The authors studied a new method of night soil treatment where suspended solids in night soil were first removed by a rotary screen to reduce BOD loading to the following biological treatment. After treatment by a cationic polymer coagulant (1.3-1.4%/dry solids), night soil was passed through a rotary screen, and separated into thickened sludge and filtrate. The thickened sludge was best dewatered by a screw press giving sludge cake with 60% moisture content. The dewatered sludge cake was combustible without supplementary fuel. The filtrate was treated by the biological nitrification-denitrification process steadily giving treated water with T-N 60mg/l (98% removal), BOD 32mg/l (99% removal), COD 530mg/l (74% removal), color 3,400 units and SS 110mg/l. Extraordinary foam trouble at the nitrification tank was not met in this process and foam was suppressed mostly by spraying nitrified liquor with the aid of antifoam agent, as little as 0.03 kg/kl-night soil. For coagulation of the effluent from the biological treatment, poly (ferric sulfate) or ferric chloride was better than alum. The optimum pH for the ferric coagulants was 4–5 and the optimum dosage was 500mg/l as Fe. The treated water was CODMn 90mg/l, color 200 unit and PO4-P less than 0.5mg/l. It was possible to reSuce CODMn and color to less than 20mg/l and 20 units respectively by activated carbon adsorption.

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