Electrodeionization (EDI) in Clean Water Production

Electrodeionization (EDI) is a water treatment method that combines ion-exchange resins and ion-selective membranes to continuously remove ions from water under the influence of an electric field. This technology has evolved in response to the increasing demand for high-purity water in various industries, as well as the growing focus on water resource efficiency and chemical waste reduction in water treatment processes.

Historical Background:

In the past, the demand for ultrapure water primarily came from industries such as pharmaceuticals, chemicals, power generation, and paper manufacturing, with relatively lower water quality requirements. During the 1960s and 1970s, water purification methods mainly involved distillation and ion exchange. Distillation was energy-intensive, while ion exchange required chemical regeneration, which was both cumbersome and uneconomical. Moreover, strong ion-exchange resins were ineffective at removing organic molecules, resulting in high levels of total organic carbon (TOC) in the treated water.

The development of the semiconductor industry in the 1980s drove the demand for higher water quality standards, prompting advancements in water purification technologies. Advanced methods, including microfiltration, ultrafiltration, electrodialysis, and reverse osmosis (RO), gained widespread adoption. The RO-mixed bed system replaced traditional ion exchange systems, addressing TOC concerns and meeting the stringent water quality requirements of industries like electronics. However, RO had limitations in desalination efficiency, and the need to reduce chemical regeneration agents for environmental reasons led to a growing interest in electrochemical-based EDI technology.

Structure and Working Principles of EDI:

EDI is often used in conjunction with RO to form RO-EDI pure water systems. Standard EDI units are modular, comprising multiple modules combined into a single unit. Each EDI module consists of several double chambers sandwiched between two electrodes (under DC voltage), forming a layered plate-and-frame structure. These double chambers consist of a dilute water chamber (D) and a concentrate water chamber (C). Separating these chambers are pairs of ion-selective membranes (either anion or cation exchange membranes), and within these membranes, there’s a mixture of anion and cation exchange resins forming the D chamber. These ion-selective membranes also create a C chamber when combined with membranes from the adjacent D chamber.

Water is introduced into the system, with a specific proportion passing through the C and D chambers. The behavior of ions in the D chamber can be explained through four processes:

  1. Ions migrate towards the concentrate water chamber under the influence of an electric field.
  2. Ions bind with the resins.
  3. Water undergoes ionization and migration, forming water molecules in the C chamber by combining H+ and OH- ions.
  4. Due to the electric field, ions continuously dissociate from the resins, regenerating the resins.

This process establishes a concentration gradient in the direction of water flow, allowing the adjustment of current (voltage) based on the influent and effluent requirements to produce ion-free, pure water. The ion-selective membranes prevent ions in the C chamber from concentrating in the adjacent D chamber.

Typically, in a standard EDI system, 90% to 95% of the influent water passes through the D chambers, while 5% to 10% passes through the C chambers. To prevent scaling, the concentrate water is forcibly circulated using pumps, passing rapidly across the membrane surface. A portion of this concentrate water is discharged, but it can also be returned for further treatment by RO.

Economic and Technical Advantages of EDI:

The primary advantage of EDI lies in its use of electric fields and ion-selective membranes to replace the chemical regeneration of ion-exchange resins. This results in significant advantages over RO-mixed bed systems in terms of equipment structure, operational simplicity, and reduced operating costs. EDI has also overcome the problem of chemical resin regeneration and subsequent wastewater discharge.

  1. EDI Compatibility with RO: EDI can be used in combination with RO, allowing for adjustable current to modify water quality and the ability to change water production by combining standard modules. Over a decade of commercial applications has shown that the system operates reliably at pressures of up to 100 pounds per square inch (7 kg/cm2). The effluent resistivity can reach over 16 MΩ·cm, with Si content below 20 ppb, meeting the stringent water quality requirements of various industries. Water production can reach up to 2000 gallons per minute (450 cubic meters per hour).
  2. No Resin Regeneration Required: EDI eliminates the need for resin chemical regeneration facilities, such as acid and alkali storage tanks, pumps, and pipelines. This simplifies the structure of pure water systems, reduces investment costs, simplifies operation, and lowers operating expenses.
  3. Adaptability to Feedwater Variability: Comparative technical and economic analyses have shown that EDI is more adaptable to changes in total dissolved solids (TDS) in the feedwater without affecting water quality. Additionally, EDI has a minimal impact on water production costs.
  4. Environmental Benefits: EDI provides significant environmental benefits, including:
    • Mitigating wastewater pollution caused by chemical resin regeneration.
    • Enabling the direct reuse of concentrate water discharged by EDI before it enters the RO stage, resulting in zero wastewater discharge from EDI units.

Future Prospects of EDI Technology:

Due to its aforementioned advantages, EDI technology and products have experienced rapid development. Currently, several international companies manufacture and sell RO-EDI systems. These systems find applications not only in pharmaceuticals, papermaking, chemicals, and power generation but also in various other fields. For instance, the Millipore company’s Elix series of pure water equipment in the United States employs RO-EDI technology for laboratory pure water preparation, meeting secondary standards for laboratory pure water quality. EDI has already established a stable market internationally and continues to expand.

With an increased focus on environmental awareness and higher environmental protection standards, EDI technology is gaining favor compared to traditional mixed bed systems, which require chemical regeneration and generate significant wastewater pollution. As membrane technology continues to advance, further improvements can be expected in RO-EDI systems. Experts predict that within the next 3 to 5 years, 85% of industrial water treatment systems will adopt RO-EDI technology.

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 ultrapure water treatment

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