A prototype of expert system which can support the operation for the municipal waste water treatment plant located at Kyoung-Ki Do, Kwang-ju Kun was developed and tested. This system provides ( i ) tracking the cause of the problem, ( ii ) analysis, and ( iii ) solution Knowledge-base consists of about 100 production-rules for the biological wastewater process, such as bio-reactor and final clarifier. Rules were obtained from the analysis of the problems such as sludge bulking. The system provides stable process control and management and effectively helps inexperienced operators with advanced and standard technologies. Future works will focus to develop a statistical process control model and associate with expert system. The control model can process the operation data statistically ; analyze the relationship between affecting factors and control variables ; and provide optimum operation parameters.