A laboratory-scale submerged membrane bioreactor (MBR) for the treatment of synthetic wastewater containing 200 mg/L Ca(2+) was operated under sludge retention time (SRT) of 10, 20 and 30 d in this study in order to verify the impacts of SRT on the performance of MBR with the addition of Ca(2+). The results showed that the effluent quality was slightly influenced by SRT, while membrane fouling was effectively mitigated by extending SRT with the addition of Ca(2+) due to the decline of fouling layer resistance. The concentration of soluble microbial products (SMP) significantly decreased as SRT prolonged from 10 to 30 d, which correlated well with the variation of membrane fouling. SMP and particle size played a more important role in membrane fouling in this study compared to other sludge properties. The Ca(2+) accumulated in MBR as the SRT extended, and the accumulation of Ca(2+) enhanced bioflocculation and resulted in the decrease of MLVSS/MLSS ratio and the increase of the average particle size. The accumulated Ca(2+) in mixed liquor was mainly located in bound extracellular polymeric substances (BEPS). It might be indicated that the negatively functional groups such as polysaccharide in SMP were adsorbed to BEPS by the bridging effects of Ca(2+), which led to the reduction of SMP and increase of BEPS at longer SRT. Crown Copyright (C) 2011 Published by Elsevier B.V. All rights reserved.