Effects of dissolved oxygen(DO)concentration on removal performance of nitrogen and phosphorus were investigated in a pilotscale anaerobicanoxicaerobic(A2/O)activated sludge process for treating municipal wastewater.During run operation

sludge recycling ratio and internal mixed liquid recirculation ratio were kept constant at 150% and 100%

respectively.The results showed that DO concentration played an important role in nutrient removal by A2/O process.Denitrifying phosphorus removal was observed when aerobic tank was kept at low DO(DO=1.0—1.5 mg·L-1)conditions.The best performance of nutrient removal was achieved when DO concentration was kept at 0.2 mg·L-1 in anoxic tank and 1.0 mg·L-1in aerobic tank.Total nitrogen(TN)and total phosphorus(TP)removal was about 64.6% and 89.6%

respectively.TN and TP concentration in the effluent was （11.9±5.3）mg·L-1 and （0.17±0.09）mg·L-1

respectively.TP removal in anoxic tank was about 48.2% of the total TP removal by the whole process.Denitrifying phosphorus removal became an important way of dephosphorization.Denitrifying phosphorus removal bacteria(DPB)accounted for 55.7% of the total phosphorus accumulating organisms(PAOs)based on the results of denitrifying phosphorus removal activity analysis.The analysis for particle diameter distributions showed that the particle diameter of activated sludge in aerobic tank was larger than those in anaerobic and anoxic tanks.Larger particle diameter resulted in the existence of anaerobic or anoxic microenvironments in the sludge particles in aerobic tank

which favored the survival and propagation of DPBs in the whole system.Therefore

the nutrient removal performance and costefficiency of conventional activated sludge processes can be enhanced by maintaining low DO in aerobic tank and improving denitrifying phosphorus removal activity in anoxic tank.