This safety manual provides design, operating, and safety personnel of municipal wastewater treatment plants which use oxygen aeration of activated sludge systems with the knowledge to prevent hazards due to the interaction of the oxygen with combustibles and other hazardous materials which may be dumped or formed in the wastewater. These hazards include fires, explosions, and toxic gases; all injurious to personnel and plant equipment alike. All potential sources of hazardous materials were considered, which led to the inclusion in the manual of (1) the wastewater train (collection, primary treatment and secondary treatment), and (2) the oxygen train (on-site generation or trucked-in supply, storage and injection into the activated sludge system). The concept followed in this manual was to discuss the sources and nature of hazardous wastes, their detection at various stages, and safe design and treatment practices to minimize adverse effects. Consideration was given to the special problems of industrial wastewater which most urban plants must handle, and the organizational side of plant safety. Safety checklists are provided for the major components of all systems in (1) and (2) above.