Electrodialysis (ED) is a separation process in which charged membranes and electrical potential difference are used to separate ionic species from an aqueous solutions and other uncharged components and is frequently used for water and wastewater desalination.
In this article, YASA ET’s engineers introduce the working principle and benefits of this technology.
What is Electrodialysis (ED)?
The technology of purifying and separating substances by selective membranes and electrodes is called Electrodialysis (ED).
The method of separating different solute particles (such as ions) using the selective permeability of a semipermeable membrane is called dialysis. When dialysis is performed under the action of an electric field, the phenomenon in which charged solute particles (such as ions) in the solution migrate through the membrane is called electrodialysis.
At the beginning, it was used for seawater desalination and is now widely used in chemical industry, light industry, metallurgy, papermaking, medicine Industry, especially the preparation of pure water and the treatment of three wastes in environmental protection are the most important, such as acid and alkali recovery, electroplating waste liquid treatment and recovery of useful substances from industrial wastewater.
What Is Electrodialysis Reversal (EDR)?
Electrodialysis Reversal (EDR) is an electrodialysis reversal water desalination membrane process that has been commercially used since the early 1960s. An electric current migrates dissolved salt ions, including fluorides, nitrates and sulphates, through an electrodialysis stack consisting of alternating layers of cationic and anionic ion exchange membranes.
The principle of Reversed Electrodialysis (EDR) is basically the same as that of electrodialysis, except that during operation, the polarities of the positive and negative electrodes of EDR are switched each other at regular intervals.
The EDR system is composed of three parts: the electrodialysis body, the rectifier and the automatic inversion system. The inverter is generally divided into the following three steps: (1) Convert the polarity of the DC power supply electrode, so that the thick and thin chambers are interchanged, and the ions flow (2) Switch the inlet and outlet valves to exchange the water supply and drainage systems of the thick and thin chambers; (3) Continue for 1 to 2 minutes after the polarity change and resumes normal operation.
The use of the inverted electrode electrodialyzer greatly improves the electrodialysis operating current and water recovery rate, and prolongs the operation period. It is widely used in drinking water purification and boiler feed water treatment.
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Introduction to Electrodialysis Process Working Principle
The electrodialysis process is a combination of the electrochemical process and the dialysis-diffusion process; driven by an external DC electric field, the selective permeability of the ion-exchange membrane (that is, cations can pass through the cation-exchange membrane, and anions can pass through the anion-exchange membrane), anions and cations move towards the anode and cathode, respectively.
In the process of ion migration, if the fixed charge of the membrane is opposite to that of the ions, the ions can pass; if they have the same charge, the ions are repelled, so as to achieve the purpose of desalination, concentration, purification or purification of the solution.
Which Membranes Does Electrodialysis (ED) Electrodialysis Reversal (EDR) Use?
The semi-permeable membranes used in Electrodialysis (ED) and Electrodialysis Reversal (EDR) are actually an ion exchange membrane. This ion exchange membrane can be divided into cation exchange membrane (positive membrane) and anion exchange membrane (negative membrane) according to the charge properties of ions.
In an aqueous electrolyte solution, the cationic membrane allows cations to pass through and rejects and blocks anions, and the anion membrane allows anions to pass through and rejects and blocks cations. This is the selective permeability of ion exchange membranes. In the process of electrodialysis, the ion exchange membrane does not exchange with certain ions in the aqueous solution like the ion exchange resin, but only selectively permeates ions of different electrical properties, that is, the ion exchange membrane does not need to be regenerated.
The electrodes and membranes of the electrodialysis process are composed of sections called electrode compartments, and the electrochemical reactions that take place in them are the same as ordinary electrode reactions. The oxidation reaction occurs in the anode chamber, the anode water is acidic, and the anode itself is easily corroded. The reduction reaction occurs in the cathode chamber, the cathode water is alkaline, and the cathode is easy to scale.
In practical applications, an electrodialyzer is not composed just of a pair of anion and cation exchange membranes (because the efficiency is very low), but a hundred pairs or even hundreds of pairs of exchange membranes are used, thus greatly improving the efficiency.
Practical Applications of Electrodialysis (ED) and Reverse Electrodialysis (EDR)
Electrodialysis (ED) and Reverse Electrodialysis (EDR) have been widely used in the desalination of brackish water and is the main method of producing fresh water in some parts of the world.
Due to the newly developed charged membranes with higher selectivity, lower membrane resistance, better thermal stability and higher mechanical strength, the electrodialysis process is not only limited to applications in desalination plants, but also in the food, pharmaceutical and chemical industries.
The electrodialysis process has many industrial applications, such as the treatment of industrial wastewater, mainly including the recovery of acids and metals from the waste liquid formed by cleaning metal surfaces with acid solutions; recovery of heavy metals from electroplating wastewater ions; recovery of sulphate from synthetic fibres wastewater; recovery of sulphite from pulp waste liquid, etc.
It is also used in the food industry, such as milk desalination to make infant milk powder; it is used in the chemical industry to separate ionic and non-ionic substances.
