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Efficient Wastewater Treatment: Design and Operation Guidelines

Learn about wastewater treatment plant layout, mechanical and biological treatment processes, effluent standards, and load variations for optimal water management.

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Efficient Wastewater Treatment: Design and Operation Guidelines

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  1. Technische Universität Dresden Peter Krebs Department of Hydro Science, Institute for Urban Water Management Urban Water Systems 11 Wastewater Treatment 11.1 Boundary conditions 11.2 Layout of a wastewater treatment plant (WWTP) 11.3 Mechanical treatment 11.4 Biological treatment 11.5 Final clarification 11 Wastewater treatment

  2. 11 Wastewater treatment 11.1 Boundary conditions 11 Wastewater treatment

  3. Wastewater treatment in Germany In 2000 more than 10‘000 communal WWTPs Class Number Total capacity in mio PE > 100’000 272 83.1 10’000 – 100’000 1’817 56.1 2’000 – 10’000 2’617 12.3 50 – 2’000 5’677 3.2 11 Wastewater treatment

  4. Effluent standards COD (mg/l) BOD5(mg/l) NH4-N (mg/l) N* (mg/l) Ptot(mg/l) Class 1 < 1000 PE 60 kg BOD5 / d 150 40 - - - 2 < 5000 PE 300 kg BOD5 / d 110 25 - - - 3 < 10000 PE 600 kg BOD5 / d 90 20 10 - - 4 < 100000 PE 6000 kg BOD5 / d 90 20 10 18 2 5 > 100000 PE 6000 kg BOD5 / d 75 15 10 13 1 * N = Sum of NH4+, NO3-, und NO2- 11 Wastewater treatment

  5. 70 7 60 6 NH -load 4 50 5 COD-load 40 4 -load (kg/h) COD-load (kg/h) 30 3 4 COD NH Daily average and 20 2 4 NH 10 1 0 0 00:00 04:00 08:00 12:00 16:00 20:00 00:00 Time (hh:mm) Load variation in WWTP inlet 11 Wastewater treatment

  6. Wet-weather load of NH4 and TSS from sewer 4 16 NH4 load 14 Flow rate ) 0 C 3 12 TSS load · d Q 10 load / (Qd·C0) 2 8 TSS load / ( 6 4 ; NH 1 4 d Q / Q 2 0 0 10:00 12:00 14:00 16:00 18:00 20:00 22:00 0:00 2:00 11 Wastewater treatment

  7. Flow rate Load Time Conc., load Concentration Time WWTP load with NH4+ at storm event 11 Wastewater treatment

  8. 11 Wastewater treatment 11.2 Layout of a wastewater treatment plant (WWTP) 11 Wastewater treatment

  9. Layout of a WWTP Mechanical treatment Biological treatment precipitation chemical Screen Grid chamber Fat chamber Primary clarifier Activated sludge tank Secondary clarifier River, filtration Sand Fat Primary sludge Return sludge Thickening Secondary sludge Excess sludge Biogas Use, dewatering, drying, incineration, Disposal disposal, incineration Wash, disposal Digestion Storage 11 Wastewater treatment

  10. Example: WWTP Chemnitz-Heinersdorf 11 Wastewater treatment

  11. Typical residence time in reactors Wastewater HRT (h) Sludge SRT (d) Mechanical pre-treatment 0.2 0.01 Primary clarifier 1.5 1 Activated sludge tank 10 10 Secondary clarifier 5 2 Sludge thickener 2 Digester 20 Storage 100 < 1 d > 100 d 11 Wastewater treatment

  12. 11 Wastewater treatment 11.3 Mechanical treatment 11 Wastewater treatment

  13. Automatically cleaned trash rack 11 Wastewater treatment

  14. Sieve Slotted sieve ≥ 0.5 mm Perforated tin ≥ 3 mm Hans Huber AG, Typ Ro9 11 Wastewater treatment

  15. Basics for rack design • Flow velocity: 0,6  v  2,5 m/s • Widen flume to compensate for racks cross section • Consider backwater effect • Hydraulics: • + due to clogging • lower bottom to compensate for backwater effect • Safety for maintenance (redundant) • Building around racks (Frost, smell) 11 Wastewater treatment

  16. Surface load: qA = Q / A(Hazen, 1904) L U U Q H VS Critical case Settling condition  VS qA  Independent of H ! 11 Wastewater treatment

  17. Grid chamber • In connection with combined systems • Effects of non-organic particles: • Abrasion of steel (e.g. pumps) • Clogging (Sludge hopper, pipes, Pumps) • Sedimentation (activated sludge tank, digester)  High maintenance requirements Sludge in sand is hindering Sand in sludge is detrimental for operation ! 11 Wastewater treatment

  18. Empirical sedimentation velocities Kalbskopf, 1966 11 Wastewater treatment

  19. Design rules • Flow velocity:  0,3 m/s • Width extending with height • Length: • Sand collector: appr. 0,2 x 0,3 m (not too large!) • Venturi flow measurement after grid chamber 11 Wastewater treatment

