High-Pressure Nanofiltration for Brine Concentration: The Critical Role of Support-Layer Compaction

Nanofiltration (NF) membranes have emerged as promising candidates for use in low-salt-rejection reverse osmosis (LSRRO) for high-salinity brine concentration, yet their performance under high operating pressures remains poorly understood. Here, we systematically evaluate the feasibility of NF membranes for LSRRO under elevated pressures using a commercial state-of-the-art membrane. Experimental results and structural characterization show that, while the selective active layer remains structurally and functionally stable, the porous support layer undergoes severe compaction, leading to a pronounced decline in water permeance. To elucidate this behavior, we develop a framework that integrates polymer-network thermodynamics, finite-deformation kinematics, and porous-media transport. The framework quantitatively predicts the pressure-dependent permeance of the membrane and clarifies transport limitations under high-pressure operation. Sensitivity analysis further reveals that membrane performance at high pressures is governed primarily by the mechanical robustness of the support layer, rather than its initial porosity or hydrophilicity. Our results indicate that membrane design for LSRRO should prioritize resistance to compaction. Overall, this study establishes a predictive and mechanistically informed framework for evaluating and designing NF membranes for minimal or zero liquid discharge (MLD/ZLD) applications.