Urban District STORAGE: Water demand calculation Water demand is calculated based on the guidelines of NBC. Total quantity of water requirement for the proposed project is estimated to be about 122.09 KLD during the operation phase. By considering 85 % of the water supplied will be converted in to sewage i.e. 103.77KLD. Wastewater generated from the proposed project will be treated in an STP of 115 KLD. Detailed design write up of the STP is in the following section. The sewage generated during the operation phase will be treated up to the tertiary level in Sewage Treatment Plants (STP) The entire (100%) treated sewage from STP of 115KLD capacity will be recycled/ reused for toilet flushing, car washing and landscaping in the project site excess will be used for avenue plantation/Sewer. Sewage Treatment Plant Total water requirement = 122.09 m3 / day. Assuming Diversity Factor of 0.85 = 103.77 m3 Say = 110 m3 / day SEWAGE TREATMENT PLANT TREATMENT SCHEME It is proposed to install STP of capacity 1,15,000 litres/day, Following is the scheme for treating 1,15,000Liters/day capacity sewage. The treated water can be utilized for Toilet Flushing & gardening. Various treatment units which have been envisaged in this sewage treatment plant are explained below. General characteristics: Characteristics of waste water: BOD 300 to 350 Mg / Ltr. COD 350 to 450 Mg / Ltr. TSS 350 to 450 Mg / Ltr. PH 7 to 8 Oil.
& Grease 100 to 150 mg / Ltr. Page 3 Characteristics of treated effluent: BOD5 < 10 Mg / Ltr. COD < 100 Mg / Ltr. TSS < 20 Mg / Ltr. PH < 6.5 to 7.5. Oil & Grease < 10 Mg / Ltr. Turbidiy =1 mg/l Page 4 Design Detail of Sewage Treatment Plant with Sequential Batch Reactor Principle The sequential batch reactor (SBR) process is a cyclic activated sludge treatment process. Multiple reactors are provided to treat the wastewater in batches. Sequencing batch reactors will be operated to oxidize carbonaceous BOD, nitrify the ammonia and denitrify to reduce total nitrogen to a level that meets the permit limits. All treatment processes including equalization, aeration, denitrification, and sedimentation and decanting occur in the SBR’s eliminating the need for separate clarification and return activated sludge systems. The typical SBR treatment sequence for nitrification – denitrification systems is as follows: 1. The reactor is allowed to fill with raw wastewater. The filling phase is often divided into stages that include aeration to reduce BOD and to nitrify ammonia and then mixing without aeration to promote denitrification (removal of nitrogen). 2. A reaction phase is generally provided to promote additional biological treatment. 3. A settling or quiescent phase the follows to allow biological solids to settle. 4. A decanting phase is provided to draw off the clarified effluent from the upper portion of the reactor 5. Often a small idle phase is provided to allow time for
miscellaneous operations that may need to occur to keep the reactors in sequence. Wasting of the biological solids that are produced by converting BOD to bacteria needs to occur periodically to maintain the design mixed liquor concentration and sludge age. Wasting can occur at any time in the process sequence. Frequently, wasting is performed following the decant phase when sludge concentration is highest or during react phase when the sludge concentration is consistent. The SBR’s will be equipped with diffused aeration and decanting facilities. The discharge from SBR systems is higher than the inflow because the same volume of water that entered the SBR is discharged over a shorter time period. The treated water from SBR is pumped through the Pressure sand filter and Activated carbon filter for removal of suspended solids, final polishing and removal of any traces of colour. For HVAC purpose part of the treated water is passed through Softener for Page 5 achieving the desired hardness reduction. The effluent is then disinfected with Chlorine by intermittent dosing system. The treated sewage can be used for chillers and flushing purposes. Excess sludge from SBR tank is pumped out periodically to the Sludge holding tank and then fed in to the Filter press for dewatering and drying during decanting phase. Data & Assumption:- Design Flow = 115 KLD Influent BOD = 400mg/L Total.
