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Activated Sludge Processes

Activated Sludge Processes. CE - 370. Basic Process. The basic AS process consists of A reactor in which the microorganisms responsible for treatment are kept in suspension and aerated Liquid-solids separation, usually sedimentation tank

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Activated Sludge Processes

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  1. Activated Sludge Processes CE - 370

  2. Basic Process • The basic AS process consists of • A reactor in which the microorganisms responsible for treatment are kept in suspension and aerated • Liquid-solids separation, usually sedimentation tank • A recycle system for returning solids removed from the liquid-solids separation unit back to the reactor • Important feature of the AS process is: • Formation of flocculent settleable solids that can be removed by gravity settling

  3. Activated Sludge process utilizes: • Fluidized microorganisms • Mixed growth microorganisms • Aerobic conditions • Microorganisms • Use organic materials in wastewater as substrates • Thus, they remove organic materials by microbial respiration and synthesis • MLSS • Ranges between 2000 and 4000 mg/l • Flows • Feed wastewater (Q) • Waste activated sludge (Qw) • Recycled activated sludge (R) • Prior to entering aeration tank • OR immediately after entering

  4. Oxygen Supply • Diffused compressed air • Mechanical surface aeration • Pure oxygen • Purposes of aeration • Provides oxygen required for aerobic bio-oxidation • Provides sufficient mixing for adequate contact between activated sludge and organic substances • In order to maintain the desired MLSS in the aeration tank, R/Q ratio must be calculated

  5. Calculate (R / Q) Ratio • Calculate the Sludge Density Index (SDI) • Sample MLSS from downstream of aeration tank • Determine SS in MLSS • Place 1 liter of the MLSS in 1-liter graduate cylinder • Settle the sludge for 30 minutes • Measure volume occupied by settled sludge • Compute SS in settled sludge in mg/l • SS represents SDI • The test approximates the settling that occurs in final clarifier • If SDI = 10,000 mg/l and MLSS must be 2,500 mg/l • Then, Q(0) + R(10,000) = (Q+R)(2500) • R/Q = (2500)/(7500) = (1/3) = 33.3 % • So, R is 33.3% of feed wastewater (Q)

  6. Sludge Volume Index (SVI) = 1/ SDI • Is the volume in ml occupied by 1 gram of settled activated sludge • It is a measure of settling characteristics of sludge • Is between 50 and 150 ml/gm, if process is operated properly • Why Qw? • Microbes utilize organic substances for respiration and synthesis of new cells • The net cell production (Qw) must be removed from the system to maintain constant MLSS • Qw is usually 1 to 6 % of feed wastewater flowrate (Q)

  7. Common organic materials in municipal wastewater are: • Carbohydrates (C, H, O0 • Fats (C, H, O) • Proteins (C, H, O, N, S, P) • Urea (C, H, O, N) • Soaps (C, H, O) • Detergents (C, H, O, P) • Traces of • Pesticides • Herbicides • Other agricultural chemicals • Activated sludge can be represented by: • C5H7O2N • Has a molecular weight of 113

  8. Design • To design of AS, the following must be determined: • Volume of reactor • Number of basins • Dimensions of each basin • Volume of reactor is determined from: • Kinetic relationships • Space loading relationships • Empirical relationships • Sludge production per day (Xw), kg/day • Oxygen required per day (Or), kg/day • Final clarifier • Number of basins

  9. Biological Kinetics • 1. Michaelis – Menten Concept • (1/X)(ds/dt) = specific rate of substrate utilization • (ds/dt) = rate of substrate utilization • ks = maximum rate of substrate utilization • Km = substrate concentration when the rate of utilization is half maximum rate • S = substrate concentration

  10. If S is very large, Km can be neglected, therefore S cancels out and the reaction is zero order in substrate. K is the rate constant for zero-order reaction. • If S is relatively small, it can be neglected in the denominator and the reaction is first-order in substrate. K is the rate constant for the first-order reaction

