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270 12. REVERSE OSMOSIS SYSTEM DESIGN AND PRETREATMENT
two to four times larger number of RO-vessel groups (banks) and a smaller number of mem- brane vessels per bank. Under this configuration the individual vessel banks are directly con- nected to the high-pressure pump-feed lines and can be taken off service one at a time for membrane replacement and cleaning.
Although the feed-water distribution piping for such membrane center configuration is more elaborate and costly than that use of individual RO trains that contain two to three times more vessels per train, what is lost in capital expenditure is gained in overall system performance reliability and availability. A reliability analysis completed for a 95,000 m3/ day (25 MGD) SWRO plant (Liberman and Wilf, 2005) indicates that the optimum number of vessels per bank for this scenario is 54 and number of RO banks per plant is 20. A typical RO train-based configuration would include two to four times more (108e216) vessels per RO train and two to four times less (5e10) RO trains. According to this analysis, the use of the three-center configuration instead of the conventional RO-train-based approach allows to improve RO-system availability from 92% to 96% (avg. 95%) to 98%, which is a significant benefit in terms of additional amount of water delivered to the customers and improvement in water supply reliability.
The centralized energy-recovery system included in the three-center configuration (Fig. 12.10) uses high-efficiency pressure exchanger-based energy-recovery technology. The proposed configuration allows to improve the overall energy-recovery efficiency of the RO system and to reduce system power, equipment, and construction costs. While typically, the energy recovery efficiency of the conventional Pelton wheel systems drops significantly when the overall SWRO plant recovery is reduced below average design conditions, the energy-recovery efficiency of the pressure-exchanger systems improves with the reduction of the plant recovery rate. This allows operating the SWRO plant cost-effectively while deliv- ering variable product water flow. For example, if the SWRO plant output has to be reduced by 40% to accommodate low diurnal demand, a SWRO system with RO train-based config- uration has to shut down 40% of its trains and if this low demand persists, it has to flush these trains to prepare them for a the next start-up.
An RO system with three-center configuration would only need to lower its overall recovery to achieve the same reduction of the diurnal demand. Although temporary opera- tion at lower recovery would result in elevated costs for pumping and pretreatment of larger volumes of source water, these extra operational expenses are typically compensated for by the improved energy-recovery efficiency that results from operating the SWRO the system at lower water recovery ratio. In addition to providing flexibility in operating cost-effectively the desalination plant at varying production flows, the three-center design yields low- fouling RO-plant operations of very high availability factor.
It is important to point out that the RO-desalination system used at the Ashkelon desali- nation plant, as that of a number of other large desalination plants, (e.g., Hadera, and Sorek SWRO plants in Israel, and the Carlsbad desalination plant in California) is a four-stage SWRO system depicted in Fig. 12.11 (Gorenflo et al., 2007).
Table 12.7 presents a summary of the source-water quality and the design and actual product-water quality of the Ashkelon SWRO plant in Israel, which was the first facility where such reverse osmosis configuration was used and, therefore, has the longest track re- cord of successful performance (Dreizin et al., 2008).