Activated Sludge Wastewater Treatment Calculations Posted on Saturday, March 12, 2011 at 2:52 PM Activated Sludge Introduction
The most widely used biological wastewater treatment process in the world is the activated sludge process. The process has only been around for about 100 years, since it was invented in England in the early 1900s. The main function of a properly operating activated sludge plant is removal of biodegradable organic matter (biochemical oxygen demand or BOD) and suspended solids from the incoming wastewater. The diagram below shows a typical activated sludge process flow diagram. Preliminary treatment usually includes screening, flow measurement and grit removal. The primary clarifier removes settleable solids before the flow goes to the aeration tank. The aeration tank and secondary clarifier function as a unit for removal of BOD and maintaining an adequate concentration of microorganisms in the aeration tank.
This article will provide details of the parameters, equations and calculations for design and operation of the aeration tank and secondary clarifier.
Parameters for Activated Sludge Calculations
The aeration tank/secondary clarifier flow diagram at the left shows some of the parameters used for activated sludge design and operational calculations. The parameters shown in the diagram, plus some more, are described with U.S. and S.I. units in the following list.
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- Qo, primary effluent flow rate, MGD (m3/day for S.I.)
- So, primary effluent biochemical oxygen demand (BOD) concentration, mg/L (g/m3 for S.I.)
- Xo, primary effluent suspended solids concentration, mg/L (g/m3 for S.I.)
- V, aeration tank volume, ft3 (m3 for S.I.)
- X, aeration tank MLSS (suspended solids concentration), mg/L (g/m3 for S.I.)
- Qe, secondary effluent flow rate, MGD, (m3/day for S.I.)
- Xe, secondary effluent suspended solids concentration, mg/L (g/m3 for S.I.)
- Se, secondary effluent biochemical oxygen demand (BOD) concentration, mg/L (g/m3 for S.I.)
- Qw, waste activated sludge flow rate, MGD (m3/day for S.I.)
- Sw, waste activated sludge biochemical oxygen demand (BOD) concentration, mg/L (g/m3 for S.I.)
- Xw waste activated sludge suspended solids concentration, mg/L (g/m3 for S.I.)
- Qr, recycle activated sludge flow rate, MGD (m3/day for S.I.)
- F:M, Food to Microorganism ratio, lb BOD/day/lb MLVSS (kg BOD/day/kg MLVSS)
- HRT, hydraulic retention time, hours for U.S. or S.I. units
- SRT, sludge retention time (also called sludge age), days for U.S. or S.I. units
- VL, volumetric loading, lb BOD/day/1000 ft3 (kg BOD/day/m3 for S.I.)
- %Vol., % volatile solids in the aeration tank mixed liquor suspended solids
Activated Sludge Design Calculations with Excel Templates
The table at the left shows typical values for three commonly used activated sludge aeration tank design parameters: volumetric loading, hydraulic retention time, and food to microorganism ratio. The table includes information for three activated sludge variations: conventional plug flow, complete mix, and extended aeration activated sludge plants.
The spreadsheet image below uses Excel formulas to calculate the required aeration tank volume based on entered values for the input parameters shown: primary effluent flow rate, Qo; primary effluent biochemical oxygen demand (BOD), So; Aeration tank MLSS, X; design volumetric loading, VL; and % volatile solids in the aeration tank, %Vol. Using input values for these parameters, the Excel formulas in the spreadsheet calculate the design value of aeration tank volume, V, and values for two other design parameters sometimes used to size the aeration tank, hydraulic retention time in the aeration tank, HRT, and food to microorganism ratio, F:M, using the following equations.
- V = [ (8.34*So*Qo)/VL ] (1000)
- VMG = V*7.48/1,000,000
- HRT = 24*VMG/Qo
- F:M = (8.34*So*Qo)/(8.34*%Vol*X*VMG)
To download an Excel spreadsheet for making activated sludge design or operational calculations go to our DOWNLOADS PAGE. These spreadsheets are available in U.S units or S.I. units.
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