Discussion on the Application of Practical Analytical Methods in Electro-deionization Water Purification Technology (Part Two)

High Salinity Operating Conditions

This operating condition refers to the use of EDI (Electronic Distillation) to prepare partially demineralized water. In this case, the EDI feedwater is tap water or raw water pretreated to remove suspended solids, with a salt content typically around 200–300 mg/L. This partially demineralized water is used as feedwater for low-pressure boilers and in general industrial applications for coating formulation and cleaning.

According to my country's low-pressure boiler water quality standards, the feedwater standard for steam boilers using external chemical water treatment should meet a hardness of ≤0.03 mmol/L, while for hot water boilers, the hardness is relaxed to ≤0.6 mmol/L. The standard does not limit the salinity of the water; this should be determined by whether using feedwater with this salinity is economically reasonable based on calculations of the boiler blowdown rate. General industrial users do not specify requirements for partially desalinated water quality; the requirements vary depending on the application, but the general requirements are comparable to those for industrial boiler feedwater. When these users use EDI (Electronic Distillation) desalination, the EDI operates under high salinity conditions.

Similar to the operating conditions at low salinity, electrodialysis migration of ions is neglected at the beginning of desalination. After the desalination chamber has been operating for a period of time, the resin layer will inevitably exhibit the ion exchange layer spectrum shown in Figure 2(b). From top to bottom, this spectrum, for cations, consists of a Fe³⁺ and Ca²⁺ (containing Mg²⁺) ineffective layer, a Ca²⁺++Na⁺ working layer, and a Na⁺ (possibly containing a small amount of H⁺) protective layer; for anions, it consists of a SO₄²⁻ and Cl⁻ ineffective layer, a Cl⁻+HCO₃⁻ (containing HSiO₃⁻) working layer, and an HCO₃⁻ (containing HSiO₃⁻ and possibly a small amount of OH⁻) protective layer.

Similar to the EDI under low salinity conditions, the self-regeneration of the EDI resin layer under high salinity conditions still relies on the ionization of water under the action of a DC electric field. During normal operation of the EDI, self-regeneration mainly takes place in the protective layer. If the EDI can be operated intermittently, the EDI that is shut down can achieve complete regeneration of the EDI resin layer by changing the operating parameters, such as increasing the voltage [9]. At this time, H+ and OH- ions are ionized by water, and the resin is completely converted into H-type and OH-type. Once the EDI is restarted, its resin layer quickly establishes the above-mentioned ion exchange layer spectrum, as shown in Figure 2(b), thereby realizing the electro-deionization treatment of water under high salinity conditions. If the ion exchange layer in the freshwater chamber can stably establish this chromatographic pattern when using EDI under high salinity conditions, then the EDI effluent is partially desalinated softened water. This EDI effluent differs from softened water obtained through Na ion exchange. From a cation perspective, it directly removes Fe³⁺ and Ca²⁺ (including Mg²⁺) from the water to be treated, without exchanging Na⁺ from the resin to replenish it. From an anion perspective, it also removes some SO₄²⁻ and Cl⁻, thus significantly reducing the total salinity of the EDI effluent.

This electro-deionization treatment differs from simple electrodialysis or reverse osmosis. Because ion exchange is involved, under normal operation, the effluent will not contain hardness ions Ca²⁺ (including Mg²⁺), whereas water from simple electrodialysis or reverse osmosis may contain small amounts of calcium and magnesium ions.

Like other membrane treatments, EDI treatment requires attention to sterilization and scale prevention. Sterilization refers to removing bacteria from the water and preventing their growth and reproduction on the membrane and resin. Ultraviolet light irradiation is commonly used. Scale prevention is to prevent scale formation on the membrane surface. Common methods include adjusting the pH value and softening the water first, and then operating the EDI in reverse polarity [10]. When the EDI is in a low-salinity condition, the influent contains very little Ca2+, so membrane scaling will not occur. To prevent membrane scaling, if the above-mentioned scale prevention treatment is to be used, it is better to use ordinary reverse polarity electrodialysis for preliminary desalination treatment. This not only solves the membrane scaling problem of EDI, but also reduces the desalination burden of EDI. Therefore, for high-salinity water with a salt content >300 mg/L, if EDI treatment is to be used, it is better to first use ordinary electrodialysis to remove most of the calcium and reduce the salt content. Electrodeionization is an ion separation method that organically combines electrodialysis and ion exchange. Based on a large amount of existing practice and theory, the chemical reactions that occur during the electrodeionization process are distinguished in terms of their primary and secondary aspects, their order and location, and a practical analytical method describing the reaction superposition of electrodeionization is derived. This method can fully explain practical problems such as the application of EDI to remove electrolyte ions from water to prepare ultrapure water, pure water, softened water and partially deionized water, thus facilitating the promotion and application of EDI.

When using EDI to remove electrolyte ions from water, EDI can work in two states: at low salt content, it relies on water ionization to generate H+ and OH- to self-regenerate the ion exchange resin. The resin works in both H- and OH- forms, and ultrapure water and pure water are produced by EDI for use in the electronics, pharmaceutical and other industries and thermal power plants. This type of water purifier is called an electrodeionized pure water purifier [4]; at high salt content, the resin is in the basic form, and softened water and partially deionized water are produced by EDI for use in industrial boilers and related industries. This type of water purifier is called an electrodeionized soft water purifier [11]. The widespread adoption of electro-deionization water purification technology will enable the regeneration of ion exchange resins without the need for acid, alkali, salt, or chemical reagents, thus completing a major transformation in the ion exchange water treatment process and turning it into an environmentally friendly process.