Reverse osmosis rejection mechanisms

Cellulose acetate was the first high-performance reverse osmosis membrane material discovered. The flux and rejection of cellulose acetate membranes have now been surpassed by interfacial composite membranes. However, cellulose acetate membranes still maintain a small fraction of the market because they are easy to make, mechanically tough, and resistant to degradation by chlorine and other oxidants, a problem with interfacial composite membranes. Cellulose acetate membranes can tolerate up to 1 ppm chlorine, so chlorination can be used to sterilize the feed water, a major advantage with feed streams having significant bacterial loading. 

The main mechanisms of membrane fouling are adsorption of feed components, clogging of pores, chemical interaction between solutes and membrane material, gel formation and bacterial growth. Fet us first consider bacterial growth on membranes. Microbiological fouling of reverse osmosis membranes is one of the main factors in flux decline and loss of salt rejection [25-29] (Table 11.1). 

Aromatic tri-functional acid and amine monomers are used to obtain reticulated polyamides, which have better mechanical and chemical stability and, for that reason, they are preferred for nanofiltration and reverse osmosis membrane materials. In these membranes, a thin polyamide layer (less than l jm thickness) is fabricated by interfacial polymerization on the top of a porous support (normally an ultrafiltration polysulfone membrane), which usually presents a non-woven reinforcement for mechanical stability as can be seen in Figure 8. Despite its small thickness, the polyamide dense layer is the main regulator of the rejection/transport of water and ions across the membrane. 

J. Kim, M. Wilf, J.-S. Park, J. Brown, Boron Rejection by Reverse Osmosis Membranes National Reconnaissance and Mechanism Study, Denver, CO Desalination and Water Purification Research (DWPR) Department of the Interior, Bureau of Reclamation, U.S., 2009. 

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