A full scale modified A(2)/O process which combined pre-anoxic selector and the staging strategy treating low strength wastewater was investigated. In South China, domestic wastewater is always low in strength due to the high level of groundwater and setting of septic tank at the beginning of wastewater collection system. The results suggested that inadequate denitrification could result in deterioration of phosphorus removal. In addition, influent phosphorus concentration had effect on phosphorus removal. The pre-anoxic selector in modified A(2)/O process changed the distribution of nitrogen denitrified in different tanks. Characteristics of 3-stage aeration tanks were also studied. The simplified design of rectangular aeration tank could also perform as plug flow as conventional channel aeration tank. In 3-stage aeration tanks, mixed liquid suspended solid (MISS) increased from one tank to another, while specific oxygen uptake rate (SOUR) of sludge, chemical oxygen demand (COD) and total phosphorus (TP) removal rate decreased, however ammonia nitrogen (NH3-N) and nitrate nitrogen (NO3-N) reaction rate remained constant. Furthermore, high MLSS concentration was not suitable for treating low strength wastewater. Waste sludge discharge could improve removal efficiency of COD. NH3-N, and TP. Without waste sludge discharge, nitrite accumulated in settler. (C) 2009 Elsevier B.V. All rights reserved.