ELECTRODIALYSIS (ED) removes impurities from water using an electrical current to draw ionic contaminants through ion selective membranes (ion exchange resin in sheet form) and away from the purified water. Used occasionally to produce potable water from clean brackish feedwater, ED is cost competitive with reverse osmosis.

To produce laboratory grade water, however, ED has several drawbacks and, as such, is rarely used in lab settings. First, the contaminants ED can remove are limited. ED cannot remove contaminants such as certain organics, pyrogens and elemental metals which have weak or nonexistent surface charges because they are not attracted to the membranes. Second, ED requires a skilled operator and routine maintenance. Large molecules which bear a significant charge such as certain colloids and detergents can plug the membranes’ pores, reducing their ionic transport ability and requiring frequent cleaning. During operation, ED liberates caustic soda which may cause scaling, and hydrogen gas which is potentially dangerous. Finally, ED is relatively expensive. As ionic contaminants are removed from the water, its electrical resistance increases, so that higher electrical current is required to continue the purification process. Purification beyond the potable level is considered uneconomical due to the increased electrical consumption. Component materials such as platinum and stainless steel are also expensive.