Electrodeionization (EDI) is a water purification technology that has become increasingly popular in the water industry. It uses a combination of semi-permeable membranes, ion exchange resins, and direct electric fields to effectively remove impurities from water. The ion exchange resins are continuously regenerated without the need for cleaning chemicals. To understand how EDI works, let’s take a closer look at how water contains cations (positively charged ions) and anions (negatively charged ions).
When an electrical current is applied to a pair of electrodes, the electric field drives the cations in the direction of the cathode and the anions in the opposite direction toward the anode. When an ion exchange membrane made of cation-selective resins is inserted, only cations can pass through to the other side, and when an anion-selective membrane is inserted, only anions can pass through to the other side. This forms the framework of an EDI module.
However, the slow speed of ions moving through water limits this process. As the product compartment water becomes purer, the electrical resistance of this water increases, resulting in the low conductivity of water impeding ion removal. EDI technology solves this problem by sandwiching a mixture of cation and anion-selective resins between the two membranes, which eliminates ion diffusion resistance, allowing ions to move more freely.
This mixture of resin beads creates a series of water purification compartments alternating with compartments where the ions are concentrated. As feed water is pumped into the system, it is diverted into separate compartments, namely the concentrating compartment and the purification or dilute compartment. Ions migrate and accumulate in the concentrating compartments where they are washed away into the reject stream. The water leaving this compartment contains a concentration of ions approximately 10-20 times higher than the original feedwater.
At the top of the purification compartments, the ion concentration is at its highest, and the surface of the resin beads acts as a conductive path, effectively moving the ions to the membrane. At the lower end of the purification compartments, where the ion concentration has been reduced to the ppt level, the electric field becomes concentrated between the resin beads and the surrounding water. When the localized voltage potential exceeds approximately eight tenths of a volt, an electrochemical reaction occurs, splitting water molecules into H+ and OH- ions, which regenerate the resin beads by replacing trace ions remaining the attachment of H+ and OH- ions.
EDI systems are used in a variety of applications requiring ultra-high purity water, such as laboratories, pharmaceuticals, power generation, boiler feed water, semiconductor production, electronics & microelectronics and food and beverage. EDI systems remove dissolved inorganics and low molecular organics effectively and produce a constant flow of reliable and consistent water quality.
EDI systems are also environmentally friendly, use no chemical regeneration, chemical disposal, or resin disposal, and have low energy consumption. They also have low operating costs, require no neutralization units, local permitting or other costs of chemicals, and have a low maintenance needs. Additionally, they utilize no hazardous chemicals, have less risk to people and accidental discharge, no heating element, and a long life-cycle because of continuous regeneration of the ion exchange resins inside the module. However, EDI systems require good quality feed water, with hardness <1, and the feed water volume is limited.
Post time: Sep-18-2024