The purpose of this manual is to provide technical guidance for persons conducting evaluations of wastewater treatment plants and serve as a model which can be used by state regulatory agencies. It furnishes the information needed to facilitate identification and classification of various types of treatment plants. It also details the processes commonly used in wastewater treatment. The common problems affecting plant operation are identified and described. Several aspects of each problem are covered: exactly what is the problem; how it is detected, what are the possible causes, and what solutions are feasible.