This Environmental Protection Agency (EPA) publication is an operations manual for activated sludge and trickling filter wastewater treatment facilities. The stated purpose of the manual is to provide an on-the-job reference for operators of these two types of treatment plants. The overall objective of the manual is to aid the operator in determining what process control and operationsl measures may be most effective in optimizing the performance of his/her particular treatment plant. The manual is presented in three major divisions: (1) activated sludge process, (2) trickling filter process, and (3) appendices. The first two divisions are each divided into four sections: (1) troubleshooting, (2) process control, (3) fundamentals, and (4) laboratory control. Each section concludes with a list of selected references. This publication assumes familiarity with these two aerobic treatment processes, but it includes fundamentals sections in the first two divisions for those who may be unfamiliar with these processes. The four appendices contain material on operational records, plant visits, laboratory equipment, and a glossary. (MR)