Extensive field investigations were conducted at two large primary treatment plants to define the variability of flow, primary effluent pollutants, and waste solids production. These data plus that collected during a pure oxygen pilot plant investigation were utilized in specially developed cost-effective computer programs for activated sludge treatment and waste solids handling to solve for the optimum design configuration as a function of the design reliability. A surprisingly high degree of design reliability was found to be justified in municipal wastewater treatment with the traditional means of operational flexibility (oxygen supply, return sludge, solids storage, etc.) exerting little impact on the total annual cost of the facility.