Membrane fouling causing increased feed channel pressure loss and flux reduction seriously hampers the application of spiral-wound RO and NF membranes for the production of water intended for human consumption or industrial purposes. In many cases accumulation of microbial biomass in the membrane elements (biofouling) is the main fouling process. The rate and extent of biofouling depend on the rate of multiplication of bacteria in the membrane elements, which in turn depends on the concentration of organic and inorganic nutrients in the 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 treatment processes applied prior to membrane filtration (pretreatment); and, the effects of chemicals added to the feed water. In many cases surface water serves as raw water, but also seawater and groundwater are used. Surface water contains biodegradable compounds originating from contamination with (treated) wastewater. Furthermore, growth of algae or cyanobacteria leads to the production of easily biodegradable compounds in surface water and in seawater. Treatment may lead to a reduction of the concentration of biodegradable compounds by physicochemical processes, e.g. coagulation/sedimentation (CS) or ultrafiltration (UF) for the removal of large molecular-weight compounds and particles, including microorganisms and by biological processes, e.g. sand filtration or granular activated carbon filtration. Application of oxidative processes, e.g. ozonation, leads to the formation of easily biodegradable low molecular-weight compounds from refractory humic and fulvic acids. Furthermore, addition of chemicals for scaling prevention, including inorganic acids and organic antiscalants (AS) may increase the concentration of biodegradable compounds (Hiemstra et al. 1997; Van der Hoek et al. 2000). Consequently, different raw water types and different water treatment schemes, including additives, lead to a large range of feed water qualities with different growth-promoting properties, i.e. biofouling characteristics. Elucidation of the effects of water composition, water treatment and addition of chemicals is required to enable decisions regarding the prevention of biofouling. 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-promoting properties of a number of AS types demonstrated that these chemicals differed strongly 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 the concentration of available phosphorus (P). Includes 10 references, tables, figures.