Deionizers (DI) remove both cations and anions, releasing hydrogen ions (H+) in exchange for the former, and hydroxyl ions (OH-) for the latter. The hydrogen and hydroxyl ions subsequently combine to form pure water.

Figure 1 illustrates the ion-exchange process for mixed-bed deionization.


Types of deionizers
Deionizers may be categorized as mixed-bed, containing both cation and anion resin in a single vessel, or dual-bed, where each resin type is in a separate vessel.

Mixed-bed deionizers produce water containing the lowest ionic concentrations. Dual-bed deionizers produce water of lesser quality, generally unacceptable for specialized medical purposes such as hemodialysis.

Because of this, dual-bed deionizers are generally employed only as a pretreatment for mixed-bed deionizers. In this configuration, the mixed-bed deionizer will polish the water to very high ionic quality and the service cycle of this unit will also be extended.

As with water softeners, deionizers may be either portable exchange or permanent. Portable exchange deionizers are provided in a fully-regenerated, ready-to-use condition by vendors. When regeneration is needed, it is done by the vendor at a central facility.

A simplified diagram of the construction of a portable mixed-bed deionizer is shown in Figure 2 with an appropriate resistivity monitor.


Applications
Deionizers are most commonly used when ionic contamination is such that reverse osmosis (RO) alone cannot be relied upon to produce water of acceptable quality. In most instances, mixed-bed deionizers may be placed downstream of the RO unit, completing the purification process.

A wide variety of public water vending machines, as well as many industrial applications in the electronics industry, operate this way.

A simplified diagram of this application is shown in Figure 3, including monitors appropriate for deionizer operation.


Reliance on ion exchange to remove aluminum should be approached with caution. In an acidic environment, aluminum exists predominantly as a hydrated cationic complex, while in an alkaline environment the predominant form is the aluminate anion.

In the pH range commonly encountered in most water supplies (between about 6.5 and 8.5), however, the bulk of aluminum is present as neutral, highly insoluble hydrated aluminum hydroxide.

Thus, in the pH range 6.5 to 8.5, ion exchange is limited in its ability to remove aluminum from the water, although the recent upward trend in municipal water pH may offer some improvement by anion exchange resins.

Deionizers may also be used as portable systems and are convenient for use as a temporary or backup treatment to RO.

While circumstances vary, it is generally not economical to use deionization alone to produce large volumes of purified water. Deionizers, like softeners, have a finite capacity for ion exchange, and the costs of regeneration are substantial.

The higher the level of supply water ionic contamination and/or the greater the water consumption rate, the greater the costs of deionization.

The combination of RO followed by deionization greatly reduces costs and RO often extends the service cycle of the deionizer by a factor of 10 or more.

To a lesser extent, costs may be reduced by using a dual-bed deionizer followed by a mixed-bed deionizer because the regeneration costs of dual-bed units are lower than mixed-bed.

Operating guidelines
Portable exchange deionizers are normally maintained by independent vendors although some companies may perform on-site regeneration or replacement functions.

For most applications, deionizer water quality is measured electrically in terms of resistivity in units of ohm-cm. As an example, the minimum resistivity for hemodialysis water produced by deionizers is 1 million ohm-cm or 1 megohm-cm.

For safety and convenience, it is preferable to utilize two mixed-bed deionizers in a series configuration. The upstream unit purifies the water to the 1 megohm-cm level, thus maintaining the downstream unit in a nearly fully-regenerated state.

Exhaustion
Deionizers have a limited capacity and it is important to understand the possible consequences of operating them beyond their limits.

If deionizers are operated to exhaustion, ions previously removed may be released, possibly at concentrations exceeding that of the incoming water — a potentially hazardous phenomenon.

Mixed-bed deionizers use cationic and anionic resins and these typically will not reach exhaustion simultaneously. Consequently, effluent water may become either extremely acidic or extremely alkaline, depending on which resin reaches exhaustion first.

In addition, the effluent water may contain high levels of previously ex-changed chemicals.

