CEE484 Decentralized and Onsite Wastewater Management and Reuse. Homework Assignment #6 Due in class on May 16, 2007

Transcription

1 CEE484 Decentralized and Onsite Wastewater Management and Reuse Homework Assignment #6 Due in class on May 16, Two activated sludge design sheets are included below. The first one is a design example for an onsite treatment system with no septic tank and a flow of 3000 gal/day. Assumed values for BOD and TKN concentrations and treatment process temperature are shown. The design procedure determines the aerobic tank volume, the oxygen transfer rate required, sludge production rate and clarifier size. Use the design worksheet provided (make copies for each of the 4 design conditions) and compare the design results for an activated sludge system with a clarifier versus a membrane bioreactor activated sludge system. The design is for a cluster of 50 homes with an average design flowrate of 15,000 gal/day. The influent BOD and TKN concentrations are 350 and 70 mg/l, respectively. The minimum biological reactor temperature is expected to be 15 0 C. Evaluate the two system designs also for the case in which a septic tank is used first for solids settling and removal prior to the secondary treatment process. In this case the septic tank effluent BOD and TKN concentrations are 140 and 59 mg/l, respectively. The aeration tank TSS (mixed liquor suspended solids) design concentration is 3000 mg/l for the conventional activated sludge system and 12,000 mg/l for the membrane bioreactor system. The systems are to be designed as extended aeration processes to better handle variable loads. Provide the comparative information from your calculation results in the following table. Assuming that the aeration tank depth is 15 ft, what is the relative area for the two systems? Conventional activated sludge Membrane bioreactor Process parameter No septic tank With septic tank No septic tank With septic tank MLSS, mg/l 3,000 3,000 12,000 12,000 Aeration tank volume, gal Detention time, hr Daily sludge wasted, lb/d Peak Oxygen rate required, lb/hr Clarifier area, ft 2 1

2 Simple Activated Sludge Design Approach Primary Treatment,YES or NO NO Row Enter Influent Conditions Units 1 Flowrate gal/day BOD mg/l TKN mg/l 70 4 Alkalinity as CaCO3 mg/l 200 Temperature deg C 15 5 Nitrate production 6 g N used for cells/g BODr g/g N used for synthesis mg/l 16.0 Row2*Row6 8 Assume effluent NH4-N mg/l NO3-N mg/l 53.0 Row3-Row7-Row8 10 Temperature deg C Select SRT days 25.0 from table per treatment and temperature 12 Aeration tank Volume needed 13 Solids yield g TSS/g BOD g/g 0.85 NO in Row 3 so use Eq B 14 Daily solids production mg/l-gal/d *BOD*flowrate(Row13*Row2*Row1) 15 solids in system mg/l-gal Row14*Row11(SRT) 16 Aeration tank TSS conc. mg/l 3500 assume value 17 Volume needed gal Aeration Time hrs 46.7 Volume*24/flow rate Oxygen Needed 21 lb O2/lb BOD lb/lb 1.34 Eq *Ln(SRT) lbo2/lb NO3-N produced lb/lb 4.6 given 23 Oxygen needed, avg/day lb/d (1.34*BOD+4.6*NO3-N)*8.34*flow/1,000, Daily sludge wasted lb/d 6.81 net yield*bodr*flowrate/ *8.34 net yield = row13 25 Peak oxygen demand ratio 1.5 should be 1.5 to 2.0 for small installations 26 Peak oxygen demand rate lb/hr 1.05 Row25*Row23/24 2

3 27 Clarifier HAR at avg flow gpd/ft assume value that considers peak flows 28 Clarifier area ft flowrate divided by Row27 29 clarifier diameter ft 3.6 consider 1 or 2 clarifiers, area of row2 area Approximate preanoxic zone ratio of anoxic/aerobic volume gal/gal % is typical of process-use higher for higher NO3 conc to remove Anoxic zone volume gal *row17 SRT Selection Table extended Temp Nitrif Temp aeration deg SRT deg SRT C days C days For Row 13 Eq A if YES primary treatment solids yield =0.9967(SRT^ ) Eq B if NO primary treatment solids yield =1.246(SRT^ ) 3

4 Simple Activated Sludge Design Approach Primary Treatment,YES or NO Row Enter Influent Conditions Units 1 Flowrate gal/day 2 BOD mg/l 3 TKN mg/l 4 Alkalinity as CaCO3 mg/l Temperature deg C 5 Nitrate production 6 g N used for cells/g BODr g/g 7 N used for synthesis mg/l 8 Assume effluent NH4-N mg/l 9 NO3-N mg/l 10 Temperature deg C 11 Select SRT days 12 Aeration tank Volume needed 13 Solids yield g TSS/g BOD g/g 14 Daily solids production mg/l-gal/d 15 solids in system mg/l-gal 16 Aeration tank TSS conc. mg/l 17 Volume needed gal 18 Aeration Time hrs Oxygen Needed 21 lb O2/lb BOD lb/lb 22 lbo2/lb NO3-N produced lb/lb 23 Oxygen needed, avg/day lb/d 24 Daily sludge wasted lb/d 25 Peak oxygen demand ratio 26 Peak oxygen demand rate lb/hr 4

5 27 Clarifier HAR at avg flow gpd/ft2 28 Clarifier area ft2 29 clarifier diameter ft Approximate preanoxic zone 32 ratio of anoxic/aerobic volume gal/gal 33 Anoxic zone volume gal SRT Selection Table extended Temp Nitrif Temp aeration deg SRT deg SRT C days C days For Row 13 Eq A if YES primary treatment solids yield =0.9967(SRT^ ) Eq B if NO primary treatment solids yield =1.246(SRT^ ) 5

6 2. SBR design elements SRT gets total volume Cycle Times Fill Volume/decant volume V F Effluent volume = fill volume V s Vs = settled volume V F = fill volume V F +V S = total tank liquidvolume V F /V T = critical design parameter What is a good value for V F /V T? Given the following for a 2 tank SBR treatment what is the fill time per cycle, the total time for a cycle (i.e. time for fill, aerate, settle, decant, and idle) and the aeration time per cycle? Average Flow = 10,000 gal/d = gal/hr Based on SRT and load, volume/tank = 10,000 gallons Assume: VF/VT = 0.20 Settle time Decant time Idle time = 1.0 hr = 0.5 hrs = 0.3 hrs 3. What are the advantages for an intermittent sand filter versus a package plant activated sludge system for wastewater treatment for a homeowner prior to an irrigation application, and vice versa what are the relative advantages of the activated sludge system? 6