Theories of the process of ultrafiltration and reverse osmosis, that involve permeation of solvent through a membrane in a sense opposite to that which obtains in osmosis, are considered. Factors that affect rates of permeation of solute and of solvent, and hence the selectivity of the membrane, are discussed.The physical structure and chemical composition of membranes are discussed, and the effect of processing variables on structure and composition, and hence on permeation rates and selectivity, are considered. The formation of boundary layers of high solute concentration at the surface of the membrane exposed to the solution has an appreciable effect on permeation; techniques for preventing their formation are mentioned. Some basic considerations relating to the power requirements of membrane processes are defined.
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