TECHNICAL PAGES
By Jeff Tate
For more information on this topic, go to www.watertechonline.com and enter keyword(s): Electrodeionization, ultrapure, ion exchange. Electrodeionization basics
EDI provides a significant advantage to ultrapure water users.
Electrodeionization (EDI) is an ultrapure water treatment process that pol- Ion exchange resin, two ion exchange mem- ishes reverse osmosis (RO) permeate without chemical regeneration. EDI has branes and two electrodes. been in existence for over 55 years, and has been commercially available for Figure 1 (page 25) shows a diagram of the more than 23 years. It has become a proven and acceptable technology for all internal process of an electrodeionization industrial water treatment users who require high purity. device. Two electrodes are on either side of The process of operating an EDI system is extremely simple. multiple EDI compartments, which are Electrodeionization is sometimes referred to as continuous electrodeionization known as diluting chambers and concentrat- (CEDI). Since electrodeionization is continuous in nature, we will refer to the ing chambers. As water flows through the technology simply as EDI. While there are several different commercially EDI module and power is applied, there are available EDI products with varying design, all operate using the same prin- three processes occurring simultaneously: ciples of chemistry. The deionization process where the water is Electrodeionization modules are electrochemical devices. Since each EDI mod- purified by ion exchange; ion migration ule is driven by electrical energy from an outside power supply, more specifical- where the ions are removed from the resin; ly it is an electrolytic cell. Each EDI module consists of five primary components: and continuous regeneration of the resin.
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typical of all high purity resin, including electrodeionization. The cation exchange resin is in the regenerated hydrogen (H+) form as the charged exchange sites are bonded with hydrogen ions. As the feed water contacts the resin, the contaminant Figure 1 cations such as calcium, magnesium, sodi- In Figure 1, we see there are two types of um, potassium and ammonium have a high- chambers in an EDI device. Diluting cham- er affinity to the site charges on a resin bead bers (D-chambers) are the portion containing than a hydrogen ion. The cation exchange mixed bed ion exchange resin where water is resin releases a hydrogen ion to bond with purified or diluted of ions. Concentrating the cation. Likewise, anion exchange resin is chambers (C-chambers) are the areas where in the regenerated hydroxide (OH-) form as water is concentrated of ions, and becomes the charged exchange sites are bonded with waste water. The D-chambers contain both hydroxide ions. cation exchange resin and anion exchange The contaminant anions — such as car- resin. There are several different concentrate bonate, bicarbonate, chloride, sulfate, chamber designs; however the most efficient nitrate, fluoride, silica, boron and carbonic EDI devices contain resin-filled concentrate acid — have a higher affinity to the anion chambers, known as an “all filled” EDI exchange resin than hydroxide. The anion design. exchange resin releases a hydroxide ion to The D and C chambers are separated by bond with the anion. The released hydro- ion exchange membranes. The membranes gen and hydroxide ions bond to form are similar in material and charge to the ion water. Water is purified, or deionized, by exchange resin. For example, cation the removal of the cations and anions as it exchange membranes only allow cations to flows through the mixed resin bed. pass, and anion exchange membranes only allow anions to pass. Water and oppositely Ion migration charged ions may not pass across the ion The second process in electrodeioniza- exchange membrane used in EDI. tion is ion migration. This differs from chemically regenerated ion exchange as Deionization EDI continuously removes the ions from Deionization is the removal of ions — the resin. Conventional ion exchange resin both positively charged cations and nega- becomes exhausted until chemical regener- tively charged anions. Cations are posi- ation occurs. EDI power sources supply a tively charged ions because they have a DC electrical current between the two elec- loss of one or more electrons, very small trodes. The electrical current is the move- negatively charged particles. For example, ment of electrons between the electrodes, the sodium ion (Na+) is positive because from the anode to the cathode. The elec- the ion lost one electron. Calcium (Ca++) trode with a negative charge is the cathode, has twice the positive charge of sodium where reduction of the oxidation number because it has lost two electrons. Anions occurs (electrons are available). The elec- contain a negative charge because they trode which oxidizes (takes electrons) is the contain one or more additional electrons. anode and has a positive charge. As the The chloride ion (Cl-) is formed when chlo- ions are removed from the feed water, rine gains one cations that have lost electrons are attracted electron, and to the negative cathode where electrons are oxide (O--) has supplied, or the oxidation number is gained two elec- reduced. Likewise, anions are attracted to trons. the negative anode where electrons leave Figure 2 shows the cell or oxidation number increases. the ion exchange Once on the resin, positively charged ions process. This is will migrate through the cation resin bed, Figure 2 (Concluded on next page) Circle 228
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(Continued from prior page) chamber is resin regeneration. With con- Therefore, EDI operation is continuous. through the cation exchange membrane and ventional ion exchange, the resin is regen- The ions are continuously removed, and into the concentrate chamber due to their erated with acid and caustic chemicals. the resin is continuously regenerated with- attraction to the cathode. Negatively Hydrochloric acid (HCl), or sometimes sul- out chemicals. This provides a significant charged ions will migrate through the anion phuric acid (H2SO4), is applied to cation advantage to ultrapure water users over resin bed, through the anion exchange exchange resin to regenerate. When this is either onsite or offsite chemically regener- membrane and into the concentrate cham- done, the massive concentration of H+ dis- ated ion exchange. The operation of the ber due to their attraction to the anode. places the cations on the resin. Sodium EDI system is as simple as operating a Once the ions are in the concentrate stream, hydroxide (NaOH) is applied to anion reverse osmosis system and the results of they are not able to continue their migration exchange resin and the massive concentra- EDI are more reliable than chemically to the electrode as they encounter an oppo- tion of OH- displaces the anions from the regenerated ion exchange. WT sitely charged ion exchange membrane resin. EDI does not require acid to regener- which does not permit entry to the adjacent ate the cation exchange resin, nor does it Jeff Tate is president of Agape Water Solutions diluting chamber. Concentrate water exits require caustic chemicals to regenerate the Inc. Based in Harleysville, Pennsylvania, Agape the EDI module and is most often sent anion resin. Instead, it takes advantage of Water Solutions is the Master Service Provider in directly to drain since this is typically only the electrical current that is applied across North America for Ionpure Technologies, a lead- 5-10 percent of the feed water. EDI waste the EDI module. ing manufacturer of electrodeionization modules. water can be recovered by sending it to a In the presence of the electrical field, Recognized worldwide as an expert in reverse ventilated raw water storage tank, some- a phenomena known as “water splitting” osmosis and electrodeionization, Tate has written times advantageous with larger systems. occurs. The electricity causes a small numerous technical articles and has delivered percentage of water molecules to split presentations at several conferences. He can be Resin regeneration into hydrogen and hydroxide ions which reached via e-mail at [email protected] or The third process that occurs as the ions continuously regenerate the resin bed: www.agapewater.com. + - are removed and migrate to the concentrate H2O → H + OH .
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