This is a series of technical reference materials on the process control of activated sludge. It describes activated sludge process procedures based on sludge quality and solids inventory. It consists of the following parts: Observations and Control Tests, I and II. The study describes the interpretation of process status and requirements from physical observations; and control tests used for process control. Calculation Procedures, III-A. It describes the calculations used with the control test results to develop the necessary process status and control decision information. Calculation Procedures for Step-feed Process Responses, III-B. Also it describes the special calculational procedures required for the step-feed configuration on the activated sludge process. The report provides an updated summary of the operational control procedures with new material on sludge quality and process control and expanded versions of return sludge flow control which considers most types of sludges encountered in actual practice.