Electrodeionization

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Electrodeionization (EDI) is a process that removes ionizable species from liquids using electrically active media and an electrical potential between an anode and a cathode to influence ion transport. A typical system has at least one product channel through which liquid to be processed is flowed. At the outer edge of one side of the product channel is an anion permeable membrane which channel defines the outer limit of the product channel on that side. The opposite side of the product channel is defined by a cation permeable membrane. Waste channels are formed on the opposite sides of the membranes from the product channel. The anion side waste channel is formed between the anion permeable membrane and the anode that is spaced outwardly and apart from the anion membrane. Likewise the cation side waste channel is formed between the cation permeable membrane and the cathode.  

The product channel is filled with a mixed bed of ion exchange resin ion exchange materials. These ion exchange materials are either anion specific or cation specific.  The liquid to be purified is flowed into the product channel while an electrical potential is applied to the system. The ion exchange materials in the product channel selectively cause the ions in the liquid to attach to the bead surfaces where they are transferred from bead to bead toward the electrode (anode or cathode) they favor. Once they pass through the ion selective membrane, they are passed to ion exchange materials in the waste channels. A liquid is also flowed through each waste channel that removes the ions from the ion exchange materials and carries them to waste.  

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Electro-Deionization, or EDI, is a process that evolved from conventional ion exchange technology.  EDI provides continuous demineralization at recovery rates of 90% or more.  In EDI, just as in conventional ion exchange, cations and anions in the feed water are exchanged for hydrogen and hydroxyl ions in the ion exchange resins, producing demineralised water.  The key operational difference is that with EDI, the ion exchange resin is regenerated continuously, while with conventional ion exchange, chemical regeneration is performed intermittently. 

Continuous electro-deionization (CEDI) is the most advanced generation of ion exchange technology and it is achieved electrochemically, by means of ion conducting membranes and an imposed electric current. CEDI is a chemical-free operation and therefore environmentally friendly, a continuous production, and has extremely low operating costs when compared to chemically regenerated ion exchange technology.

 

Advantages of EDI

Conventional ion exchange is a viable treatment option for applications where high-flow and high conductivity requirements are not critical. In other situations, however, cost comparisons between ion exchange and continuous deionization may reveal potential economic advantages.

The biggest difference is that continuous electrodeionization eliminates all chemical regeneration and waste neutralization steps. While capital equipment costs may be higher with continuous deionization systems, operating expenses are lower because there are no regeneration chemicals, and labor or maintenance costs are less. There are significant tangible cost benefits associated with the elimination of regeneration. The costs of regeneration labor and chemicals are replaced with a small amount of electrical consumption.

Electrical requirements are nominal. A typical system uses one kilowatt-hour (kWh) of electricity to deionize 1,000 gallons, based on a feed conductivity of 50 micromhos/cm and a product water resistivity of 10 megohm-cm.

Continuous electrodeionization systems do not require duplexing (two separate treatment units) which can increase the cost, complexity and size of the system.

Both conventional ion exchange and continuous electrodeionization may require pretreated feedwater to prevent scale formation and plugging by colloids and particles. Pretreatment is also required to reduce high levels of eventual free chlorine and organic foulants. The type of pretreatment required is determined by product water quality requirements. For most high-purity water needs, a reverse osmosis (RO) pretreatment step is sufficient. With RO pretreatment, continuous electrodeionization systems can achieve better than 99.5 percent salt removal, reduce the levels of individual ionic species to parts-per-billion or even parts-per-trillion levels, and produce high-purity water with resistivities of 10 to 18 megohm-cm (or 0.1 to 0.055 micromhos/cm conductivity).

Continuously regenerating the ion exchange resins also removes the possibility of exhausted or improperly regenerated resins contaminating the product water.

Continuous electrodeionization systems typically convert 80 to 95 percent of the feedwater into product water. The waste stream can be discharged without treatment or recycled back to the RO pretreatment unit. It can also reduce facilities costs because waste neutralization equipment and hazardous fumes ventilation equipment are not required. The elimination of regeneration chemicals can help improve workplace health and safety, as well as prevent corrosion from hydrochloric acid fumes.

There are also less tangible cost reductions, which are harder to quantify, but usually favor the use of EDI systems. By eliminating hazardous chemicals wherever possible, workplace health and safety conditions can be improved. With today's increasing regulatory influence on the workplace, the storage, use, neutralization, and disposal of hazardous chemicals result in hidden costs associated with monitoring and paperwork to conform to safety and environmental requirements. In addition, the fumes, particularly from acid, often cause corrosive structural damage to facilities and equipment. 

Therefore proper pretreatment is even more important with a EDI device, in order to prevent fouling or scaling. This is one of the reasons that RO pretreatment is normally required upstream of a EDI system. In general, the feed water requirements for EDI systems are more stringent than for chemically regenerated demineralizers

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