Laboratory-scale filtration tests utilizing wastewater treatment plants (WWTP) effluent were conducted to investigate fouling and filtration behaviour, especially the influence of colloidal stability on nanofiltration performance. Acidification and coagulant dosage were used to create unstable colloidal conditions. Colloidal stability of the effluent was analysed on by zeta potential measurements. A statistical design method, full factorial design with blocking, has been used to account for effluent composition variations and to account for interaction between experimental parameters. The results show a high correlation between unstable colloidal natural organic matter and the formation of dense fouling layers. The reversibility of the fouling process was shown to be independent from colloidal stability. Irreversible fouling was show to be promoted by mild acidic (pH 5) effluent conditions, whereas coagulant addition showed an increase in the reversibility of the fouling.