With the increasing cost of chemical regeneration and waste neutralization, as well as the liabilities of storing and handling hazardous chemicals, many industries are looking at alternatives to on-site regenerable ion exchange for producing high-purity water.

One alternative is continuous electrodeionization (CEDI), which uses a combination of ion exchange resins and membranes and direct current to continuously deionize the water without the need for chemicals.

CEDI vs Conventional Ion Exchange
Conventional ion exchange technology uses polymer resin beads to attract and remove certain contaminant ions, such as dissolved inorganics and some dissolved organics, from water. Eventually, the resin beads become saturated with contaminants and must be regenerated off-line with hydrochloric acid and caustic.

As with conventional ion exchange, CEDI removes dissolved inorganics and some dissolved organics, but also removes weakly ionized contaminants such as dissolved silica and carbon dioxide. Because CEDI resins are continuously regenerated by direct current, they do not become exhausted. This continuous electrodeionization allows CEDI systems to produce multi-megohm quality water without chemical regeneration.

How CEDI Works
There are several types of CEDI modules available today. Most often, these are plate-and-frame devices, providing flow rates from 0.5 gallons per minute (gpm) to greater than 1,000 gpm. One CEDI device, the patented CDI System from USFilter, consists of anion and cation permeable ion exchange membranes, with mixed bed ion exchange resin packed between them, as shown in the figure. The membranes are ion exchange resins manufactured in sheet form. Resin compartments alternate between product water (diluting) and waste water (concentrating) compartments. Direct current is applied to the anode (positive electrode) on one end of the module, and to the cathode (negative electrode) on the other end. The electric potential drives the ions captured by the ion exchange resins through the membrane, and continuously regenerates the resins in the module.

Advantages
CEDI systems are typically pretreated with reverse osmosis (RO) to produce ultra-high-purity, mixed-bed-quality water. RO-pretreated CEDI systems achieve greater than 99.5 percent salt removal, and produce up to 18 megohm-cm resistivity (0.055 micromho/cm conductivity) water-comparable to mixed-bed ion exchange. Because they never need chemical regeneration, these systems operate continuously with no downtime, making duplexing (two separate systems) unnecessary. The waste stream can be safely discharged or recycled to the RO system. There are no liability issues related to bulk chemical storage and handling, no corrosion problems from hydrochloric acid fumes and no paperwork related to hazardous waste disposal. In addition, users save money on chemical and waste neutralization or waste disposal costs. Operating expenses are also lower, as CEDI systems are maintenance-free and use very little electricity. For example, when fed with 50 micromho water, a typical CDI system uses one Kilowatt-hour (KWH) of electricity to deionize 1,000 gallons. Because CEDI systems convert 80 to 95 percent of the feedwater into product water, there is less water sent to drain.

Another advantage of many CEDI systems is their ability to remove silica, carbon dioxide (CO2) and total organic carbon (TOC). For example, some systems have been shown to remove from 95 percent to more than 98 percent of feedwater silica, depending on the system operating ranges. CDI Systems remove up to 30 ppm of CO2, which is not removed by reverse osmosis, and also can remove 90 percent of the feedwater TOC, when effectively pretreated with 185-nanometer ultraviolet light.

Applications
CEDI systems produce high-purity water for industrial processes such as food and beverage manufacturing, electronics rinsing, surface finishing and optical glass applications. Because CEDI systems remove silica, they are being chosen for boiler makeup in power and cogeneration, as well as for semiconductor manufacturing. CEDI systems are popular with the pharmaceutical and biotechnology industries, as they produce water that meets United States Pharmacopoeia (USP) purified water specifications. CEDI systems can also be used for general industrial applications requiring low to medium quality deionized water, such as bottle washing, electrocoat painting, electroplating and chemical manufacturing. In this case, RO pretreatment is not required, since CEDI can remove 95 to 99 percent of the dissolved ions from pretreated tap water.

Is CEDI for You?
For companies that require high-purity water, but don't want the costs and problems associated with bulk chemical handling and storage or waste neutralization, CEDI is an economical alternative. CEDI systems are ideal for those who need a continuous supply of water with no downtime. In addition, CEDI is particularly well suited for high-flow-rate applications and thoserequiring silica, CO2 and/or TOC reduction. And for those wishing to reduce water usage and costs, the system's high-water-recovery rate is a distinct advantage. ·

About the Author:
Trude Witham is senior technical writer for the ultrapure water group at USFilter. She has been with the company for 16 years, and has authored and co-authored a number of articles on water treatment topics.