Electrodeionization (EDI) is a water treatment technology that utilizes DC power, ion exchange membranes, and ion exchange resin to deionize water, thereby removing trace minerals that function as anions and cations. By itself, it does not remove other contaminants, including pathogens, organic compounds, or dissolved gases. EDI is typically employed as a polishing treatment following reverse osmosis (RO), and is used in the production of ultrapure water. It differs from other RO polishing methods, like chemically regenerated mixed beds, by operating continuously without chemical regeneration.

Electrodeionization can be used to produce high purity water, reaching electrical resistivity values as high as 18.2 MΩ/cm.

Electrodeionization (EDI) integrates three distinct processes:

EDI is sometimes labeled "continuous electrodeionization" (CEDI) because the electric current continually regenerates the ion exchange resin mass.

To maximize the purity of product water, EDI feedwater needs pre-treatment, usually done via reverse osmosis. When fed with feedwater that is low in total dissolved solids (e.g., purified by RO), the product can reach very high purity levels. The contents of the feedwater must be kept within certain parameters to prevent damage to the EDI instrument.

Common feedwater quality concerns are:

Electrodeionization was developed in the early 1950s to eliminate or minimize the concentration polarization phenomenon present in electrolysis systems of the time. A patent on the technology was filed in 1953, and subsequent publications popularized the technology.

The technology was limited in application because of the low tolerance of total dissolved solids, hardness and organics. During the 1970s and 1980s, reverse osmosis became a preferred technology to ion exchange resin for high TDS waters. As RO gained popularity, EDI emerged as a suitable polishing technology. Packaged RO and EDI systems began to displace chemically regenerated ion exchange systems.