ABSTRACT
Membrane separation processes are based on the ability of semipermeable membranes of the appropriate physical and chemical nature to discriminate between molecules primarily on the basis of size, and to a lesser extent, on shape and chemical composition. A membrane’s role is to act as a selective barrier, enriching certain components in a feed stream, and depleting it of others. In this regard, the phenomenon is very similar to osmosis, which has been observed and studied for more than 250 years, beginning with the efforts of the French scientist Abbe Nollet in 1748. However, there was little interest in the osmosis process outside the academic, medical, and photographic fields until the early 1950s, mostly because membranes capable of withstanding the high pressures necessary (>20 bar), and to give high flux and high rejections, were not available. Serious study of reverse osmosis (RO) as a practical tool for the production of potable water from brackish or saline water began in 1953 when the U.S. Department of the Interior, Office of Saline Water, began supporting research projects aimed at developing RO technology for desalination. The first breakthrough was made by Reid and Breton (1959) who, while screening membrane materials for desalination, discovered that cellulose acetate membranes gave high rejections and reasonable fluxes (dewatering rates). Shortly thereafter, Loeb and Sourirajan (1960, 1962) developed the casting procedure for asymmetric cellulose acetate membranes and demonstrated that flux could be greatly improved by making asymmetric rather than homogeneous membranes.
