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12.4 ALTERNATIVE SWRO-MEMBRANE SYSTEMS AND PRETREATMENT 265
TABLE 12.5 Reverse Osmosis Permeate Water Quality Seawater Source e Red Sea Permeate Water Quality
Water-Quality Parameter
Temperature, C pH
Ca2þ, mg/L Mg2þ, mg/L Naþ, mg/L
Kþ, mg/L CO32, mg/L
HCO3  , mg/L
SO42, mg/L Cl, mg/L F, mg/L NO3, mg/L B, mg/L Br, mg/L TDS, mg/L
Red Sea Source Seawater Quality
22e33 7.0e8.0 500 1,540 13,300 489
2.4
142.4 3,100
22,840 0.9 0.00 5.3
80 42,000
Single Pass Split Partial Two SWRO System Pass RO System
23e34 24e35 6.8e7.8 7.6e8.0 1.1e2.1 0.5e0.7 1.8e3.4 0.7e1.0 142e220 20e38 3.2e6.5 1.2e1.8 0.0 0.0
1.4e2.0 0.5e1.0 2.8e6.2 1.9e2.6
195e276 29e58 0.5e0.7 0.3e0.5 0.00 0.00 1.2e1.7 0.45e0.80 1.0e1.4 0.45e0.60 350e520 55e105
The first pass of the Fujairah plant consists of 17 duty and 1 standby RO trains using stan- dard rejection SWRO membranes producing permeate of TDS concentration of 400e500 mg/ L. The overall recovery of the first-pass SWRO system is 43%. The second pass has 8 BWRO trains with a total recovery rate of 90%. The second-pass BWRO trains have two stages and treat approximately 80% of the first-pass permeate to TDS concentration of 10e20 mg/L. The rest (i.e., 20%) of the first-pass permeate is blended with the second-pass permeate to produce finished water of TDS concentration of less than 120 mg/L. Concentrate produced by the second-pass RO system has salinity lower than that of the source seawater and is recycled to the feed of the first-pass RO system (see Fig. 12.8). Despite of its complexity, the two- pass/two-stage RO system in Fujairah performs very reliably.
An example of a two-pass/two-stage SWRO plant is the 146,000 m3/day (38 MGD) Point Lisas facility in Trinidad (Fig. 12.9). This plant produces high-quality desalinated seawater of TDS concentration of 85 mg/L or less, which is predominantly used for industrial applica- tions. The first pass of the Point Lisas SWRO system consists of six two-stage RO units. Each of the first-stage RO trains uses SWRO membranes and is coupled with Pelton wheel energy-recovery device. The second-stage trains of the first pass are equipped with brackish water RO elements.