Page 278 - ro membanes

12.4 ALTERNATIVE SWRO-MEMBRANE SYSTEMS AND PRETREATMENT 261
smaller than the energy savings of processing the entire volume of the first-pass permeate through the second pass. Under the split partial two pass RO system configuration the vol- ume of permeate pumped to the second RO-pass and the size of this pass are typically 25%e50% smaller than the volume pumped to the second RO-pass under conventional once-trough operation. Since pumping energy is directly proportional to flow, the energy costs for the second-pass feed pumps are reduced proportionally, i.e., with 25%e50%. For SWRO system operating at 45% recovery, such savings will amount to 14%e22% of the en- ergy of the first-pass RO pump.
The concentrate returned from the second pass carries only 1%e2% of additional salinity to the first-pass RO feed, which reduces the energy benefit from such recovery proportion- ally, i.e., by 1%e2% only. As a result, the overall energy savings of the use of split partial two-pass RO system as compared to conventional two-pass RO system are between 12% and 20%. Practical experience with large SWRO desalination plants indicates that the average total RO system life-cycle cost savings associated with applying such SWRO system config- uration are typically between 14% and 16%.
At present, most new SWRO desalination systems are designed with split partial second pass configuration because this configuration allows reducing the size of the second-pass RO system and the overall freshwater production costs. It should be pointed out that split-partial second RO-pass systems can be configured in several alternatives, which may involve the use of the same or different type of membrane elements within the first-pass SWRO system.
Example, of plant with partial-second pass configuration is the 95,000 m3/day (25 MGD) Tampa Bay seawater desalination facility in Florida. The second pass at this facility is designed to treat up to 30% of the permeate produced by the first-pass SWRO system as needed in order to maintain the concentration of chlorides in the plant-product water always below 100 mg/L.
The partial second pass at the Tampa Bay water seawater desalination plant was installed to provide operational flexibility and to accommodate the wide fluctuations of source water salinity (16e32 ppt) and temperature (18e40C/64e104F). Typically, the product water quality target chloride concentration of 100 mg/L at this plant is achieved by only operating the first pass of the system. However, when source water TDS concentra- tion exceeds 28 ppt and/or the source water temperature exceeds 35C (95F), the second pass is activated to maintain adequate product water quality. The percent of first-pass permeate directed for additional treatment through the second pass is a function of the actual combination of source water TDS and temperature and is adjusted based on the plant product water chloride level.
Conventional full two-pass RO systems and split partial second-pass systems are equally vulnerable to accelerated fouling due to underperforming pretreatment especially during pe- riods of high-intensity and severe algal blooms. However, the product water quality of partial second-pass RO systems is usually more affected by algal bloom events than that of full two- pass systems, because these systems have more flexibility to handle the permeate water qual- ity impacts from fouling.
12.4.2.3 Product Water Quality of Single- and Two-Pass SWRO Systems
Tables 12.2 through 12.6 present summary of the range of permeate water quality pro- duced by typical single-pass and partial two-pass seawater desalination systems processing