Spatial Heterogeneity of Reverse Osmosis Membrane Fouling During Long-Term Reclaimed Water Treatment: A 3.5-Year Field Study
Reverse osmosis (RO) membrane fouling remains a critical bottleneck in reclaimed water production, yet its spatial heterogeneity over extended operation is poorly understood. This study investigated fouling characteristics and microbial community dynamics on RO membranes after 3.5 years of operation in a full-scale microfiltration-reverse osmosis (MF-RO) system treating reclaimed water. Long-term monitoring showed stable effluent quality (turbidity <0.1 NTU, conductivity <400 μS/cm), but RO inlet pressure exhibited seasonal fluctuations of 15%–22% between summer and winter, attributed to water viscosity changes. Membrane autopsies revealed distinct fouling layers at the inlet (RO1) and outlet (RO2) ends. RO1 featured a dense bio-inorganic composite fouling layer with CaSO4 crystals and rod-shaped microbial aggregates (5–10 μm), dominated by Proteobacteria (77.11%), particularly Alphaproteobacteria (71.49%) and Xanthobacteraceae (35.29%), which secreted extracellular polymeric substances (EPS) to form biofilms. In contrast, RO2, exposed to higher salinity, showed reduced microbial abundance (Proteobacteria decreased to 64.79%) and a shift toward halotolerant taxa, including Microbacteriaceae (23.73%) and Actinobacteriota (24.76%), with EPS secretion increased by 42%. Alphaproteobacteria relative abundance dropped by 19.3%, while Gammaproteobacteria rose to 12.54%. These findings elucidate salinity-driven microbial succession and spatial heterogeneity of fouling, providing a basis for targeted antifouling strategies and 'zonal-graded' cleaning protocols in reclaimed water plants.