Advanced treatment of the wastewater from a small city consists of aeration tanks without primary sedimentation with the addition of alum for phosphorus precipitation, final clarifiers with sludge recirculation back to the aeration tanks, tertiary granular-media filters with a backwash storage tank that drains back to the head of the plant, effluent chlorine-contact chambers for disinfection, aerobic digesters with supernatant return to the head of the plant, and a belt filter press for dewatering digested sludge withdrawn from the digesters with filtrate returned to the head of the plant. The influent wastewater is 2000 m3/d with 200 mg/l BOD, 240 mg/l SS, and 10.0 mg/l P. The effluent characteristics are 3 mg/l BOD, 3 mg/l SS, and 0.5 mg/l P. The fecal coliform count is less than 200 per 100 ml. The activated-sludge process is operated at F/M = 0.15, and the alum addition is 190 mg/l. Aerobic digestion reduces the organic content of the waste-activated sludge by 25% and thickens the digested sludge to 2.0% organic-chemical solids. The dewatered filter cake has 18% solids.
(a) Draw a flow diagram of the treatment system.
(b) Estimate the weight of filter cake produced per day by calculating the following: the organic solids produced by aeration, organic solids removed by tertiary filtration, organic solids remaining after digestion, chemical precipitates, and weight of total solids at 18% concentration.
(c) What is the principal form of chlorine residual formed in chlorination of the waste-water? What chemical can be used for dechlorination?
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