Closed-concentrate circulation for high recovery and energy efficiency in small-scale brackish reverse osmosis

There is a need for small-scale desalination of brackish water to meet the water needs of scattered small communities and households in the United States and other parts of the world. Reverse osmosis (RO) would be a viable means to meet this need if it could be adapted to small-scale applications in an energy-efficient manner. However, small-scale RO desalination usually operates with low recovery and/or low energy efficiency because the design features typically used in large-scale RO desalination to improve recovery and energy efficiency cannot be adapted to small-scale applications due to the limited number of membrane elements. In this study, the concept of closed concentrate circulation was investigated as a means to increase the recovery and energy efficiency of small-scale RO systems with a limited number of membrane elements. Experiments were conducted on an experimental small-scale RO system employing closed concentrate circulation and parallel single-membrane elements under various operating conditions, such as feedwater salt concentration, permeate flux, cross-flow velocity, and recovery. Results demonstrated the potential of the small-scale RO system to produce permeate at energy efficiencies comparable to those published for conventional large-scale RO systems at recoveries greater than 75%.