Electro Dialysis (ED) is a membrane process, during which ions are transported through semi permeable membrane, under the influence of an electric potential.<./p>

The membranes are cation- or anion-selective, which basically means that either positive ions or negative ions will flow through. Cation-selective membranes are polyelectrolytes with negatively charged matter, which rejects negatively charged ions and allows positively charged ions to flow through. By placing multiple membranes in a row, which alternately allow positively or negatively charged ions to flow through, the ions can be removed from wastewater.

In some columns concentration of ions will take place and in other columns ions will be removed. The concentrated saltwater flow is circulated until it has reached a value that enables precipitation. At this point the flow is discharged.

This technique can be applied to remove ions from water. Particles that do not carry an electrical charge are not removed. Cation-selective membranes consist of sulphonated polystyrene, while anion-selective membranes consist of polystyrene with quaternary ammonia. Sometimes pre-treatment is necessary before the electro dialysis can take place. Suspended solids with a diameter that exceeds 10 µm need to be removed, or else they will plug the membrane pores. There are also substances that are able to neutralize a membrane, such as large organic anions, colloids, iron oxides and manganese oxide. These disturb the selective effect of the membrane.

Pre-treatment methods, which aid the prevention of these effects are active carbon filtration (for organic matter), flocculation (for colloids) and filtration techniques

The electrolyte solution used to ensure the conductivity and remove gases produced by electrode reactions in the system.
The degree of electrodialysis depends on

• Ion exchange membrane structure,
• Feed water ion concentration,
• Current density
• pH
• Flow rate
• ED cell structure

ED is an electromembrane separation process that ions are transferred through ion exchange membranes under electrical current (DC voltage).
When electrical current is applied

• Cations migrate to the cathode, or negative electrode.
• Anions migrate to the anode, or positive electrode.
• Water molecules break down and produce two hydroxyl ions and hydrogen gas at the cathode.
• Water molecules dissociate to four hydrogen ions, one molecule of oxygen, and four electrons at the anode
• Chlorine gas may be formed at the anode.

Electrodialysis (ED) processes are used on a large scale in commercially. ED systems, corresponding processes and their applications are summarized in table 1. ED has been widely used to demineralize, concentrate and/or convert salt-containing solutions. ED contains ion exchange membranes that ions are transported from one solution through ion selective membranes to another solution under influence of an applied electric potential difference. Performance of the system depends on operating conditions and ED cell structure.