Toward Rational Design of Low-Salt-Rejection Reverse Osmosis for Minimal and Zero Liquid Discharge

Low-salt-rejection reverse osmosis (LSRRO) has emerged as a promising technology for concentrating high-salinity brines in minimal- and zero-liquid-discharge (MLD/ZLD) applications. However, rational LSRRO design remains hindered by uncertainties about how existing modeling frameworks should be interpreted and applied. In this Perspective, we critically examine the limitations of process modeling and module-scale analysis in LSRRO design. Conventional process models can yield misleading predictions for spiral-wound LSRRO modules because the stagewise concentration cutoff embedded in these models is based on the cumulative permeate composition rather than the local permeate conditions that govern water transport. Experimental results and local transport calculations show that the brine concentration in spiral-wound LSRRO stages is not constrained by a fixed thermodynamic concentration cutoff. Nevertheless, standalone process models remain inherently limited in assessing practical feasibility because they do not resolve kinetic constraints, such as declining water flux along the module. Module-scale analysis is therefore essential for linking local transport behavior to practical design metrics, including membrane area requirements, productivity, and achievable brine concentration. However, its predictive accuracy is constrained because transport parameters derived from conventional local transport models vary with salinity, which challenges their use as constants across the module. This Perspective therefore highlights the need for condition-dependent and experimentally validated local transport descriptions to enable rational LSRRO design for future MLD/ZLD applications.