A novel fouling control strategy for forward osmosis membrane during sludge thickening via self-forming protective layer

Severe membrane fouling significantly limited forward osmosis (FO) process for sludge thickening (FST). Here, a novel antifouling strategy by loading a self-forming protective layer on FO membrane surface was proposed in this study. The protective layer was coated on FO membrane surface via a short-term FO process using activated sludge as the feed solution. Results indicated that the self-forming protective layer made FO membrane surface more negatively charged and more hydrophilic. As for the control FO (C-FO) membrane, the water flux decreased approximately 66% of the initial flux in Cycle 2 of FST process and thus resulting in the sludge concentration only increased to 40 g/L. In contrast, the flux decline rate of modified FO (M-FO) membrane was about 22%, and the corresponding sludge concentration still rose to 50 g/L. It suggested that the M-FO membrane with a protective layer had a better sludge thickening efficiency and a lower flux decline during sludge thickening compared to the C-FO membrane. Furthermore, less deposited foulants and better fouling reversibility were observed for M-FO membrane after two cycles of sludge thickening, implying that the protective layer effectively mitigated FO membrane fouling during sludge thickening. This phenomenon could be attributed to the excellent barrier effect of the protective layer and the enhanced surface hydrophilicity and negative charge, which improved the antifouling performance of the FO membrane. The findings of this study were conducive to better understanding of FO membrane fouling mechanisms and further development of fouling mitigation strategy via a protective layer.