Water purification and recycling using the CDI process

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Abstract
Continuous Deionization (CDI)—also known as electrodeionization—is a hybrid separation process combining ion‐exchange resins and ion‐exchange membranes. A voltage gradient is used as the driving force for separations. The process is capable of acieving high levels of purification and concentration of dissolved ionic solutes without the use of chemical regenerants. Early research in the field focussed on the removal and concentration of trace metals from effluent streams. In CDI equipment, the electrical conductivity of only the diluting compartments is enhanced by the ion‐exchange packing. For this reason, purification levels often improve as the water recovery increases. Examples of improved performance are provided as measured by water conductivity, and levels of dissolved carbon dioxide.In high purity water production, concentration ratios of 1,000 to over 10,000 are achieved at high electrical efficiency. The process is therefore well‐suited for liquid purification applications requiring a high extent of deionization coupled with a high concentration, low volume concentrate stream with no net chemical additives. Using a municipal water source, purification levels, as measured by electrical conductivity, of 10 to 0.1 microsiemens/cm are achievable at 75–80% water recovery. Recovery is limited primarily by the precipitation of solids in the concentrating stream. In specialized applications, e.g., for production of less than 0.1 microsiemens/cm ultra‐pure water downstream of reverse osmosis (RO), CDI product water recoveries of 95% are achievable. Typical flow diagrams are provided and described.

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