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254 12. REVERSE OSMOSIS SYSTEM DESIGN AND PRETREATMENT
The maximum recovery that can be achieved by a given pressure-driven membrane desa- lination system mainly depends on the source water salinity and is limited by the magnitude of the osmotic pressure to be overcome by the RO system high-pressure feed pumps, and by the fouling and scaling potential of the source seawater. It should be pointed out that higher the salinity and fouling potential of the saline source water, the lower the target design recov- ery should be. For example, relatively lower (33e35 ppt) salinity of the Pacific Ocean allows the SWRO systems treating such water to be designed for recovery of 50%, if the seawater has relatively low fouling potential. For the Red Sea and Persian Gulf seawaters, which have sa- linities of 42e46 ppt, the sustainable design SWRO system recovery is 40%e45%. For desali- nation plants in the Middle East exposed to severe algal blooms, the sustainable design recovery is even lowerd36%e38%. Higher recoveries (40%e45%) could be achieved, howev- er, if the desalination plant is equipped with more robust pretreatment as that recommended in Chapter 11 for such seawaters.
As indicated in previously, scaling occurs when the minerals left behind on the rejection side of the RO membrane are concentrated to a level at which they begin to form precipitates (crystalline compounds) which in turn plug the membrane surface and interfere with fresh- water transport through the membrane. Typically, seawater desalination plants can only turn 40%e60% of the source water into low-salinity permeate.
Membrane performance tends to naturally deteriorate over time due to combination of material wear-and-tear and irreversible fouling of the membrane elements. Typically, mem- brane elements have to be replaced every 5e7 years to maintain their performance in terms of water quality and power demand for salt separation.
Improvements of membrane element polymer chemistry and production process have made the membranes more durable and have extended their useful life. Use of conservatively designed dual or tri-media granular filtration systems and MF or UF-filtration pretreatment prior to RO desalination would allow to extend the membrane useful life beyond 7 years and further beyond. On the other hand, practical experience shows that MF- and UF-membrane pretreatment systems designed for very high fluxes (80 lmh or more) usually result in accel- erated aging of the downstream SWRO membrane elements and reduction of their useful life below 3 years.
12.2.1 Classification of Thin-Film Composite SWRO Membranes
As indicated previously, 8-in. thin-film composite SWRO membranes are the most widely used types of membranes at present. The three key performance parameters of all RO mem- branes are: salt rejection, flux/productivity, and operating pressure. Currently, there are a number of commercially available SWRO membrane elements designed with special features allowing to optimize their performance around one or more of these three key performance parameters. Commercially available RO and NF elements at present can be classified in the following key groups:
1. Standard Rejection;
2. High Rejection;
3. High Productivity (or Low-Energy); and 4. High Pressure.