Within this research project different treatments (ozonation, coagulation) of secondary effluent and the fouling behavior during subsequent ultrafiltration were investigated at labscale. Coagulation with 4 mg Fe3+/L leads to a significant removal of fouling resistance and moreover pre-ozonation up to a dosage of 15 mg O3/L can clearly enhance the filtration process. In contrast rising ozone dosages produce more hydraulically irreversible fouling. The subsequent coagulation can only compensate ozone-induced stronger irreversible fouling to some extent.