Fouling in submerged membrane bioreactors (MBRs) was studied under two calcium levels (280 and 830 mg/L). Two MBRs were operated on a synthetic municipal wastewater at a solids retention time (SRT) of 15 days, a control at an influent calcium level of around 35 mg/L and a test reactor at two influent calcium concentrations of 280 and 830 mg/L. Permeability, soluble extracellular polymeric substances (EPS), Ca2+ concentration and particle size distribution were measured regularly in both reactors to investigate the cause of membrane fouling. The test reactor showed 35% higher permeability than the control at 280 mg/L of calcium. However, at 830 mg Ca2+/L feed, the permeability in the test reactor stabilized at 0.15 LMH/kPa approximately 50% lower than the control. The lower membrane fouling rate in the Ca2+ fed MBR at an influent calcium concentration of similar to 280 mg/L can be attributed to cationic bridges with EPS within the floc. Thus, the reduction in membrane permeability at higher calcium concentration (similar to 830 mg/L) was the result of very high Ca2+ concentrations in the sludge that contributed to significant inorganic fouling. (C) 2008 Elsevier B.V. All rights reserved.