Electrodialysis with brine solutions oversaturated with calcium sulfate

Electrodialysis (ED) was applied to concentrate brine solutions of various concentrations — similar to effluents from 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.