Dual-purpose desalination–salt production electrodialysis

Electrodialysis (ED) is applied in Japan for seawater concentrating to ca. TDS 200 followed by thermal concentration and salt crystallization. A partially desalinated water (ED diluate) is obtained at the same time. Its further desalination for potable water production may be considered to decrease costs of ED by obtaining two valuable products instead of mere salt. Univalent ion permeable ion-exchange membranes are however applied in ED process and as a result the contents of calcium, magnesium and sulfate ions in diluate are relatively high (in relation to chloride), which makes any further processing difficult. Electrodialysis reversal (EDR) may overcome these shortcomings. The salinity of partially desalinated water is as high as TDS 22.9. If dual-purpose desalination– salt production electrodialysis concern is high degree of desalination, which is however necessary, this implicates many problems in the ED process that are the result of major differences in ED concentrate and diluate concentrations and the small conductivity of potable water grade diluate. Electrodialysis of seawater was investigated in a two-step stand with ED-MSF-Crystallization system being the first stage. Univalent ion permeable Tokuyama Co. ACS and CMS membranes were applied in the first step and normal grade CMV, AMV Asahi Glass membranes in the second step (for the first step diluate treating). ED stack developed by the author and equipped with 0.19 mm spacer was applied. A single-pass low residence time mode of operation was applied to avoid gypsum crystallization in the second step ED concentrate. The first step diluate containing, in g/L, Ca2+ — 0.38, Mg2+ — 1.26, Cl– — 4.08, SO42– — 2.78 was treated by EDR yielding 90% recovery. Potable water grade EDR diluate was obtained. No gypsum crystallization in the EDR concentrate was observed despite its 360% saturation. The total water recovery of EDMSF-crystallization system is equal to 90%. As there are two valuable products in the described process, the cost of each of them is obviously related to their selling price. If $30 per one ton of salt is assumed, the potable water cost is estimated to be only $0.44/m3.

ED-MSF-crystallization was proposed for a dual-purpose desalination–salt production system. Two-step electrodialysis in countercurrent flow mode was proposed for seawater desalination. Electrodialysers with small intermembrane distances were applied. Applied solutions allowed diminishing problems that are the result of major differences in ED concentrate and diluate concentrations and the small conductivity of potable water grade diluate. Electrodialysis reversal was used in the second step of electrodialysis stage with 90% recovery. No gypsum crystallization in the EDR concentrate was observed despite its 360% saturation. In the suggested systems not only desalinated water, but salt as well is a valuable product of plant supplied by seawater. From the data presented one can see that by integrating membrane and thermal operations in dual-purpose system it is possible to improve the performance of seawater desalination. Thus 90% water recovery is obtained. As there are two valuable products, the cost of each of them is obviously directly 0.8 0.7 0.6 0.5 0.4 ACS, CMS and AMV, CMV 0.3 membranes 0.2 Low-resistance membranes 0.1 250 300 350 400 450 500 550 600 650 700 750 Current density, A/m Fig. 5. Dependence of water cost in ED-MSF-crystallization system on first ED step current density.