  20. Aerated grid chamber 11 Wastewater treatment

  21. Efficiency of primary clarifier 100 90 settleable solids 80 70 TSS 60 50 Efficiency (%) 40 30 BOD 5 20 10 0 0 1 2 3 4 5 (h) Residence time HRT 11 Wastewater treatment

  22. Effects of primary clarifier on wastewater Compound Unit Inlet Outlet* TSS g TSS / m3 360 180 0.5 BOD5 g O2 / m3 300 230 0.23 COD g O2 / m3 600 450 0.25 TKN g N/ m3 60 56 0.067 NH4-N g N/ m3 40 40 0 NO2-N g N/ m3 0 0 0 NO3-N g N/ m3 1 1 0 Ptot g P/ m3 10 9 0.1 Alkalinity mol HCO3- / m3 = f(Drinking water) + NH4-N * Short residence time 11 Wastewater treatment

  23. Rectangular sedimentation tank Primary / secondary clarifier Flight scraper Effluent weir Inlet Effluent to activated sludge tank or to receiving water Sludge withdrawal Surface: Primary clarifier qA = 2 to 6 m/h Secondary clarifier qA = 0.5 to 1.5 m/h 11 Wastewater treatment

  24. 11 Wastewater treatment 11.4 Biological treatment 11 Wastewater treatment

  25. Biological treatment Suspended biomass  Activated sludge system • suspended through turbulence • sludge flocs with 0.1 – 1 mm diameter • degradation related to biomass  Increase suspended biomass concentration Sessile biomass  Biofilm system • biofilm on carrier • little erosion • degradation related to biofilm surface area  Increase of specific surface area 11 Wastewater treatment

  26. Important micro-biological processes Growth Implementation of C, N, P in biomass Decay If external nutrients are rare Hydrolysis Heavily  easily degradable substances, through encymes Aerobic degradation organic compounds CH2O + O2 CO2 + H2O Nitrification NH4+ + 2 O2 NO3- + H2O + 2 H+ Denitrification 5 CH2O + 4 NO3- + 4 H+ 2 N2 + 5 CO2 + 7 H2O 11 Wastewater treatment

  27. Growth of bacteria Doubling time tD 0·tD 1·tD 2·tD 3·tD i·tD ... n·tD 20 21 22 23 2i ... 2n Activated sludge: tD = 6 h Sludge age = 10 d 11 Wastewater treatment

  28. ) -1 (T m Substrate, nutrients, O 2 Growth of bacteria  Growth is limited through nutrients and oxygen Growth max,2/2 max,1/2 Specific growth rate KS,1 KS,2 XB = Biomass concentration max = Maximum specific growth rate S = Substrate concentration KS = Half saturation constant 11 Wastewater treatment

  29. Activated sludge system Activated sludge tank Secondary clarifier Air, O2 Sedimentation Effluent Inlet Nutrients Bacteria Return sludge Excess sludge 11 Wastewater treatment

  30. Sludge inventory in activated sludge system Activated sludge tank Secondary clarifier XAST Q Q Q + QRAS XAST Xe (QWAS) QRAS= R·Q XRAS (XWAS) Sludge balance in equilibrium mit 11 Wastewater treatment

  31. Flow scheme of activated sludge system Hydraulic wash-out of sludge to secondary clarifier  sludge must be returned to activated sludge tank Activated sludge is circled 20 to 50 times  sludge concentration in activated sludge tank is maintained Excess sludge is withdrawn from system  equivalent to sludge production Through increased hydraulic loading (wet-weather condition) sludge is shifted to secondary clarifier 11 Wastewater treatment

  32. 3000 2500 2000 1500 Sludge mass (kg COD) 1000 500 Activated sludge tank 0 0 0,5 1 1,5 2 Time (d) 1400 1200 Sludge bed 1000 800 Sludge mass (kg COD) 600 400 200 0 0 0,5 1 1,5 2 Time (d) Dynamic sludge shift 11 Wastewater treatment

  33. Aeration in an actvated sludge tank Oxygen Bacteria Sludge Organic compounds Effluent, water and sludge Air Inflow from mecha-nical treatment Return sludge Dimension: communal wastewater treatment 2 m3 wastewater per m3 reactor volume and day 11 Wastewater treatment

  34. Dimensioning on sludge loading Sludge loading  BOD5 inlet load is related to sludge mass in activated sludge tank (AST) F/M BOD5 loading related to dry sludge mass (Food/Microorganisms) Q Inflow to WWTP (m3/d) BOD5,in BOD5 concentration in inlet (kg BOD5 / m3) VAST Volume of activated sludge tank (m3) XAST Sludge concentration in activated sludge tank (kg TSS / m3) 11 Wastewater treatment

  35. Dimensioning on sludge age Sludge age  Sludge production is related to sludge mass in activated sludge tank SRT Sludge age in (d), 3 – 15 d ESPBOD Specific excess sludge production per BOD5 converted (kg TSS / kg BOD5) SP Sludge production (kg TSS / d) 11 Wastewater treatment

