Performance of A2O Facilities for Rural Domestic Sewage Treatment in Yangtze River Delta Plain

Anaerobic-anoxic-oxic (A²O) facilities are widely applied in rural domestic sewage treatment in the Yangtze River Delta plain. Pollutants removal performance and influent flow rate of 39 A²O facilities in five towns were investigated. The influent concentrations of COD, ammonia nitrogen (NH ₃ -N),total nitrogen (TN)and total phosphorus (TP) were (95±80) mg/L,(56.2±40.0) mg/L,(61.6±40.1) mg/L and (4.6±2.8) mg/L, respectively. The very low influent concentration in some facilities indicated that there was a problem in sewage collection. Due to the unstable influent flow rate, the A²O facilities were greatly affected by rainfall and deviated from the design capacity.The removal rates of COD, NH₃-N, TN and TP by the A²O facilities were (46.9±30.5)%, (46.0±37.3)%, (19.7±18.7)% and (25.1±21.9)%, respectively,and the removal performances varied greatly among facilities. The standard achieving rate of effluent COD was high, but those of effluent nitrogen and phosphorus were low. Important reasons for the poor performance of the facilities were as follows: the too low organic load and influent COD/TN ratio caused by the low concentration of influent COD, high shocking load caused by incomplete diversion of rain and sewage, dramatic fluctuation in quality and quantity of the influent, as well as poor biological removal of nitrogen and phosphorus caused by inadequate operation and maintenance of the facilities.