Analysis of Factors Affecting the Quality of Electro-Deionized Water

Li Qingxue, Li Fuqin, Wang Dongyun (Department of Urban Construction, Hebei University of Architecture and Technology, Handan, Hebei 056038, China)

China Water & Wastewater 2002 No.11

Abstract: An experiment was conducted on a raw water electrodialysis followed by an electrodeionization (EDI) unit. The effects of influent flow rate and water quality on the quality of EDI permeate were investigated, and the optimal operating parameters for EDI ion removal were explored. The experimental results show that reducing the conductivity of the influent, appropriately increasing the operating voltage of the membrane stack, and increasing the influent flow rate can all improve the quality of the permeate. Keywords: Electrodeionization; Electrodialysis; Ion exchange; High-purity water

CLC Number: TU991.2

Document Code: C

Article Number: 1000-4602(2002)11-0036-03

Funding Project: Hebei Provincial Science and Technology Research and Development Project (00213093)

Received Date: 2002-05-22

Electrodeionization (EDI) technology is a novel membrane separation technology that combines electrodialysis and ion exchange. Its main characteristics are: ① Resin is regenerated electrically without the need for acids or alkalis, achieving clean production; ② The equipment regenerates itself during operation, thus it is equivalent to a continuously regenerated mixed-bed ion exchange column, enabling continuous deep desalination; ③ The produced water has good quality, low water production cost, and convenient daily operation and management.

Basic Principle of EDI

The desalination chamber of an 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 the resin particles due to exchange. Then, under the influence of an electric field, they migrate to the membrane surface through 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.

Experimental Apparatus and Procedure

The EDI device employs a two-stage, five-section design. The desalination chamber is filled with a mixture of anion and cation exchange resins (anion:cation = 2:1), with four membrane pairs per section. Desalination chamber partition: 280mm × 120mm × 5mm (rigid PVC board; the four-chamber, non-circuit-connected, closed-channel inlet and outlet are self-made); Concentration chamber partition: 280mm × 120mm × 5mm (rubber board, non-circuit-connected); Ion exchange resins: 001×7 cation and 201×7 anion resins produced by Tianjin Nankai University Chemical Plant; Ion exchange membranes: 3361-BW cation and 3362-BW anion membranes produced by Shanghai Chemical Plant; Electrodes: the anode is titanium-coated ruthenium, and the cathode is stainless steel.

Results and Analysis

Under the same operating current, the conductivity of the EDI effluent increases with increasing raw water conductivity. This is because low raw water conductivity indicates low ion content, which in turn leads to a larger potential gradient on the resin and membrane surfaces in the desalination chamber. This results in enhanced water dissociation, a higher limiting current, and a greater quantity of H+ and OH- ions produced, leading to better regeneration of the anion and cation exchange resins in the desalination chamber.

When the raw water conductivity is 21.5 μS/cm, the EDI effluent conductivity remains very low (0.1–0.05 μS/cm) with increasing operating current. This is because lower raw water conductivity results in more intense water dissociation, producing more H+ and OH- ions, thus improving resin electroregeneration (maintaining good exchange performance). As the operating current continues to increase, H+ and OH- are used not only for resin regeneration but also for load current. Therefore, the degree of water dissociation in the dilute chamber continues to increase, gradually bringing ion exchange and resin regeneration into equilibrium, and stabilizing the product water conductivity. Thus, the raw water conductivity is one of the most important factors affecting product water quality. When the influent conductivity is high, the product water quality decreases with increasing operating current. Taking the curve with a raw water conductivity of 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 approximately 0.5 μS/cm (the water quality decreases). This is because at high salinity, concentration polarization is small, water dissociation is weak, and the resin hardly regenerates. At this point, ion exchange plays a major role, and the resin is saturated with salt ions in a short time. The resin then primarily enhances ion migration.

Regardless of the salinity of the influent, the two-stage, five-stage EDI system exhibits excellent desalination performance (desalination rate >99%), and the conductivity of the effluent meets high-purity water standards (conductivity <1 μS/cm).

The conductivity of the EDI effluent changes very little with the operating current at different influent flow rates. This is because, in the circuit, the solution phase and resin phase in the dilute chamber are connected in parallel. Since the conductivity of the ion exchange resin is much higher than that of the electrodialysis permeate, the resin phase resistance becomes the determining factor for the dilute chamber resistance. Ion transport mainly occurs through the resin phase, and within a certain dilute flow rate range, the flow rate has little effect on the resin phase resistance. Therefore, the total current of the membrane stack does not change significantly, and the permeate conductivity changes very little. Consequently, the influent flow rate has little impact on the degree of water dissociation.

The quality of the EDI effluent is closely related to the operating voltage. If the operating voltage is too low, it is insufficient to remove ions from the dilute chamber before the pure water is discharged. The electrodialysis process and the resin electroregeneration process are relatively weak, and the ion exchange process is the primary process. As the operating voltage increases, the degree of water dissociation increases, the resin regeneration effect improves, and the conductivity of the freshwater decreases. When the operating voltage increases to a certain level, the ion exchange process and the resin regeneration process reach equilibrium, and the conductivity of the product water further decreases and tends to stabilize. However, excessively high operating voltage will cause excessive water ionization and ion back diffusion, thus reducing the quality of the product water. Therefore, it is recommended that EDI be operated at an appropriate voltage.

Conclusion

Increasing the operating voltage of the EDI membrane stack can yield high-quality pure water, but from the perspective of improving the current efficiency of the membrane stack, the operating voltage should not be too high. The lower the conductivity of the EDI membrane stack feed water (i.e., the better the pretreatment effect), the lower the conductivity of the EDI permeate, and the better the permeate quality. In addition, appropriately increasing the feed water flow rate, i.e., increasing the compartment flow velocity, can improve the permeate quality.