For example, exhausted anion resin may release fluoride ions which, when combined with hydrogen ions from the unexhausted cation resin, would form hydrofluoric acid, an extremely toxic substance.

It is essential that the deionizers are properly sized and carefully and continuously monitored.

Monitors
Accurate, temperature-compensated monitors are mandatory following deionizers, but when a series of deionizers are employed, less accurate monitors may be used for all but the final unit.

Monitors, such as lights that are illuminated at specified resistivities, are economical and, while typically not temperature compensated, are acceptable for all but the final deionizer and permit maximum utilization of the ion exchange resin.

Resins
A variety of ion-exchange resins are available. If water is being produced for consumption or medical applications, be certain to specify that, at a minimum, only food-grade materials are used.

Additionally, deionizers are often used in industrial applications involving reclamation of heavy metals or exposure to hazardous organic chemicals. When using portable exchange deionizers, dealers should specify that during regeneration resin used for critical medical applications must not be mixed with resins used for anything other than potable water purification.

Limitations
It has also been reported that, unless preceded by carbon adsorption, deionizer effluent may contain carcinogenic nitrosamines. For this reason, deionizers must always be used in combination with carbon adsorption beds.

While deionizers produce water of high ionic quality, they do not remove bacteria or endotoxin (pyrogens). In fact, deionizer resin provides an environment that is conducive to bacterial proliferation.

For this reason, it is prudent to follow deionization purification with equipment that removes bacteria and/or endotoxin, such as ultrafiltration (UF), submicron filtration, steam distillation or even ultraviolet (UV) irradiation.

Aqua Technology Water Stores, with headquaters in California, distribute residential, medical and industrial water treatment systems.

References
Owens D, Practical Principles of Ion Exchange Water Treatment. Tall Oaks Publishing, Inc., 1985.
Nickey WA, Chinitz VL, Kim KE, Onesti G and Swartz C: Hypernatremia from water softener malfunction during home dialysis [letter]. JAMA 214:915, 1970.
Otten G and Brown G: Bactena and pyrogens in water treatment. Amer Lab 5:49-60, 1973.
Favero MS, Petersen NJ, Boyer KM, Carson LA and Bond WW: Microbial contamination of renal dialysis systems and associated health nsks. Trans Am Soc Artif Intern Organs 20:175-183, 1974.
Chapman K, Alegnani G, Heinze G, Flemming C, Kochling J, Croll D, Kladko M,Lehman D, Smith D, Adair F, Amos R, Enzinger D, Grant D and Soli T: Protection of water treatment systems, Part I: The problem. Pharm Technol 7(5):48-57, 1983.
Gacek EM, Babb AL, Uvelli DA, Fry DL and Scribner BH: Dialysis dementia: The role of dialysate pH in altering the dialyzability of al’,minum. Trans Am Soc Artif Int Organs 25:409-415, 1979.
Rahman H, Channon SM, Parkinson IS, Skillen AW, Ward MK and Kerr DNS: Aluminum in the dialysis field. Clin Nephrol 24(Suppl 1):S78-S83, 1985.
American National Standard for Hemodialysis Systems (RD-5), Association for the Advancement of Medical Instrumentation, 1982.
Johnson WJ and Taves DR: Exposure to excessive Duoride during hemodialysis. Kidney Int 5:451-454, 1974.
Keshaviah P, Luehmann D, Shapiro F and Comty C: Investigation of the Risks and Hazards Associated with Hemodialysis Systems. (Technical Report, Contract 223-785046), U.S. Department of Health and Human Services, Public Health Service, Food and Drug Administration, Bureau of Medical Devices, June, 1980.
Dorson W: Evaluation and selection of water treatment equipment. ~ Issues in Hemodialysis, Association for the Advancement of Medical Instrumentation, 1981, pp 49-54.
Kirkwood RG, Dunn S, Thomasson L and Simenhoff ML: Generation of the precarcinogen dimethylaitrosamine (DMNA) in dialysate water. Trans Am Soc.Artif Intern Organs 27:168-171, 1981.


From the April 2005 edition of Water Technology magazine. For a free introductory subscription, click here.



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