EDI principle and its application in pure water clean production

1. Development of EDI technology

Electrodeionization (EDI-electrodeionisation) is a water treatment method that combines ion exchange resins and ion membranes to continuously remove ions under the action of an electric field. This technology is gradually developed with the continuous improvement of industrial production’s requirements for pure water quality and the improvement of environmental protection’s requirements for water utilization rate and chemical discharge control in water treatment.
Historically, the early demand for pure water mainly came from industries such as medicine, chemical industry, power generation, and papermaking, and the water quality requirements were relatively low. In the 1960s and 1970s, distillation and ion exchange were mainly used for the preparation of pure water. The former consumes a lot of energy, while the latter requires the regeneration of chemical agents, which is troublesome and uneconomical, and because the strong resin has a poor removal effect on general organic molecules, the TOC content in the effluent is high. With the development of the semiconductor industry, the quality requirements for pure water are constantly increasing, which greatly promotes the development of pure water technology. By the 1980s, membrane technology had been widely used, and advanced water treatment technologies such as microfiltration, ultrafiltration, electrodialysis and reverse osmosis (RO) had made great progress. The RO-mixed bed system replaces the traditional ion exchange system, solves the TOC problem, and meets the pure water quality requirements of industries such as electronics. However, due to the limited desalination rate of RO, the problem of the regeneration of chemical reagents in the mixed bed is still unsolved, and for the sake of environmental protection, the voice of reducing the use of chemical regeneration reagents is getting louder and louder, so the EDI technology based on electrochemistry has been obtained. Pay attention to.
As early as forty years ago, EDI was used in the laboratory as a water treatment method without regeneration of chemicals. The rapid development of EDI technology is in the past ten years, especially in recent years. The initial EDI system design was not perfect, the reliability was problematic, and the price was high, so it was only suitable for small flow users. Now foreign companies such as E-CELL in the United States have successfully commercialized EDI equipment, and the water quality is comparable to that of mixed bed water; EDI is designed as a standard module like RO, which can be mass-produced and combined on a large scale, and the water volume can also meet Industrial water consumption requirements.

2. EDI structure and working principle

EDI is often used in conjunction with RO to form a RO-EDI pure water system. As mentioned above, EDI has been designed as a standard module, and the EDI unit is composed of several modules. The structure of each EDI module is shown in Figure 1. There are several double chambers sandwiched between two electrodes (with direct current) in a stacked frame structure; the double chambers include a fresh water chamber (indicated by D) and a concentrated water chamber. (Denoted by C); the two chambers are separated by a pair of anion and cation membranes (also known as anion membranes or anion membranes), and a mixed bed of anion and yang resins is filled between the anion and cation membranes to form room D; the anion and cation membranes are respectively connected to The male and female membranes in the other D room form the C room.
The difference from the ordinary mixed bed is that the anions and cations entering the D chamber are first combined with the resin, and then continuously dissociated from the resin under the action of a DC electric field, and move to the anode and cathode through the anion and cation membranes respectively, and the resin is regenerated at the same time. . Due to the above-mentioned balance effect, a concentration gradient is formed in the direction of water flow, and the current (voltage) can be adjusted according to the water inlet situation and water outlet requirements, so that the outflow water is pure water without anions and cations; due to the selection of anions and cations by the membrane Permeability, the ions entering the C chamber cannot pass through the other polar membrane and concentrate in the C chamber.
In a typical EDI system, 90% to 95% of the incoming water passes through the D chamber, and 5% to 10% of the incoming water passes through the C chamber. In order to prevent scaling, the concentrated water is forced to circulate through the pump and passes through the membrane surface at high speed. Part of the concentrated water is discharged; the discharged concentrated water can be returned to RO for reprocessing.

3. EDI’s economic and technical characteristics and environmental protection value

The biggest feature of EDI technology is to replace the chemical regeneration of ion exchange resin with electric field and ion membrane, so that RO-EDI pure water system has obvious advantages compared with RO2 mixed bed in terms of equipment structure, operation and operation cost; Units that have solved the problem of wastewater discharge generated by recycled resins and have wastewater that needs to be treated can also consult companies with similar wastewater treatment experience on the Sewage Bao project service platform.

  1.  EDI is used together with RO, the current can be adjusted to change the water quality, and the standard module combination can be used to change the water output. More than ten years of commercial application have shown that the system operates stably under a pressure of 100 pounds per square inch (7kg/cm2), the resistivity of the effluent water can reach more than 16M8 cm, and the Si content is below 20ppb. The water quality is reliable and can meet the current most Strict industrial water quality requirements, the water output can be as high as 2000 gallons per minute (450 cubic meters per hour).
  2. EDI does not need to regenerate resin, and eliminates resin chemical regeneration supporting facilities (such as acid-base storage tanks, pumps and pipelines), which simplifies the structure of pure water system equipment, saves investment, simplifies operation, and reduces operating costs. In recent years, foreign E2CELL companies and ECODYNE companies have made technical and economical analysis of RO-EDI and RO2 mixed bed pure water systems under different flow rates (50, 200, 600 gallons per minute) and different water qualities (low, medium, and high TDS). Performance made a special comparative study. The results show that the labor of the former is 0.5 man-hours/day, and that of the latter is 2 man-hours/day; the installation cost of EDI is much lower than that of the mixed bed. Equipment investment, installation and operation costs are calculated on the basis of a 10-year amortization period, and the EDI investment coefficient is 10%. EDI is much more economical than the mixed-bed system at low flow rates; it is equivalent at high flow rates. The cost of land occupation and environmental protection management is not considered in the above comparison, and EDI has a clear advantage in this respect.
  3. The technical and economic comparison also shows that EDI is more adaptable to the change of TDS in the influent than the mixed bed system without affecting the quality of the effluent, and has little impact on the cost of water production.
  4. The environmental benefits of EDI are remarkable, manifested in two aspects: ①It overcomes the wastewater pollution caused by resin chemical regeneration; ②The concentrated water discharged from EDI can be directly returned to the RO for reuse, so that the EDI unit can achieve no wastewater discharge.

4. EDI Technology Application Prospects

Due to the above advantages of EDI, EDI technology and products have developed rapidly. At present, many companies in the world have produced and sold RO-EDI systems. Its application is not only in pharmaceutical, papermaking, chemical industry, power generation and other industrial sectors, but also in other fields. The quality of its EDI effluent meets the secondary standard for pure water used in analytical laboratories. In fact, EDI has formed a stable international market and is constantly expanding. With the strengthening of environmental awareness and the improvement of environmental protection requirements, compared with the traditional mixed bed which requires chemical regeneration and produces a large amount of wastewater pollution, EDI technology will be favored. And with the continuous development of membrane technology, RO-EDI system is expected to have further improvement. Relevant experts predict that 85% of industrial water treatment systems will use RO-EDI technology in the next 3 to 5 years.

At present, there is no large-scale application of EDI technology in our country. Compared with the research of membrane technologies such as UF and RO, the research of EDI technology lags behind. The domestic water treatment technology market is very large and is developing rapidly. Therefore, research, development and application of EDI technology is very necessary.

Xi’an CHIWATEC Water Treatment Technology is a high-tech enterprise specialized in various water processing devices. Aside from these individual products, which cover a number of types and series, we can also help with related comprehensive engineering projects. Thanks to our hard work and dedication upon our founding, we are now one of the fastest-developing water treatment equipment manufacturers in Western China.

Further reading

EDI module CP-3600S unit

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