EDI Ultra-Pure Water Equipment

In the diagram, ion exchange membranes are represented by vertical lines and labeled with the type of ions they allow to pass through. These membranes are impermeable to water, thereby separating the dilution water and concentrated water streams. Ion exchange membranes and ion exchange resins operate on similar principles, enabling selective ion passage: an anion exchange membrane only permits anions to pass (blocking cations), while a cation exchange membrane only allows cations to pass (blocking anions). An EDI unit is constructed by filling the space between a pair of cation and anion exchange membranes with mixed ion exchange resins.

The space occupied by mixed ion exchange resins between cation and anion exchange membranes is called the dilution water chamber. When a certain number of EDI units are arranged in sequence (with alternating cation and anion exchange membranes) and special ion exchange resins are added between the membranes, the formed space is termed the concentrated water chamber. Under a specified DC voltage, cations and anions in the ion exchange resins within the dilution water chamber migrate toward the positive and negative electrodes, respectively, passing through cation and anion exchange membranes into the concentrated water chamber. Meanwhile, ions in the feedwater are adsorbed by the ion exchange resins, filling the gaps left by the migrated ions. In practice, ion translocation and adsorption occur simultaneously and continuously. Through this process, ions in the feedwater pass through ion exchange membranes into the concentrated water chamber, where they are removed, resulting in desalinated water.

Negatively charged anions (e.g., OH⁻, Cl⁻) are attracted to the positive electrode (+) and pass through the anion exchange membrane into the adjacent concentrated water chamber. As these ions continue migrating toward the positive electrode, they encounter the adjacent cation exchange membrane (which blocks anions), trapping them in the concentrated water. Cations in the dilution water stream (e.g., Na⁺, H⁺) are similarly trapped in the concentrated water chamber. In the concentrated water chamber, ions that have passed through cation and anion membranes maintain electrical neutrality. The electrical current in the EDI unit is proportional to ion migration and consists of two parts: one from the translocation of removed ions, and the other from the migration of H⁺ and OH⁻ ions generated by the self-ionization of water.

Within the EDI unit, a high voltage gradient induces water electrolysis, producing large quantities of H⁺ and OH⁻. These locally generated ions continuously regenerate the ion exchange resins. The ion exchange resins in an EDI unit can be divided into two parts: "working resins" and "polishing resins," with their boundary known as the "working front." Working resins are responsible for removing most ions, while polishing resins are tasked with eliminating weak electrolytes and other hard-to-remove ions.

Pretreatment of EDI feedwater is a primary condition for optimizing EDI performance and reducing equipment failures. Pollutants in feedwater can adversely affect the desalination unit, increasing maintenance needs and shortening the service life of membrane components.