Dynamic evolution of membrane biofouling in feed channels affected by spacer-membrane clearance and the induced hydrodynamic conditions

Membrane biofouling is regarded as a Gordian knot in applying reverse osmosis and nanofiltration for water treatment. It occurs in the feed spacer channels of spiral wound membrane elements. The influence of feed spacer structure on hydrodynamic condition and biofouling development remains yet to be fully understood. This study aims to quantify the dynamic evolution and spatial distribution of biomass in feed spacer channels, and reveal the great importance of spacer-membrane clearance in the biofouling development. Four kinds of feed spacers with the same thickness (28 mil) but different clearance width, by varying filament diameter (0.4, 0.45, 0.55 and 0.6 mm), were used. A relatively long-term (20 d) biofouling experiment was conducted by using a feed solution of lower bacterial load and nutrient dosage. Results showed that the shear stress effect dominated the early stage of biofouling as biofilm was first attached to the low-shear regions, regardless of the spacer-membrane clearance width. In the middle and late stage, the blocking effect emerged with the continuous accumulation of biomass, as was especially the case in the feed channel with narrow spacer-membrane clearance. In this period, biofilm was prone to deposit and block the narrow flow passage between the spacer filaments and membrane surface, which decreased the uniformity of flow field, expanded the flow dead zone area, boosted the feed channel pressure (FCP) drop, and resultantly accelerated the rate of membrane biofouling. Enhancement of the shear stress effect under an appropriate range of spacer-membrane clearance width and thus channel porosity, as well as customed design of feed spacer geometries according to specific fouling locations are suggested for the future optimization of feed spacer.

Automated summary

The influence of feed spacer structure on biofouling development was investigated, and it was found that the spacer-membrane clearance width plays a crucial role in the biomass accumulation and channel blockage. In the early stage, biofilm attached to low-shear regions, while in the middle and late stage, narrow clearance width led to flow passage blockage and accelerated membrane biofouling. Future optimization of feed spacer design should focus on enhancing the shear stress effect and customizing spacer geometries based on specific fouling locations.