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 differs from other RO polishing methods, like chemically regenerated mixed beds, by operating continuously without 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:
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. 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. 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
To maximize the purity of the product, EDI feedwater needs pre-treatment, usually reverse osmosis. 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. 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.
History
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.
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.
Electronics manufacturing
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.
Pharmaceutical industry
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.
Power generation
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.
Other applications
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.