A fifteen-month laboratory program has shown that all grades of municipal waste water may be significantly improved by the reverse osmosis process. Comparisons are provided on the behavior and response of the reverse osmosis process to carbon-treated secondary sewage, alum-treated secondary sewage, secondary sewage, primary settled sewage, raw sewage, and digester supernatant. High removals of dissolved minerals, organic substances, and suspended matter have all been achieved in the same treatment. The effects of a flocculant, dispersant, chelating agent, enzyme, and acid on reducing product water flux decline are compared. The relative effects of reverse osmosis test-cell geometry on solids deposition and membrane performance are presented. A phenomenological model is postulated describing the role of undissolved solids and organic substances in producing product water flux decline and the subsequent maintenance of constant product water fluxes. A computer model of the reverse osmosis process has been developed to provide an accurate and rapid method of determining the design and cost of reverse osmosis facilities. (Author)