What is Electrodeionization (EDI)?

EDI technology has grown in popularity in water purification industry. It effectively removes impurities from water using a unique combination of semi-permeable membranes, ion exchange resins and a direct electric field. The ion exchange resins are continuously regenerated without the need of cleaning chemicals. To understand how the EDI purification technology actually works, let’s go down to the molecular level and take a look at some water containing ions.

How does Electrondeionizaton (EDI) work?

Water contains positively charged cations (plus signs) and negatively charged anions (minus signs).

When a pair of electrodes is inserted and an electrical current applied, the electric field drives the cations in the direction of the cathode and at the same time drives the anions in the opposite direction toward the anode.

When we insert an ion exchange membrane made of cation selectively resins, it will block the flow of anions and water molecules. Only cations will be able to pass through to the other side. When we insert another ion exchange membrane made of anion selectively resins, it will block the flow of cations and water molecules. Only anions will be able to pass through to the other side. This configuration membranes and electrodes forms the framework of an EDI module.

This process is limited however by the slow speed at which ions move through water. In effect, the low conductivity of water impedes ion removal. In other word, as the product compartment water becomes more and more pure, the electrical resistance of this water increases. EDI technology solves this problem by sandwiching a mixture of cation and anion selective resins between the two membranes. The large surface area now offered by the resin beads effectively eliminates the ion diffusion resistance and allows the ions to move freely. Even in ultrapure water solution, the surface of the resin beads acts as a conductive path, effective acting as a bridge for the ions to move quicker toward the membrane surface faster than they were in water alone.

Take a step back to look how an EDI module is constructed, we can see that if we move the electrodes farther apart and continue to add resin beads sandwiched between the cation and anion selectively membranes, we create a series of water purification compartments alternating with compartments where the ions are concentrated.

As feed water is pumped into the system, it is diverted into separate compartments, namely the concentrating compartment and the purification or dilute compartment. These water streams remain separated throughout the process because only ions can pass through the membranes. Ions migrate and accumulate in the concentrating compartments where they are washed away into the reject stream. The water leaving this compartment contains a concentration of ions approximately 10-20 times higher than the original feedwater. This water may be drained or reclaim for further treatment.

At the top of the purification compartments, the ion concentration is at its highest. Here the surface of the resin beads acts as a conductive path effectively moving the ions to the membrane. At the lower end of the purification compartments, where the ion concentration has been reduced to the ppt level, the electric field becomes concentrated between the resin beads and the surrounding water.

When this localized voltage potential exceeds approximately eight tenths of a volt, an electrochemical reaction occurs and water molecules are split into H⁺ and OH^– ions, which are essentially acid and caustic. The acid and caustic generated in the module regenerate resin beads by replacing other trace ions remaining the attachment of H+ and OH- ions. This is exactly what happens to resin beads when acid and caustic are used for regeneration in conventional deionization systems.

As a result, an EDI system does not need chemicals for regeneration. The electric potential does all the work. You essentially have a worker and a polisher combined into one unit which constantly regenerates.

In sum, an EDI system can consistently and predictably remove dissolved materials such as salts, acids and bases, especially weakly ionized materials such as low molecular organics, carbon dioxide, silica, and boron. The continuous regeneration process also eliminates pH shock and greatly extends the life of the resins.

Applications
EDI water (Type II water) is often selected for projects requiring ultra-high purity water, typically used in the following fields:

• Laboratories
• Pharmaceuticals
• Power generation
• Boiler feed water
• Semiconductor production
• Electronics & Microelectronics
• Food and Beverage

Benefits

• Removes dissolved inorganics/low molecular organics effectively
  • A constant flow of Type II water. Reliable and consistent water quality.
  • Resistivity: 5-16MΩ • cm @ 25 °C， typically >15MΩ • cm
  • TOC < 30 ppb
• Reliable and consistent water quality
  • Produce a constant flow of Type II water
• Environmentally friendly
  • No chemical regeneration.
  • No chemical disposal.
  • No resin disposal.
  • Low energy consumption.
• Inexpensive Operation Cost:
  • No need for neutralization units
  • No need for local permitting
  • No need for other costs of chemical
  • Low power consumption
• Low Maintenance Needs
  • Long life-cycle because of continuous regeneration of the ion exchange resins inside the module (little exhaustion)
• Safe Operation
  • No hazardous chemical is required
  • Less risk to people and accidental discharge
  • No heating element

Limitations

• Feed water limited
  • Requires good quality water of feed water (hardness < 1; RO water is recommended)

RephiLe lab water purification products utilizing EDI technology 

Among products of RephiLe, Genie G, Genie E, Super-Genie G and Super-Genie E are equipped with best-in-class Ion-pure EDI module. Take Genie G as an example, Genie G produces ultrapure water (Type I water) with the resistivity of 18.2 MΩ·cm at 25°C and TOC < 5 ppb and EDI water (Type II water) with the resistivity of > 5 MΩ·cm at 25°C and TOC < 30 ppb from tap water directly. It offers desired solutions for research professionals who work with varieties of critical applications using Type I ultrapure and Type II pure water in the lab.

For more information, please visit: https://rephile.com/genie-g-ultrapure-edi-lab-water-systems/

About RephiLe:
Driven by innovation and quality, RephiLe is a dedicated provider of water purification systems and laboratory filtration products. At the same time, RephiLe produces comprehensive consumables that can be used in Millipore lab water systems with reliable performance. It is RephiLe’s commitment to becoming a partner of choice for customers in the area of life science and biotechnology. The company is striving to bring superior quality, high value and innovative purification tools to enable and accelerate the advancement of the life sciences and technologies. Products are being sold to over 90 countries worldwide.

For more information, please visit：https://rephile.com/