Development and application of EDI technology

Development and application of EDI technology

An important news for industrial companies that produce high-purity water is that high-purity water is now a reality without the use of chemical regenerating agents. The purity of water produced by recently developed EDI technology can reach the limit and bring a range of other benefits.

    EDI technology without chemical regenerating agents

The electrodeionization method called EDI in the water treatment industry is not a new term. In fact, commercialized EDI has appeared more than a decade ago. Despite the low output of the early EDI system and the poor reliability of operation, today's EDI has been able to meet the wide range of requirements for water treatment in the industrial sector.

The current RO (Reverse Osmosis)-EDI system is undergoing a revolution in the way water is cleaned, but there is still a long way to go before the industry is widely accepted by EDI.

From the pharmaceutical, paper, petrochemical and electric power of more than forty years ago to today's semiconductor industry, water has always been the lifeblood of the industrial sector, and it is these sectors that have revolutionized ultrapure water treatment technology. Although the basic characteristics required by industrial users such as less chemicals, less wastewater discharge, simple operation and lower operating costs are basically the same, many changes have taken place in the original water treatment technology. .

Major developments in water treatment technology

In order to meet the needs of the industrial sector for high-purity water, in recent years, two technologies have been developed in industrial water treatment, and each of these new technologies has made a breakthrough change in the water treatment system.

In the 1960s and 1970s, the water quality required by the industrial sector was met by chemically regenerated ion exchange technology. Due to the small application at the time, there was less concern about the long-term effects of using chemical agents.

In the early water treatment systems, the mixed bed ion exchange stage was placed as a separate unit in a subsequent unit after the cation and anion exchange. As the demand for applications increases, chemically regenerated ion exchange systems are clearly limited, with the focus of the problem being that they have a higher TOC content. Compared with the recent technology, a large number of chemical regenerating agents are used in these systems, and chemical wastewater is required to be continuously processed, and the operation thereof is complicated and the running cost is high.

In the 1970s and 1980s, as people became less aware of the use of chemical agents, people began to seek new processes in industrial water treatment, which led to new applications for reverse osmosis technology. Reverse osmosis replaces the cation/anion exchange unit in the pre-desalting system using membrane technology, but this new technology is not successful in the initial application, RO has higher requirements for pretreatment, and the water treatment system as a whole It tends to be simplified.

As the electronics industry is increasingly demanding pure water quality (including lower TOC content), water treatment technology continues to move forward, and RO is seen as a solution. As the pretreatment process improves and more advanced RO membranes are developed, the problems encountered by the RO in the early stages of application are gradually overcome.

With the passage of time, RO has gradually been accepted by the world, and subsequent ion exchange technologies such as countercurrent regeneration design, full bed ion exchange and special resin development have also been developed accordingly. Due to the wide application of these new processes, the cost has been reduced, but the RO/mixed bed system is still more economical than the current chemically regenerated ion exchange system, and there is still a certain demand for these aforementioned technologies. .
The RO/mixed bed system meets the multifaceted requirements of the industrial sector for high purity water quality, which treats insoluble impurities to parts per billion and also reduces TOC content. In any case, the industry still needs to continue to rely on mixed bed technology as the final stage of desalination. The use of chemical agents in the mixed bed stage and the requirements of related facilities mean that the benefits of RO are not fully reflected, further reducing chemistry. The use of pharmaceuticals contributed to the second technological revolution.

The electrodeionization process, commonly referred to as EDI, was first developed as a non-chemical process in the laboratory more than 40 years ago. The recent development of EDI technology has made it possible to completely eliminate the dependence on chemical regenerating agents, and it can also bring a series of Other benefits.

How EDI works

A typical EDI system involves such a process: pretreatment - RO-EDI. EDI uses conventional ion exchange resins to continuously remove ions from water, but since it uses current to continuously regenerate the resin, it does not require periodic chemical regeneration.

A typical EDI film stack consists of a certain number of cells sandwiched between two electrodes (see Figure 1 for a working schematic of EDI). There are two different types of chambers in each unit: a fresh water chamber to be desalted, that is, a D chamber, and a concentrated water chamber in which impurity ions are removed, that is, a chamber C. The D chamber is filled with a mixed cation and anion exchange resin. These resins are located between the two membranes: a cation exchange membrane that only allows cation permeation and an anion exchange membrane that only allows anion to permeate.

The resin bed is continuously regenerated by direct current applied to both ends of the chamber, and the voltage decomposes the water molecules in the influent into H+ and OH-. These ions in the water are attracted by the corresponding electrodes, and pass through the anode and the anion exchange resin to the corresponding membrane. In the direction of migration, when these ions pass through the exchange membrane and enter the concentration chamber, H+ and OH- combine to form water. The generation and migration of such H+ and OH- is the mechanism by which the resin can be continuously regenerated.

When impurity ions such as Na+ and CI- in the influent water are sucked onto the corresponding ion exchange resin, the impurity ions undergo the same ion exchange reaction as in the ordinary mixed bed, and the H+ and OH- are replaced accordingly. Once the impurity ions in the ion exchange resin are also added to the migration of H+ and OH- toward 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 migrate further in the direction of the corresponding electrode due to the blocking action of the adjacent compartment exchange membrane, so that the impurity ions are concentrated in the concentrated water chamber, and then the concentrated water containing the impurity ions can be discharged from the membrane stack.