Increased Support Layer Hydraulic Resistance Partially Explains Permeability Loss in High-Pressure Reverse Osmosis

High-pressure reverse osmosis (HPRO) is emerging as a critical technology for energy-efficient minimal- and zero-liquid-discharge (MLD/ZLD) treatment of hypersaline brines. However, membrane compaction at elevated pressures substantially reduces water permeability, and the mechanisms governing this loss remain incompletely understood. Here, we investigate compaction-induced transport and structural changes in a commercial seawater thin-film composite (TFC) membrane (Veolia AD series) operated up to 150 bar. Water permeability declined from 2.44 to 0.96 L m-2 h-1 bar-1 (61% reduction). Selective removal of the polyamide active layer enabled direct measurement of compacted support permeability. Despite a substantial drop (90% reduction) in support permeability, a resistance-in-series analysis suggested that bulk support densification accounted for only 32% of the total TFC permeability decline. Incorporating the effects of 20-30% elastic deformation, measured through uniaxial compression testing, increased the estimated support-layer contribution but still could not fully explain the observed permeability decline, suggesting a persistent resistance beyond bulk deformation. Support surface characterization revealed a 41% decrease in mean surface pore diameter. A resistance analysis suggests that residual permeability loss (∼57% at 100 bar) may stem from polyamide nodules becoming disconnected from underlying open pores (i.e., increased interfacial resistance between the selective and support layers), though direct experimental confirmation of this mechanism remains a direction for future work. These findings suggest that permeability loss under HPRO compaction is governed by both support hydraulic resistance and an interlayer transport limitation, requiring membrane designs that preserve bulk integrity and surface pore accessibility under high pressure.