This manual was designed to furnish information for upgrading the design of wastewater treatment plant facilities and to serve as a resource for establishing criteria for upgrading these plants. The manual also furnishes information for modifying plant design to compensate for current organic and hydraulic overloads and/or to meet more stringent future treatment requirements. In addition, the importance of modifying existing plants and designing new plant facilities to accommodate operation, maintenance, and other routine plant functions is emphasized. Information is presented in 17 units, with objectives, text material/figures, and resources provided for each unit. Unit topics include: maximizing plant operations feedback in wastewater treatment plant design; plans and specifications reviews; general design considerations; unit process design for preliminary treatment, primary sedimentation, aeration basins, secondary sedimentation, disinfection, effluent discharge, sludge treatment, additional biological processes, and other factors (safety, energy conservation, and emergencies); systems approach to unit process selection; equipment selection/placement (for operability and maintainability); and operator/design engineer relationship. A FORTRAN computer program (with abridged users guide) for the preliminary design of wastewater treatment systems is included in unit 15. It is assumed that the user of this manual has an engineering background and is familiar with wastewater treatment plant design. (Author/JN)