Global water scarcity persists as one of the greatest challenges of our time, already affecting two-thirds of the world population and expanding at an alarming rate. Effectively combatting water scarcity will require the harnessing of unconventional, typically saline, water sources through the use of desalination processes. Hence, as our reliance on desalinated water grows, the identification and implementation of the most energy efficient technology is critical towards ensuring sustainable water production in the coming decades. In a recent series of works, we focused on determining the energy consumption of desalinating brackish waters (salinity range of approximately 1 g L-1 to 10 g L-1) with the two most prevalent technologies — reverse osmosis (RO) and electrodialysis (ED) — as well as the emerging electro-driven technology, membrane capacitive deionization (MCDI). In our analyses, we used rigorous mechanistic models to perform a systematic and direct comparison of each technology across a wide range of desalination conditions. Notably, by maintaining the same separation parameters (i.e., feed salinity, salt removal, water recovery) and productivity among each technology, we ensure that our comparisons are valid and fair. Our results indicate that while ED and RO are capable of operating with high energy efficiency (&gt;30%), the energy efficiency of MCDI is substantially lower, requiring nearly an order of magnitude more energy in many cases. Furthermore, we demonstrate that that even with complete energy recovery in MCDI, its energy efficiency can not approach that of ED or RO due to inherent fundamental limitations of the process. Overall, our analyses clearly indicate that electro-driven desalination technologies like MCDI and ED show optimal performance for low feed salinities and relatively small extents of salt removal, whereas RO is dominant for the treatment of higher feed salinities (&gt; 5 g L-1) which require extensive degrees of water-salt separation. Specifically, we determine that ED is more energy efficient than RO for feed salinities &lt; 3 g L-1, though its widespread implementation remains hindered by the high cost of ion-exchange membranes. We therefore suggest that future studies further extend on the performed energy consumption comparisons with detailed technoeconomic analyses to provide a more holistic assessment of each technology.

Energy efficiency of brackish water desalination: Comparison of reverse osmosis, electrodialysis, and capacitive deionization

ACS Fall 2021

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