Electrodeionization (EDI) is a water treatment process that removes ionizable species from liquids using electrically active media and an electrical potential to effect ion transport. It differs from other water purification technologies such as conventional ion exchange in that it is does not require the use of chemicals such as acid and caustic. EDI is commonly used as a polishing process to further deionize Reverse Osmosis (RO) permeate to multi-megohm-cm quality water.
The continuous electrodeionization (EDI) process, is distinguished from other electrochemical collection/discharge processes such as electrochemical ion exchange (EIX) or capacitive deionization (CapDI), in that EDI performance is determined by the ionic transport properties of the active media, not the ionic capacity of the media. EDI devices typically contain semi-permeable ion-exchange membranes and permanently charged media such as ion-exchange resin. The EDI process is essentially a hybrid of two well-known separation processes - ion exchange deionization and electrodialysis, and is sometimes referred to as filled-cell electrodialysis.
EDI theory and practice have been advanced by a large number of researchers throughout the world. It is believed that EDI was first described in a publication by scientists at Argonne Labs in January 1955 as a method for removal of trace radioactive materials from water (Walters, et al.). One of the earliest known patents describing a EDI device and process was awarded in 1957 (Kollsman). It is thought that the first pilot device incorporating mixed resins was developed by Permutit Company in the United Kingdom in the late 1950’s for the Harwell Atomic Energy Authority, as described in a paper (Gittens and Watts) and in more than one patent (Kressman; Tye). One of the first detailed theoretical discussions of EDI was written in December 1959 (Glueckauf). In April 1971, a Czechoslovakian researcher reported results of his experimental and theoretical work that advanced the theory of ionic transport within a EDI device (Matejka). The use of electroactive materials (resins) in the concentrate streams for pH control and scale prevention was proposed in 1984 (O'Hare). Layered bed devices were described in the patent literature in the early 1980’s (Kunz).
EDI devices and systems were first fully commercialized in early 1987 by a division of Millipore that is now part of Siemens Water Technologies (Ganzi et al., 1987). Since then, the theory and practice of EDI has advanced worldwide, and commercial EDI devices are now manufactured by a number of companies (Towe et al.; Parsi et al.; Rychen et al., Stewart and Darbouret). There are now thousands of EDI systems in commercial operation for the production of high purity water at capacities ranging from less than 0.01 to more than 1500 m3/h. This includes EDI systems that have been in continuous operation for over 15 years, producing makeup water for high pressure boilers (Layton).
The electrically active media in EDI devices may function to alternately collect and discharge ionizable species, or to facilitate the transport of ions continuously by ionic or electronic substitution mechanisms. EDI devices may comprise media of permanent or temporary charge, and may be operated batchwise, intermittently, or continuously.
There are two distinct operating regimes for EDI devices: enhanced transfer and electroregeneration (Ganzi, 1988). In the enhanced transfer regime, the resins within the device remain in the salt forms. In low conductivity solutions the ion exchange resin is orders of magnitude more conductive than the solution, and act as a medium for transport of ions across the compartments to the surface of the ion exchange membranes. This mode of ion removal is only applicable in devices that allow simultaneous removal of both anions and cations, in order to maintain electroneutrality.
The second operating regime for EDI devices is known as the electroregeneration regime. This regime is characterised by the continuous regeneration of resins by electrically produced hydrogen and hydroxide ions. The dissociation of water preferentially occurs at bipolar interfaces in the ion-depleting compartment where localized conditions of low solute concentrations are most likely to occur (Simons). The two primary types of interfaces in EDI devices are resin/resin and resin/membrane. The optimum location for water splitting depends on the configuration of the resin filler. For mixed-bed devices water splitting at both types of interface can result in effective resin regeneration, while in layered bed devices water is dissociated primarily at the resin/membrane interface (Ganzi et al., 1997).
"Regenerating" the resins to their H+ and OH- forms allows EDI devices to remove weakly ionized compounds such as carbonic and silicic acids, and to remove weakly ionized organic compounds. This mode of ion removal occurs in all EDI devices that produce ultrapure water.
Under Direct Current (DC) electrical potential, Water (H2O) behaves as follows:
H2O -> H+ + OH-
A typical EDI device contains alternating semipermeable anion and cation ion-exchange membranes. The spaces between the membranes are configured to create liquid flow compartments with inlets and outlets. A transverse DC electrical field is applied by an external power source using electrodes at the ends of the membranes and compartments.
When the compartments are subjected to an electric field, ions in the liquid are attracted to their respective counterelectrodes. The result is that the compartments bounded by the anion membrane facing the anode and the cation membrane facing the cathode become depleted of ions and are thus called purifying (or sometimes, diluting) compartments. The compartments bounded by the anion membrane facing the cathode and cation membrane facing the anode will then “trap” ions that have transferred in from the purifying compartments. Since the concentration of ions in these compartments increases relative to the feed, they are called concentrating compartments, and the water flowing through them is referred to as the concentrate stream (or sometimes, the reject stream).
