Electrodialysis

 

Water is known to be an essential resource for Humanity. People around the world are convinced about the need of providing new and improved means of maintaining a healthy environment. Despite its abundance on the planet, the fact is that only around 0.8% of the total earth's water is fresh water.As a result, the present surface water resources will no longer be sufficient to meet the future needs for mankind and water tends to become scarce because of disordered consumption, which contaminates our reserves and make them unsuitable for use, either for human consumption, or for industrial use.

Due to the attention toward the importance of water in life and the water resource shortage, water desalination has been give noticeable importance.Desalination the water is one of the ways of purifying water which makes it possible to be used.In the last few years, several wastewater treatment processes have been proposed that would enable water reuse, with special emphasis on waste minimization and hazardous waste treatment. But still, decreasing the energy requirements and infrastructure costs of existing desalination technologies remains a challenge. In general, desalination technologies can be categorized into two different mechanism separations, i.e. thermal and membrane-based desalination. Membrane technologies are getting more and more attention nowadays due to their reliable contaminant removal without production of any harmful by-products, especially in water and wastewater treatment.

Electrodialysis is one of the membrane methods for desalinationwhich because of less energy-consuming is a desirablemethod in comparison with other ones such as thermal method. This process offers an electrochemical technique that removes ionic pollutants from an aqueous solution, producing two new solutions: one concentrate of ions and another consisting of almost pure water. The first solution can be reintroduced to the industrial process and the water can be reused. Associated to another field of research, such as membrane technology, this technique can be powerful in the treatment of industrial effluents.

Electrodialysis (ED) uses direct current power to remove salts and other ionized species through cation and anion ion selective membranes to a concentrate collecting stream. In almost all practical processes, multiple electrodialysis cells are connected in series to form an electrodialysis stack, with alternating anion (AM) and cation (CM) exchange membranes forming the multiple electrodialysis cells (Fig.1).

 

Figure 1Schematic diagram of a electrodialysis cell

 

A simple electrodialysis cell consists of (i) a feed (diluate) compartment, (ii) a concentrate (brine) compartment and (iii) an electrode compartment. The diluate (D) feed stream (from which salt is eliminated), a brine or concentrate (C) stream (where salt is concentrated) an and electrode (E) stream (where electrode processes are taking place) are allowed to flow through the appropriate cell compartments formed by the pair of AM and CM ion exchange membranes. When an electrical potential difference is applied between the two end-electrodes, the negatively charged ions (e.g. chloride) in the diluate stream migrate towards the positively charged anode, passing through the positively charged anion exchange membrane (AM). They are prevented from further migration toward the anode by the negatively charged cation exchange membrane (CM) and therefore stay in the C stream, which becomes concentrated with anions. The positively charged species (e.g. sodium) in the D stream migrate toward the negatively charged cathode, passing through the negatively charged cation exchange membrane. These cations also stay in the C stream, prevented from further migration toward the cathode by the positively charged anion exchange membrane.

The overall result of the electrodialysis process is that the salt concentration increases in the concentrate stream with a depletion of salt in the diluate solution feed stream.

Electrodialysis is widely used for desalination of brackish water. In some areas of the world it is the main process for the production of potable water. Other applications, such as the treatment of industrial effluents, the production of boiler feed water, demineralization of whey and deacidifying of fruit juices are gaining increasing importance and are found in large-scale industrial installations. Another application of electrodialysis, limited to Japan and Kuwait, that as gained considerable commercial importance is the production of table salt from sea-water. Japan has no natural deposits of NaCl and had formerly obtained the salt by evaporation of sea-water. The use of ED to produce a concentrated brine up to 20% TDS has substantially reduced energy costs (less than 200 kwh per ton of salt) and a salt purity of 97% has been obtained.

PNF company is now in three areas has used this technology: desalination of brackish water with high TDS (such as seawater), denitrification and desalination of Glycerine.