BACKGROUND: Membrane scaling is an area of research interest because it can deteriorate membrane performance. The extent to which membrane scaling is produced varies depending upon the concentration of scale-forming species such as calcium on the membrane surface. Bench-scale tests have been conducted to better understand membrane scaling in submerged membrane reactors (MBR). However, relatively few studies of membrane scaling in pilot-scale, submerged MBR have been reported. The objective of this study was to perform membrane autopsy work to analyze membrane scaling in a submerged MBR treating calcium-rich wastewater. RESULTS: Membrane autopsy work provided evidence that deposition of calcium carbonate (CaCO3) scale occurred on the membrane surface at the completion of pilot-scale, submerged MBR operation. The CaCO3 scaling resulted in significant external fouling on the surface of the membrane. The membrane scaling increased the rejection of calcium with MF membranes. However, the level of CaCO3 scaling as internal fouling (in the pores) was almost negligible. This autopsy work also showed that aeration did not play a major role in controlling CaCO3 scaling at the membrane surface in a submerged MBR. Chemical cleaning using citric acid solution efficiently removed CaCO3 scale from the membrane. Combining citric acid with sodium hypochlorite pretreatment provided synergistic effects, further reducing CaCO3 scale formation. CONCLUSION: The carbonate salt of calcium leads to precipitation resulting in surface fouling of membranes, and this cannot be removed physically by aeration in a submerged MBR treating calcium-rich wastewater. It is necessary to combine properly-selected cleaning strategies with submerged MBR treating wastewater containing a high potential for inorganic chemical precipitates. (c) 2009 Society of Chemical Industry