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Urban Water Systems11 Wastewater treatment© PK, 2005 – page 1 11Wastewater Treatment 11.1 Boundary conditions 11.2 Layout of a wastewater treatment plant.

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Presentation on theme: "Urban Water Systems11 Wastewater treatment© PK, 2005 – page 1 11Wastewater Treatment 11.1 Boundary conditions 11.2 Layout of a wastewater treatment plant."— Presentation transcript:

1 Urban Water Systems11 Wastewater treatment© PK, 2005 – page 1 11Wastewater 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 Technische Universität Dresden Department of Hydro Science, Institute for Urban Water Management Peter Krebs Urban Water Systems

2 11 Wastewater treatment© PK, 2005 – page Boundary conditions 11 Wastewater treatment

3 Urban Water Systems11 Wastewater treatment© PK, 2005 – page 3 Wastewater treatment in Germany In 2000 more than 10‘000 communal WWTPs ClassNumberTotal capacity in mio PE 10’000 – 100’000 > 100’000 2’000 – 10’ – 2’ ’817 2’617 5’

4 Urban Water Systems11 Wastewater treatment© PK, 2005 – page 4 Effluent standards Class COD (mg/l) 2 < 5000 PE 300 kg BOD 5 / d 1 < 1000 PE 60 kg BOD 5 / d 3 < PE 600 kg BOD 5 / d 5 > PE 6000 kg BOD 5 / d 4 < PE 6000 kg BOD 5 / d BOD 5 (mg/l) NH 4 -N (mg/l) N* (mg/l) P tot (mg/l) * N = Sum of NH 4 +, NO 3 -, und NO 2 -

5 Urban Water Systems11 Wastewater treatment© PK, 2005 – page 5 Load variation in WWTP inlet :0004:0008:0012:0016:0020:0000:00 Time (hh:mm) COD-load (kg/h) NH 4 -load (kg/h) Daily average COD and NH 4 4 -load COD-load

6 Urban Water Systems11 Wastewater treatment© PK, 2005 – page 6 Wet-weather load of NH 4 and TSS from sewer :0012:0014:0016:0018:0020:0022:000:002:00 NH 4 load / (Q d ·C 0 ) Q / Q d ; TSS load / ( Q d · C 0 ) NH4 load Flow rate TSS load

7 Urban Water Systems11 Wastewater treatment© PK, 2005 – page 7 Time Flow rate WWTP load with NH 4 + at storm event Time Conc., load Load Concentration

8 Urban Water Systems11 Wastewater treatment© PK, 2005 – page Layout of a wastewater treatment plant (WWTP) 11 Wastewater treatment

9 Urban Water Systems11 Wastewater treatment© PK, 2005 – page 9 ScreenGrid chamber Fat chamber Primary clarifier Activated sludge tank Secondary clarifier Thickening DigestionStorage Return sludge Primary sludge Fat Sand Secondary sludge Excess sludge Biogas disposal, incineration Wash, disposal Use, dewatering, drying, incineration, Disposal River, filtration Mechanical treatmentBiological treatment precipitation chemical Layout of a WWTP

10 Urban Water Systems11 Wastewater treatment© PK, 2005 – page 10 Example: WWTP Chemnitz-Heinersdorf

11 Urban Water Systems11 Wastewater treatment© PK, 2005 – page 11 Typical residence time in reactors Mechanical pre-treatment Primary clarifier Activated sludge tank Secondary clarifier Sludge thickener Digester Storage Wastewater HRT (h) Sludge SRT (d) < 1 d> 100 d

12 Urban Water Systems11 Wastewater treatment© PK, 2005 – page Mechanical treatment 11 Wastewater treatment

13 Urban Water Systems11 Wastewater treatment© PK, 2005 – page 13 Automatically cleaned trash rack

14 Urban Water Systems11 Wastewater treatment© PK, 2005 – page 14 Sieve Hans Huber AG, Typ Ro9 Slotted sieve ≥ 0.5 mm Perforated tin ≥ 3 mm

15 Urban Water Systems11 Wastewater treatment© PK, 2005 – page 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)

16 Urban Water Systems11 Wastewater treatment© PK, 2005 – page 16 Surface load: q A = Q / A (Hazen, 1904) L H U VSVS U Q Critical case Settling condition  V S  q A  Independent of H !

