In this study, the impact of sludge characteristics on membrane fouling in a submerged membrane bioreactor (MBR) was investigated. Bulking sludge due to excess growth of filamentous bacteria was changed to normal sludge by use of an aerobic selector. Excellent effluent quality was achieved in the MBR regardless of the quality of the sludge of the bioreactor. The removal rates of total organic carbon (TOC) and ammonia nitrogen were maintained at around 98 and 99%, respectively, during the entire experiment. However, serious fouling was observed during the occurrence of the bulking sludge despite having larger particle size distribution. Capillary suction time (CST) was found highly related to the amount of soluble microbial products (SMP) and membrane fouling. Therefore, it could be used as a simple indicator for SMP and fouling potential. Filamentous bacteria were found to produce more SMP including soluble polysaccharides and soluble proteins in the mixed liquor, which resulted in severe fouling. In particular, the release of high concentration of soluble polysaccharides resulted in serious fouling in bulking sludge. Bound EPS which was found similar in the normal sludge and bulking sludge was not the main cause for membrane fouling. In conclusion, solutes in mixed liquor such as SMP are responsible for membrane fouling, which cannot be prevented by increasing shear stress. (C) 2009 Elsevier B.V. All rights reserved.