Membrane autopsy to investigate CaCO₃ scale formation in pilot-scale, submerged membrane bioreactor treating calcium-rich wastewater

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 themembrane. Themembrane 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.