Activated sludge is a complex mixture of solid, colloidal and soluble components. Their respective contributions to membrane fouling process in membrane bioreactor (MBR) have been investigated by the fractioning method. The synergistic effects among these components on membrane fouling are neglected by using the method. A mathematical model was developed to simulate fouling that included the mixed liquor suspended solids (MLSS), soluble and colloidal components, the activated sludge floc size distribution (FSD) and aeration intensity. The membrane fouling was divided into cake layer formation and pore fouling. The cake layer is assumed to be formatted by MLSS and consolidated by the entrapment of colloidal components, resulting in the decreasing in cake porosity and increasing in specific cake resistance. The addition of activated sludge FSD in the model development allows the probability and deposition rate of individual solid sizes to be considered. The developed model was calibrated and validated using experimental data from different aeration intensities. It provides a valuable tool for analyzing the synergistic fouling effects from solids, colloidal and soluble components within activated sludge. (C) 2012 Elsevier B.V. All rights reserved.