Industrial Applications of Electrodialysis (ED) and Reversed Electrodialysis Reversal (EDR)
Electrodialysis (ED) and Reversed Electrodialysis (EDR) are suitable for industrial applications such as water supply treatment in electronics, medicine, chemical industry, thermal power generation, food, beer, beverage, printing and dyeing and coating industries. It can also be used in physical and chemical processes such as concentration, purification and separation of materials.
Electrodialysis can also be used for the treatment of wastewater and waste liquid and the recovery of precious metals, such as the recovery of nickel from electroplating waste liquid.
Electrodialysis (ED) and Electrodialysis Reversal (EDR) Process Applications
Electrodialysis process of water is used for different applications. The main application of electrodialysis has historically been the desalination of brackish water or seawater as an alternative to RO for potable water production and seawater concentration for salt production.
The electrodialysis process is used for many different applications:
Electrodialysis (ED) and Electrodialysis Reversal (EDR) Applications
Small and medium scale drinking water production (e.g., towns & villages, construction & military camps, nitrate reduction, hotels & hospitals)
Pre-demineralization (e.g., boiler makeup & pre-treatment, ultrapure water pre-treatment for electro deionization, etc.)
Food processing industries
Water reuse (e.g., desalination brine treatment, industrial laundry wastewater, etc.)
Agricultural water (e.g., water for greenhouses, hydroponics, irrigation, livestock, etc.)
Glycol desalting (e.g., antifreeze / engine-coolants, capacitor electrolyte fluids, oil and gas dehydration, etc.)
Automatic Electrodialysis (ED) System Recovery and Desalination Rate
Automatic control of reverse electrodialysis (EDR), allows for smooth and simple operation management. The raw water utilization rate can reach 80%, and the general raw water recovery rate is between 45-70%.
Electrodialysis is mainly used for primary desalination of water, and the desalination rate is between 45-90%. It is widely used in seawater and brackish water desalination; primary desalination when preparing pure water, desalination and softening of boiler and power equipment feedwater, etc.
Shortly, electrodialysis can be said to be a desalination technology, because there is a certain amount of salt in all kinds of water (including natural water, tap water, and industrial wastewater), and the anions and cations that make up these salts act on the DC electric field.
The bottom layer will move to the electrodes in the opposite direction. If an anion and a cation exchange membrane are inserted into an electrodialyzer, due to the selective permeability of the ion exchange membrane, that is, the cation exchange membrane only allows cations to pass freely, and the anion exchange membrane only allows anions to pass through. In the middle compartment, the concentration of salt will decrease due to the directional migration of ions, while the two compartments near the electrode are the concentration chambers for anions and cations, and finally the desalination chamber in the middle achieves the purpose of desalination.
Electrodialysis (ED) and Electrodialysis Reversal (EDR) Advantages
Electrodialysis (ED) and Electrodialysis Reversal (EDR) have many advantages:
ED and EDR can desalinate, concentrate, separate and purify the aqueous electrolyte solution at the same time;
It can be used for the purification of non-electrolytes such as sucrose to remove the electrolytes;
In principle, the electrodialyzer is an electrolytic cell with a diaphragm, which can utilize the high redox efficiency on the electrode.
For the migration of the ions with the same kind, the selective permeability of the ion exchange membrane is often impossible to be 100%, so there will always be a small amount of opposite ions permeating the exchange membrane;
Diffusion of ion concentration, due to the concentration difference in the solution in the concentration chamber and the desalination chamber, there will always be a small amount of ions diffused and migrated from the concentration chamber to the desalination chamber, thus reducing the dialysis efficiency;
Infiltration of water, although the exchange membrane does not allow solvent molecules to penetrate, but due to the concentration difference between the desalination chamber and the concentration chamber, some solvent molecules (water) will penetrate into the concentration chamber;
In the electrodialysis of water, due to the hydration of ions and the formation of an electric double layer, water molecules can also migrate from the desalination chamber to the concentration chamber under the action of a DC electric field;
The polarization ionization of water, sometimes due to poor working conditions, will force the water to ionize into hydrogen ions and hydroxide ions, which can enter the concentration chamber through the exchange membrane;
Pressure osmosis of water, due to the difference in fluid pressure between the concentration chamber and the desalination chamber, water molecules are forced to infiltrate from the side with higher pressure to the side with lower pressure. Obviously, these secondary processes are detrimental to electrodialysis, but they can all be avoided or controlled by changing the operating conditions.
What Is the Best Treatment Application for Electrodialysis (ED) and Electrodialysis Reversal (EDR)?
The best treatment system made by Electrodialysis (ED) and Electrodialysis Reversal (EDR) equipment often involves various stages and technologies. The correct equipment needs to be selected and carefully manufactured to reach optimal results to reach the project goals.
The first step in planning a treatment system is to analyze the chemical composition of the waters. Secondly, extensive testing needs to be undertaken to find the correct parameters for treating the specific components. Finally, the right equipment for the treatment system can be manufactured and the system can be installed at the client site.
For the right electrodialysis treatment system, you need the right expertise. For more information about our solutions you can get in touch with us at:
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Sources Systematic research on the bipolar membrane reverse electrodialysis - ScienceDirect