Most of wastewater treatment plants (WWTP) are operated under low loading both in water quality and water capacity (flow rate) in Taiwan. Because various treatment methods used in WWTP would cause different power consumptions. In general, the flow rate of wastewater treated is proportional to the power consumption. The purpose of this study is simulating water quality and water capacity with a case of municipal wastewater treatment plant, using standard activated sludge method. In this work we will investigate the feasibility of save power using operation and parameters adjustment in wastewater system. A further step is to conduct the save energy in current WWTP under normal wastewater treatment capacity and national effluent standard. Results showed the major save energy was at inflow pump and aeration system of biological treatment stage. Both power consumptions of two were about 60 ~70 % of total power consumption in the plant. Thus we should focus the operating conditions to plan the save energy project, and calculate the total power system and energy consumption of all unit facilities before we are going to improve the energy save in WWTP. Important note is firstly to select units and facilities having high energy consumption for evaluating the possibility of energy save. Secondly is develop and using a good management system to attain the goal of save energy.