Wastewater Technology Part 2: Wastewater treatment Exercise: Basic calculations of loads, wastewater streams and tank d

1. Wastewater TechnologyPart 2: Wastewater treatment Exercise: Basic calculations of loads, wastewater streams and tank dimensions Iosif Mariakakis (adopted from the lecture of Prof. Dr.-Ing. Heidrun Steinmetz) Institute of Sanitary Engineering, Water Quality and Solid Waste Management - Sanitary Engineering and Water Recycling -

2. Contents • Calculation of wastewater inflow into a WWTP • Calculation of wastewater load into a WWTP for dry weather conditions and for design inflow • Dimensioning principles of primary sedimentation • Short description of the Activated Sludge biological treatment • Fundamental parameters of the Activated Sludge biological treatment • Dimensioning principles of the aeration tank of the Activated Sludge system • Dimensioning principles of the sedimentation tank (secondary sedimentation) of the Activated Sludge system

3. Basic Flow Scheme of a WWTP Biological treatment Primary treatment Grit rem. Grease rem. Primary sediment. Aeration tank Secondary sediment. Screening Influent Effluent Return sludge Primary sludge Grease Screenings Sand Supernatant Secondary sludge = Excess sludge Thickener Gas storage Biogas Dewatering and -agricultural use -landfill -incineration Raw sludge Gujer, 1999 Digester 35°C Thickener and storage tank Sludge treatment

4. Design inflow of a WWTP Gujer, 1999

5. Definitions • Wastewater (Sewage) • Dry weather flow (DWF) • Combined wastewater flow (CWF) (about100times the dry weather flow) Qww = Qd + Qi + Qi QDW = QWW + Qinf Source: ATV-DVWK-A 198E Qcomb = QDW + QR

6. Daily Variations of Wastewater Flow • Determination of yearly wastewater flow (sewage flow on all days) • Determination of yearly dry weather flow (dry weather flow on days without rain) • Determination of peak flow during dry weather [ l/s ] Source: ATV-DVWK-Arbeitsblatt A 198 (April 2003) Source: ATV-DVWK-A 198E

7. Design inflow of a WWTP Source: ATV-DVWK-A 198E

8. Specific Loads and Concentrations per Inhabitant Typical effluent values of a WWTP with100,000 EW < 15 mg/l < 75 mg/l < 13 mg/l Source: ATV-DVWK-A 131E < 1 mg/l < 20 mg/l e.g.

9. Primary sedimentation - dimensioning

10. Efficiency of Primary Sedimentation Settleable Solids Filterable Solids BOD5 and COD Source: ATV-Handbuch, 1997a Nitrogen

11. Activated Sludge Process (scheme) Aeration tank Final sedimentation Influent Aeration Effluent Return sludge Excess sludge Gujer, 1999

12. Parameters of Activated Sludge System • Sludge Loading (kgBOD/(kgTSS*d) • Sludge Age, Sludge retention time; Mean Cell Residence Time (d) • Biomass Concentration = Total suspended solids (TSS)MLSS = Mixed Liquor Suspended Solids (kg/m³) • SV = Sludge Volume (mL/L) • SVI = Sludge Volume Index (mL/g) • Return Sludge Ratio (%) • Excess Sludge Production (kgTSS/d) • Oxygen Concentration and –Consumption (mg/L) • Volumetric Loading (kgBOD/(m³*d))

13. Sludge Volume Index 1000 1000 800 600 200 400 800 600 400 200 Start of Test • In this Example • Sludge Volume Index SVI = 110 ml/g or l/kg • (Diluted) Sludge Volume SV=330 ml/l or l/m³ After 30 min 1 liter of Act. Sludge SSAT =3.0 g/l Volume: 330 ml/l SVI=330/3.0 =110 ml/g

14. Sludge Loading BX BSS= Sludge Loading kgBOD5/(kgSS*d)Bd,BOD = Daily BOD5 loadin influent kg/(m3*d) VAT = Volume of aeration tank m3 SSAT= Dry solids in aeration tank kg SS/m3 Source: ATV-DVWK-A 131E

15. Sludge Loading BSS and Treatment Efficiency 100 BOD - Elimination [%] 80 > 13°C 60 < 11°C 40 20 0,1 0,2 1,0 10 Sludge Loading BSS Gujer, 1999

16. Sludge Retention Time SRT / Sludge Age tSS • Sludge Retention Time (SRT) = Sludge Age = Mean Cell Residence Time (MCRT) • The SRT is the average retention time of every sludge floc in the system • The SRT controls the microbial population in the activated sludge • High SRT – slowly growing organisms can stay in the system (e.g. nitrifying bacteria) • The SRT is difficult to measure, but it can be calculated as: • Mass of TSS in the aeration tank/daily sludge production Source: ATV-DVWK-A 131E

17. Sludge Age tSS (SRT) • Selection of sludge age according to treatment purpose Source: ATV-DVWK-A 131E

18. Return Sludge • With return sludge, thickened activated sludge is pumped back into the aeration reactor • RS = Return Sludge Ratio QRS/Q • SSRS = Suspended Solids Concentration in Return Sludge • SSAT = Suspended Solids Concentration in the Aeration Tank Source: ATV-DVWK-A 131E

19. Specific excess sludge production (kg SS/kg BOD5) Excess Sludge Production Source: ATV-DVWK-A 131E X/C = Suspended solids / BOD5 in the influent

20. Required mass of suspended solids in the biological reactor (kg) Volume of aeration tank • Volume of the biological reactor (m³) Source: ATV-DVWK-A 131E

21. Main Parameter of Activated Sludge Processes DIN EN 12255-6 (4/2002)

22. Secondary settling tank • Surface loading important • Dependant from the sludge characteristics (SVI) • Calculation of the tank surface [m/h] Source: ATV-DVWK-A 131E [m²]

23. Secondary settling tank • Depth of the sedimentation tank • Dependant from the sludge characteristics (SVI) • Dimensioning of the different zones according DWA A- 131 Source: ATV-DVWK-A 131E • h1 = 0.5 m (min. for safety reasons) • h2 = [0.5  qA  (1 + RS)] / (1-SV / 1000) • h3 = 0.45  qSV (1 + RS) / 500 • h4 = [XTSS,AT qA  (1 + RS)  tTh] / XTSS,SST