This paper concerns the development and use of a computer-based expert system (ASCA) to advise operators of activated sludge wastewater treatment plants in the event of an actual or potential effluent problem. The system provides a diagnosis of problems and gives advice on remedial actions. An incident database facility permits the recording and later inspection of problematic events. The system contains a general knowledge base applicable to all such treatment works. This is supplemented by site-specific knowledge when customised to particular plant. Development required close collaboration with users. On-site trials are being performed at two sites. Early results indicate both usability and usefulness, and demonstrate that the system can be readily customised for further sites. Further possible enhancements are discussed.