In work package 4 the influence of different treatments (ozonation, coagulation) on macromolecular organic substances (biopolymers) in secondary effluent and the effects on subsequent ultrafiltration were investigated at lab-scale. Furthermore, fouling mechanisms were intensively investigated and an analytical method was developed to observe the formation of ozonation by-products. Analyses with LC-OCD showed a significant reduction of major organic foulants (biopolymers) for coagulation while ozonation appears to transform macromolecules into compounds smaller than approx. 50 nm. With ultrafiltration tests (PES membranes) it could be shown that coagulation is capable to reduce total fouling resistance to some extent and additional ozonation can further enhance the membrane filtration process. However ozonation as a pretreatment step caused more irreversible fouling. The lowest irreversible fouling was achieved with coagulation. LC-OCD analyses showed that the transformation of organic matter by ozonation is mainly responsible for the observed increased irreversible fouling of ultrafiltration membranes. Tests with different membranes showed comparable results for pretreated secondary effluent concerning total fouling resistance. Total fouling resistance was reduced with additional ozonation compared to coagulation without ozonation. In contrast to the observations with all tested UF membranes, for the tested microfiltration membranes irreversible fouling was reduced with additional ozonation. In general, the pore size seems to be strongly influencing irreversible fouling if ozonation is used for pretreatment of membrane filtration. Intensive investigations of fouling mechanisms using filtration laws identified cake filtration as the dominant filtration process for coagulation while additional ozonation leads to increased pore blocking/in pore fouling. Experiments with secondary effluents from different sewage treatment plants in Berlin showed comparable fouling behavior for all observed pretreatments. Thus membrane filtration results generated with samples from WWTP Ruhleben seem to be transferable to other WWTPs in Berlin. MALDI-TOF-MS analyses of secondary effluent were not suitable to identify major organic foulants, neither in solution nor on top of the membrane after filtration. Consequently, MALDI-TOF-MS was primarily used for investigations of theoretical aspects of fouling by using model fouling substances. An analytical procedure for bromate was successfully developed with LC-MS/MS at TUB. With the procedure it was possible to quantify samples up to a limit of quantification of 0.5 µg bromate per liter. Higher concentrations of bromate (> 10 µg/L) were produced only at specific ozone consumptions higher than 0.9 mgO3/mgDOC0.