In an EDI device, the space within the ion depleting compartments (and in some cases in the ion concentrating compartments) is filled with electrically active media such as ion exchange resin. The ion-exchange resin enhances the transport of ions and can also participate as a substrate for electrochemical reactions, such as splitting of water into hydrogen (H+) and hydroxyl (OH-) ions. Different media configurations are possible, such as intimately mixed anion and cation exchange resins (mixed bed or MB) or separate sections of ion-exchange resin, each section substantially comprised of resins of the same polarity: e.g., either anion or cation resin.
pH is a measure of the acidity or alkalinity of a solution. Aqueous solutions at 25°C with a pH less than seven are considered acidic, while those with a pH greater than seven are considered basic (alkaline). When a pH level is 7.0, it is defined as 'neutral' at 25°C because at this pH the concentration of H3O+ equals the concentration of OH− in pure water. pH is formally dependent upon the activity of hydronium ions (H3O+),(H+ is often used as a synonym for H3O+.)
Oxidation Reduction Potential (ORP) quantifies the potential for all chemical reactions in which atoms have their oxidation number (oxidation state) changed via a redox (oxidation/reduction) reaction.
This can be either a simple redox process such as the oxidation of carbon to yield carbon dioxide, or the reduction of carbon by hydrogen to yield methane (CH4), or it can be a complex process such as the oxidation of sugar in the human body through a series of very complex electron transfer processes.
The term redox comes from the two concepts of reduction and oxidation. It can be explained in simple terms:
Oxidation describes the loss of electrons by a molecule, atom or ion Reduction describes the gain of electrons by a molecule, atom or ion
However, these descriptions (though sufficient for many purposes) are not truly correct. Oxidation and reduction properly refer to a change in oxidation number—the actual transfer of electrons may never occur. Thus, oxidation is better defined as an increase in oxidation number, and reduction as a decrease in oxidation number. In practice, the transfer of electrons will always cause a change in oxidation number, but there are many reactions which are classed as "redox" even though no electron transfer occurs (such as those involving covalent bonds).
When compared with conventional resin-based, chemically regenerated deionization equipment, EDI systems offer a variety of benefits. Most obvious is the elimination of the regeneration process and its associated hazardous regeneration chemicals - acid and caustic. Since EDI operates through a combination of ion-transfer across the resins and membranes, as well as electrochemical regeneration of a portion of the bed, the resins and membranes are always functional as long as the DC voltage is applied. The resin in a conventional deionizer only purifies water when in its active (regenerated) form. As a result, the EDI system product water quality stays constant over time, whereas in regenerable deionization, product water quality degrades as the resins approach exhaustion. For those processes requiring DI water on a continuous basis, conventional systems must be duplexed so that one system can provide water while the other is regenerated. Duplexing adds cost, complexity, and size to conventional DI systems. Because EDI is continuous, and not a batch process, duplexing is not necessary. As a result of this, as well as the avoidance of regenerant chemical storage and transfer equipment, EDI system footprints are often one half of the size of their conventional counterparts.
There are significant tangible cost benefits associated with the elimination of regeneration. The costs of regeneration labor and chemicals are replaced with a small amount of electrical consumption. A typical EDI system will use approximately 1 kW-hr of electricity to deionize 1000 gallons from a feed conductivity of 50 microsiemen /cm to 0.1 µS/cm product conductivity. Since the EDI concentrate (or reject) stream contains only the feed water contaminants at 5-20 times higher concentration, it can usually be discharged without treatment, or used for another process. Thus facility costs can also be reduced since waste neutralisation equipment and ventilation for hazardous fumes are not necessary.
There are also less tangible cost reductions, which are harder to quantify, but usually favor the use of EDI systems. By eliminating hazardous chemicals wherever possible, workplace health and safety conditions can be improved. With today's increasing regulatory influence on the workplace, the storage, use, neutralisation, and disposal of hazardous chemicals result in hidden costs associated with monitoring and paperwork to conform to EPA and OSHA requirements as well as the "Right To Know" laws. In addition, the fumes, particularly from acid, often cause corrosive structural damage to facilities and equipment.
For the most part the elimination of regenerant chemicals is considered advantageous, but the chemicals do offer at least one benefit. In conventional demineralisers, acid and caustic is typically applied to the ion exchange resins at concentrations of 2-8% by weight. At these concentrations the chemicals not only regenerate the resins but clean them as well. The electrochemical regeneration that occurs in a EDI device does not provide the same level of resin cleaning. Therefore proper pretreatment is even more important with a EDI device, in order to prevent fouling or scaling. This is one of the reasons that RO pretreatment is normally required upstream of a EDI system. In general the feed water requirements for EDI systems are stricter than for a chemically regenerated demineraliser.