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272 12. REVERSE OSMOSIS SYSTEM DESIGN AND PRETREATMENT
periodically impacted by moderate algal blooms, the source water of which is successfully pre- treated by conventional gravity granular media filtration systems of medium depth of 1.4e1.6 m (4.6e5.3 ft).
12.5 ALTERNATIVES FOR CONTROL OF MICROBIAL FOULING
As discussed in Chapter 2, marine microorganisms that naturally occur in the source seawater and their biological waste products deposited on the surface of the membrane elements can decrease RO system productivity over time. For the microbial fouling (biofouling) to occur the following three key factors need to be in place: (1) the source seawater would need to contain metabolically active marine organisms that can produce biofilm that allows them to attach to the membranes and to colonize the membrane surface over time; (2) the source water will need to have relatively large concentration of easily biodegradable organics which serve as a food source for these organisms that can sustain their growth on the membrane surface; (3) favorable hydrodynamic conditions within the membrane elements that facilitate microbial attachment and biofilm accumulation. Of these three factors, however, the most important factor governing the occurrence and extend of microbial fouling is the content of easily biodegradable organics in the source seawater. It is important to note that marine microorganisms inhabit the source seawater at all times, but unless they are “activated” by the presence of large amount of biode- gradable organics, they would typically behave as particles and would pass through the SWRO membranes and exit the treatment process with the concentrate without causing measur- able biofouling and significant impact on RO membrane fresh water production capacity.
Taking under consideration these three factors, the microbial fouling process could be controlled by a combination of one or more of the following methods: (1) reduction of biode- gradable organics in the seawater; (2) reduction of bacterial content in the source seawater; and (3) creation of hydrodynamic conditions in the membrane elements that do not allow the active bacteria to attach to the membrane surface and form biofilm. The alternatives avail- able to control biofouling based on the three biofouling control methods are discussed below.
12.5.1 Reduction of Bacterial Food Sources in Source Water
Microbiological fouling can be effectively controlled by reduction of source-water constituents that accelerate microorganism growth. Since food content is the most important factor for micro- bial fouling, controlling this content is typically the most efficient biofouling protection strategy.
As indicated in Chapters 6, 7, and 8, controlling the content of easily biodegradable organics could be achieved by reduction of their content ahead of the RO-membrane system in the pre- treatment facilities. The organic content of the source seawater could be reduced effectively by one or more of the following approaches: (1) biodegradation upstream of the SWRO system; (2) coagulation; (3) adsorption of organics upstream of the SWRO system; and (4) gentle removal of algal biomass from the source water by DAF and/or gravity filtration.
12.5.1.1 Biodegradation of Organics Upstream of the RO System
Source-water organics biodegradation and removal upstream of the RO system are the most efficient of the four approaches listed above, because the total content of organics could