In this study, two different wastewater treatment systems, a conventional activated sludge process and pure oxygen activated sludge process, used at a municipal wastewater treatment facility in California were investigated. Their BOD and NH₃ removal efficiencies, sludge productions, and economic efficiencies were compared. The BOD removal efficiency of the conventional activated sludge process (92.4~95.4%) is similar to that of the pure oxygen activated sludge process (91.9~96.2%). Further, both processes showed poor NH3 removal efficiencies; 9~18% for the conventional activated sludge process and 0~11.1% for the pure oxygen activated sludge process. Such poor NH₃ removal efficiencies are probably because these processes are optimized for BOD removal. The pure oxygen activated sludge process produced 2~3 times more sludge than the conventional activated sludge process. The pure oxygen activated sludge process was slightly more economically efficient than the conventional activated sludge process (saved about 0.3 million $/yr).