As originally constructed, a conventional activated-sludge plant cannot thicken the waste-activated sludge before mixing with the primary sludge.The combined sludge has an average solids concentration of 2.5%, which is too low for good operation of anaerobic digestion. The proposed solution is to modify the sludge-processing system by installing gravity belt thickeners to concentrate solids in the waste-activated sludge before mixing with the primary sludge, which has a solids concentration averaging 3.5%. If the waste-activated sludge were thickened to 6%, the mixed sludge would have a solids concentration of 4.0% to 4.5%, which is considered the minimum for satisfactory anaerobic digestion. Currently, during the maximum month, the average plant wastewater load is 41,000 m3/d. Normal operation of the activated-sludge process produces an average of 270 m3/d, with a solids concentration of 1.12% (11,200 mg/l). Poor operation, which may periodically last for one or two weeks, results in an average of 380 m3/d with 0.81% solids. Determine the size of two gravity belt thickeners to increase solids content of the waste-activated sludge to the 6% at the plant design load of 57,000 m3/d of wastewater.
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