Electrodeionization

To offer all its potential, EDI feed water needs pre-treatment, usually reverse osmosis. Feed water must follow certain requirements, these parameters are fixed before to prevent damage to the equipment. Some parameters are:

• In most cases hardness of feed water must be < 1ppm as CaCO_3. In some cases hardness can be as high as 2 or 4 ppm as CaCO3_.
• Silica content (SiO₂) must be < 1ppm or < 2 ppm in the CDIT thin cell modules.
• CO₂ must be monitored to prevent excessive loading of anion exchange resin.
• TOC, which can foul resins and membranes
• Chlorine, ozone and other oxidizers oxidize resins and membranes and create permanent damage.

To eliminate or minimize the concentration polarization phenomenon present in electrolysis systems, electrodeionization originated in the early 1950s. Klansman developed the first descriptions of electrodeionization in a 1953 patent and subsequently published many papers following the subject.[1]

The technology was limited in application because of the low tolerance of total dissolved solids, hardness and organics. During the 1970s and 1980s, reverse osmosis became a preferred technology to ion exchange resin for high TDS waters. As RO gained popularity, it was determined that EDI would be a suitable polishing technology. Packaged RO and EDI systems were used to displace chemically regenerated ion exchange systems.

In 1986 and 1989, companies like Millipore, Ionpure, HOH Water Technologies, and Ionics Inc. developed electrodeionization devices. The initial devices were large, costly, and often unreliable. In 1995, Glegg Water Conditioning introduced E-Cell brand electrodeionization. The new technology reduced cost and improved reliability, based on a modular design standard. E-Cell was also offered to many OEMS and revolutionized the industry. Competitors like Electroplate and Snow Pure soon followed with modular leak-free designs.

Presently, this technology is widely available from much water treatment companies, but should only be applied by experts who understand the limitations and use top-quality products.[2][3]

When fed with low total dissolved solids (TDS) feed (e.g., feed purified by RO), the product can reach very high purity levels (e.g., 18 megohms/cm, Resistivity / Conductivity Measurement of Purified Water). The ion exchange resins act to keep the ions, allowing these to be transported across the ion exchange membranes. The main applications of EDI technology, such as that supplied by Ionpure, E-cell, and Snow Pure, are in electronics, pharmaceuticals, and power generation. Concerning electronics, deionized water is used to rinse components during manufacturing. The electronic chips are very small, so the components are very condensed inside. If ions are present between the components, a short circuit can occur and make the chip unusable. Electrodeionisation systems have been applied for the removal of heavy metals from different types of wastewater such as mining, electroplating, and nuclear processes. The ions primarily present are those related to chromium, cobalt, nickel, copper and others.

An electrode in an electrochemical cell is referred to as either an anode or a cathode, a term that was coined by Michael Faraday. The anode is defined as the electrode at which electrons leave the cell and oxidation occurs, and the cathode is the electrode at which electrons enter the cell and reduction occurs. Each electrode may become either the anode or the cathode, depending on the voltage applied to the cell. A bipolar electrode is an electrode that functions as the anode of one cell and the cathode of another cell.

Each cell comprises an electrode and an electrolyte with ions that undergo either oxidation or reduction. An electrolyte is a substance containing free ions that behave as an electrically conductive medium. Because they comprise ions in solution, electrolytes are also known as ionic solutions, but molten electrolytes and solid electrolytes are also possible. They are sometimes referred to in abbreviated jargon as lytes.

Water is passed between an anode (positive electrode) and a cathode (negative electrode). Ion-selective membranes allow the positive ions to separate from the water toward the negative electrode and the negative ions toward the positive electrode. High purity deionized water results.[3]

When using an excess of current that is higher than the necessary for the movement of the ions. A portion of the water will be split, forming OH- and H+. This species will replace the impurity anions and cations in the resin. This process is called regeneration in situ of the resin. And because it occurs during the process itself, there is no need to stop the installation and use chemicals in other techniques.[4]

Electrodeionization installation scheme

The typical EDI installation has the following components: anode and cathode, anion exchange membrane, cation exchange membrane, and resin. The most simplified configuration comprises 3 compartments. To increase production, the number of compartments or cells can be increased.

The cations flow toward the cathode and the anions flow toward the anode. Only anions can go through the anion exchange membrane and only cations can go through the cation exchange membrane. This configuration allows anions and cations to only flow in one direction because of the membranes and the electric force, leaving the feed water free of ions (deionized water).

The concentration flows (right and left of the feed flow) are rejected and they can be wasted, recycled, or used in another process.

The purpose of the ion exchange resin is to maintain stable conductance of the feed water. Without the resins, the conductance will drop dramatically as the concentration of ions is decreasing. Such drop-off of conductance makes it very difficult to eliminate 100% of the ions, but using resins makes it possible.

• video.
• Continuous Electrodeionization (CEDI/EDI) Ionpure CEDI Products
• Bộ lọc nước phèn
• Electrodeionization Technology
• EDI History
• Electrodeionization Systems, Electrodeionization Systems
• Advanced Electrodeionization Technology for Product Desalting, Argonne National Laboratory