17 Urban Water Systems11 Wastewater treatment© PK, 2005 – page 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 !

18 Urban Water Systems11 Wastewater treatment© PK, 2005 – page 18 Empirical sedimentation velocities Particle diameter Sedimentation velocity [mm] = 100% [cm/s] = 90% [cm/s] = 85% [cm/s] 0,125 0,160 0,200 0,250 0,315 0,17 0,29 0,46 0,74 1,23 0,26 0,44 0,78 1,25 2,00 0,31 0,56 0,99 1,60 2,35 Kalbskopf, 1966

19 Urban Water Systems11 Wastewater treatment© PK, 2005 – page 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

20 Urban Water Systems11 Wastewater treatment© PK, 2005 – page 20 Aerated grid chamber

21 Urban Water Systems11 Wastewater treatment© PK, 2005 – page 21 Efficiency of primary clarifier Residence time HRT (h) Efficiency (%) settleable solids TSS BOD 5

22 Urban Water Systems11 Wastewater treatment© PK, 2005 – page 22 Effects of primary clarifier on wastewater CompoundUnitInletOutlet * TSS BOD 5 COD TKN NH 4 -N NO 2 -N NO 3 -N P tot Alkalinity g TSS / m 3 g O 2 / m 3 g N / m 3 g P / m 3 mol HCO 3 - / m = f(Drinking water) + NH 4 -N * Short residence time

23 Urban Water Systems11 Wastewater treatment© PK, 2005 – page 23 Rectangular sedimentation tank Primary / secondary clarifier Flight scraper Effluent weir Inlet Sludge withdrawal Effluent to activated sludge tank or to receiving water Surface: Primary clarifier q A = 2 to 6 m/h Secondary clarifier q A = 0.5 to 1.5 m/h

24 Urban Water Systems11 Wastewater treatment© PK, 2005 – page Biological treatment 11 Wastewater treatment

25 Urban Water Systems11 Wastewater treatment© PK, 2005 – page 25 Biological treatment Suspended biomass  Activated sludge system Sessile biomass  Biofilm system suspended through turbulence sludge flocs with 0.1 – 1 mm diameter degradation related to biomass biofilm on carrier little erosion degradation related to biofilm surface area  Increase suspended biomass concentration  Increase of specific surface area

26 Urban Water Systems11 Wastewater treatment© PK, 2005 – page 26 Important micro-biological processes Growth Decay Hydrolysis Aerobic degradation Nitrification Denitrification Implementation of C, N, P in biomass If external nutrients are rare Heavily  easily degradable substances, through encymes organic compounds CH 2 O + O 2  CO 2 + H 2 O NH O 2  NO H 2 O + 2 H + 5 CH 2 O + 4 NO H +  2 N CO H 2 O

27 Urban Water Systems11 Wastewater treatment© PK, 2005 – page 27 Growth of bacteria Doubling time t D 0·tD0·tD 1·tD1·tD 2·tD2·tD 3·tD3·tD i·tDi·tD...n·tDn·tD i2i 2n2n Activated sludge: t D = 6 h Sludge age = 10 d

28 Urban Water Systems11 Wastewater treatment© PK, 2005 – page 28 Growth of bacteria  Growth is limited through nutrients and oxygen  max,2 /2  max,1 /2 K S,2 K S,1 Growth Specific growth rate X B = Biomass concentration  max = Maximum specific growth rate S = Substrate concentration K S = Half saturation constant

29 Urban Water Systems11 Wastewater treatment© PK, 2005 – page 29 Inlet Activated sludge tank Secondary clarifier Effluent Return sludge Excess sludge Nutrients Bacteria Air, O 2 Sedimentation Activated sludge system

30 Urban Water Systems11 Wastewater treatment© PK, 2005 – page 30 Sludge inventory in activated sludge system Activated sludge tank Secondary clarifier QQ + Q RAS X AST Q XeXe Q RAS = R·Q X RAS (Q WAS ) (X WAS ) Sludge balance in equilibrium mit

31 Urban Water Systems11 Wastewater treatment© PK, 2005 – page 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

32 Urban Water Systems11 Wastewater treatment© PK, 2005 – page 32 Dynamic sludge shift ,511,52 Time (d) Sludge mass (kg COD) Activated sludge tank ,511,52 Time (d) Sludge mass (kg COD) Sludge bed

