Hybrid semi-batch/batch reverse osmosis (HSBRO) for use in zero liquid discharge (ZLD) applications

Hybrid semi-batch/batch reverse osmosis (HSBRO) is a new method of high-recovery desalination that provides low Specific Energy Consumption (SEC) in a compact design. In this first experimental study on HSBRO, we report SEC over a range of operating parameters using brackish feed water. For example, at 500–1500 mg/L feed concentration, and recovery of 0.94, we measured hydraulic and electrical SEC of 0.20–0.31 kWh/m3 and 0.42–0.54 kWh/m3 respectively, using a flux of 18.9 L/m2/h and obtaining an output of 17.5 m3/day. Second law efficiency was 7.8–17.8 % thus improving on available multistage and semi-batch RO systems. A model of the system, that includes the effect of finite salt rejection, predicts SEC with accuracy of 1–3 %. With improvements in membrane permeability, valves, and pump efficiency, we predict electrical SEC lowered to 0.14–0.28 kWh/m3 and second law efficiency elevated to 22.8–34.5 %. Though simple batch RO can achieve comparable SEC to HSBRO, for recovery as high as 0.94 it would require an impractically large work exchanger. The model shows that feed of salinity up to 6000 mg/L may be treated with recovery of 0.95 and peak system pressure < 120 bar, indicating great potential in ZLD and MLD applications.