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12.2 SWRO MEMBRANE ELEMENTSdKEY TYPES AND PRETREATMENT CONSIDERATIONS 253
wider distance between the envelopes (0.87-mm/34-mil). The wider the feed channel be- tween the envelopes, the more foulants it could accumulate before the membranes need cleaning. Therefore, the RO elements with wider (34-mil) spacers are usually preferred for sa- line source waters with high fouling potential and less conservatively designed pretreatment filtration systems.
The most widely used commercially available SWRO-membrane elements have diameter of 20 cm (8-in.), length of 100 cm (40-in.), and produce 11e14 m3/day of permeate. As shown in Fig. 12.2, the SWRO membrane elements are connected in series inside the pressure vessel. Typically, one pressure vessel houses from six to eight SWRO-membrane elements. Most RO- membrane systems are designed with seven elements per vessel.
A recent design trend for RO feed waters that have very low turbidity (<0.1 NTU), silt content (SDI15 < 3), and organic concentration (TOC < 1.0 mg/L) at all times is to use eight elements per vessel. Such high RO feed-water quality can be achieved either if the source water originates from well-designed subsurface intake or if it is collected by open intake and is pretreated via conservatively designed single or two-stage filtration system. Typi- cally, the higher the source seawater total dissolved salts (TDS) and fouling potential, and the lower the design membrane flux the fewer elements are used per vessel. SWRO systems designed around lower salinity feed water (e.g., TDS < 35 ppt), with low fouling potential are suitable for eight membrane elements per vessel configuration. Such SWRO systems can be designed for relatively high average permeate flux of 15 lmh/9 gfd or more.
In the majority of existing seawater RO installations, the entire pretreated seawater flow is fed at the front end of the membrane vessel and collected at the rear end. However, most recent RO-system designs incorporate permeate flow collection from both front and back ends of the membrane vessel as shown in Fig. 12.2. Removal of some of the permeate from the front end is practiced when the desired concentrations of TDS, chloride, and boron in the desalinated water are targeted to be below 250, 100, and 0.75 mg/L, respectively, because the permeate collected from the front end of the membrane vessel (usually the first two to three elements) already meets these water quality targets.
If left in the RO-membrane vessel, the high-quality permeate generated from the front-end membrane elements will blend with the permeate form the remaining membranes in the cen- tral collection pipe of the RO vessel, whose permeate has inferior water quality, and then the entire permeate volume produced in the RO vessel will need to be retreated in a second RO pass to meet the same product water target. The ability to collect permeate from both ends also allows to achieve a better control over the water quality of permeate produced by the desa- lination system and, to some extend, to control the fouling of the RO system membranes.
Commercially available membrane RO elements are of standardized diameters and length, and salt rejection efficiency. Standard membrane elements have limitations with respect to a num- ber of performance parameters such as: feed water temperature (45 C/104 F); pH (minimum of 2 and maximum of 12); silt density index (less than 4); chlorine content (not tolerant to chlorine in measurable amounts); and feed water pressure (maximum of 83e100 bars). Some producers offer 16- and 18-in. spiral-wound SWRO- and BWRO-membrane elements but they have found very limited application in large-scale installations due to their relatively higher costs and elevated fouling propensity.
The ratio between the volume of the product water produced by the membrane desalina- tion system and the volume of the source water used for its production is commonly defined as recovery and is presented in percent of the plant RO-system feed water volume.