A free-piston batch reverse osmosis (RO) system for brackish water desalination: experimental study and model validation

Batch RO is designed to achieve high energy efficiency and high recovery in desalination. However, so far relatively few experiments on batch RO have been reported. Here we present an extensive experimental study of a single-acting, free-piston batch RO system using an 8-inch spiral wound membrane. The system was tested in the laboratory with brackish feed water containing up to 5 g/L NaCl. The objective was to quantify system performance in terms of Specific Energy Consumption (SEC), recovery, rejection, and output. Sensitivity to permeate flux and recirculation flow rate was also investigated. Performance was compared against the predictions of a theoretical model that accounts for salt retention, concentration polarization, and longitudinal concentration gradient in the RO module. For the first time, osmotic backflow was measured and incorporated into the model. For feed concentrations ranging from 1–5 g/L and recovery of 0.8, hydraulic SEC was measured in the range 0.22–0.48 kWh/m³ and electrical SEC in the range 0.48–0.83 kWh/m³. With improvements to the membrane permeability from 4.4 to 8 LMH/bar, selection of more efficient pumps, and reduction of valve friction losses, the model predicts that hydraulic SEC will be lowered to 0.14–0.39 kWh/m³.