Investigating the impact of low-pressure reverse osmosis membrane aging and heterogeneity on performance: A study of virus and microplastic retention

The increasing demand for freshwater, alongside increasing microplastics and persistent microbiological contaminants, is degrading water quality and highlighting the need for advanced treatment processes with better retention performance. Low-pressure reverse osmosis (LPRO) presents a promising solution for producing drinking water from freshwater, offering an optimal balance between water quality and production costs. However, uncertainties remain regarding the long-term evolution of these performance metrics, which are primarily based on data from reverse osmosis membranes used in seawater desalination. This study investigates the impact of membrane aging on LPRO performance and the role of membrane heterogeneity within a spiral-wound module. New and long-term aged membranes were compared in terms of permeability, salt retention, and virus retention, with additional analysis of microplastic retention. Natural membrane aging led to increased permeability and decreased salt retention, without affecting virus and microplastic retention, for which retention remained total across the tested coupons. Performance heterogeneity was observed depending on the position of the coupons within the module, with a more significant degradation at the beginning of the module and winding, likely due to mechanical effects associated with higher pressure and deformation. In contrast, coupons located at the end of the module and winding showed minimal performance impact. No impact was observed on virus and microplastic retention, which remained complete, confirming that viruses do not pass through an intact membrane but rather through membrane defects and seals. Autopsy of the LPRO modules revealed membrane defects (glue patches, minor and major folds) that affected membrane performance. In particular, the presence of apparent major folds led to significant performance degradation, with lower permeability and salt retention. The folds also caused the passage of viruses into the permeate, while microplastic retention remained unaffected. The virus passing through the permeate of an LPRO pilot would therefore be due to defects in the membrane and O-rings of the modules.