Impacts of inorganic draw solutes on the performance of thin-film composite forward osmosis membrane in a microfiltration assisted anaerobic osmotic membrane bioreactor

Abstract

The influences of inorganic draw solutes on the performance of the thin-film composite forward osmosis (TFC–FO) membrane in microfiltration (MF) assisted anaerobic osmotic membrane bioreactors (AnMF–OMBRs) were investigated in this study. The results indicated that compared to sodium chloride (NaCl) at the same osmotic pressure, magnesium chloride (MgCl₂) led to a higher flux decline of the TFC–FO membrane, induced by more severe membrane fouling. In addition, the NaCl and MgCl₂ had no impacts on the rejection for organic matters by the TFC–FO membrane. However, the NH₄⁺–N rejection of TFC–FO membrane was neglected in the AnMF–OMBR with NaCl as draw solute, while it was enhanced to a range of 57.5–87.6% for the draw solute MgCl₂. The different NH₄⁺–N rejection and membrane fouling of TFC–FO membrane with draw solutes NaCl and MgCl₂ could be attributed to the Donnan potential. As for NaCl, more Na⁺ diffused into the mixed liquor resulting in NH₄⁺–N passing through the TFC–FO membrane to the draw solution for keeping a charge balance. With regard to MgCl₂, more Cl⁻ passing through the FO membrane to the mixed liquor led to an accumulation of NH₄⁺–N in the reactor. Moreover, Mg²⁺ passing from the draw solution to the mixed liquor enhanced the biofouling on the active layer of the FO membrane, and in the meanwhile more anions passing to the draw solution aggravated the inorganic fouling of the support layer.