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Operating conditions at low salinity:
This operating condition refers to the conditions under which EDI is used to prepare ultrapure water and pure water. Ultrapure water refers to water in which electrolytes have been almost completely removed, and non-dissociated colloidal substances, gases, and organic matter have also been removed to very low levels. The residual salinity of ultrapure water should be below 0.1 mg/L, and the conductivity of water at 25°C should be less than 0.1 μS/cm. Pure water refers to water in which easily removable strong electrolytes have been removed, and weak electrolytes such as silicic acid and carbon dioxide, which are difficult to remove, have been removed to a certain extent. The residual salinity of pure water should be below 0.1 mg/L, and the conductivity of water at 25°C should be 0.1–1 μS/cm. Pure water is also known as deionized water or deeply desalinated water.
Any desalination system for producing ultrapure and pure water is generally equipped with a mixed bed as a final purification device to control water quality. Numerous studies have shown that the combination of RO (Rotation Oxidation) and EDI (Electronic Distillation Irrigation) as an alternative to the mixed bed offers optimal process performance. Because EDI eliminates the need for chemical regeneration, allows for unattended operation and continuous water output, it is best suited for producing ultrapure and pure water.
There are two types of users requiring ultrapure and pure water. One type is used in industries such as semiconductors, electronics, and pharmaceuticals, as well as in scientific research. These industries have very high water quality requirements, mostly requiring ultrapure water, but the capacity of a single water purification unit is relatively small, generally below 5 m³/h. The other type is used in thermal power plants, which require pure water as feedwater for high-pressure boilers. Due to the large volume of boiler feedwater, they prefer a single water purification unit with a capacity of around 100 m³/h. When thermal power plants use EDI (Electrodeionization), the EDI influent should be primary chemically demineralized water or RO (Regenerative Thermal Oxidation) effluent, with a SiO₂ < 100 μg/L and a conductivity < 5 μS/cm at 25℃. The EDI effluent should at least meet the standards for primary chemical demineralization-mixed bed systems: SiO₂ < 20 μg/L and a conductivity < 0.2 μS/cm at 25℃. In this case, the EDI operates under low salinity conditions.
At this point, the salinity of the EDI feed water was very low, much lower than that of water treated by conventional electrodialysis. Therefore, to discuss the operating conditions of EDI when used with low salinity, the electrodialysis process can initially be ignored, and only the ion exchange process needs to be considered. As a result, shortly after the EDI was put into operation, the resin layer in the desalination chamber exhibited the ion exchange layer spectrum shown in Figure 2(a). From top to bottom, this spectrum shows, for cations, a layer of ineffective Fe3+, Ca2+ (including Mg2+), and Na+, a working layer of Na++H+, and a protective layer of H+; for anions, a layer of ineffective SO42-, Cl-, and HCO3- (including HSiO3-), and a protective layer of OH-.
If the ion exchange chromatography pattern formed in the freshwater chamber stabilizes without significant changes (elongation or contraction), it indicates that the ions in the water to be treated, flowing from the top of the degraded resin layer to the bottom of the working layer, have continuously migrated from the freshwater chamber to the concentrate chamber. This is because the resin in the degraded layer is saturated and can no longer participate in ion exchange. The ions in the water are not adsorbed as they pass through the degraded resin layer but undergo electrodialysis and lateral migration under the influence of the DC electric field. By the time they reach the bottom of the working layer, all ions have migrated out of the freshwater chamber. It's important to note that under the influence of the electric field, the resin continuously undergoes ion desorption and adsorption. Both longitudinal ion exchange and lateral electrodialysis migration are dynamic equilibrium processes, with ions constantly entering and leaving the chamber, carrying out ion exchange.
After the water flows through the working layer, all electrolyte ions are removed, resulting in high-quality ultrapure or pure water. The protective layer below the working layer protects the quality of the effluent. It prevents ions from penetrating the protective layer if they occasionally do, thus preventing ion penetration. Under stable operating conditions, significant resin self-regeneration should not occur in either the failed layer or the working layer. Because the ion concentration in the aqueous solution of these layers is relatively high, concentration polarization is not easily achieved at the interface between the aqueous solution and the particle surface or membrane, making water ionization difficult and thus resin self-regeneration insignificant. Only in the protective layer, where electrolyte ions are extremely scarce and concentration polarization is easily achieved, will water ionize to produce H+ and OH- ions, thereby maintaining the H- and OH- forms of the resin in the protective layer.
If the system transitions from a stable to an unstable operating condition, such as when the amount of electrolyte ions entering the desalination chamber decreases, the dead and working layers in the ion exchange chromatography layer shrink. The vacated working layer then becomes capable of ionizing water, enabling resin self-regeneration and transforming into a protective layer. Once the working layer moves downwards until the protective layer disappears, electrolyte ions will penetrate, leading to water quality deterioration. This indicates that excessive electrolyte ions are entering the desalination chamber, exceeding the EDI equipment's operating capacity.
