EDI Principles and Influencing Factors

Electrodeionization (EDI) is a novel membrane separation technology that combines electrodialysis and ion exchange. Its main features include: ① Resin regeneration via electricity, eliminating the need for acids or alkalis and achieving clean production; ② Self-regeneration during equipment operation, making it equivalent to a continuously regenerating mixed-bed ion exchange column, enabling continuous deep desalination; ③ High-quality produced water, low water production costs, and convenient daily operation and management.

The schematic diagram of the EDI principle shows that the desalination chamber of the electrodialysis unit is filled with a mixture of anion and cation exchange resins, organically combining electrodialysis and ion exchange within a single container. Ions in the water are first adsorbed onto resin particles through exchange. Then, under the influence of an electric field, they migrate to the membrane surface via the ion transport channels formed by the resin particles and pass through the ion exchange membrane into the concentration chamber. Polarization in the diffusion layer at the contact point between the resin, membrane, and water phase causes water to dissociate into H+ and OH-. While some of these ions participate in the load current, most contribute to resin regeneration. Thus, ion exchange, ion migration, and electroregeneration occur simultaneously and mutually promote each other, achieving continuous ion removal.

The effect of raw water conductivity on desalination efficiency: At an influent flow rate of 120 L/h, changing the raw water conductivity yields the relationship between the effluent conductivity and the raw water conductivity. The effect of raw water conductivity on product water quality shows that, under the same operating current, the conductivity of the EDI effluent also increases with increasing raw water conductivity. Because the raw water has low conductivity, its ion content is also low. Simultaneously, the low ion concentration results in a large potential gradient on the resin and membrane surfaces in the desalination chamber. This leads to enhanced water dissociation, a higher limiting current, and a greater production of H₂ and OH⁻, resulting in good regeneration of the anion and cation exchange resins filling the desalination chamber. Figure 3 also shows that when the raw water conductivity is 21.5 μS/cm, the conductivity of the EDI effluent remains very low (0.1–0.05 μS/cm) as the operating current increases. This is because the lower the raw water conductivity, the more intense the water dissociation, resulting in more H₂ and OH⁻ and better resin electroregeneration (maintaining good exchange performance). As the operating current continues to increase, H₂ and OH⁻ are used not only for resin regeneration but also for loading the current. Therefore, the degree of water dissociation in the desalination chamber continues to increase, gradually bringing ion exchange and resin regeneration into equilibrium, and the product water conductivity tends to stabilize. Therefore, raw water conductivity is one of the most important factors affecting product water quality. When the influent conductivity is high, the quality of the product water decreases with the increase of the operating current. Taking the curve when the raw water conductivity is 100 μS/cm as an example, as the operating current gradually increases from 0 to 5A, the conductivity of the EDI effluent rises from 0.17 μS/cm to about 0.5 μS/cm (the water quality decreases). This is because at high salinity, concentration polarization is small and water dissociation is weak, and the resin hardly regenerates. At this time, ion exchange plays a major role, and the resin is saturated with salt ions in a short time. At this time, the resin mainly plays the role of enhancing ion migration. As can be seen from the figure, regardless of whether the influent salinity is high or low, the two-stage five-stage EDI equipment has a good desalination effect (desalination rate of 99%), and the conductivity of the effluent can reach the high-purity water standard (conductivity <1 μS/cm).