Electrodeionization (EDI)

What is EDI? High purity water production has traditionally used a combination of membrane separation and ion exchange processes. EDI is a process that combines semi-impermeable membrane technology with ion-exchange media to provide a high-efficiency demineralization process. Electrodialysis employs electrical current and specially-prepared membranes which are semi-permeable to ions based on their charge, electrical current, and ability to reduce the ions based on their charge. Through electrodialysis, an electrical potential transports and segregates charged aqueous species. The electrical current is used to continuously regenerate the resin, eliminating the need for periodical regeneration. The EDI process produces industrial process water of very high purity, using less than 95% of the chemical products used in the conventional ion exchange processes. With EDI system membranes and electricity replace a million gallons of acid and caustic chemicals that the old processes required daily. How does it work? An EDI stack has the basic structure of a deionization chamber. The chamber contains an ion exchange resin, packed between a cationic exchange membrane and an anionic exchange membrane. Only the ions can pass through the membrane, the water is blocked. When flow enters the resin-filled diluting compartment, several processes are set in motion. Strong ions are scavenged out of the feed stream by the mixed bed resins. Under the influence of the strong direct current field applied across the stack of components, charged ions are pulled off the resin and drawn towards the respective, oppositely-charged electrodes. In this way, these charged strong-ion species are continuously removed and transferred into the adjacent concentrating compartments. As the ions go towards the membrane, they can pass through the concentration chamber (see figure) but they cannot reach the electrode. They are blocked by the contiguous membrane, that contains a resin with the same charge. As the strong ions are removed from the process stream, the conductivity of the stream becomes quite low. The strong, applied electrical potential splits water at the surface of the resin beads, producing hydrogen and hydroxyl ions. These act as continuous regenerating agents of the ion-exchange resin. These regenerated resins allow ionization of neutral or weakly-ionized aqueous species such as carbon dioxide or silica. Ionization is followed by removal through the direct current and the ion exchange membranes. The ionization reactions occurring in the resin in hydrogen or hydroxide forms for the removal of weakly ionized compounds are listed below:
CO2 + OH- ==> HCO3- HCO3- + OH- ==> CO32- SiO2 + OH- ==> HSiO3- H3BO3 + OH- ==> B(OH)4- NH3 + H+ ==> NH4+
Applications EDI is useful for any application that requires constant and economic removal of water impurities without using dangerous chemicals. Some examples are: Reuse of residual water in the food and beverages industry Chemical production Biotechnology Electronics Cosmetic Laboratories Pharmaceutical industry Boiler Feed Water Reduction of ionizable SiO2 and TOC (total organic carbon) Since installation EDI units perform quite reliably, providing the customers with high purity production water for either power plant boiler feed or microchip rinse water. The water produced has met or exceeded customer high-purity water specifications. In addition, when a dilute stream cleaning was required as a result of fouling, product quality was completely recovered. Advantages As a substitute for the more traditional ion-exchange process, EDI brings advances in both energy and operating expenses to the high purity water treatment train. By eliminating the periodic regeneration requirement of ion exchange resin, environmental benefits are also realized by avoiding the handling and processing of acid and caustic chemicals brought to the site. Some of the advantages of the EDI, as opposed to the conventional systems of ionic interchange, are: Simple and continuous operation Chemicals for regeneration completely eliminated Cost-effective operation and maintenance Low power consumption Non-pollution, safety, and reliability It requires very few automatic valves or complex control sequences that need supervision by an operator It requires little space It produces high purity water in a constant flow It provides complete removal of dissolved inorganic particles In combination with reverse osmosis pre-treatment, it removes more than 99.9% of ions from the water Disadvantages EDI cannot be used for water having a hardness higher than 1, since the calcium carbonate would create a scab in the camera of the concentrated one, limiting the operation It requires purification pretreatment Carbon Dioxide will freely pass through an RO membrane, dissociating and raising the conductivity of water. Any ionic species formed from the carbon dioxide gas will lower the outlet resistivity of the water produced by EDI. The management of CO2 in water is typically handled in one or two ways: the pH of the water can be adjusted to allow the RO membrane to reject the ionic species or the carbon dioxide can be removed from the water using a stripping gas.