Limiting power density in pressure-retarded osmosis: Observation and implications

Pressure-retarded osmosis (PRO) shows promise for mitigating energy and environmental concerns associated with seawater reverse osmosis (RO) desalination. A systematic investigation of PRO membrane fouling and limiting flux was conducted for the RO-PRO scenario, where increasingly complex feed and draw solutions were used to represent treated wastewater and seawater RO brine, respectively. Four membranes were tested for water flux and power density. When there were no foulants in the feed solution, the membranes had distinctly different water flux and power density values. When there were foulants in the feed and draw solution, all membranes converged to similar water flux and power density values regardless of membrane transport parameters. For the first time, this study identified limiting water flux for multiple membranes in PRO and also, a limiting power density. To increase water flux and power density in PRO, it was suggested to first focus efforts on mitigating fouling on the feed side (support layer) of the membrane. If that can be done, then improvement of membrane transport parameters would be important for increasing limiting water flux and power density.