  11. Rearrange and integrate Equation (2) • X = average cell mass concentration during the biochemical reaction, that is X = (X0 + Xt)/2 • St = substrate concentration at time t • S0 = substrate concentration at time t = 0

  12. Rearrange and integrate Equation (3) • X = average cell mass concentration during the biochemical reaction, that is X = (X0 + Xt)/2 • St = substrate concentration at time t • S0 = substrate concentration at time t = 0

  13. Equations (4) and (5) are in the form of • y = mx + b • Plotting St on y-axis versus Xt on the x-axis on arithmetical paper produce a straight line with a slope of –K • Plotting St on y-axis versus Xt on the x-axis on semilog paper produce a straight line with a slope of -K • The substrate could be • The BOD5 • Biodegradable part of COD • Biodegradable fraction of TOC • Biodegradable of any other organic matter

  14. Rate Constant, K • Depends on the specific wastewater • For domestic wastewater, it ranges between 0.1 to 1.25 liter/(gram MLSS)(hr) using BOD5 • Should be determined using lab-scale or pilot-scale studies • In the absence of studies, K between 0.1 and 0.4 liter/(gram MLSS)(hr) is recommended

  15. Example on Biochemical Kinetics

  16. Food to Microorganism Ratio (F/M) • F/M ratio is equal to the specific rate of substrate utilization (1/X)(dS/dt) • The units of F/M ratio are (mass substrate) / (mass microbes)  (time) • (kg BOD5/kg MLVSS-day)

  17. Mean Cell Residence Time (c) • It is defined as: • X = active biological solids in the reactor • X = active biological solids in the waste activated sludge flow • Units of c is days • Mean cell residence time is sometimes referred to as sludge age

  18. F/M Ratio and c • Both parameters are used characterize the performance of the activated sludge process • A high F/M ratio and a low c produce filamentous growth that have poor settling characteristics • A low F/M ratio and a high c can cause the biological solids to undergo excessive endogenous degradation and cell dispersion • For municipal wastewater • c should be at least 3 to 4 days • If nitrification is required, c should be at least 10 days

  19. F/M Ratio and c • Relationship between c and F/M ratio can be derived by starting with the equation of cell production, as follows: • (X/t) = rate of cell production, mass/time • Y = cell yield coefficient, mass cell created/mass substrate removed • ke = endogenous decay, mass cells/(total mass cells)  (time) • X = average cell concentration, mass

  20. F/M Ratio and c • Divide by X • c is the average time a cell remains in the system, thus

  21. F/M Ratio and c • The F/M ratio is the rate of substrate removal per unit weight of the cells, thus • Thus

  22. F/M Ratio and c • Since F/M was also expressed as: • Then,

  23. Types of Reactors • Plug-flow reactors • Dispersed plug-flow reactors • Completely-mixed reactors

  24. Plug-flow and Dispersed-flow Reactors • In plug-flow reactors, there is negligible diffusion along the flow path through the reactor • In dispersed-flow reactors, there is significant diffusion along the flow path through the reactor • Both types of reactors are used in conventional and tapered aeration activated sludge

  25. Conventional Activated Sludge • Rectangular aeration tank • F/M = 0.2 to 0.4 (kg BOD5/kg MLSS-day) • Space loading = 0.3 to 0.6 (kg BOD5/day-m3) • c = 5 to 15 (days) • Retention time (aeration tank) = 4 to 8 (hours) • MLSS = 1500 to 3000 (mg/l) • Recycle ratio (R/Q) = 0.25 to 1.0 • Plug-flow and Dispersed-flow • BOD removal = 85 to 95 (%)

  26. Tapered Aeration • It is a modification of the conventional process • F/M = 0.2 to 0.4 (kg BOD5/kg MLSS-day) • Space loading = 0.3 to 0.6 (kg BOD5/day-m3) • c = 5 to 15 (days) • Retention time (aeration tank) = 4 to 8 (hours) • MLSS = 1500 to 3000 (mg/l) • Recycle ratio (R/Q) = 0.25 to 1.0 • Plug-flow and Dispersed-flow • BOD removal = 85 to 95 (%)

