Which is better: IX or EDI?

Compiled by Mike Henley



Editor’s note: This column is based on a recent discussion on the LinkedIn Ultrapure Water Group. This column seeks to accurately reflect comments from each contributor. On occasion, there may be the need to edit contributor comments for clarity or length. Readers are invited to join the Ultrapure Water Group and to participate in discussions. An important purpose of the group is to provide a forum for practical discussion of issues facing endusers of high-purity water.


Deionization (DI) is a key element in achieving high-purity water quality. Ion exchange (IX) has long been the key form of DI treatment, and since its introduction in the 1940s, IX has taken many forms: separate anion and cation beds, mixed beds, polishing beds, and packed beds, among others. Over the years, high-purity water systems evolved with complementary treatment steps designed to work with IX.

In the 1980s, electrodeionization (EDI) emerged as an alternative form of DI treatment. Early systems had small capacities that worked well for laboratory water purification, but over time EDI system manufacturers have developed equipment that will produce the purified water quantities needed for a power plant or other industrial endusers. And, water system designers have found the needed pretreatment steps, namely the use of reverse osmosis prior to EDI, to ensure optimum operation.

One advantage EDI brings to the table is that it does not require chemical regeneration, which is an attractive feature. The benefit of IX? A long-standing, proven technology that when properly operated is capable of producing the high-purity water demanded by power, microelectronics, and pharmaceutical plants.

EDI versus IX

Mike: “Which is the better approach—EDI or conventional IX? Or, does it matter? Since first introduced commercially in the 1980s, EDI has been developed to the point that systems are available for everything from laboratory applications up to large volumes such as a power plant would need. So, is it practical for all cases where DI-quality water is needed, or does conventional IX still offer the better choice in some applications? Your thoughts?”

Craig: “This is a very interesting conversation. We have excellent results with the utilization of our Liqui-Cel technology to improve the performance of both technologies with the reduction of CO2. I believe it really comes down to economics, especially if regen is accomplished onsite.”

Fritz: “Well, I can tell something concerning pharmaceutical and lab applications. The main advantages of EDI instead of ion exchange are the high quality (permanent regeneration) and the non-existing need for chemicals. If the EDI module is hot water sanitizable, you don't even need chemical agents for sanitization.

But in most labs, hot water sanitization is no option. Although EDI systems are more expensive (due to patents), operating costs are much lower in sum than using conventional ion exchange.

My last point: people in the pharma industry desire constant quality. That's something only EDI technology can offer.

Falk: “Those in need of a consistently high quality water should prefer EDI. At lower quality needs, or with low water requirements, ion exchangers can be more advantageous.”

Denis-François: “Condensate polishing is an example where conventional IX has still some advantages over EDI.”

Qiu: “or the most of large UPW systems in the electronics industry, which is more than 500 tons per hour, IX is still the first choice.”

Jim:If we are talking about pharma and power— I would definitely say that EDI is the better choice. As already stated, consistency in quality, lower operating costs, and reduced human interaction allow a performance advantage. Outside of high quality needs— such as a specific ion removal, like arsenic, IX is a much better choice as it operates as a filter as opposed to a polishing bed. The processing of the IX resins in these cases calls for the removal of the resins as opposed to chemical regeneration, which makes EDI unacceptable to use.”

John:A comment from the analytical perspective for EDI devices. We have done extensive work with analytical-scale EDI devices using ion chromatography for trace and ultra-trace analysis of anions and cations. For analytical applications, the EDI approaches consistently produce water with lower ionic contaminants compared to conventional mixed-bed columns (traps). Using the EDI device in ion chromatography, trace anions can be determined to the low and sub-parts-per-trillion levels. Carbonate in DI water, which is problematic in anion ion chromatography, is greatly reduced using the point-of-use EDI approach.”

Rob: For general applications in life sciences, pharmaceuticals, and university research, EDI will be superior overall to IX. It will provide more consistent water quality (usually in the 12 to 15 megohm-cm level), reduced bacteria and TOC levels, conserve space, and save a considerable amount of operating costs over the life of the system.

Also, it is wise to consider it as a supplement to IX where ultra-high-purity water is required in the electronics industry by extending the "life" of the IX resin between exchanges.”

Jonathan: “The more crucial the application, in terms of quality or uninterrupted supply, the more IX seems to be paired with EDI. Using both regenerable and non-regenerable IX options, EDI alone can give less "back-up redundancy" if there are operational problems than the extra time you're required to use if also running the flow through an IX set-up (polishing or otherwise). A large storage tank, or slightly less demand for high-purity water, like in a power plant setting, lets you get away with EDI only. This is not so true within a semiconductor setting where the consequences of a loss of quality or quantity are going to get much more expensive.”

Matt: “There are many factors to look at before making a decision on which is the best application for the job. EDI is not the best fit for all applications and has its limits. Influent water parameters need to be met for a successful EDI application. Effluent water quality demands also play into EDI application. You also have the ROI to apply to the job. Even my EDI applications have SDI hook ups for back up or polishing.”


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