1.Description of electrodialysis water treatment seawater desalination plant

Dialysis is a kind of natural physical phenomenon. The phenomenon is dialysis when water of two kinds of concentration are separated by a permeable membrane and the electrolytic ions with higher salt content would permeate through the membrane to the water with lower salt content. Since the dialysis is caused by the difference in concentration, it is called dialysis caused by different concentration. The dialysis process is related to the concentration difference. The larger the concentration difference, the faster the dialysis process and otherwise the slower the process. Since dialysis is realized with concentration difference as the driving force, the speed of permeation is relatively slow. If the speed is to be increased, a DC field can be applied on both sides of the membrane. The electrolytic ions would transfer rapidly through the membrane. This is called electrodialysis. Dialysis membrane is a film made of polymeric materials, on which there are active groups of ion exchange. A membrane with acid active groups is called cation membrane and a membrane with alkali active group is called anion membrane. Analyzed in terms of structure, membranes can be classified into three kinds: heterogeneous membranehomogeneous membrane and semi-homogeneous membrane.

2.Flow of electrodialysis water treatment seawater desalination plant

In an electrodialysis stack, the dilute (D) feed stream, brine or concentrate (C) stream, and electrode (E) stream are allowed to flow through the appropriate cell compartments formed by the ion exchange membranes. Under the influence of an electrical potential difference, the negatively charged ions (e.g., chloride) in the dilute stream migrate toward the positively charged anode. These ions pass through the positively charged anion exchange membrane, but are prevented from further migration toward the anode by the negatively charged cation exchange membrane and therefore stay in the C stream, which becomes concentrated with the anions. The positively charged species (e.g., sodium) in the D stream migrate toward the negatively charged cathode and pass 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. As a result of the anion and cation migration, electric current flows between the cathode and anode. Only an equal number of anion and cation charge equivalents are transferred from the D stream into the C stream and so the charge balance is maintained in each stream. The overall result of the electrodialysis process is an ion concentration increase in the concentrate stream with a depletion of ions in the dilute solution feed stream see video.

The E stream is the electrode stream that flows past each electrode in the stack. This stream may consist of the same composition as the feed stream (e.g., sodium chloride) or may be a separate solution containing a different species (e.g., sodium sulfate).Depending on the stack configuration, anions and cations from the electrode stream may be transported into the C stream, or anions and cations from the D stream may be transported into the E stream. In each case, this transport is necessary to carry current across the stack and maintain electrically neutral stack solutions