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9.5 FILTER PERFORMANCE 203
membrane modules from several different manufacturers such as PALL, Dow, GE Zenon, Toray, and Hydranautics. Such membrane pretreatment systems are referred to as “Universal MF/UF Systems” because they allow the owner of the desalination plant/membrane pre- treatment system to use interchangeably and to procure competitively membranes from a number of well-established membrane manufacturers with comparable products. It should be pointed out that the capital costs for the universal membrane systems are usually 20%e30% higher than these for a membrane system tailored to a specific membrane product. However, the additional capital costs are likely to be compensated by operation and mainte- nance savings from membrane replacement.
9.5 FILTER PERFORMANCE 9.5.1 Removal of Solids
MF and UF membrane systems have proven to be very effective for turbidity removal as well as the removal of nonsoluble and colloidal organics contained in the source water. Turbidity can be lowered consistently below 0.1 NTU and filter effluent SDI15 levels are usu- ally below 3 over 90% of the time. The suspended solids removal efficiency of membrane pre- treatment systems is not sensitive to variations in turbidity and silt content of the saline source water. However, the amount of particulate foulants in the source water may impact the cleaning frequency and length of the filtration cycle.
Typically, membrane pretreatment systems can handle saline source waters with turbidity lower than 50 NTU and SDI5 lower than 16. If the saline source water turbidity and/or silt content exceed these thresholds, an upstream pretreatment system such as lamella settler or dissolved air flotation clarifier would need to be installed to reduce the feed water turbidity and/or silt below the threshold levels.
9.5.2 Removal of Organics
Membrane pretreatment does not remove a significant amount of dissolved organics and aquatic microorganisms, which typically cause RO membrane biofouling. Because of the very short water retention time of the membrane pretreatment systems, they do not provide measurable biofiltration effect, unless designed as membrane bioreactors. For comparison, granular media filters, depending on their configuration, loading rate, and depth, could remove 20%e40% of the soluble organics contained in the source water. In addition, UF and MF membrane filtration systems have very limited oil and grease removal efficiency and usually do not provide adequate protection of the downstream RO membranes from the destructive impact of these contaminants.
9.5.3 Removal of Microorganisms
9.5.3.1 Algae
UF and MF membranes can remove practically all algae from the saline source water. Their operation will typically not be affected by mild or moderate algal blooms, when the content