Modification of conventional activated sludge process by adding microfiltration type hollow fiber membrane module offers various advantages. The membrane module is immersed directly in the bioreactor to separate the treated water from the sludge and the modified process is called submerged membrane activated sludge process (SMASP). Bench scale SMASP units were operated at constant flux to study the possibility of minimizing the excess sludge production and stable operation at different SRT and BOD loading. The long SRT developed high stabilized MLSS concentration in the bioreactor and decreased the sludge loading rate [kg-BOD/kg-MLSS-d]. In SMASP, very low sludge yield coefficients (0.04-0.09 kg-MLSS/kg-BOD) were obtained at long SRT (500 d) and small BOD loading (< or =0.5 kg/(m3 x d)) as compared to conventional activated sludge process and these coefficients showed considerable minimization in excess activated sludge production. At the same operation conditions, suction pressure was also maintained low (<25 kPa), and then SMASP was operated stably for long time (approximately 200 d).