Multistage-batch electrodialysis to concentrate high-salinity solutions: Process optimisation, water transport, and energy consumption

Zero discharge of high-salinity solutions has become a hot topic due to its significance for environmental protection. Vacuum evaporation can be used to treat the abovementioned solutions to produce freshwater and coarse salts, but it consumes a large amount of power. Multistage-batch electrodialysis (ED) has been proposed to concentrate the high-salinity solutions prior to being treated by an evaporative crystalliser, because of its superior concentrating performance and low energy consumption. The results indicate that the concentration gradient between the dilute and the concentrated solutions can be maintained at a low value in each stage-batch of ED, and that the flux ratio of water to salt decreases as the stage-batch advances. The salt content of the concentrated solution can be increased from 3.5% to 17.9% and 20.6% by two- and three-stage-batch ED, respectively. The total energy consumption based on the material balance for two- and three-stage-batch ED is 0.31 and 0.45 kW h/kg, respectively. Therefore, high-salinity solutions can be efficiently concentrated by multistage-batch ED, with relatively low energy consumption.