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9.7 OVERVIEW OF MEMBRANE PRODUCTS USED FOR SALINE WATER PRETREATMENT 207
the United States (Atassi et. al, 2007), which employ MF or UF membrane filtration systems, has identified a number of challenges these plants have experienced during their startup, acceptance testing, and full-scale operations (Table 9.4). Although the MF and UF systems were used for fresh water treatment, most of the lessons learned from these applications are relevant to saline water applications as well.
Overview of the information presented in Table 9.4 points out that for the use of MF/UF systems for pretreatment to be successful, such systems have to be designed carefully and conservatively and have to be operated by highly trained staff that often is not readily avail- able in many desalination plants worldwide. Because MF/UF membranes are uncommodi- tized proprietary products, the plant owner and operators will have to rely on the membrane manufacturer and supplier to provide expert guidance and technical support, which typically are not necessary if conventional granular media filtration is used for pretreatment.
9.7 OVERVIEW OF MEMBRANE PRODUCTS USED FOR SALINE WATER PRETREATMENT
9.7.1 X-Flow (Pentair)
X-Flow (Pentair), formerly known as Norit, has two UF membrane products, which are specifically developed for membrane pretreatment of saline water, Seaguard and Seaflex (see Figs. 9.7 and 9.8). Both products are available in three types of modules, which differ by the size of their filtration area per moduled64, 55, and 40 m2, respectively. To date, the X-Flow UF membrane product that has found the widest full-scale application for seawater pretreatment is the Seaguard 40 module.
The main difference between the Seaguard and Seaflex modules is their maximum TMP and the configuration of the racks in which they are installed. Seaguard UF membranes can withstand higher maximum TMP (8 bars vs. 3 bars for Seaflex modules), which makes them more suitable for desalination plants with UF/RO direct coupling configuration, where the saline water from the intake is pumped directly through the UF membrane system and into the suction header of the high-pressure RO feed pumps. This type of UF- RO system configuration allows to eliminate the need for an interim tank and pump station between the pretreatment and RO systems and results in capital cost savings. However, its successful use also requires the pretreatment membranes to be able to withstand higher working pressures because they have to carry through not only the pressure needed for normal operation of the UF membranes (0.1e0.8 bar) but also the suction pressure needed for the high-pressure pumps of the RO system (2.0e3.5 bars). In addition, these membranes have to be able to withstand elevated pressures caused by hydraulic surges.
Seaguard UF membrane system racks have horizontal configuration (similar to that of RO racks) with two to four membranes per module (vessel) and up to 120 modules per rack. For comparison, Seaflex UF system racks have vertical configuration with only one membrane module and can accommodate a maximum of 18 modules per rackdthese racks are also referred to as X-line Racks. The main reason of the difference of the two configurations is that the horizontal configuration is less sensitive to hydraulic surges/pressure spikes and