerobic and anaerobic) nThis will lead to the release of phosphates from the microbial cells. In some instances, phosphates may be precipitated during anaerobic digestion. If the liquid is to be returned

87 ChangKyoo Yoo - 87 Operator Skills nGreater operator the necessary skills are easily learned and applied. nA retraining program for the operators should be part of any retrofit project.

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