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than is required at lower temperatures. To a certain extent, salt flux is also increased as temperature is increased, but the effect is not important at normal operating temperatures. Membrane manufacturers generally provide a table or formula for determining TCF (temperature correction factors). The TCF is proportional to the change in pressure needed to maintain the 25 “C flux rate and is equal to 1.00 at 25 “C. The TCF increases at temperatures less than 25 “C and decreases at higher temperatures. Most cellulosic and thin film composite membranes listed in chapter 10 have maximum temperature limits of 40 to 45 “C, which should be adequate for most surface and ground water sources. Streams at higher ambient temperatures should be used in a heat transfer process before treatment. Excessive heat in the RO system can cause a variety of problems. Carbonate scaling is more likely at higher temperatures, membrane compaction is enhanced, and fouling caused by increased water flux is also more likely. Most membranes can handle feed water at temperatures as low as 1 “C without problem. The membrane matrix becomes more rigid at low temperatures and water flux decreases; thus, higher pressure is needed to maintain a standard 25 “C flux rate. 4.4.3 Effect of pH. - The pH of the feed water can affect the membrane structure and the scale formation potential of the brine stream. Cellulosic membranes have a narrow operational pH range of 4 to 6. Some membranes have pH ranges as narrow as 5.5 to 6.0 (see tables 10.5 and 10.6). If exposed to a pH outside this range, hydrolysis occurs. Rends in the membrane matrix are broken and replaced with hydroxyl ions, leaving holes in the matrix (Murphy, 1990). Thin fdm composite membranes generally have a much broader operational pH range, some as large as 2 to 11. Ceramics and metallics are unaffected by PH. The pH of the feed water may need adjustment to control scaling by the brine concentrate conveyance system. For example, silica solubility increases dramatically above pH 7.7, and at higher temperatures. A silica scaling problem could be controlled by either raising the pH or the temperature of the feed water. Calcium carbonate, on the other hand, is more soluble at low temperatures and at a pH less than 8.0. A carbonate scaling problem can be relieved by lowering the pH or the temperature. However, if the brine is saturated in both silica and carbonate, changes in temperature or pH can cause one or the other to precipitate. Care must be taken to find the best condition to prevent scaling. 4.4.4 Tolerance to Oxidation. - Oxidants are added to water supplies to control biological growth, to improve taste and odor, to remove iron and manganese, and to speed the decomposition of vegetable and animal matter (Weber, 1972). In the past, chlorine has been used very reliably. The cellulose acetate membranes are chlorine tolerant. Non-cellulosic thin film composite membranes are not tolerant to oxidation; yet some method of biologicalPDF Image | DESALTING AND WATER TREATMENT MEMBRANE
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