Page 274 - ro membanes

12.4 ALTERNATIVE SWRO-MEMBRANE SYSTEMS AND PRETREATMENT 257
FIGURE 12.3 Internally staged membrane configuration.
The first (lead) element (Fig. 12.3), which receives the entire seawater feed flow of the vessel, is a low-permeability/high salt-rejection element (Dow Filmtec SW30 XHR-400i). Because of its low permeability, this element produces only 10%e14% (instead of 25%) of the permeate flow produced by the entire vessel and, thereby, preserving the feed energy for more effective separation by the downstream RO-membrane elements in the vessel.
The second RO element in the pressure vessel is of a standard (average) rejection perme- ability and salt rejection (Dow Filmtec SW30 XLE-400i), and produces approximately 14%e16% of the total flow, while the remaining five elements in the vessel are of the same high-permeability/low-rejection model (Dow Filmtec SW30 ULE-400i). This 1e1e5 combina- tion of low-permeability/high-rejection and high-permeability/low-rejection elements results in a more even distribution of flux and pressure along the vessel typically, which yields 5%e10% energy savings and reduces the fouling rate of all membrane elements.
The more even distribution of flux achieved by the hybrid membrane element configuration not only results in lower overall energy use but also reduces the fouling of the first element as compared to standard configuration with all of the RO membranes in the vessels being of the same model. Therefore, the use of inter-stage membrane configuration would impose less strin- gent requirements on the upstream pretreatment system as compared to conventional RO mem- brane configuration using the same model of elements in all positions within the vessels.
12.4 ALTERNATIVE SWRO-MEMBRANE SYSTEMS AND PRETREATMENT
The SWRO-system configurations that are most widely applied at present include: single- pass treatment, where the source water is processed by reverse osmosis only once (Fig. 12.4); and two-pass RO treatment, where the seawater is first processed through a SWRO system and then permeate produced by this system is reprocessed by brackish RO membranes (see Figs. 12.5 and 12.6).