33 Urban Water Systems11 Wastewater treatment© PK, 2005 – page 33 Aeration in an actvated sludge tank Oxygen Organic compounds Bacteria Sludge Effluent, water and sludge Dimension: communal wastewater treatment 2 m 3 wastewater per m 3 reactor volume and day Air Inflow from mecha- nical treatment Return sludge

34 Urban Water Systems11 Wastewater treatment© PK, 2005 – page 34 Dimensioning on sludge loading Sludge loading F/MBOD 5 loading related to dry sludge mass (Food/Microorganisms) Q Inflow to WWTP (m 3 /d) BOD 5,in BOD 5 concentration in inlet (kg BOD 5 / m 3 ) V AST Volume of activated sludge tank (m 3 ) X AST Sludge concentration in activated sludge tank (kg TSS / m 3 )  BOD 5 inlet load is related to sludge mass in activated sludge tank (AST)

35 Urban Water Systems11 Wastewater treatment© PK, 2005 – page 35 Dimensioning on sludge age Sludge age SRTSludge age in (d), 3 – 15 d SP Sludge production (kg TSS / d) ESP BOD Specific excess sludge production per BOD 5 converted (kg TSS / kg BOD 5 )  Sludge production is related to sludge mass in activated sludge tank

36 Urban Water Systems11 Wastewater treatment© PK, 2005 – page 36 Nutrients demand of micro-organisms Nitrogen i N = 0.04 – 0.05 (g N / g BOD 5 ) Phosphorous i P = 0.01 – 0.02 (g P / g BOD 5 )  Partial elimination of nutrients Wastewater composition in the inlet 300 (g BOD 5 /m 3 ) 60 (g TKN/m 3 ) 12 (g TP/m 3 ) Effluent concentrations after 100% BOD 5 degradation TKN eff = TKN in – i N ·BOD 5,in = 60 – 0.045·300 = 46.5(g N / m 3 ) TP eff = TP in – i P ·BOD 5,in = 12 – 0.015·300 = 7.5 (g P / m 3 )  Enhanced nutrients removal is necessary!

37 Urban Water Systems11 Wastewater treatment© PK, 2005 – page 37 Nitrification NH 4 +  NO 3 - Nitrifying organisms („autotrophic biomass“ X A ) have a low growth rate  A  high sludge age necessary to omit nitrifiers from being washed out of the system With production of autotrophic biomass and a safety factor SF the necessary sludge age yields  Volume of activated sludge tank must be large

38 Urban Water Systems11 Wastewater treatment© PK, 2005 – page 38 anoxicaerobic C-Elimination Nitrification De-nitrification „Bio-P“ aerobic anaerobic anoxicaerobic anoxic Development of activated sludge system

39 Urban Water Systems11 Wastewater treatment© PK, 2005 – page 39 Oxygen consumption SOC Specific O 2 -consumption (kg O 2 / kg BOD 5 ) Oxygen introduction c s O 2 saturation concentration (g O 2 / m 3 ) c O 2 concentration (g O 2 / m 3 ) f Peak factor for variations (-) SOI Spec. O 2 introduction rate to clean water (kg O 2 / (m 3 ·h))  Reduction factor for wastewater (0.4) 0.6 – 0.8 ConsumptionIntroduction  The smaller the actual oxygen concentration, the more efficient is the oxygen introduction

40 Urban Water Systems11 Wastewater treatment© PK, 2005 – page 40 Specific oxygen consumption SOC (kg O 2 / kg BOD 5 ) TSludge age in d (°C) Peak factors for C und N degradation fCfC fNfN < 20‘000 PE > 100‘000 PE

41 Urban Water Systems11 Wastewater treatment© PK, 2005 – page 41 Design rules Type of biological reactor Without nitrification Nitrification >10°C Denitrifi- cation Simultaneous aerobic sludge stabilisation SRT< 20‘000 PE > 100‘000 PE ESP BOD F/M – – – – – – 0.05 (kg BOD 5 / (kg TS · d)) (kg TS / kg BOD 5 )

42 Urban Water Systems11 Wastewater treatment© PK, 2005 – page 42 Concentrations in a plug flow areation tank Q RS Q in NH 4 + BSB 5 O2O2 NO 3 2- Q ES

43 Urban Water Systems11 Wastewater treatment© PK, 2005 – page 43 Trickling filter Primary clarification Trickling filter Secondary clarification Recirculation Return sludge Sludge withdrawal Biofilm grown on internal surface

