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Concept
Electrodeionization
Résumé
Electrodeionization (EDI) is a water treatment technology that uses DC power, ion exchange membranes, and ion exchange resin to deionize water. EDI is usually a polishing treatment for reverse osmosis (RO). EDI differs from other RO polishing technologies, such as chemically regenerated mixed beds, in that it is continuous and requires no chemical regeneration.
EDI is sometimes referred to as "continuous electrodeionization" (CEDI) since the electric current regenerates the ion exchange resin mass continuously. The CEDI technique can achieve high purity, with product conductivity on the order of 0.1 S/cm and sometimes with resistivity as high as 18.2 MΩ/cm.
Electrodeionization (EDI) is a combination of three different processes: electrolysis, where with a continuously applied electric DC current, both positive and negative ions are directed to the electrode that has an opposing electrical charge; the electrical potential pulls anions and cations from diluting chambers, through cation or anion exchange membranes, into concentrating chambers. Ion exchange, where ion exchange resin makes up the diluting chambers and the cations and anions are affixed to resin sites as water passes through the resin bed; and chemical regeneration, which occurs by water splitting. In a chemically regenerated mixed bed, the hydrogen (H+) of acid regenerates cation resin. The hydroxide (OH-) of sodium hydroxide (caustic soda) regenerates anion resin. In EDI, the electric current causes water to split from H2O to H+ and OH-, regenerating the resin without the need for the addition of external chemicals.
To maximize the purity of the product, EDI feedwater needs pre-treatment, usually reverse osmosis. Feedwater must follow certain requirements to prevent damage to the instrument.
Common parameters are:
Hardness of feedwater: 1 ppm as CaCO3, with limited exceptions up to 2 ppm.
Silica content (SiO2) must be 1 ppm in most EDI cells or 2 ppm in thin-cell modules.
CO2 must be monitored to prevent excessive loading of anion exchange resin.
Source officielle
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