Electrodialysis (ED) and reverse electrodialysis (RED)

Condorchem Envitech has electrodialysis and reverse electrodialysis experts among its staff and extensive experience in using the different variants of this technique. It uses high efficiency modules in its designs that can treat a variable flow rate between 1.7 and 30 m³/h in each module. These can operate in parallel; so treated flows can reach hundreds of cubic meters per hour.

Electrodialysis processes (ED, RED, SED, EDBM and EDM) applications are constantly increasing.

Among the most used are those for obtaining ultra pure water, concentrating salt currents and desalinating brackish waters. The energy consumption in all these applications is much lower than for other membrane processes, such as reverse osmosis.

In addition, they work at low pressures, require no great food pre-treatment and there are no problems of fouling or scaling with the membrane. Finally, unlike reverse osmosis, EDR can achieve zero discharge, which is a highly appreciated objective in numerous industrial sectors.

In conclusion, electrodialysis is an alternative technique to reverse osmosis and is usually the more financially competitive.

Electrodialysis (ED) is a process in which ions are transported through an ion exchange membrane using electrical energy as the driving force.

The membranes contain a high density of ionic groups which allow selective transport of ions through the membrane depending on their charge. Counterions (oppositely charged ions) are allowed to pass while coions (those of the same charge) are prevented by the Donnan effect. This process is carried out between two electrodes under the influence of an electric field.

The applied electrical energy moves the ions from the least concentrated solution to the most concentrated solution. Thus, solvent purification occurs by removing the solute. This is different to what happens in reverse osmosis or ultrafiltration, where transport of the solvent occurs through the membrane at the same time as the passage of the solute is prevented.

Inverse or reverse electrodialysis (RED) works by using the same mechanism as electrodialysis, except that in RED the polarity of the electrodes is periodically reversed (approximately 3 to 4 times per hour); and the output of the concentrated and diluted solutions are exchanged automatically by means of valves. This moves the ions in the opposite directions, and makes the formation of scale difficult while washing the membrane.

The benefits of electrodialysis in relation to reverse osmosis are less rejects, lower sensitivity to suspended solids, a longer membrane life, no need for complete pre-treatment, easier operation and low electricity consumption.

Electrodialysis is widely used for the desalination of brackish water and for the production of drinking water. As this is its main application, it is also used on a smaller scale in the following industries: food (e.g. in the desalination of whey, removal of tannic acid from wine and the recovery of citric acid from fruit juice); pharmaceutical (production of ultra-purified water); biotechnology (protein production); surface treatment, textiles, mineral recovery, power generation, electronics and wastewater treatment (e.g. removal of heavy metals or salts for water reuse and reverse osmosis reject effluent concentration).

Different processes have been developed within this technology that give ambitious results with more specific applications by adjusting the membrane type and arrangements. The following are notable among these developments:

• Selective electrodialysis or selectrodialysis (SED)

SED applies the principles of conventional ED, while using monovalent cationic permselective membranes (MCPSM) and monovalent anionic permselective membranes (MAPSM) which are membranes selective to ion charge; they allow monovalent ions to be separated from their polyvalent equivalents.

However, a limitation to this technique is the need to treat the ions present in the solution: there can be no divalent cations in the food when SED is used to separate anions based on their charge; and, conversely, the food cannot contain divalent anions when SED is used to separate monovalent cations from polyvalent ones.

This technique separates the ions according to their charge, and is useful in applications where the fractionation of the ions is of special interest; this is the case with the pre-treatment of phosphate-containing currents for subsequent recovery or the removal of salts from solutions of high salinity.

• Bipolar Electrodialysis (EDBM)

Ion exchange membranes are used to separate and concentrate acids and bases in a salt stream in the EDBM process. The differentiating part of this process is the bipolar membrane, which is formed by two different layers selective to oppositely charged ions.

The main advantage of this technique is that acid and base solutions can be formed directly with the ions from a starting salt and the H⁺ and OH^– ions from the water in the different electrolytic cell compartments.

Under the influence of an electric field, water diffuses at the membrane interface where it dissociates into its constituent H⁺ and OH^– , ions, which are then transported through the anionic and cationic membrane layers to the different chambers. The result is the concentration of these species in their respective chambers.

Salts present in industrial effluents, such as sodium acetate, chloride, sulfate and nitrate and potassium fluoride can be converted into their respective acids and bases by EDBM.

The most relevant applications of this process are the following:

• Regeneration of liquors used in the manufacture of stainless steel (recovery of HF, HN0₃ , KOH).
• Desulfurization of fumes and gases, producing sodium sulphite
• Recovery of organic amino acids and acids
• Ion exchange regenerant recovery
• Acid and base purification

• Electrodialysis metathesis (EDM)

The concept of electrodialysis metathesis (EDM) was developed to desalinate brackish waters without producing any residual current; that is, with zero liquid discharge (ZLD).

To achieve this purpose, cationic exchange membranes (CEM) and anionic exchange membranes (AEM) are generally used; with monovalent cationic permselective membranes (MCPSM) and monovalent anionic permselective membranes (MAPSM) also being used in certain applications.

By arranging these membranes appropriately, it is possible to separate and concentrate monovalent and divalent anions and cations, all at the same time. Thus, a stream containing monovalent cations with anions and another stream containing monovalent anions with cations are obtained.

This prevents poorly soluble salts, such as CaCO₃ , MgSO₄ and CaSO₄ , from being formed inside the equipment.

Subsequently, the two concentrated streams can be mixed to recover their components for re-use in the ZLD unit, if appropriate.

• Monovalent electrodialysis (mEDR)

The main objective of monovalent electrodialysis (mEDR) is the selective removal of ions or salts, especially with complex wastewater. It is based on conventional electrodialysis principles, but one of the membranes (anionic or cationic) is replaced by a selective membrane (anionic or cationic, as appropriate).

There are no limitations to the types of ions present in food, but an additional solution containing monovalent ions (anions or cations, depending on the specific application) is required.