Novel low-salt-rejection reverse osmosis configuration for continuous brine concentration: Pilot-scale demonstration

Low-salt-rejection reverse osmosis (LSRRO) applies a cascade of RO stages to achieve high water recoveries from brines, while overcoming high osmotic pressures. Here, we report the implementation of a small-scale pilot of an LSRRO system that employs intra-stage brine recycling combined with pressure intensifiers. When brine volumes are reduced, the final concentrate flow rates may fall below the operating limits of the RO modules. Recycling the brine within each stage enables overcoming this limitation, and incorporating pressure exchangers reduces the energy penalty associated with the recycle stream. Following this novel principle, the small LSRRO pilot system allowed to concentrate 45 L/h of synthetic brackish water RO brine (26.9–33.3 g/L NaCl) threefold. Importantly, hydraulic limitations that may render LSRRO implementation challenging were overcome by adopting a flexible process configuration. Notably, only three stages, equipped with one 2540 spiral-wound element of seawater RO (SWRO), nanofiltration (TS80), and chlorinated SWRO, respectively, were needed to achieve target water recoveries and desired concentration factors. While final salt concentrations exceeded those commonly attainable with conventional RO systems, rejections for the elements were not optimal to realize the full concentration potential of the staged LSRRO design. Further work is being conducted to demonstrate viable pathways for improving system performance metrics.