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12.5 ALTERNATIVES FOR CONTROL OF MICROBIAL FOULING 273
be reduced below the threshold level of 1 mg/L of TOC, which the rate of membrane biofouling slows down significantly.
Biodegradation could be achieved either using deep-gravity granular media filters designed to be operated as biofilters, or in a membrane filtration system configured to function as a mem- brane bioreactor (MBR) rather than as a particulate removal system. At present, biofiltration in gravity granular media filters is a well-understood process, and this pretreatment technology is widely used and viable. For comparison, the development of MBR technology for seawater pre- treatment is in its infancy and at present is not practiced in full-scale desalination plants.
The biodegradation approach for control of RO-membrane biofouling is based on the fact that if the easily biodegradable organics contained in the saline source water are consumed by the microorganisms in the pretreatment system, then this food source would no longer be available for bacteria to grow on the RO-membrane surface and to form sustainable bio- film, and therefore to cause biofouling.
It should be pointed out that biodegradation of seawater organics practically always oc- curs in conventional granular media filtration systems, but unless these filters are designed to provide adequate contact time and favorable conditions for steady biogrowth, the removal of easily biodegradable organics in gravity filters is typically lower than 30% (measured at TOC reduction). Gravity filters designed for controlled biofiltration could yield organics removal of over 60%, which in the case of moderate or low-intensity algal blooms will be adequate to sustain stable and reliable performance of the RO system.
For comparison, UF and MF membrane pretreatment systems yield significantly lower organic removal efficiency (10%e15% of TOC reduction) because they are designed for a very short con- tact time and for frequent and vigorous agitation of the membrane surface, which does not allow significant biomass to be formed and maintained in the pretreatment membrane modules.
12.5.1.2 Coagulation
As discussed in Chapter 6, coagulation allows to enhance the removal of particulate organic matter in the saline-source water by agglomeration of the organic particles and their subsequent removal by sedimentation and/or filtration. Removal of source-seawater organics is usually be- tween 5% and 20% (measured as reduction of the TOC concentration). In most cases however, such removal would not allow to bring the concentration of TOC in the feed water to the RO system below 1 mg/L and, therefore, to prevent accelerated biofouling of the RO membranes. Coagulant addition could be beneficial for both granular media and membrane filtration, espe- cially if the organics in the source water mainly consist of NOM.
However, coagulant addition is not likely to enhance the removal of dissolved organics such as these that typically occur during algal blooms.
The MF and UF systems typically remove only organics in particulate form. Because dur- ing algal blooms over 85% of the organics in the source water are usually in dissolved form, existing MF and UF systems currently available on the market are ineffective for controlling RO-membrane biofouling. Practical experience from full-scale installations shows that these systems could provide higher removal of organics by enhanced coagulation, but only if the main type of organics in the saline source water is NOM. In most SWRO desalination plants with open intakes, however, NOM contributes only very small portion of the organic sub- stances in the waterdthe main source of organics are algae, dead and living bacteria, and exopolymer products excreted from bacteria.