44 Urban Water Systems11 Wastewater treatment© PK, 2005 – page 44 Dimensioning of trickling filter Surface loading B Su Surface loading (g BOD 5 / (m 2 ·d)) without nitrification 4 (g BOD 5 / (m 2 ·d)), with nitrifi. 2 (g BOD 5 / (m 2 ·d)) Q Inflow to trickling filter (m 3 /d) BOD 5,in BOD 5 inlet concentration (kg BOD 5 / m 3 ) V TF Volume of trickling filter (m 3 ) a Specific biofilm surface per volume of trickling filter (m 2 / m 3 TF) 100 – 140 – 180 (m 2 / m 3 TF)

45 Urban Water Systems11 Wastewater treatment© PK, 2005 – page 45 Degradation in trickling filter Concentration BOD 5 NH 4 + N0 3 - Trickling filter  C-degradation and nitrification are separated in space

46 Urban Water Systems11 Wastewater treatment© PK, 2005 – page Final clarification 11 Wastewater treatment

47 Urban Water Systems11 Wastewater treatment© PK, 2005 – page 47 Functions of final clarifiers Separation Clarification Storage Thickening of sludge and cleaned wastewater through Sedimentation  Low effluent concentration of sludge shifted from actibated sludge tank, namely under wet-weather conditions  High return sludge concentration Geometry Circular, flow from centre to periphery Rectangular, longitudinal flow Rectangular, lateral flow Vertical, upward flow

48 Urban Water Systems11 Wastewater treatment© PK, 2005 – page 48 Concentration Particle-Interaction Thickening Hindered settling Flocculating settling Free settling noneflocculating high low Sedimentation Primary clarifierFinal clarifier, separation zone Final clarifier, sludge bed Final clarifier, bottom

49 Urban Water Systems11 Wastewater treatment© PK, 2005 – page 49 Sludge volume index SVI SVI is an indicator for volume of sludge flocs and their settling characteristics 0.5 h H hShS X0X0 V Sludge volume (ml/l) (ml / g TSS)

50 Urban Water Systems11 Wastewater treatment© PK, 2005 – page 50 Final clarifier, idealised functions Clear water layer Separation layer Storage layer Thickening layer Inlet zoneEffective zone > 3 m ATV A131 (2000)

51 Urban Water Systems11 Wastewater treatment© PK, 2005 – page 51 Dimensioning of surface Surface overflow rate Sludge overflow rate Limit values qAqA q SV (m/h)(l/(m 2 ·h) Horizontal flow tanks Vertical flow tanks ATV A131 (2000)

52 Urban Water Systems11 Wastewater treatment© PK, 2005 – page 52 Dimensioning of water depth Clear water layer Separation layer Storage layer Thickening layer X BS Sludge concentration at bottom t th Thickening time 1.5 – 2.0 hwithout nitrification 1.0 – 1.5 hwith nitrification 2.0 – (2.5) hwith denitrification ATV A131 (2000)

53 Urban Water Systems11 Wastewater treatment© PK, 2005 – page 53 Calculation procedure (plant without nitrogen elimination) estimate or measure inflow load choose system estimate sludge volume index choose thickening time calculate concentration of bottom sludge calculate return sludge concentration choose recycle ratio  estimate X AST calculate surface of secondary clarifier calculate h 1, h 2, h 3, h 4 of secondary clarifier check thickening time calculate volume of activated sludge tank is balance between AST and SC optimised? Variation X AST

54 Urban Water Systems11 Wastewater treatment© PK, 2005 – page 54 Circular tank With scraper or suction removal device Inlet Flocculation chamber Scum skimmer Sludge scraper

55 Urban Water Systems11 Wastewater treatment© PK, 2005 – page 55 Rectangular tank, longitudinal flow, flight scraper system Sludge hopper Inlet Sludge Chain motor Water level Effluent launder Effluent

56 Urban Water Systems11 Wastewater treatment© PK, 2005 – page 56 Rectangular tank, lateral flow, suction system Removal bridge Inlet channel Scum skimmer Inlet baffle

57 Urban Water Systems11 Wastewater treatment© PK, 2005 – page 57 Vertical flow tank Effluent Filter Thickening Inlet Return sludge Effluent launder Pump


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