We consider nonlinear model predictive control for an enhanced wastewater treatment plant(EWWTP) conducting the anaerobic anoxic oxic (A2O) process to improve effluent water quality.The A2O process is modeled with an activated sludge model (ASM3+Bio-P), for which nonlinear model predictive control is applied. After formulating the cost function, we conduct numerical simulation and compare the proposed method with the conventional method, which consists of PI control and feed-forward control. Finally, it is shown that the performance of the proposed method is superior to that of the conventional method. It is also shown that real time optimization for the nonlinear model is achieved.