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256 12. REVERSE OSMOSIS SYSTEM DESIGN AND PRETREATMENT
5000e6000gpd at a salt rejection of 99.6%. In 2003, several membrane manufacturers introduced high-productivity seawater membrane elements that are capable of producing 7500 gpd at salt rejection of 99.75%. Just 1 year later, even higher productivity (9000 gpd at 99.7% rejection) seawater membrane elements were released on the market.
Some of the newest high-productivity SWRO-membrane elements have unit production capacity of 12,000e16,000 gpd, provide flexibility and choice, and allow to trade productivity and pressure/power costs. The same water product quality goals can be achieved either by (1) reducing the system footprint/construction costs by designing the system at higher pro- ductivity or by (2) reducing the system overall power demand by using more membrane el- ements, designing the system at lower flux and recovery, and taking advantage of newest energy-recovery technologies that further minimize energy use if the system is operated at lower (35%e45%) recovery.
12.2.5 High-Pressure SWRO Elements
The main purpose of this type of SWRO elements is to produce freshwater from concen- trated seawater with salinity of 50,000e60,000 mg/L and are used to maximize water recov- ery form a given source water volume. While a standard membrane element can only allow to recover up to 50% of the source seawater, the high-pressure SWRO are suitable to obtain recoveries of 60% and higher.
The high-pressure membrane elements are specifically designed to operate at 20%e40% higher pressure than that of the other types of membrane elements listed above and to treat high-salinity concentrate produced by the first stage of a two-stage SWRO system. While these high-pressure elements are commercially available from a number of manufacturers, they have not found widespread use, because they also typically have a higher fouling rate and elevated cleaning costs as well as reduced useful life. It should be pointed out that higher the feed pressure of the RO elements, the higher the fouling rate, and the faster the increase of transmembrane pressure for the same saline source water.
12.3 INTERNALLY STAGED MEMBRANE CONFIGURATIONdFOULING IMPLICATIONS
Ideally, redistributing and evening out the feed pressure and flux of all seven RO elements in the vessel to near-equal level can achieve the most energy-efficient desalina- tion process with lowest fouling within the RO vessels. A widely used membrane config- uration design allows to achieve such more even flux distribution by combining three different models of membranes with different permeability within the same vessel instead of using the same model of RO elements throughout the vessel (which is a typical configuration for conventional SWRO systems). This membrane vessel configuration was developed by Dow Filmtec (Mickols et al., 2005) and is known as Inter-Stage Design (ISD) (Fig. 12.3).