Brine-split osmotically assisted reverse osmosis for high-salinity brine concentration: stagewise projection and experimental evaluation

Osmotically assisted reverse osmosis (OARO) extends membrane-based brine concentration by reducing the effective osmotic pressure difference across the membrane. This study develops a process-level stagewise projection framework for brine-split OARO using a commercial Toyobo hollow-fibre membrane module and experimentally assesses selected local operating conditions. In this configuration, part of the final brine is redirected to the sweep side. The framework calculates feed- and sweep-side concentrations, flow rates, membrane-transferred water flow, salt transport and pressure requirements. The projection was applied to a 70 g/L NaCl feed at 230 L/h with a target concentration of 316 g/L. At a brine-split factor of 55%, the framework yielded a nine-stage configuration operating at about 67 bar with a feed-side pumping specific energy consumption of 2.40 kWh/m 3 of process feed. Seven- and eight-stage configurations reached approximately 291 and 307 g/L, respectively, while operating at the 70 bar module pressure limit. Increasing the brine-split factor from 45 to 60% reduced feed-side pumping specific energy consumption from 2.51 to 2.34 kWh/m 3 but increased sweep-outlet salinity from approximately 37 to 47 g/L. Six selected local stage conditions were reproduced experimentally using a single OARO module. The framework captured the main trends in membrane-transferred water flow, concentration development and flow redistribution, with a root-mean-square error of 3.24 L/h for water transfer. Deviations were larger in the high-concentration region. The results support the framework as a computationally efficient tool for preliminary stagewise sizing of brine-split OARO systems approaching NaCl saturation.