This study investigated membrane fouling mechanisms of various fractions of denitrifying granular sludge mixed liquors developed in three sequencing batch reactors (SBRs) operated at respective calcium concentrations of 0, 50 and 100 mg/L. Results showed that supernatants caused the lower fluxes and more severe membrane fouling than their corresponding mixed liquors and granule solutions, indicating supernatants were the major contributors to membrane fouling in microfiltration of granular sludge mixed liquors. Cake layer formed by denitrifying granules on membrane surface was observed to serve as a prefilter which could reduce membrane fouling effectively by entrapping fine particles, colloids and soluble extracellular polymeric substances (sEPS). Such observation was further confirmed by the fact that cake, specific cake and total resistances of mixed liquors all decreased with increase of granule concentrations from 0 to 5000 mg SS/L, and remained unchanged when granule concentrations were further increased to 13,000 mg SS/L. The resistances of granular sludge mixed liquors were increased with concentrations of fine particle, colloid and sEPS in supernatant when granule concentration was kept constant. It was also revealed that the supplementation of calcium of 100 mg/L in granulation process could greatly reduce the contents of fine particles, colloids and sEPS, leading to less membrane fouling. (C) 2012 Elsevier B. V. All rights reserved.