An Overview of Deionization Technology

Principles of Operation

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. The following Figure illustrates the ion exchange process for mixed bed deionization.

Figure 1 - Schematic represntation of ion exchange showing exchange of sodium and chloride for hydrogen and hydroxyl ions. The latter combine to form water.

It should be noted that while deionizers produce water of high ionic quality, they do not remove bacteria or endotoxin (pyrogens). In fact, deionizers often worsen quality in terms of bacteria and endotoxin, the resin bed providing an environment which is conducive to bacterial proliferation (3-5).

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

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, if used at all, 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.

Portable exchange deionizers offer maximum convenience, with maintenance and service being provided by the vendor. More importantly, portable exchange deionizers offer greater safety to personnel, since strong acids and bases are used for regeneration.

Because of the hazardous nature of these chemicals, permanent deionizers are seldom used in hemodialysis facilities. A simplified diagram of the construction of a portable, mixed bed deionizer is shown in the Figure below, together with an appropriate resistivity monitor.

Applications

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

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

A highly simplified disgram of this type of application is shown below, including monitors appropriate for deionizer operation.

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Typical configuration for deionization combined with reverse osmosis. Note an inline resistivity indicators at (1) and (2) which lights or sets alarm or lights at purity levels less than specified requirements.

Reliance on ion exchange to remove aluminum should be approached withgreat caution. Aluminum is amphoteric in nature. 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 (6). Thus, in the pH range 6.5 to 8.5, ion exchange is limited in its ability to remove aluminum from the water (7), 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 such applications as a temporary or backup purification process to reverse osmosis. While circumstances vary, it is generally not economical to utilize deionization alone to produce large volumes of purified water.

This is because 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 will be the costs of deionization. The combination of reverse osmosis, followed by deionization, greatly reduces costs and reverse osmosis 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. This is economical because the regeneration costs of dual bed units are lower than the mixed bed type.

Equipment Selection

As with water conditioners/softeners, deionizer capacities are also rated in terms of "grains of total dissolved solids as calcium carbonate". The term, total dissolved solids (TDS), includes all ionized substances.

Dividing the exchange capacity of the deionizer by the TDS of the supply water gives an indication of the volume of water which may be purified before the unit is exhausted. The utility of such calculations is as a guide to estimating the frequency (and cost) of portable exchange unit replacement, on-line, continuous-reading monitors being relied on to indicate when the resistivity of the deionizer effluent has degraded to 1 megohm-cm(for example), the minimum value for a medical application such as hemodialysis.

SAMPLE CALCULATION

Determine the volume of water which can be deionized with a deionizer having a capacity of 12,000 grains with supply water having a total dissolved solids (TDS) of 120 mg/L, expressed as CaCO3.

1. Convert the supply water TDS from mg/L (sometimes reported on water analyses as parts per million or ppm) to grains/gal from 1 grain/gal = 17.1 mg/L.

(120 mg/L)/17.1 = 7.0 grains/gal

2. Divide the capacity of the deionizer, rated in terms of grains of TDS as CaCO3, by the TDS of the supply water as determined in step 1 above.

(12 000 grains)/7.0 grains/gal = 1710 gallons

Thus, the estimated volume of water which can be deionized is approximately 1700 gallons.

Deionizer manufacturers also rate their equipment in terms of the maximum flow rates which may be achieved. Exceeding these flow rates can result in inadequate contact time between the supply water and resin, causing water quality to be unacceptable, and/or loss of water pressure which can adversely affect downstream equipment.

As most deionizers are of the portable exchange type, which are limited in size because of the need to transport them to and from the regeneration plant, facilities having high flow or volume requirements may have to arrange the units in a parallel configuration or employ an on-site DI regeneration capability.

Operating Guidelines

Portable exchange deionizers are normally maintained by independent vendors, although as noted above, some companies may perform on-site regeneration or replacement functions. On site regeneration and replacement is readily accomplished as the portable exchange units are typically equipped with "quick-disconnect" couplings for this purpose.

For most applications, deionizer water quality is measured electrically in terms of "resistivity," in units of "ohm-cm". Again, as an example, the minimum resistivity for hemodialysis water produced by deionizers is 1 million ohm-cm or 1 megohm-cm (8).

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

Like other ion exchangers, 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(9-11).

Mixed bed deionizers utilize both 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 exchanged 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. Because of these characteristics, it is essential that the deionizers be both properly sized and carefully and continuously monitored.

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 (8,10). Monitors such as "lights" (shown in the preceeding figures) which 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.

A variety of ion exchange resins are available, not all of which are suitable for hemodialysis applications. 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 (8,10). When using portable exchange deionizers be certain to specify that, during regeneration, resin used for critical, medical applications such as hemodialysis must not be mixed with resins used for anything other than potable water purification.

It has also been reported that, unless preceded by carbon adsorption, deionizer effluent may contain carcinogenic nitrosamines (12). For this reason, deionizers must always be used in combination with carbon adsorption beds.

As described earlier, deionizer resin provides an environment which is conducive to bacterial proliferation. For this reason, deionizers, including those which are preceded by reverse osmosis, should be expected to produce water which may contain excessive levels of bacteria and/or endotoxin and further means to ensure effluent of adequate biologic quality should be employed.

It should be recognized that of the types of equipment previously listed for this purpose, ultrafiltration, submicron filtration and ultraviolet irradiation, only ultrafiltration has the capability of endotoxin removal.

Technical References for Water Conditioning & Deionization

1. Owens D: Practical Principles of Ion Exchange Water Treatment. Tall Oaks Publishing, Inc., Voorhees, NJ, 1985.

2. Nickey WA, Chinitz VL, Kim KE, Onesti G and Swartz C: Hypernatremia from water softener malfunction during home dialysis [letter]. JAMA 214:915, 1970.

3. Otten G and Brown G: Bactena and pyrogens in water treatment. Amer Lab 5:49-60, 1973.

4. 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.

5. 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.

6. 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.

7. 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.

8.American National Standard for Hemodialysis Systems (RD-5), Association for the Advancement of Medical Instrumentation, Arlington, VA, 1982.

9. Johnson WJ and Taves DR: Exposure to excessive Duoride during hemodialysis. Kidney Int 5:451-454, 1974.

10. 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, Silver Spring, MD, June, 1980.

11. Dorson W: Evaluation and selection of water treatment equipment. ~ Issues in Hemodialysis, Association for the Advancement of Medical Instrumentation, Arlington, VA, 1981, pp 49-54.

12. 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.