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2.6 MICROBIAL FOULANTS 31
In order for biological fouling to occur, aquatic bacteria need to have suitable low flow- through velocity conditions so they can attach to the RO membrane surface or to the surface of facilities upstream of the membrane system, such as cartridge filters. Besides on the desa- lination plant cartridge filters and RO membranes, bacteria would also attach to surfaces with low-velocity conditions such as cavities with stagnant water or high-retention time vessels along the feed water route to the RO membrane system (e.g., dead-end valves and pipes, pipe fittings, and oversized or hydraulically flawed cartridge filter housings).
The formation of a permanent biofilm layer occurs when membrane flux (flow through the membranes) exceeds a certain level (critical flux) at which cross-flow velocity through the membrane elements is low-enough to allow aquatic microorganisms to attach to the RO- membrane surface (Winters et al., 2007). The critical flux for aquatic bacteria is dependent upon the cross-flow velocity over the surface of the membranes and increases with the in- crease of this velocity. In practical terms, this means that the lower the permeate membrane flux (e.g., RO system recovery) for the same membrane feed flow, the higher the cross-flow velocity and the lower the rate of biofilm formation.
The critical flux is also a function of the concentration of active bacteria in the saline source water and it decreases as the concentration of bacteria rises. The concentration of active aquatic bacteria in turn mainly depends on the type of bacteria, the availability of easily biodegradable organic matter in the source saline water, and the water temperature. For a given RO system, decreasing the recovery from 50% to 35% would result in approximately two times lower fouling potential for the RO system operating in a typical flux range of 13.5e18.0 L/m2 h (8.0e10.5 g/ft2 day).
Although operation at low recovery may be attractive from the point of view of minimizing membrane biofouling, designing RO plants for low recovery is usually not cost-effective because of the associated increased size of the desalination plant intake, pretreatment, and RO systems, and the 30%e40% higher capital costs. Therefore, other approaches for biofouling reductiondsuch as control of the organic content in the source water and inactivation of aquatic bacteria by disinfection or UV irradiationdhave found wider practical application.
2.6.3 Factors Impacting Biofouling Potential of Saline Source Waters
The biofouling potential of a given saline source water depends on many factors, including:
1. Concentration and speciation of microorganisms contained in the source water;
2. Content of easily biodegradable compounds in the source water;
3. Concentration of nutrients and the balance (ratio) between organic compounds and the
biologically available nitrogen and phosphorus in the source water; and
4. Temperature of the saline source water.
2.6.3.1 Concentration and Speciation of Microorganisms
Most saline water sources contain microorganisms (bacteria, viruses, algae). The larger the concentration of microorganisms in the source water, the higher the potential for membrane biofouling, assuming that all other ambient conditions (e.g., food content, temperature, etc.) are conducive to biofouling. Shallow and warm surface waters may have several orders of