Investigation of spatial heterogeneity in reverse osmosis membrane fouling during long-term operation of reclaimed water treatment

With the increasingly severe global water scarcity， the utilization of reclaimed water has become an effective strategy to ensure water supply. Reverse osmosis （RO） technology is crucial for reclaimed water production， but membrane fouling severely constrains its efficiency. To address the spatial heterogeneity of RO membrane fouling after long-term operation in dual-membrane reclaimed water treatment processes， this study employs multi-dimensional characterization techniques to reveal fouling characteristics and microbial community differences at the inlet （RO1） and outlet （RO2） ends of RO membranes after 3.5 years of operation. Long-term monitoring shows that the microfiltration-reverse osmosis （MF-RO） coupled process stabilizes effluent turbidity below 0.1 NTU and conductivity under 400 μS/cm. However， the RO system's inlet pressure exhibits significant seasonal fluctuations （15%~22% amplitude in summer and winter）， which may be closely correlated to water viscosity changes induced by temperature variations. Simultaneously， while membrane surface fouling is predominantly characterized by calcium sulfate crystallization and rod-shaped microbial symbiotic structures， significant spatial heterogeneity exists in biofouling. The inlet end （RO1） features dense bio-inorganic composite fouling dominated by Proteobacteria （77.11%）， particularly Alphaproteobacteria （71.49%） and Microbacteriaceae. In contrast， the outlet end （RO2） shows reduced microbial abundance and altered community structure， likely driven by salinity gradients along the flow direction. This study elucidates the spatial heterogeneity of microbial fouling and community dynamics in long-term RO operation， providing critical theoretical and practical insights for optimizing RO management and developing targeted antifouling strategies in reclaimed water plants.