Suspended Solids = 300mg/L F/M Ratio = 0.18/day MLSS = 4000mg/L VSS/TSS = 0.8 Maximum Volume of BOD Loading = 3.2 Kg/m3.d Minimum Aeration Time = 2 Hr Minimum Mean Cell Residence Time (MCRT) = 3 days Design: 1. Bar Screen Chamber: Max Flow = 115 KLD Detention Time = 6.0 min Bar Screen Chamber Volume = 0.25 m3 Let Side water depth (SWD) be 1.0 m BSC Size = 2.0 m x 1.25m x 1.0 Screen is made out of MS Flat of Size 10mm x 50mm (10mm facing the flow) Clear spacing between bars = 20mm Inclination of bars with horizontal = 60° (For Manual Cleaning) 2. Equalization Tank: Design average flow = 115 KLD Detention Time = 12 Hr Volume of Equalization tank = 140.0 m3 Equalization tank dimension = 4.5m x 4.5m x 3.0m Page 6 3. Bio Reactor: No. of. Tanks = 1 No. No. of. Sequential Batch = 1 Nos. Cycle duration = 16 Hr each Average flow to the reactor, Q = 115 KLD Total BOD entering STP, Y0 = 400mg/L BOD of the Effluent, YE = 10mg/L BOD removal to be achieved = 390mg/L MLSS, X = 4000mg/L Influent VSS, Xt = 4000 x 0.8 = 3200mg/L F/M ratio = 0.18/day F = Q x Y0 M V X Bio reactor volume, V= 105.0 m3 Each Bio reactor dimension = 5.5m x 5.5m x 3.5m BOD Loading = Q x Y0 = 0.72 Kg/m3.d V Hydraulic Retention Time = V = 105 m3 x 16 Hrs = 14.60 Hrs Q 115m3 Mean Cell Residence Time, θc (Sludge Age) Influent VSS, Xt = 0.8 x 400mg/L = 320mg/L Θc = VX = 105 x 4000 = 11.4 days (OK since >3 days) QXt 115 x 320 4. Decant Tank: Unlike Other treatment processes, in SBR the Clarified water tank is designed to hold decanted supernatant from each batch, i.e., 105.0 m3/batch. Thus Clarified Water Tank dimension = 5.5m x 5.5m x 3.5m 5. Sludge Holding Tank: Reactor Volume = 12 m3 MLSS = 4000mg/L Weight of solid = 12m3 x 4kg/L = 48 Kg Sludge retention time for 9 days, SRT9 = 48 Kg = 5.33 Kg Sludge/day 9 Wasting during Aeration phase = 25 Sludge holding tank dimension = 2.0m x 2.0m x 3.5m Page 7 6. Tertiary treatment units
Thewastewater after biological treatment still contains some solids, colour, odour and harmful micro-organisms. The pressure sand filter and activated carbon filter are used to remove the solids and colour. The disinfection process is used to remove the micro-organisms and odour. 6.1 Pressure sand filter Pressure sand filter with various grades of pebbles and sand media help in the removal of residual suspended solids. The filter will be operating in the pressure range of 3-3.5 kg/cm2. Residual suspended solids. The filter will be operating in the pressure range of 3-3.5 kg/cm2. Assuming a loading rate of 6.0 m3/ hr Design flow = 115 m3/day and considering 20 hr/day of operations of PSF The recommended PSF dimension is 1.2 m dia and height 3 m. Suitable designed backwashing system is adopted. 6.2 Activated Carbon Filter Considering the same parameters for designing the activated carbon filter, the recommended dimensions for ACF is 1.2 m Dia and height 3.0 m 7. Pre Aeration Tank: Detention time = 4.0 Hrs Volume of Final collection tank = 25.0 m3 Final Collection tank dimension = 3.0m x 3.0m x 3.5m 8. Final Collection Tank: Detention time = 11.5 Hrs Volume of Final collection tank = 70.0 m3 Final Collection tank dimension = 4.5m x 5.0m x 3.5m 9. Disinfection In order to disinfect the treated effluent various disinfection methods like chlorination, ozonationetc can be used. However chlorination is proved to be cheaper and relatively effective. Hence the same system is adopted. The Chlorine can be administered in the form of liquid or solid (bleaching powder) into the treated effluent channel. An automatic electronic metering system can be adopted. The chlorine demand is calculated based on the ‘break point