Like conventional ion exchange, continuous electrodeionization removes dissolved, ionizable materials such as salts, acids and bases. It can also remove weakly ionized materials such as dissolved silica, carbon dioxide and some organics. Contaminants too large to pass through the ion exchange membrane -- such as large particles and large organic molecules -- are not removed.
Continuous electrodeionization modules consist of mixed-bed resins sandwiched between alternating anion and cation membranes. These membranes are actually ion exchange resins manufactured in sheet form. Resin compartments in this "sandwich" construction alternate between diluting and concentrating compartments. Compartment sets are called cell pairs and form the basic element in a module.
In the module, direct current is applied to the anode (positive electrode) on one end of the module, and to the cathode (negative electrode) on the other end. This electric potential drives the ions captured by the ion exchange resins through the membrane.
Because the resins in the module are continuously regenerated by the electric current, they do not become exhausted. This continuous electro-regeneration enables continuous electrodeionization systems to produce multi-megohm water without the need for chemical regeneration or downtime.
Commercially available continuous electrodeionization modules are normally plate-and-frame devices with varying numbers of cell pairs to accommodate flow rates from 0.5 gallons per minute (gpm) to 1,000 gpm. Customized systems can produce even higher flow rates.
Where it Works
Continuous electrodeionization can be chosen for new projects that require ultra-high purity water or stringent wastewater discharge requirements. It can also be a cost-effective way to upgrade existing ion exchange systems.
Applications include creating process water for the biotechnology and food and beverage industries, and providing high-quality rinse water for electronics, surface finishing and optical-glass processes. For electronics applications, continuous electrodeionization can reduce total organic carbon (TOC) levels in the water.
Because it is able to meet United States Pharmacopoeia (USP) purified water specifications -- including those for bacteria and pyrogens -- continuous electrodeionization systems are frequently used in the pharmaceutical industry, hospitals, university research facilities and dialysis centers.
Electrodeionization (EDI) is a process that removes ionizable species from liquids using electrically active media and an electrical potential to effect ion transport. 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.
The continuous electrodeionization (EDI) process, a subset of EDI, is distinguished from the EDI 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.
This section will focus on EDI devices, and more specifically on the use of such devices in conjunction with reverse osmosis to produce water of sufficient quality to feed to high pressure boilers.