Electrodeionization

EDI is typically employed as a polishing treatment following reverse osmosis (RO), and is used in the production of ultrapure water.

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.

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.

[2] 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.

Electrodeionization systems have also been applied to the removal of heavy metals from different types of wastewater from mining, electroplating, and nuclear processes.

[4] When using a current that is higher than necessary for the movement of the ions, a portion of the incident water will be split, forming hydroxide (OH−) anions and hydrogen (H+) cations.

With a resin, therefore, the final remaining ion concentrations in the processed water can be lower by orders of magnitude.

This configuration allows anions and cations to flow in only one direction because of the selectivity of the membranes and the electrical forces, rendering the feedwater relatively free of ions.