Deionized water also known as demineralized water is water that has had its mineral ions removed. Mineral ions such as cations of sodium, calcium, iron, copper, etc and anions such as chloride, sulphate, nitrate, etc are common ions present in water. Deionization is a physical process which uses specially-manufactured ion exchange resins which provides ion exchange site for the replacement of the mineral salts in water with water forming H+ and OH- ions. Because the majority of water impurities are dissolved salts, deionization produces a high purity water that is generally similar to distilled water, and this process is quick and without scale buildup.
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 does not require the use of chemicals such as acid and caustic for reactivation. EDI is commonly used as a polishing process to further remove trace ionic salts of the Reverse Osmosis (RO) permeate to high purity water of multi-megohm-cm quality.
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.
How it works
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.
"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
Comparing New and Old Technology
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.