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Electrodeionization Water Purification

Wed, 10/31/2007 - 8:00pm

Chemical-Free, Simple and Economical Method for Producing Type 2 Water
by Jamie Grossi, Associate Product ManagerWater Purification, Sartorius Stedim Biotech

As analytical laboratory processes evolve, the need for water purification processes for the small scale production of highly purified and cost effective water in the laboratory has grown. Over the past 20 years, one of these emerging water purification technologies has been electrodeionization (EDI).

EDI, also referred to as continuous deionization (CDI), is a chemical-free process that removes ionized and ionizable species from solution using ion exchange resins that are continuously regenerated by an electric current. Purified water produced by EDI is commonly referred to as type 2 water. To understand the process of EDI, it is necessary to first understand the configuration of a typical EDI module.

Click to enlarge.
EDI modules, also referred to as stacks, are comprised of cell pairs. Each cell pair contains an anode on one end and a cathode on the other end. (Figure 1) In between the anode and cathode are 3 compartments, all filled with mixed bed ion exchange resins. Each compartment is separated by alternating selectively permeable anion or cation membranes that are manufactured from ion exchange resins. The center compartment is termed the dilute (product water) compartment and the two compartments on either end are termed the concentrating (waste) compartments.

Water enters both the dilute and concentrating compartments. When a direct current (DC) is passed through the cell the cations migrate from the diluting compartment through the cation permeable membrane towards the cathode, the anions migrate towards the anode through the anion permeable membrane and both are removed in the concentrating streams. The cations from the left hand concentrate compartment are impeded from passing into the dilute stream towards the cathode by the selectively permeable anion membrane. Likewise, in the right hand side concentrate compartment anions are impeded from passing into the dilute stream towards the anode by the selectively permeable cation membrane. The end result is deionized water passing through the dilute stream in the range of 10-15 MΩ× cm.

The importance of the resin inside the cell pair compartments is that it acts as a conductor, enabling the electrical current to coerce the captured cations and anions through the resin and selectively permeable membranes for concentration and removal in the concentrate streams. This electrical current also serves to split the water molecules into hydrogen and hydroxyl ions in the dilute stream, which act to continuously regenerate the mixed bed resins, ensuring they do not become exhausted and require replacement or regeneration.

Electrodeionization as Part of a Process
As previously mentioned, EDI is a process that acts to remove charged species from water. Though EDI modules remove charged species at an economical and continuous rate, they are somewhat ineffective at removing other species such as bacteria, pyrogens, colloids, particulates and organics, and thus require appropriate pretreatment steps to perform correctly. A common order of pretreatment steps is illustrated in Figure 2, and includes activated carbon, depth filtration, reverse osmosis and softening.

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Activated carbon and a depth filter are included in the pretreatment cartridge of Figure 2. The activated carbon will act to remove chlorine and some organics from water. The importance of removing chlorine is that high levels will act to oxidize the anion exchange resin within the EDI module making it more susceptible to scaling. The depth filter will act to remove any particulates or colloids coming in from the general feed water source. The RO modules will act to remove a broad spectrum of contaminants including inorganic and organic contaminants, particulates or colloids too small for the depth filter to remove and microorganisms and pyrogens. All of these contaminants can be removed with an efficiency of up to 99%. Finally the softener cartridge will act to remove divalent cations from solution, such as calcium and magnesium. Divalent cations, at high enough concentrations, will cause scaling on the mixed bed resins of the EDI module decreasing the effective lifetime of the module and its performance.

Conclusion
EDI is an emerging technology that offers significant benefits to the laboratory. It provides a high quality of water in the range of 10-15 MΩ× cm with typically < 30 ppb of TOC and controllable flow rates. It can replace mixed bed deionization resin tanks and avoid the need for periodic costly replacement. It also negates the cost and labor involved with the regeneration of mixed bed resins, which requires the use of costly chemicals that need to be disposed of in a manner safe to the environment. From an economic perspective, EDI modules normally only require replacement every 3 to 5 years, with longer life spans being common. Further, the water from EDI systems can be used for a variety of applications within the laboratory, including feeding ultrapure water systems, feeding instrumentation such as autoclaves and dishwashers and for general laboratory applications such as buffer and reagent preparation. EDI is quickly proving itself to be an economical and simple way of producing high quality purified water for the laboratory.

For more information, contact Jamie Grossi, Associate Product ManagerWater Purification, Sartorius Stedim Biotech at Jamie.Grossi@sartorius-stedim.com or by phone at 877-452-2345 x8827 or 631-245-3361

AT A GLANCE
• Purified water produced by EDI is commonly referred to as type 2 water
• EDI modules are comprised of cell pairs, each of which contains an anode on one end and a cathode on the other end
• Resin inside the cell pair compartments acts as a conductor
• EDI modules are ineffective at removing species such as bacteria and thus require pretreatment steps• EDI provides a high quality of water in the range of 10-15 MΩ× cm with typically < 30 ppb of TOC and controllable flow rates

ONLINE
For additional information on the technology discussed in this article, see Laboratory Equipment magazine online at www.LaboratoryEquipment.com or the following Web sites:
www.sartorius-stedim.com
www.cediuniversity.com
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