In this paper, membrane fouling in three parallel MBRs operated under different aeration intensities (150, 400 and 800L/h) was studied to have a better understanding of the membrane fouling mechanism. The impact of aeration on membrane fouling was interpreted from two aspects: evolution of biomass characteristics and formation mechanism of the cake layer. The results showed that either small or large aeration intensity had a negative influence on membrane permeability. The large aeration intensity resulted in a severe breakup of sludge flocs, and promoted the release of colloids and solutes from the microbial flocs to the bulk solution. The sludge supernatant would become heterogeneous as the aeration intensity increased. As the MBR operated under high aeration intensity of 800L/h, colloids and solutes became the major foulants. In addition., the back transport mechanism of membrane foulants in the three MBRs was different from each other. Aeration had a positive effect on cake layer removal, but pore blocking became severe as aeration intensity increased to 800 L/h. The main components of organic matters in the membrane foulants were identified as proteins, polysaccharide materials and lipids by the Fourier transform infrared spectroscopy (FTIR). (C) 2007 Published by Elsevier B.V.