Application of flux enhancers might be an effective alternative to retard fouling in membrane bioreactors (MBRs). In a comprehensive study, the specific removal of different organic fractions (humic substances, organic acids, biopolymers and SMP) from sludge supernatant and the impact on filtration performance was investigated in lab and pilot scale for two cationic polymers, two powdered activated carbons (lab tests only) and a starch. It was found that the activated carbons are able to remove the whole range of molecular weight compounds, while the polymers and the starch eliminated mainly larger molecules (biopolymers). Good removal of soluble microbial products (SMP) was achieved by both flocculants and activated carbons. The capillary suction time (CST) was reduced by all additives. The two polymers and the starch were then studied over several months in a pilot MBR plant. They had no negative impact on the biological performance in terms of COD- and N-elimination. While both polymers increased the particle size by 17-19% and showed a positive effect on membrane performance, the starch led to an accelerated fouling behaviour and only slightly increased floc sizes (+6.5%). The improvements found in lab tests were much higher than the effects in the pilot system. (C) 2009 Elsevier B.V. All rights reserved.