Energy efficiency of RO and FO-RO system for high-salinity seawater treatment

Forward osmosis (FO) has been proposed as an alternative method for seawater desalination, wherein reverse osmosis (RO) membrane technology is used for regeneration of the draw solution. Previous studies have indicated that a standalone RO unit is more energy efficient than an FO-RO system, and as such it was recommended that an FO-RO system is best employed only for the desalination of high-salinity seawaters. This study examined FO-RO applicability in more detail by examining the impact of seawater salinity, impact of an energy recovery device (ERD), and the effect of membrane fouling. For comparison purposes, the performance of the FO process was improved to minimize the impact of concentration polarization and optimize the concentration of draw solution. Model calculations revealed that FO-RO is more energy efficient than RO when no ERD was employed. However, results showed that there was no significant difference in the power consumption between the FO-RO system and the RO unit at high seawater salinities particularly when a high-efficiency ERD was installed. Moreover, the FO-RO system required more membrane area than a conventional RO unit which may further compromise the FO-RO desalination cost.