Integrating Divalent-Selective Electrodialysis in Brackish Water Desalination

Brackish water desalination is imperative for meeting water demands in arid regions far from the seashore. Reverse osmosis, the leading desalination technology, removes nearly all calcium and magnesium ions, which are essential in drinking and irrigation water. Multistep process schemes combining reverse osmosis with ion-selective membrane processes can maintain or reintroduce these minerals without external chemical addition. Previous efforts emphasized membrane processes that retain multivalent ions, focusing primarily on nanofiltration and monovalent-selective electrodialysis. The potential of processes where monovalent ions are retained and divalent preferentially transported through the membrane has not been studied systematically. Here, we explored applying divalent-selective electrodialysis to transfer calcium and magnesium from the influent into the brackish water reverse osmosis permeate. This novel concept enables chemical-free remineralization of the reverse osmosis permeate while reducing membrane scaling by sparingly soluble calcium salts. We tested this concept experimentally using commercial membranes and natural brackish water, evaluated the product water based on quality criteria for domestic and agricultural use, and assessed the techno-economic feasibility. We found that water suitable for potable use and irrigation can be attained at a reasonable cost, depending on current density. These findings highlight the need for more research on divalent-selective electrodialysis and offer future directions.