To prepare purified water with the resistivity of 5–18 MΩ.cm (i.e. up to the quality of theoretically clear water) on industrial scale, only ion-exchange columns were used until recently. One disadvantage of these is their gradual depletion during the process. As soon as unwanted ions permeate, the water treatment process must be stopped and ion-exchangers regenerated. This requires handling aggressive chemicals – acids and lyes, neutralization of their surplus and subsequent elimination of highly salinized waste water.
Electrodeionization (EDI) is an alternative separation process, replacing the conventional demineralization of water on a mixed bed of ion-exchangers (so-called mixbed, a mixture of highly acidic cation-exchanger and highly alkaline anion-exchanger). EDI combines electrodialysis (ED) with ion-exchangers in one device and benefits from favourable electro-mechanical properties of ion-exchangers in an environment of very diluted, or little conductive solutions. The separation of ions and little dissociated constituents from water is achieved by the application of direct electric current. The consumption of electric power is low, usually 0.1 to 0.2 kWh per m³ of the product. Unlike conventional technology, EDI is a continuous process that does not require interrupting the operation to regenerate ion-exchangers. The processed liquid is usually a permeate from one- or two-step reverse osmosis (RO) or demineralised water. EDI works with a high water yield, typically 90 to 95 percent. In the area of the EDI process, the company MemBrain s.r.o. develops modules (process devices), researches materials for ion-exchange membranes and ion-exchangers, seeks new application possibilities not only in the preparation of purified water and performs pilot studies as required by customers. For these purposes, MemBrain s.r.o. owns a mobile EDI unit with the capacity of 2 m3/h, which can be combined with a reverse osmosis unit or units of reverse osmosis and ultrafiltration.