  36. Nutrients demand of micro-organisms Nitrogen iN = 0.04 – 0.05 (g N / g BOD5) Phosphorous iP = 0.01 – 0.02 (g P / g BOD5)  Partial elimination of nutrients Wastewater composition in the inlet 300 (g BOD5/m3) 60 (g TKN/m3) 12 (g TP/m3) Effluent concentrations after 100% BOD5 degradation TKNeff = TKNin – iN·BOD5,in = 60 – 0.045·300 = 46.5 (g N / m3) TPeff = TPin – iP·BOD5,in = 12 – 0.015·300 = 7.5 (g P / m3)  Enhanced nutrients removal is necessary! 11 Wastewater treatment

  37. Nitrification NH4+ NO3- Nitrifying organisms („autotrophic biomass“ XA) have a low growth rate A With production of autotrophic biomass and a safety factor SF the necessary sludge age yields  high sludge age necessary to omit nitrifiers from being washed out of the system  Volume of activated sludge tank must be large 11 Wastewater treatment

  38. aerobic aerobic anoxic aerobic anoxic anoxic aerobic anaerobic Development of activated sludge system C-Elimination Nitrification De-nitrification „Bio-P“ 11 Wastewater treatment

  39. Oxygen consumption Oxygen introduction Introduction Consumption SOC Specific O2-consumption (kg O2 / kg BOD5) cs O2 saturation concentration (g O2 / m3) c O2 concentration (g O2 / m3) f Peak factor for variations (-) SOI Spec. O2 introduction rate to clean water (kg O2 / (m3·h))  Reduction factor for wastewater (0.4) 0.6 – 0.8  The smaller the actual oxygen concentration, the more efficient is the oxygen introduction 11 Wastewater treatment

  40. Specific oxygen consumption SOC(kg O2 / kg BOD5) T Sludge age in d (°C) 4 8 10 15 20 25 10 0.85 0.99 1.04 1.13 1.18 1.22 12 0.87 1.02 1.07 1.15 1.21 1.24 15 0.92 1.07 1.12 1.19 1.24 1.27 18 0.96 1.11 1.16 1.23 1.27 1.30 20 0.99 1.14 1.18 1.25 1.29 1.32 Peak factors for C und N degradation fC 1.30 1.25 1.20 1.20 1.15 1.10 fN < 20‘000 PE - - - 2.50 2.00 1.50 - - 2.00 1.80 1.50 - > 100‘000 PE 11 Wastewater treatment

  41. Design rules Type of biological reactor Without nitrification Nitrification >10°C Denitrifi-cation Simultaneous aerobic sludge stabilisation SRT < 20‘000 PE 5 10 12 – 18 25 > 100‘000 PE 4 8 10 – 16 – F/M (kg BOD5 / (kg TS · d)) 0.30 0.15 0.12 0.05 ESPBOD 0.9 – 1.2 0.8 – 1.1 0.7 – 1.0 1.0 (kg TS / kg BOD5) 11 Wastewater treatment

  42. Concentrations in a plug flow areation tank Qin QRS QES O2 BSB5 NH4+ NO32- 11 Wastewater treatment

  43. Trickling filter Biofilm grown on internal surface Trickling filter Primary clarification Secondary clarification Recirculation Return sludge Sludge withdrawal 11 Wastewater treatment

  44. Dimensioning of trickling filter Surface loading BSu Surface loading (g BOD5 / (m2·d)) without nitrification 4 (g BOD5 / (m2·d)), with nitrifi. 2 (g BOD5 / (m2·d)) Q Inflow to trickling filter (m3/d) BOD5,in BOD5 inlet concentration (kg BOD5 / m3) VTF Volume of trickling filter (m3) a Specific biofilm surface per volume of trickling filter (m2 / m3 TF) 100 – 140 – 180 (m2 / m3 TF) 11 Wastewater treatment

  45. Degradation in trickling filter Concentration BOD5 Trickling filter NH4+ N03-  C-degradation and nitrification are separated in space 11 Wastewater treatment

  46. 11 Wastewater treatment 11.5 Final clarification 11 Wastewater treatment

  47. Functions of final clarifiers Separation of sludge and cleaned wastewater through Sedimentation Clarification  Low effluent concentration Storage of sludge shifted from actibated sludge tank, namely under wet-weather conditions Thickening  High return sludge concentration Geometry • Circular, flow from centre to periphery • Rectangular, longitudinal flow • Rectangular, lateral flow • Vertical, upward flow 11 Wastewater treatment

  48. Sedimentation Primary clarifier Final clarifier, separation zone low Free settling Flocculating settling Final clarifier, sludge bed Concentration Hindered settling Final clarifier, bottom Thickening high none flocculating Particle-Interaction 11 Wastewater treatment

  49. 0.5 h X0 H V hS Sludge volume index SVI SVI is an indicator for volume of sludge flocs and their settling characteristics Sludge volume (ml/l) (ml / g TSS) 11 Wastewater treatment

  50. Final clarifier, idealised functions Inlet zone Effective zone Clear water layer Separation layer > 3 m Storage layer Thickening layer ATV A131 (2000) 11 Wastewater treatment

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