Electrodialysis — involves separating water from dissolved solids by passing the dissolved solids (ions) through a semi-permeable membrane which is relatively impervious to water. This is accomplished by means of a direct current electrical field which transports the ions through the membranes. Following Fig. shows a basic electrodialysis system with alternating cation-selective and anion-selective membranes. Recovery of product (deionized) water with electrodialysis units ranges from 50 to 90% of the feedwater depending upon the number of stages and degree of recirculation utilized. Operating costs consist mainly of power costs (typically 6-10 kwh/1000 gallons of product water) and membrane cleaning and replacement costs. Based upon combined capital and operating costs, the electrodialysis process is most economical when used to desalt brackish water (1000 to 5000 ppmw dissolved solids) to a product water concentration of about 500 ppmw dissolved solids. A pretreatment system is usually needed to prevent fouling or degradation of the membranes. Electrodialysis units can operate over a pH of 1 to 13 and at temperatures up to about 110°F.
