Electrodialysis with brine solutions oversaturated with calcium sulfate

Electrodialysis (ED) was applied to concentrate brine solutions of various concentrations - similar to effluentsfrom the desalination of brackish and industrial water - to about 20%. The consequent reduction in effluent volumes would facilitate a reduction in disposal costs. The energy requirement for concentrating solutions from 70 to 300 mN (0.4-1.8%) to 3.9 N (20%) was in the range 1.5-7.1 kWh/m³, in contrast to approximately 25 kWh/m³ by thermal evaporation. With the aim of preventing precipitation of CaSO₄ on the membranes, laboratory-scale ED experiments were carried out on solutions of different compositions to determine the conditions that would lead to the precipitation of excess gypsum on gypsum seeds in a separate precipitator. The results were then applied in a pilot-scale ED unit in which the brine that circulated through the ED brine cells passed through a separate CaSO₄ precipitator containing gypsum seeds.