Donnan dialysis with anion-exchange membranes in a water desalination system

Donnan dialysis with an anion-exchange membrane is proposed as a pretreatment step in a water desalination system. The process replaces anions that cause the precipitation of sediments with neutral chloride anions. Water after Donnan dialysis mainly contains soluble chloride salts that should cause no scaling problems during electrodialytic desalination. The first part of the paper deals with Donnan dialysis of one- and multi-component solutions. Two anion-exchange membranes were examined with different concentrations of the receiver and varying volume ratio of the streams. The results show high efficiency of anion removal. Then, results of electrodialysis of the “raw” multi-component solution and the solution previously treated with Donann dialysis are presented and compared. Preliminary replacement of sulphates and bicarbonates with chloride anions ensures lower energy consumption and higher fluxes during subsequent electrodialysis. It also results in a shorter process time.