Electro-Deionization (EDI) is a technology applicable to the wastewater industry for the removal of dissolved impurities. Systems consist of a series of Ion Exchange Membranes, within the membranes are Electrodes, which have alternating charges applied. The contaminated waste water is passed between a series of Anode (positive + electrode) and a Cathode (negative – electrode) Electrodes. This alternating current allows a Reduction – Oxidation (Re-Dox) reaction to occur. The Anode is defined as the electrode at which electrons leave the cell and oxidation occurs, and the cathode as 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 membrane.
A Bi-Polar electrode is an electrode that functions as the anode of one cell and the cathode of another cell. By alternating the current across the membrane it is possible to increase the longevity of the unit as material will fall away from the membrane.
Electro-Deionization (EDI) has been found to be potentially useful in the removed of Dissolved Solids, such as water salts in hard water areas, for example Calcium (Ca) and Magnesium (Mg) and in industrial applications where discharge waste water might be high in dissolved solids such as sugars in the food and drinks industry.
Some of the main advantages with the EDI systems are that they can be designed for low flow and hence can have small footprint sizes, this is particularly useful if dealing with a local heavily contaminated waste-stream rather than a collective amount. Another advantage is that in general, EDI does not use chemicals reactants.
Systems can be used on their own, or in conjunction with other systems such as after Reverse Osmosis (RO) where High Purity Water is required
In standard EDI, the membranes and resins have to be regularly cleaned and replaced. Therefore, unless units are built for the provision of units to be taken out of operation e.g. by supplying in series, a provision for maintenance and cleaning has to be included.
For some applications where hard dissolved solids are present or the available footprint is low then chemicals such as Sodium Hypochlorite can be used as an accelerant to increase the reaction and efficiency times. Sodium Hypochlorite is a chemical compound with the formula NaOCl. It is composed of a Sodium (Na) cation and a Hypochlorite (OCl) anion The down-side to using this type of chemical, being that in general this does lend itself to more regular cleaning of the membranes due to scaling.
Continuous Electro-deionization (CEDI) are as the name suggests units where a continuous flow can be achieved. These tend to be units which are Bi-Polar electrodes, as previously described where the current is alternating. They are particularly good when used on flow with loads of low Total Dissolved Solids (TDS), such as post Reverse Osmosis.
High Purity Water from such systems can be achieved, normally levels of Conductivity of <0.1 µS/cm are not unusual. However, in these systems it is important to ensure that the feed-water is free from impurities such as Carbon Dioxide (CO2) and other Dissolved Gases. These impurities will be caught up in the Re-Dox reaction going on in the EDI or CEDI unit, and will decrease efficiencies.
Condorchem Envitech has successfully supplied EDI and CEDI to industries such as. The Electronics Industry, Pharmaceutical, and for Power Generation applications.