EDI Working Principle

A typical EDI system involves the following process: pretreatment – RO – EDI. EDI uses ordinary ion exchange resins to continuously remove ions from water, but because it uses electric current to continuously regenerate the resin, it eliminates the need for periodic chemical regeneration.

A typical EDI membrane stack consists of a certain number of units sandwiched between two electrodes (see Figure 1, EDI working principle diagram). Each unit contains two different types of chambers: the desalinated water chamber (chamber D) and the concentrate chamber (chamber C) for collecting the removed impurity ions. Chamber D is filled with a mixture of cation and anion exchange resins located between two membranes: a cation exchange membrane that allows only cations to pass through and an anion exchange membrane that allows only anions to pass through.

The resin bed is continuously regenerated using a direct current applied across the chamber. The voltage causes water molecules in the feed water to decompose into H+ and OH-. These ions are attracted by their respective electrodes and migrate through the cation and anion exchange resins towards their corresponding membranes. When these ions pass through the exchange membranes into the concentrate chamber, H+ and OH- combine to form water. The generation and migration of H+ and OH- are the mechanism by which the resin achieves continuous regeneration.

When impurity ions such as Na+ and Cl- in the feed water are adsorbed onto the corresponding ion exchange resin, these impurity ions undergo ion exchange reactions similar to those in a conventional mixed bed, displacing H+ and OH- accordingly. Once the impurity ions in the ion exchange resin also participate in the migration of H+ and OH- towards the exchange membrane, these ions continuously pass through the resin until they permeate through the exchange membrane and enter the concentrate chamber. Due to the blocking effect of the exchange membranes in adjacent compartments, these impurity ions cannot migrate further towards the corresponding electrode, thus concentrating in the concentrate chamber. This concentrate containing impurity ions can then be discharged from the membrane stack.

In a typical EDI system, 90-95% of the feed water passes directly through chamber D, while 5-10% is distributed to chamber C. The concentrate is pumped and circulated within the membrane stack to achieve a high flow rate, which improves desalination efficiency, promotes water mixing, and reduces potential scaling. Concentrated ions are removed from the membrane stack by discharging a certain proportion of water from the concentrate circulation loop; this water, with a pH of 5-8, can be recycled or directly returned to the inlet of the pretreatment system.

During the electro-deionization process, high-quality demineralized water is produced by removing impurity ions from the feed water.