EDI Ultra-Pure Water Equipment

In the diagram, ion exchange membranes are represented by vertical lines and are labeled with the types of ions they allow to pass through. These membranes do not permit water to pass through, thus separating the dilute water and concentrate water streams. Ion exchange membranes and ion exchange resins work on similar principles, selectively allowing ions to pass through. An anion exchange membrane only permits anions to pass through and blocks cations, while a cation exchange membrane only allows cations to pass through and blocks anions. An EDI unit is formed by filling the space between a pair of cation and anion exchange membranes with mixed ion exchange resins.

The space occupied by the mixed ion exchange resins between the cation and anion exchange membranes is referred to as the dilute water chamber. When a certain number of EDI units are arranged in sequence, with cation and anion exchange membranes alternating, and special ion exchange resins are added between the membranes, the space formed is called the concentrate water chamber. Under the influence of a given direct current voltage, in the dilute water chamber, the cations and anions in the ion exchange resins migrate towards the positive and negative electrodes, respectively, and pass through the cation and anion exchange membranes into the concentrate water chamber. Meanwhile, ions in the feedwater are adsorbed by the ion exchange resins, filling the void lefts by the migrating ions. In fact, ion migration and adsorption occur simultaneously and continuously. Through this process, ions in the feedwater pass through the ion exchange membranes into the concentrate water chamber, where they are removed, resulting in desalinated water.

Negatively charged anions (such as OH⁻, Cl⁻) are attracted to the positive electrode (+) and pass through the anion exchange membrane into the adjacent concentrate water chamber. These ions, continuing to migrate towards the positive electrode, encounter the adjacent cation exchange membrane, which does not allow anions to pass through, thus trapping them in the concentrate water. Cations in the dilute water stream (such as Na⁺, H⁺) are similarly trapped in the concentrate water chamber. In the concentrate water chamber, ions that have passed through the cation and anion membranes maintain electrical neutrality. The electrical current in the EDI unit is proportional to the ion migration and is composed of two parts: one part originates from the migration of removed ions, and the other part is from the migration of H⁺ and OH⁻ ions produced by the self-ionization of water.

In the EDI unit, a high voltage gradient exists, causes which the electrolysis of water, producing a large amount of H⁺ and OH⁻. These locally generated H⁺ and OH⁻ continuously regenerate the ion exchange resins. The ion exchange resins in the EDI unit can be divided into two parts: one part is called the working resin, and the other part is called the polishing resin. The boundary between them is known as the working front. The working resin is responsible for removing most of the ions, while the polishing resin is tasked with removing weak electrolytes and other more difficult-to-remove ions.

The pre-treatment of EDI feedwater is the primary condition for achieving optimal EDI performance and reducing equipment failure. Pollutants in the feedwater can have a negative impact on the desalination unit, increasing maintenance requirements and reducing the lifespan of the membrane components.