Analysis of Factors Affecting the Quality of Electro-Deionized Water

Effect of Flow Rate on Effluent Conductivity

The conductivity of EDI effluent changes very little with 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. Thus, the influent flow rate has little effect on the degree of water dissociation.

Effect of Operating Voltage on Effluent Conductivity

The quality of 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 operate 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 product water, and the better the product water quality. In addition, appropriately increasing the feed water flow rate, i.e., increasing the compartment flow velocity, can improve the product water quality.