Electrodionization (EDI) is a state-of-the-art technology for the production of ultra-pure water and is used in applications where very low conductivity and low residual mineralization are required. The EDI process combines elements of ion exchange technology and electrodialysis and enables continuous water treatment without chemical regeneration.
Table of contents
Technical basics of electrodeionization (EDI)
The EDI system works by using an electric field generated by electrodes to remove ions from the water. The essential structure of an EDI cell consists of ion exchange membranes (cation and anion exchange membranes) arranged alternately in the cell and ion exchange resins arranged between the membranes. The raw water is passed through the resin bed, which absorbs the ions and transports them to the corresponding membranes for discharge into the concentrate chambers. Automatic regeneration of the resins is achieved by the continuous application of an electric field, eliminating the need for chemical regeneration agents.
Structure of an EDI cell
Ion exchange membranes:
- Cation exchange membranes only allow positively charged ions (e.g. calcium, magnesium) to pass through, while anion exchange membranes only allow negatively charged ions (e.g. chloride, sulphate) to pass through. They divert the ions into the concentrate chambers, where they are flushed out of the system.
Ion exchange resins:
- These special resins, which absorb both cations and anions, provide a large contact surface for ion separation and serve as a transport medium for the ions. In the EDI, the resins ensure the continuous ion exchange capacity and contribute to the high purity of the treated water.
Electrodes:
- By applying a direct current between the electrodes, the ions are drawn into the electric field. Positive ions migrate to the cathode, negative ions to the anode, whereby the water is almost completely desalinated.
How electrodionization works
EDI is typically used as a post-treatment step after reverse osmosis (RO) to remove residual ions and produce water with very low conductivity (usually below 0.1 μS/cm). The process includes the following steps:
Pre-cleaning:
- The raw water is first treated in a reverse osmosis system to remove most of the ions. The RO system reduces the water conductivity and prepares it optimally for the EDI.
Ion separation and transportation:
- In the EDI cell, the remaining ions are absorbed by the ion exchange resins through the electric field and transferred to the concentrate chambers. This leads to a very low conductivity in the pure water.
Continuous resin regeneration:
- The electric field continuously regenerates the resins as the ions are discharged into the concentrate chambers, eliminating the need for chemical regeneration and making the EDI process continuous and environmentally friendly.
Photo: Ultrapure water system with EDI ALMA OSMO VE
Advantages of electrodeionization
EDI is a powerful and environmentally friendly technology that offers significant advantages over traditional water desalination processes:
Chemical-free regeneration:
- In contrast to conventional ion exchange processes, which require acids and alkalis for resin regeneration, regeneration in the EDI is carried out by the electric field. This significantly reduces operating costs and environmental pollution.
Continuous operation:
- EDI can be operated around the clock without the need for interruptions due to regeneration cycles, which significantly increases the efficiency and availability of the system.
High purity water:
- By removing the last remaining ions, EDI achieves extremely low conductivities, which is essential for applications in the pharmaceutical industry, electronics production and power plant technology.
Compact and space-saving design:
- The EDI cells have a modular design, which makes them very space-saving and suitable for flexible system concepts. Continuous operation reduces the need for storage tanks for regeneration-related shutdown times.
Areas of application for electrodeionization
EDI is mainly used in areas that place particularly high demands on water quality:
pharmaceutical industry:
- High-purity water is of key importance for pharmaceutical processes, as impurities could impair the quality of the products. Thanks to its chemical-free operation, EDI technology is an environmentally friendly and efficient solution for the continuous supply of ultrapure water.
Electronics and semiconductor production:
- In microelectronics and semiconductor production, ultra-pure water is required to clean sensitive components. Even the smallest residues could jeopardize the functionality of the products, which is why EDI is established here as the standard technology for ultrapure water production.
Power plants and steam generation:
- In power plant technology, the use of high-purity water for steam generators and boiler feed water is crucial to prevent corrosion and deposits. EDI is an optimal solution for providing water for use in high-pressure boilers.
Food and beverage industry:
- In beverage and food production, EDI is used to produce pure water to ensure the taste and quality of the end products. The high water purity also enables longer production cycles without interruption due to system cleaning.
Challenges and technical requirements of electrodeionization
Requirements for feed water quality:
- As the EDI reacts sensitively to high ion concentrations, pre-treatment of the water (e.g. by reverse osmosis) is necessary in order to meet the feed water quality requirements and ensure the efficiency of the system.
Operational stability and energy supply:
- As the electric field is crucial for the continuous regeneration of the resins, a constant power supply must be ensured. Fluctuations in the power supply can impair the efficiency of the process.
Fouling and regular maintenance:
- Although EDI technology requires less maintenance than conventional ion exchangers, fouling and deposits can affect the performance of the system. Regular cleaning and maintenance are necessary to protect the membranes and resins from fouling and biofouling.
Conclusion
Electrodionization (EDI) is a state-of-the-art and environmentally friendly technology for the production of ultra-pure water. Thanks to the combination of ion exchange and electrodialysis, EDI systems can be operated continuously and do not require chemical regeneration, which reduces operating costs and minimizes environmental impact. Applications range from the pharmaceutical and electronics industries to power plant engineering, where high water purity requirements are essential. EDI systems offer an excellent solution for many industrial applications due to their easy maintenance, high efficiency and excellent water quality.
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