First, lets start with a container of water with salts (cations and anions) in solution.
A typical EDI device contains alternating semipermeable anion and cation ion-exchange membranes. The spaces between the membranes are configured to create liquid flow compartments with inlets and outlets. A transverse DC electrical field is applied by an external power source using electrodes at the ends of the membranes and compartments.
When the compartments are subjected to an electric field, ions in the liquid are attracted to their respective counter-electrodes. The result is that the compartments bounded by the anion membrane facing the anode and the cation membrane facing the cathode become depleted of ions and are thus called diluting compartments. The compartments bounded by the anion membrane facing the cathode and cation membrane facing the anode will then ?trap? ions that have transferred in from the diluting compartments. Since the concentration of ions in these compartments increases relative to the feed, they are called concentrating compartments, and the water flowing through them is referred to as the concentrate stream (or sometimes, the reject stream).
Ion Exchange Membranes
Now let's add some ion exchange membranes to direct the ions into different flow channels as shown in the animation below. The red membranes are cation-selective membranes and the blue membranes are anion-selective membranes. The negatively-charged anions (e.g., Cl-) are attracted to the anode (+) and repelled by the cathode (-). The anions pass through the anion-selective membrane and into the adjacent concentrate stream where they are blocked by the cation-selective membrane on the far side of the chamber, and are thus trapped and carried away by the carrier water in the concentrate stream. The positively-charged cations (e.g., Na+) in the purifying stream are attracted to the cathode (-) and repelled by the anode (+). The cations pass through the cation-selective membrane and into the adjacent concentrate stream where they are blocked by the anion-selective membrane and are carried away.
In the concentrate stream, electrical neutrality is maintained. Transported ions from the two directions neutralize one another's charge. The current draw from the power supply is proportional to the number of ions moved. Both the "split" water (H+ and OH-) and the intended ions are transported and add to the current demand.