A typical EDI system involves such a processing procedure: pretreatment-RO-EDI. EDI uses ordinary ion exchange resin to continuously remove ions from water, but because it uses electricity to continuously regenerate the resin, it does not need to perform regular chemical regeneration at all.
A typical EDI membrane stack is composed of a certain number of cells sandwiched between two electrodes (see the working principle diagram of EDI in Figure 1). There are two different types of chambers in each unit: the fresh water chamber to be desalted, that is, the D chamber, and the concentrated water chamber to remove the impurity ions, that is, the C chamber. The chamber D is filled with mixed positive and negative ion exchange resins. These resins are located between two membranes: a cation exchange membrane that only allows cations to pass through and an anion exchange membrane that only allows ions to pass through.
The resin bed is continuously regenerated by direct current applied to the two ends of the chamber. The voltage decomposes the water molecules into H+ and OH-. These ions in the water are attracted by the corresponding electrodes and pass through the positive and negative ion exchange resins to the corresponding membranes. The direction of migration, when these ions enter the concentration chamber through the exchange membrane, H + and OH-combine to form water. This generation and migration of H+ and OH- is precisely the mechanism by which the resin can be continuously regenerated.
When impurity ions such as Na+ and CI- in the intake water are adsorbed on the corresponding ion exchange resin, these impurity ions will undergo the same ion exchange reaction as in the ordinary mixed bed, and H+ and OH- will be replaced accordingly. Once the impurity ions in the ion exchange resin are also added to the migration of H+ and OH- in the direction of the exchange membrane, these ions will continuously pass through the resin until they pass through the exchange membrane and enter the concentrated water chamber. These impurity ions cannot be further migrated in the direction of the corresponding electrode due to the blocking effect of the exchange membrane of the adjacent compartment, so the impurity ions can be concentrated in the concentrated water chamber, and then the concentrated water containing the impurity ions can be discharged out of the membrane stack.
In a typical EDI system, 90-95% of the incoming water directly passes through the D room, and 5-10% of the incoming water is distributed into the C room. The concentrated water is circulated by a pump and allowed to reach a higher flow rate in the membrane stack, which can play a role in improving the salt removal efficiency, promoting the mixing of the water flow, and reducing the possible scaling. Concentrated ions can be removed from the membrane stack by excluding a certain percentage of water from the concentrated water circulation circuit. This pH 5-8 water can be recovered or directly returned to the inlet of the pretreatment system.
In the process of electrodeionization, high-quality demineralized water is prepared after removing the impurity ions in the influent water.
Packed bed electrodialysis is also called Electrodeio?nization, or EDI for short. It uses the polarization phenomenon in the process of electrodialysis to regenerate the ion-exchanged packed bed electrochemically, focusing on the advantages of electrodialysis and ion exchange. Overcome the shortcomings of both. EDI technology combines two mature water treatment technologies-electrodialysis technology and ion exchange technology, which is called packed bed electrodialysis or electrodeionization technology in my country. It mainly replaces the traditional ion-exchange mixed bed to produce high-purity water. This technology has been promoted and applied abroad in the preparation of high-purity water in industries such as electronics, power, and chemical. It can be expected that this water treatment product will become the mainstream equipment in the process of preparing high-purity water in this century. The application of this technology and related technologies will bring about some fundamental changes in the original water treatment technology, thereby achieving better environmental protection and economic benefits.