Pilot plant studies were carried out using actual wastewater to investigate the applicability of a membrane separation activated sludge (MSAS) process to municipal wastewater treatment. A small-scale pilot plant (6.7 m3/day) with immersed flat sheet membrane was operated at the flux of 0.4 m3/m2/day. Continuous operation for 140 days without chemical cleaning was possible. Average Sludge production ratio was about 0.6. No significant difference was observed in the dewaterability between membrane separation activated sludge and conventional activated sludge at the CST test. Large-scale pilot plants (30–70 m3/day) with five types of membrane were also operated. In these plants nitrogen removal by nitrification and denitrification, and phosphorus removal by coagulant addition were carried out. Stable operation with HRT of six hours, flux of 0.4–0.8 m3/m2/day was possible, the average nitrogen and phosphorus removal efficiency being more than 80 and 95%, respectively.

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