THE EVOLUTION OF DEIONIZATION
What is EDI?
High purity water production has traditionally used a combination of membrane separation and ion exchange processes. EDI is a process which combines semi-impermeable membrane technology with ion-exchange media to provide a high efficiency demineralization process. Electro dialysis employ 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 to their charge. Through electro dialysis 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 the million gallons of acid and caustic chemicals that the old processes required daily.
The process of continuous electrodeionization has been in commercial use for 17 years, but is still not as well understood as more mature water purification processes such as reverse osmosis and ion exchange deionization. To remove residual cations and anions and producing demineralized water with maximum conductivity of 0.2µs/cm, we can use EDI system instead of mixed bed.
Electrodeionization (EDI) is a continuous process of removing ionizable species from feed water using DC power. EDI modules offer a cost-effective and chemical-free alternative to mixed-bed ion exchange resins for polishing RO permeate and eliminate the need to store and handle hazardous chemicals used for resin regeneration.
EDI module designs utilize a distinct, leak-free, low-maintenance spiral-wound design containing a membrane and IX resins, sealed in a high-strength fiberglass-reinforced plastic pressure vessel, making them an ideal choice for high purity water needs.
Feed water (dilute stream) enters from the bottom of the EDI module and is diverted into vertically spiraled cells known as the “D” (dilute chamber). The dilute stream ﬂows vertically through ion-exchange resins located between two membranes (an anion membrane speciﬁcally designed to allow migration of only anions and a cation membrane speciﬁcally designed to allow migration of only cations). Concentrate enters the bottom of the module through the center pipe and is diverted into spirally ﬂowing cells “C” (concentrate) chambers.
DC current is applied across the cells. The DC electrical ﬁeld splits a small percentage of water molecules (H2O) into hydrogen (H+) and hydroxyl (OH) ions. The H+ and OH- ions attach to the cation and anion resin sites, continuously regenerating the resin. Hydrogen ions have a positive charge and hydroxyl ions have a negative charge. Each will migrate through its respective resin, then through its respective permeable membrane and into the concentrate chamber due to its respective attraction to the cathode or anode. Cation membranes are permeable only to cations and will not allow anions or water to pass, and anion membranes are permeable only to anions and will not allow cations or water to pass. Contaminate ions, dissolved in the feed water, attach to their respective ionexchange resin, displacing H+ and OH- ions. Once within the resin bed, the ions join in the migration of other ions and permeate the membrane into the “C” chambers. The contaminant ions are trapped in the “C” chamber and are recirculated and bled out of the system.
The feed water continues to pass through the dilute chamber and is puriﬁed and is collected on the outlet of the “D” chambers and exits the EDI module.
How does it work?
An EDI stack has the basic structure of a deionization chamber. The chamber contains a ion exchange resin, packed between a cationic exchange membrane and a anionic exchange membrane. Only the ions can pass through the membrane, the water is blocked.
When flow enters the resin filled diluiting 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 in to the adiacent 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+
• The EDI modules utilizes a unique, leak free, low maintenance spiral wound design containing membrane and ion exchange resins, sealed in a high strength (FRP) pressure vessel. EDI modules are the ﬁrst truly cost-effective alternative to post-RO deionization applications.
EDI is useful for any application that requires constant and economic removal of water impurities without using dangerous chemical. Some examples are:
• Reuse of residual water in food and beverages industry
• Chemical production
• 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 diluite stream cleaning was required as result of fouling, product quality was completely recovered.
EDI System Feed Water Quality
Below Table summarizes the limits of quality parameters of the feed water. It is recommended to respect these limits to ensure successful operation of the EDI system.
EDI System Operation Condition
The EDI modules use electrical current to force a continuous migration of contaminant ions out of the feed water and into the reject stream while continuously regenerating the resin bed with H+ (hydrogen) and OH- (hydroxyl) ions that are derived from water splitting. The patented ﬂow process of the dilute and concentrate streams make the EDI module completely unique. Feed water (dilute stream) enters from the bottom of the EDI module and is diverted into vertically spiraled cells. The dilute stream ﬂows vertically through ion-exchange resins located between two membranes (an anion membrane speciﬁcally designed to allow migration of only anions, and a cation membrane speciﬁcally designed to allow migration of only cations). Concentrate enters the bottom of the module through the center pipe and is diverted into spirally ﬂowing cells (concentrate) chambers.
Operating condition in EDI modules is as follow;
DOW Electrodeionization Modules
EDI modules are made using a patented spiral wound design containing membrane and ion exchange resins sealed in a high strength fiber glassed reinforced plastic (FRP) pressure vessel. The modules can be used in place of conventional mixed bed ion exchange for polishing of reverse osmosis (RO) permeate eliminating the need to store and handle hazardous chemicals. EDI modules optimize performance, maintain continuous product quality and can produce up to 18 megohm-cm product water for high purity and ultrapure industrial water applications.
• Once through concentrate flow path eliminates brine injection and recirculation greatly simplifying system designs.
• Distinct spiral design prevents internal and external leaks common with compression style plate & frame stacks.
• Easy to clean, non-resin filled concentrate chambers.
• Lightweight modules require no special lifting equipment allowing for easy access modular designs.
• Built in sample port for dilute product water sampling.
• Cost effective-spiral wound EDI modules allow system integrators to build systems that have both lower capital and operating costs when compared to conventional mixed bed ion-exchange.
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 reliablility
• It requires very few automatic valves or complex control sequences that need supervision by an operator
• It requires little space
• It produces high pure 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.
• EDI cannot be used for water having 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 rejuect the ionic species or the carbon dioxied can be removed from the water using a strip gas