Energy Consumption by Recirculation: A Missing Parameter When Evaluating Forward Osmosis.

■ INTRODUCTION Membrane-based water reclamation technologies have demonstrated great potential to address the water-energy nexus. Among these technologies, forward osmosis (FO) can contribute to the recovery of high-quality water by taking advantage of osmotic pressure gradient across a semipermeable membrane. The major applications of the FO process lie in two realms. The first pertains to systems where FO essentially serves as high-quality pretreatment for a downstream desalination process (e.g., a reverse osmosis (RO) or distillation process), which reconcentrates the draw solution and provides high-quality product water. The second realm pertains to systems that do not require a reconcentration process, including osmotic concentration of complex feed solutions for volume reduction (e.g., domestic and industrial wastewater, landfill leachate and digester effluent) and osmotic dilution of application-specific draw solutes (e.g., fertilizers). FO offers operation with low or no hydraulic pressure requirements, high rejection of undesired compounds, and reversible fouling with prolonged membrane lifespan. Significant efforts have been invested in developing specialized membranes to enhance water flux and reduce solute leakage and in designing efficient methods to reconcentrate the draw solution. As some of the technical barriers have been overcome, commercial FO systems are being piloted in a growing number of applications where low energy consumption is a priority.