Free-piston batch reverse osmosis (RO): Modelling and scale-up

Free-piston batch RO achieves excellent energy efficiency at high recovery, but current systems have outputs <25 m3/day. Designing larger systems poses challenges like designing an appropriate work exchanger and optimizing membrane arrangements in series and/or parallel. This study calibrates a batch RO model against brackish and seawater experiments up to 112 bar and uses the model to predict performance on scale-up. Modelling up to 6 membranes in series predicts SEC of 1.11–1.5 kWh/m3 at recirculation flow Qr/Qf = 4 with brackish water, with SEC becoming more sensitive to Qr/Qf as the number of membranes increases. Using a 500-L work exchanger, a batch RO system with twelve 8-in. membranes is designed. For brackish water (3000 mg/L) this system gives outputs of 165 and 128 m3/day with SEC of 0.69 and 1.19 kWh/m3 at recoveries of 0.93–0.97 with low- and high-pressure RO membranes, respectively - representing SEC reductions of 33 and 45 % compared to semi-batch RO. For seawater, it gives 105 m3/day with SEC of 2.37 kWh/m3 at recovery of 0.66. High-permeability membranes would significantly enhance batch RO performance; for example, increasing permeability from 0.8 to 2 L/m2/h/bar would boost output to 320 and 207 m3/day in brackish and seawater desalination respectively.