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.

Donnan dialysis of a one-component solution yields a very high efficiency of anion removal, depending on anion species (anionic size and charge), concentration of the receiver and the volume ratio of the feed and the receiver. The best effects were observed for nitrates (87–97% of the removal) and sulphates (94–100% of the removal), while bicarbonates were replaced with chlorides with the lowest efficiency (77–94%). The higher the concentration of the receiver, the higher the anionic flux and anion removal efficiency. However, an increase in the concentration of this stream also results in greater salt leakage to the feed and greater salinity of the waste stream of the receiver. Properties of anion-exchange membranes significantly influence the rate of Donnan dialysis. The AFN membrane (with high ion-exchange capacity and high water content) yields higher anionic fluxes, and therefore comparable removal efficiencies can be reached much faster (for bicarbonate ion removal efficiency is about 30% higher than that observed with the AMX membrane). The anionic composition of the solution after Donnan dialysis changes significantly. Chlorides compose over 90% of all anions, while primarily the solution contained equal amount of sulphates, bicarbonates, nitrates and chlorides. Due to this change of anionic composition, salts present in water that is going to be desalinated have much greater solubility.