Characterization of reverse osmosis membranes over their lifespan: Pre-operational defects, surface scaling, and cross-sectional morphological changes

Over the operational lifespan of reverse osmosis (RO) membranes, changes at the membrane surface and within the membrane matrix contribute to gradual performance deterioration. These performance changes are commonly evaluated through loss of salt rejection and water flux, but such operational metrics provide limited insight into the underlying morphology and can overlook pre-operational defects introduced during membrane fabrication and module embodiment. In this study, multi-scale RO membrane characterization was performed using pristine flat-sheet membranes, unused embodied membranes, and field-operated spiral-wound membranes after one, two, and four years of service in a potable water reuse facility. Surface morphology and chemistry were examined using scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and atomic force microscopy; cross-sectional morphology was characterized using focused ion-beam SEM. It was found that module embodiment introduced localized surface defects including spacer imprints and dents. Spacer-imprinted regions showed selective-layer densification with an approximately 67% reduction in void-space diameter; however, overall membrane thickness, selective-layer thickness, and surface roughness were not measurably altered. Residual silica scaling, cluster formation, crack propagation, and surface roughening were observed in a nonuniform manner across the module. EDS mapping showed sulfur exposure in cracked regions, indicating possible loss of selective-layer integrity. Selective layer void-space diameter decreased by approximately 26% in the unused membrane than in the pristine membrane. Overall membrane compaction reached approximately 23% after four years of operation and became more uniform across the module regions over time. The greatest overall membrane compaction occurred during the first year of operation. These results show that RO membrane degradation includes region-dependent surface degradation and more uniform cross-sectional compaction. The membrane degradation results were also consistent with field performance data, which showed a progressive decline in salt rejection and increased salt passage after clean-in-place events during long-term operation.