Eliminating adverse effects of fouling layer using composite membranes with dense surface layers for sustainable membrane distillation of high-strength wastewater

This study explored composite membranes (CMs) with dense and water-permeable surface layers for the deep-concentration of high-strength wastewater using membrane distillation (MD). Compared to conventional microporous MD membranes, coating a dense PVA surface layer avoided the detrimental effects of landfill leachate (LFL) foulants (e.g., membrane pore blockage and deformation) during MD concentration. The hydrophilicity and water/salt separation capability of surface layers enhanced the dispersion of water diffusion on membrane surfaces and altered the solute distribution on membrane cross-sections, resulting in inorganic crystallization mitigation and transforming the fouling layer structure to a more porous and water-permeable state. This ensured the membrane efficiency was little affected by membrane fouling even as LFL was highly concentrated (water flux decline: PVDF ∼49 % vs. CM ∼14 % as concentration factor increased to 20). The resistance of foulant intrusion and in-pore deposition, which easily occurred on microporous membranes, and the decrease in membrane-foulant interactions by the surface layer endowed the CM with a high foulant-release capability (clean efficiency: ∼97 %). The surface layer also exhibited an ammonia/water sieving effect which enhanced volatile ammonia rejection (∼1.6 times). This work provides insight into the construction of high-sustainable MD membranes for high-strength wastewater treatment, promoting the practical implementation of MD.