Forward Osmosis—Reverse Osmosis Process Offers a Novel Hybrid Solution for Water Purification and Reuse

Forward osmosis (FO) is an osmotically driven membrane process that uses osmotic pressure of concentrated solutions, including seawater, to extract clean water from low salinity solutions. In a new approach, FO uses the difference in salinity of seawater and impaired water as the driving force to dilute seawater with reclaimed water through a tight FO membrane. By diluting the seawater feed stream to a reverse osmosis (RO) desalination plant, the energy demand of desalination is reduced, and two tight barriers are in place to enhance rejection of contaminants that might be present in the impaired water feed stream. Bench- and pilot-scale osmotic dilution tests were conducted with synthetic seawater as a draw solution and with secondary and tertiary effluent from a domestic wastewater treatment plant as feed streams. Impaired surface water from the South Platte River in Colorado was also tested as a feed stream to the osmotic dilution process. Although water flux was generally low, flux decline caused by fouling was minimal during weeks of continuous operation. Multiple membrane barriers provided more than 90 percent rejection of organic and inorganic solutes. In addition, the hybrid FO–RO process was found to be economically and technically feasible across a broad range of operating conditions.