Role of Transmembrane Pressure and Water Flux in Reverse Osmosis Composite Membrane Compaction and Performance

This study explores the compaction behavior of thin-film composite reverse osmosis (TFC RO) membranes for different combinations of transmembrane pressure (TMP) and transmembrane water flux. Operating a crossflow system at constant feed pressure (60 bar) but different feed solution osmotic pressures enabled adjusting the TMP─the difference between hydraulic and osmotic pressure─and water flux. The extent of membrane compaction increases as TMP (and flux) increases. Both commercial and hand-cast TFC RO membranes showed substantial compaction at high TMP (up to 30% compaction at 50 bar TMP) compared to less than 10% at 10 bar TMP. Scanning electron microscope (SEM) images reveal a direct relationship between TMP and polysulfone (PSU) support layer compaction, while molecular dynamics (MD) simulations confirmed decreased porosity and reduced thickness in the polyamide (PA) active layer as TMP increases. Combined findings from wet-testing and MD simulations confirm a hydraulic pressure drop occurs across both the PA active layer and the meso-to-macro-porous support layer; higher TMP exacerbates compaction in both layers resulting in lower water permeability but higher water flux, observed salt rejection, and salt permeability. Transitioning from high TMP to low TMP or vice versa did not notably alter the extent of membrane compaction. This observation is attributed to the highly cross-linked PA active layer's ability to recover after pressure is released, whereas the compaction in the PSU support layer is largely irreversible. While TMP dictates the overall pressure gradient, our findings suggest that flux-induced frictional forces play a crucial role in compaction dynamics. Specifically, higher flux generates additional drag forces on the polymer matrix of both the PSU support layer and the PA selective layer, intensifying structural deformation. Overall, our findings offer critical insights into the mechanisms of membrane compaction, providing a foundation for optimizing RO membrane performance and advancing next-generation membrane technologies.