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I II III Risk of biofouling Low Moderate / Unclear High G rowth potential of a ntiscalants A O C 1 and BPP 2 <0.1 >0.1-<1 >1 1, AOC expressed as lag C/mg AS; 2, BPP expressed as ng ATP/mg AS T able 3 proposes a classification for AS p roducts with respect to the risk of causing biob iofouling in three classes on the basis of the g rowth potential. AS types in class I do not cause o perational problems such as increased NPD or r educed MTC values and include AS M, N and O , respectively. Class III includes AS types c ausing severe biofouling problems. The position o f the AS types in class II is not yet clear. For p revention of biofouling AS types with very low v alues for AOC, BPP and BFR are preferred. F eed water itself may play a major role in ( bio)fouling and extended pretreated is needed to r emove biodegradable compounds to a low c oncentration. Water treatment with ozone i ncreases the growth potential of the water and b iological filtration generally reduces the AOC v alues [7]. The optimum pretreatment for a s pecific situation also depends on treatment costs a nd the costs of operating the membrane f iltration unit, including cleaning procedures. At t his stage it is not possible to define the required p retreatment. The AOC and BFR test, which can b e used for (semi) continuous monitoring, are u seful tools to detect effects of water quality, s easonal effects and fluctuations in treatment and t o define the relationship between water quality a nd rate of biofouling. Such studies may lead to q uantitative criteria for the biofouling potential T he results presented in this paper lead to the f ollowing conclusions: (i) Chemicals used to prevent scaling differ g reatly in their ability to promote growth of m icroorganisms. (ii) The described AOC, BPP and BFR tests can b e used to determine the biofouling potent ial of chemicals including organic antis calants and mineral acids, but also the feed w ater. DOC analysis is not suited for d etermining the growth potential. (iii) Low concentrations of nutrients (btg/l) in the f eed water can lead to biofouling. ( iv) Classes of antiscalants with respect to risks o n biofouling are proposed, but further i nvestigations are needed to substantiate and e ventually refine this classification.