Maximum allowable retention for low-salt-rejection reverse osmosis membranes and its effect on concentrating undersaturated NaCl solutions to saturation

For conventional reverse osmosis (RO), the osmotic pressure difference limits the allowable concentration factor for concentrating NaCl solutions. The use of low-salt-rejection RO (LSRRO) membranes could allow much higher concentration factors, provided that these membranes show a sufficient decrease in NaCl retention with increasing NaCl concentration in the retentate. This work quantifies allowable NaCl retentions for LSRRO as function of the retentate concentration for different pressures. For a pressure of 70 bar allowable NaCl retentions start to deviate from retentions reported for commercial RO membranes at NaCl weight fractions below 0.1. NaCl retentions for commercial nanofiltration (NF) membranes are much lower than allowed for LSRRO, leading to high NaCl losses in permeate. The required NaCl retention for LSRRO as function of the NaCl retentate concentration therefore falls in between those reported for commercial RO and NF membranes, implying that LSRRO membrane development is required. Based on the obtained relations required concentration factors for the concentration of purified seawater RO retentate and depleted brine from chlor/alkali production have been determined. The concentration factor for LSRRO for concentrating solutions with a NaCl concentration of either 0.07 kg·kg⁻¹ or 0.18 kg·kg⁻¹ to saturation at 120 bar pressure is 4 – 5 times higher than for a case where full retention would have been allowed. By optimizing the unit lay-out to minimize NaCl loss in permeate, approximately 1.2 m²·kg⁻¹·h membrane area per saturated NaCl solution flow produced would be required for concentrating a 0.18 kg·kg⁻¹ NaCl solution to saturation, assuming a permeance of 0.36 kg·m⁻²·h⁻¹·bar⁻¹.