Electrodialysis

The process of Electrodialysis Explained

In an electro-dialysis plant a number of ionselective membrances, made from approximately the same material as the resins, is applied in form of cells. Each cell - consisting of one cationic and one anionic membrane - is arranged in a stack analogous to a plate heat exchanger. The spacing between each membrane is about 1 mm. Between each of the above mentioned cells flows a diluent salt solution.

The entire stack is placed between a series of DC electrodes providing an electric field. Under influence of the electrical field, the cations will migrate in the direction of the electric current - i.e. towards the - pole - and pass through the cation membrane into the flushing salt solution which is recycled during operation. The anions will migrate in the opposite direction of the electric current - i.e. toward the +pole - and will pass through the anion membrane into the flushing salt solution. Like the flushing salt solution the whey is recycled during operation via a batch tank with a conductivity meter attached.

To obtain sufficient conductivity of the whey to be treated a preconcentration to 20-28% solids, as well as a processing temperature of 30-45°C, is necessary. The recirculation time depends on the required demineralization level and may reach up to 6 hours, if 90% demineralization is aimed at. Naturally, this results in bacteria growth. To avoid this, the temperature may be lowered, which, however, will result in an increased plant size, and thus higher investment, or formaldehyde may be added. As the salt content decreases in the whey the conductivity decreases, and it is necessary to increase the applied voltage to compensate for this. To keep the salt content in the flushing water at an optimum level it is diluted as the process continues. Excess salt solution is passed to the drain. The cleaning of the membranes is done by passing acid and alkaline through the plant, an operation lasting 2 to 6 hours depending on the plant manufacturers.