Impacts of Calcium on the Forward Osmosis Membrane Fouling in Osmotic Membrane Bioreactors Treating Municipal Wastewater

Membrane fouling of forward osmosis (FO) membrane remains as one of major obstacles of osmotic membrane bioreactors (OMBRs); however, the literature on calcium induced inorganic fouling of FO membrane is still limited. In order to better understand the fouling mechanisms of FO membrane in OMBRs, it is necessary to investigate the impacts of calcium on FO membrane fouling. Based on the possible interference of high salinity, novel OMBRs combined with micro-filtration (MF) membrane (MF-OMBRs) for salinity control were applied in this study to evaluate the impacts of high calcium concentration on FO membrane fouling. The results indicated that the calcium concentration of 12.5 mM had no impacts on the removals of total organic carbon (TOC) and ammonium nitrogen (NH3- N) in MF-OMBRs, but resulting in a serious flux decline of FO membrane from approximately 9.2 LMH to 5 LMH. The high calcium concentration level could reduce the thickness of biofouling layer and the adsorption of EPS to the FO membrane surface due to the EPS decrease in the activated sludge through the bridging effect of Ca2+, thus mitigating the biofouling of FO membrane in MF-OMBRs. However, due to the severer calcium induced inorganic fouling at Ca2+ concentration of 12.5 mM, the high calcium concentration level would absolutely deteriorate the FO membrane fouling including reversible and irreversible fouling in MF-OMBRs.