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204 9. MEMBRANE FILTRATION
of algae in the saline source water is less than 20,000 cells/L. Depending on the membrane type, product, and operating TMP, higher algal content could cause varying degrees of reduc- tion of membrane productivity and could result in shorter filtration cycles and higher CEB and CIP membrane cleaning frequencies.
As indicated previously, the operation of the membrane pretreatment systems causes breakage of the cells of the algae contained in the saline source water that results in the release of easily biodegradable organics in the filtered water, which may cause accelerated biofouling of the downstream RO membranes if the organic content exceeds certain threshold (e.g., total organic carbon (TOC) > 2 mg/L).
9.5.3.2 Bacteria and Viruses
Both MF and UF systems can remove four or more logs of pathogens such as Giardia and Cryptosporidium. Usually, typical UF membrane elements with a pore size of 0.01e0.02 mm can remove over 4 logs (99.99%) of viruses. MF elements with pore sizes of 0.03 mm or less can achieve 3 logs virus removal. Older generation MF membranes (pore openings of 0.1e0.2 mm) do not provide effective virus removal.
In general, both MF and UF membranes do not remove marine bacteria completely and cannot be considered effective barriers for preventing biofouling of the downstream RO elements.
9.6 PLANNING AND DESIGN CONSIDERATIONS
To date, UF membranes have found wider application for saline water pretreatment than MF membranes. Results from a comparative MF/UF study (Kumar et al., 2006) indicate that “tight” (20 kDa) UF membranes can produce filtrate of lower SWRO membrane fouling po- tential than 0.1 mm MF membranes. For comparison, these MF membranes produce filter effluent of water quality similar to that of 100 kDa UF membranes.
Under conditions, where large amounts of silt particles of size comparable to the MF mem- brane pore are brought into suspension by naval ship traffic or ocean bottom dredging near the area of the intake, the silt particles may lodge into the MF membrane pores during the filtration process and ultimately may cause irreversible MF membrane fouling. Because UF membrane pores are smaller and fiber membrane structure differs from that of MF mem- branes, typically UF membrane pretreatment systems do not face this problem. Potential problem of this nature can usually be identified by side-by-side pilot testing of MF and UF membrane systems during periods of elevated silt content.
The two most important parameters associated with the design of any membrane pre- treatment system are the design flux and feed water recovery. Membrane flux determines the amount of total membrane area and modules/elements needed to produce a certain volume of saline water. Feed water recovery indicates that the fraction of the saline source water that is converted into filtrate suitable for saline water desalination. A number of factors can impact the selection of these two parameters and ultimately can influence the size and configuration of the membrane pretreatment system. These factors are discussed below.