Energy Consumption of Brackish Water Desalination: Identifying the Sweet Spots for Electrodialysis and Reverse Osmosis

Abstract

Though electrodialysis (ED) and reverse osmosis (RO) are both mature, proven technologies for brackish water desalination, RO is currently utilized to desalinate over an order of magnitude more brackish water than ED. This large discrepancy in the adoption of each technology has yet to be thoroughly justified in the literature, particularly from the perspective of energy consumption. Hence, in this study, we performed a direct and systematic comparison of the energy consumption of RO and ED for brackish water desalination, precisely mapping out the ideal operational space of each technology for the first time. Using rigorous system-scale models for RO and ED, we determine the specific energy consumption and energy efficiency of each process over a wide range of brackish water conditions. Specifically, we investigate the effects of varying feed salinity, extent of salt removal, water recovery, and productivity to ultimately identify the operational sweet spots of each technology. By maintaining the same separation parameters (i.e., feed salinity, salt removal, water recovery) and productivity between RO and ED throughout the study, we ensure that our comparison of the technologies is valid and fair. Our results indicate that both RO and ED are capable of operating with high energy efficiency (>30%) for brackish water desalination, though for differing conditions. Particularly, we show that whereas ED excels for low feed salinities (<3 g L–1) and extents of salt removal, RO operates optimally for high salinity feeds (>5 g L–1), which require more extensive desalination. Through our in-depth energetic analysis, we provide guidance for future applications of RO and ED, emphasizing that increased implementation of ED will require significant reduction in the cost of ion-exchange membranes.