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Electrodeionization (EDI) is a water treatment technology that utilizes DC power, ion exchange membranes, and ion exchange resin to deionize water. EDI is typically employed as a polishing treatment following reverse osmosis (RO). It distinguishes itself from other RO polishing methods, like chemically regenerated mixed beds, by operating continuously without the need for chemical regeneration.

EDI is sometimes denoted as "continuous electrodeionization" (CEDI) because the electric current continually regenerates the ion exchange resin mass. The CEDI approach can achieve high purity, with product conductivity around 0.1 S/cm and, in some cases, resistivity as high as 18.2 MΩ/cm.

Electrodeionization (EDI) integrates three distinct processes:

  1. Electrolysis: A continuous DC current is applied, directing positive and negative ions toward electrodes with opposing electrical charges. This electrical potential draws anions and cations from diluting chambers, through cation or anion exchange membranes, into concentrating chambers.
  2. Ion exchange: In this stage, ion exchange resin fills the diluting chambers, and as water flows through the resin bed, cations and anions become affixed to resin sites.
  3. Chemical regeneration: Unlike chemically regenerated mixed beds, EDI accomplishes regeneration through water splitting induced by the electric current. Water is split from H2O into H+ and OH-, effectively regenerating the resin without the need for external chemical additives.

Quality of the feed[edit]

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.
  • TOC, which can foul resins and membranes, must be minimized.
  • Chlorine, ozone, and other oxidizers can oxidize resins and membranes and create permanent damage, which must be minimized.


To eliminate or minimize the concentration polarization phenomenon present in electrolysis systems of the time, electrodeionization was developed in the early 1950s. A patent on the technology was filed in 1953, and subsequent publications popularized the technology.[1]

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, it was determined that EDI would be a suitable polishing technology. Packaged RO and EDI systems were used to displace chemically regenerated ion exchange systems.

In 1986 and 1989, several companies developed electrodeionization devices. The initial devices were large, costly, and often unreliable. However, in the 1990s, smaller and less costly modular designs were introduced, some of which minimized leakage. Nonetheless, these designs and their contemporary descendants still face limitations such as cost and limited operational envelope.[2][3]


When fed with low total dissolved solids feedwater (e.g., purified by RO), the product can reach very high purity levels, with conductivity on the order of 0.5 S/cm. The ion exchange resins act to hold the ions, allowing them to be transported across the ion exchange membranes. The main applications of EDI technology are in electronics, pharmaceuticals, and power generation. In electronics, deionized water is used to rinse components during manufacturing. The electronic chips are very small, with little free space between component elements. Therefore, very low numbers of ions are necessary to conduct unwanted electricity between components. If this occurs, a short circuit may result and make the chip unusable. In the pharmaceutical industry, the presence of ions in the water used in drug development can lead to unwanted side reactions and introduce harmful impurities. In power generation, the presence of ions in boiler feedwater can lead to the buildup of solids or the degradation of boiler walls, both of which can lower boiler efficiency and present safety hazards. Due to the large financial and safety concerns present in these industries, they provide the bulk of the revenue for EDI developers. Electrodeionization systems have also been applied to the removal of heavy metals from different types of wastewater from mining, electroplating, and nuclear processes. The primary ions removed in these processes are chromium, copper, cobalt, and caesium, though EDI sees use in the removal of others as well. [4]


Electrodes in an electrochemical cell are referred to as either anodes or cathodes. The anode is defined as the electrode at which electrons leave the cell and oxidation occurs, and the cathode is the electrode at which electrons enter the cell and reduction occurs. Each electrode may become either the anode or the cathode, depending on the voltage applied to the cell.

Each deionization cell consists of an electrode and an electrolyte with ions that undergo either oxidation or reduction. Because they commonly consist of ions in solution, the electrolytes are often known as "ionic solutions", but molten and solid electrolytes are also possible.

Water is passed between an anode and a cathode. Ion-selective membranes allow the positive ions to separate from the water toward the negative electrode and the negative ions toward the positive electrode. As a result, the ions cannot escape the cell, and deionized water is produced. [3]

In situ regeneration[edit]

When using a current that is higher than what is necessary for the movement of the ions, a portion of the incident water will be split, forming hydrogen (OH-) and oxygen (H+) ions. This species will replace the impurity anions and cations in the resin. This process is called "in situ regeneration" of the resin. As it occurs during the deionization process, it has the benefit of allowing for continuous purification as opposed to requiring a pause in operation to manually chemically regenerate ion exchange resin in other deionization techniques. [5]

Installation scheme[edit]

Electrodeionization installation scheme

The typical EDI installation has the following components: electrodes, anion exchange membranes, cation exchange membranes, and resin. The most simplified configuration comprises three compartments. To increase production, the number of compartments or cells can be increased as desired.

Once the system is installed and feedwater begins to flow through it, cations flow toward the cathode and anions flow toward the anode. Only anions can go through the anion exchange membrane, and only cations can go through the cation exchange membrane. This configuration allows anions and cations to flow in only one direction because of the selectivity of the membranes and the electric force, leaving the feedwater free of ions. It also allows for the separate collection of cation and anion concentration flows, creating the opportunity for more selective waste disposal; this is especially useful in the removal of heavy metal cations.

The concentration flows (right and left of the feed flow) are rejected, and they can be wasted, recycled, or used in another process.

The purpose of the ion exchange resin is to maintain the stable conductance of the feedwater. Without the resin, ions can be removed initially, but the conductance will drop dramatically as the concentration of ions decreases. With lower conductance, the electrodes are less able to direct the flow of electrons and remove them; with the addition of resins, a steady rate of removal is possible, and the remaining ion concentration in the processed water is lower by orders of magnitude. [4]

See also[edit]


  1. ^ Kollsman, Paul (1953-10-23). Method of and apparatus for treating ionic fluids by dialysis. United States Patent Office.
  2. ^ "Fundamentals of Electrodeionization (EDI) Technology". WCP Online. 2007-03-10. Retrieved 2022-08-05.
  3. ^ a b Rathi, B. Senthil; Kumar, P. Senthil (July 2020). "Electrodeionization theory, mechanism, and environmental applications. A review". Environmental Chemistry Letters. 18 (4): 1209–1227. doi:10.1007/s10311-020-01006-9. ISSN 1610-3653. S2CID 216031814.
  4. ^ a b Wardani, Anita Kusuma; Hakim, Ahmad Nurul; Khoiruddin, null; Wenten, I. Gede (June 2017). "Combined ultrafiltration-electrodeionization technique for production of high purity water". Water Science and Technology. 75 (12): 2891–2899. doi:10.2166/wst.2017.173. ISSN 0273-1223. PMID 28659529.
  5. ^ Alvarado, Lucía; Chen, Aicheng (2014-06-20). "Electrodeionization: Principles, Strategies and Applications". Electrochimica Acta. 132: 583–597. doi:10.1016/j.electacta.2014.03.165. ISSN 0013-4686.

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