Electro dialysis is a method in which the ions are pulled out of the salt water by passing direct current, using electrodes and thin rigid plastic membrane pair (natural or synthetic).

When direct electric current is passed through saline water the sodium ions $(\text{Na}^+)$ start moving towards negative pole (cathode); while the chloride ions $(\text{Cl}^-)$ start moving towards the positive pole (anode), through the membrane.

As a result, the concentration of brine decreases in the central compartment; while it increases in two side compartments.

Desalinated brine is removed from the central compartment from time to time, while concentrated brine is replaced by fresh brine/sea water.