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190 9. MEMBRANE FILTRATION
FIGURE 9.2 Ultrafiltration (UF) pretreatment system of the Adelaide desalination plant, Australia.
Although at present less than 10% of all existing desalination plants worldwide have UF or MF pretreatment, application of membrane filtration for saline water pretreatment is gaining a wider acceptance over the past 10 years (Busch et al., 2009; Lazaredes and Broom, 2011). The number of medium and large desalination plants with membrane pretreatment has increased from less than a half a dozen in 2002 to over 40 in 2011 (Gasia-Brush et al., 2011).
Practical experience at the 40,000 m3/day (10.6 MGD) Addur seawater desalination plant in Bahrain (Burashid and Hussain, 2004) and the 20,000 m3/day (5.3 MGD) Sohar RO1 desa- lination plant (Shahid and Al Sadi, 2015) shows that membrane pretreatment alone may not always provide competitive solution, especially for challenging saline source waters of high, organic content, and biofouling potential.
A number of desalination plants with relatively shallow intakes and source water of high turbidity, algal content, and organic loads (i.e., Shuwaikh, Bekton, Kindasa) have additional pretreatment steps before the membrane filtration system to cope with these source water quality challenges. However, in most existing desalination installations worldwide, mem- brane filtration is the only pretreatment system before reverse osmosis (RO) desalination.
9.2 THE MEMBRANE FILTRATION PROCESS
All membrane pretreatment processes operate in cycles that include the following four ac- tivities: (1) processing, (2) backwash, (3) cleaning, and (4) integrity testing. The four opera- tional modes are typically monitored and controlled by a programmable logic controller.
9.2.1 Source Water Processing (Filtration)
The membrane filtration of the saline source water takes place during the processing phase. Depending on the specific membrane product and the membrane configuration, the filtration process can either occur in direct-flow or in cross-flow mode. In direct flow mode, all of the source water passes through the membranes. In cross-flow operations,