The solution is to operate membranes in the crossflow mode. By doing so, rejected contaminants are continuously carried away from the membrane surface, thereby minimizing contaminant buildup, leaving it free to reject incoming material and to allow free flow of purified water. Although membrane cleaning is periodically required, the self-cleaning nature of crossflow filtration lengthens membrane life enough to make it economically attractive. The most common of the three is RO because of its comprehensive capability to reduce levels of dissolved impurities. Because the membrane accounts for 15 to 40 percent of the price of an RO or UF machine and because membranes must be replaced periodically, careful membrane selection is essential.
Many types are available and each has unique characteristics.
Selection criteria should include chemical tolerance, mechanical suitability, cleanability, separation performance, flow performance and price. As with other mechanical devices, crossflow filtration machines function best when design and materials enhance one another. Once the correct membrane is chosen, a design that provides for appropriate crossflow rates, pressures and permeate recoveries is critical. Good design should manifest itself in a machine that has consistent performance, needs less frequent membrane cleaning or replacement, consumes reasonable amounts of power and requires little operational attention. Much of this can also be achieved be pretreating the crossflow unit's feedwater.