The concept of RO was first demonstrated in the late 1950s with cellulose acetate membranes. These membranes are asymmetric, composed of a thin dense surface layer (0.2-0.5 mm) and a thick porous substructure. Solute rejection is accomplished by the thin dense layer and the porous substructure provides structural strength. Cellulose acetate membranes can be cast in sheets or as hollow fibers.
Cellulose acetate membranes are inexpensive and easy to manufacture but suffer from several limitations. Their asymmetric structure makes them susceptible to compaction under high operating pressures, especially at elevated temperatures. Compaction occurs when the thin dense layer of the membrane thickens by merging with the thicker porous substructure, leading to a reduction in product flux. Cellulose acetate membranes are susceptible to hydrolysis and can only be used over a limited pH range (low pH 3-5 and high pH 6-8, depending on the manufacturers). They also undergo degradation at temperatures above 35 °C. They are vulnerable to attack by bacteria. Cellulose acetate membranes have high water permeability but reject low-molecular-weight contaminants poorly. Cellulose triacetate membranes have been developed with improved salt rejection characteristics and reduced susceptibility to pH, high temperature, and microbial attack. However, cellulose triacetate membranes have lower water permeability than cellulose acetate membranes. Blends of cellulose triacetate and cellulose acetate have been developed to take advantage of the desirable characteristics of both membranes.
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