Understanding roughness - fouling relationship in reverse osmosis: Mechanism and implications.

This paper investigates the relationship between roughness and fouling in reverse osmosis (RO) through specially designed experimental protocols and computational fluid dynamics (CFD) studies. Conventional polyamide thin film composite (TFC) membranes with ridge-and-valley structure and an emerging type of smooth membranes are prepared. A wide range of foulants is used, and fouling under static, crossflow and RO conditions are tested in sequence. Feature size on membrane surface is important when foulants and microscale structure show similar size, and otherwise membrane-foulant interactions govern the static attachment. Under crossflow mode, fouling on ridge-and-valley surface is not reduced to the same extent as that on smooth membranes, with the insufficient vortex in the valley region being identified as the key factor by CFD studies. In RO, uneven flux distribution as confirmed by gold nanoparticle filtration is also found to account for the much higher fouling rate of conventional membranes. Our study then suggests two strategies to design next-generation fouling-resistant RO membranes via structural optimization: first, a smooth selective layer should be maintained to ensure uniform flux distribution; second, one may mimic nature to fabricate patterned porous membranes as the support, so that it optimizes hydrodynamics while maintaining even fluxes.