Mass transfer coefficients (k(L)a) of activated sludge process are measured over a Wide range of mixed liquor suspended solids (MLSS). Total three sets of experiments have been used (with steady state dissolved oxygen (DO) of 1.0, 2.5 mg L-1 with varying agitator speeds and constant agitator speeds with varying-unsteady state DO beginning from 1.0 mg L-1). The results show rise in k(L)a-with MLSS. The rise in DO is observed when the agitator speeds are kept constant throughout because of reduction in chemical oxygen demand (COD). Attempts have also been made to treat the entrapped/adsorbed COD on the biomass.