  27. Oxygen Demand versus Reactor Length for Municipal Wastewater

  28. Performance •  is the detention time for the plug-flow reactor • The volume of the plug-flow or dispersed-flow reactor is given by:

  29. Completely Mixed Reactors • Usually circular and square aeration tanks • F/M = 0.1 to 0.6 (kg BOD5/kg MLSS-day) • Space loading = 0.8 to 2.0 (kg BOD5/day-m3) • c = 5 to 30 (days) • Retention time (aeration tank) = 3 to 6 (hours) • MLSS = 2500 to 4000 (mg/l) • Recycle ratio (R/Q) = 0.25 to 1.5 • Completely mixed • BOD removal = 85 to 95 (%)

  30. Design Parameters • The retention time and reactor volume for completely mixed reactors can be determined by:

  31. Process Modifications • Objective of modifications • Modifications • Step aeration • Modified aeration • Contact stabilization • High-rate aeration • Extended aeration

  32. Objectives of Modification Several modifications of the activated sludge process were made to attain a particular or design objective

  33. Step Aeration • It was developed to even out the oxygen demand of the MLSS throughout the length of the reactor • It uses plug-flow and dispersed plug-flow reactors with step inputs of the feed flow (Q) • Design Parameters •  = 3-5 hrs; c = 5-15 days; R/Q = 25-75%; MLSS = 2000-3500 mg/l; BOD5 and SS removal = 85-95%; F/M = 0.2-0.4 kg/kg-day; space loading = 0.6-1.0 kg BOD5/day-m3

  34. Modified Aeration • Designed to provide a lower degree of treatment than the other activated sludge processes • It uses plug-flow and dispersed plug-flow reactors • Design Parameters •  = 1.5-3 hrs; c = 0.2-0.5 days; R/Q = 5-15%; MLSS = 200-500 mg/l; BOD5 and SS removal = 60-75%; F/M = 1.5-5.0 kg/kg-day; space loading = 1.2-2.4 kg BOD5/day-m3

  35. Contact Stabilization • Designed to provide two reactors, one for the sorption of organic matter and for the bio-oxidation of the sorbed materials • It uses plug-flow and dispersed plug-flow reactors • Design Parameters •  = 0.5-6 hrs; c = 5-15 days; R/Q = 50-150%; MLSS = 1000-10000 mg/l; BOD5 and SS removal = 80-90%; F/M = 0.2-0.6 kg/kg-day; space loading = 1.0-1.2 kg BOD5/day-m3

  36. High-Rate Aeration • Designed to provide a lower degree of treatment than the other activated sludge processes • It uses completely mixed reactor • Design Parameters •  = 2-4 hrs; c = 5-10 days; R/Q = 100-500%; MLSS = 4000-10000 mg/l; BOD5 and SS removal = 75-90%; F/M = 0.4-1.5 kg/kg-day; space loading = 1.6-16 kg BOD5/day-m3

  37. Extended Aeration • Designed to minimize waste activated sludge production by providing a large endogenous decay of the sludge mass • It uses plug-flow and dispersed plug-flow reactors • Design Parameters •  = 18-36 hrs; c = 20-30 days; R/Q = 75-150%; MLSS = 3000-6000 mg/l; BOD5 and SS removal = 75-95%; F/M = 0.05-0.15 kg/kg-day; space loading = 0.16-0.4 kg BOD5/day-m3

  38. Pure Oxygen Process • Designed to reduce retention time, decrease the amount of waste activated sludge, increase sludge settling characteristics and reduce land requirement • It uses completely mixed reactors • Design Parameters •  = 1-3 hrs; c = 8-20 days; R/Q = 25-50%; MLSS = 3000-8000 mg/l; BOD5 and SS removal = 85-95%; F/M = 0.25-1.0 kg/kg-day; space loading = 1.6-3.2 kg BOD5/day-m3

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