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Membrane fouling causing increased feed channel pressure loss and flux reduction seriouslyhampers the application of spiral-wound RO and NF membranes for the production of waterintended for human consumption or industrial purposes. In many cases accumulation ofmicrobial biomass in the membrane elements (biofouling) is the main fouling process. Therate and extent of biofouling depend on the rate of multiplication of bacteria in the membraneelements, which in turn depends on the concentration of organic and inorganic nutrients inthe feed water. The concentration and nature of growth-promoting (biodegradable)compounds is defined by several factors that include: the nature of the raw water; the effects of treatmentprocesses applied prior to membrane filtration (pretreatment); and, the effects ofchemicals added to the feed water. In many cases surface water serves as raw water, but alsoseawater and groundwater are used. Surface water contains biodegradable compoundsoriginating from contamination with (treated) wastewater. Furthermore, growth of algae orcyanobacteria leads to the production of easily biodegradable compounds in surface waterand in seawater. Treatment may lead to a reduction of the concentration of biodegradablecompounds by physicochemical processes, e.g. coagulation/sedimentation (CS) orultrafiltration (UF) for the removal of large molecular-weight compounds and particles,including microorganisms and by biological processes, e.g. sand filtration or granularactivated carbon filtration. Application of oxidative processes, e.g. ozonation, leads to theformation of easily biodegradable low molecular-weight compounds from refractory humicand fulvic acids. Furthermore, addition of chemicals for scaling prevention, includinginorganic acids and organic antiscalants (AS) may increase the concentration ofbiodegradable compounds (Hiemstra et al. 1997; Van der Hoek et al. 2000). Consequently,different raw water types and different water treatment schemes, including additives, lead toa large range of feed water qualities with different growth-promoting properties, i.e.biofouling characteristics. Elucidation of the effects of water composition, water treatmentand addition of chemicals is required to enable decisions regarding the prevention ofbiofouling. For this purpose, bioassays for characterization and quantification of the growth-promoting properties of water in various stages can be used. A study on the growth-promotingproperties of a number of AS types demonstrated that these chemicals differedstrongly in the potential to produce biomass in batch tests or biofilms in a dynamic test(Vrouwenvelder et al. 2000). This study demonstrates that AS dosage may also increase theconcentration of available phosphorus (P). Includes 10 references, tables, figures.

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Edition: Vol. - No. Published: 03/01/2007 Number of Pages: 6File Size: 1 file , 300 KB