EDI Principle and Influencing Factors

Comprehensive explanation of the EDI principle and influencing factors, including how electrodeionization works, key performance variables, resin regeneration mechanisms, and how raw water conductivity affects desalination efficiency in high-purity water systems.

Electrodeionization (EDI) is an advanced water purification technology that integrates electrodialysis (ED) and ion exchange (IX) into a single, continuous, chemical-free process. It is widely used in industries requiring ultrapure water such as pharmaceuticals, electronics, power generation, and laboratory systems.

• Chemical-free resin regeneration
  No acids or alkalis are required for regeneration, making EDI an environmentally friendly and clean water purification method.
• Continuous, self-regenerating operation
  The system regenerates resin during operation, functioning like a continuously regenerated mixed-bed ion exchange system capable of deep, continuous desalination.
• High water quality and low operational cost
  Stable effluent quality, reduced maintenance, and simplified operation make EDI cost-effective for long-term use.

Inside the EDI module, the dilute (fresh water) chamber is filled with a mixture of cation and anion exchange resins. These resins, combined with an applied electric field, form a system where ion exchange, electrodialysis, and resin regeneration occur simultaneously.

• Ion Adsorption
  Dissolved ions in the feed water are first captured by the mixed-bed resin through ion exchange.
• Ion Migration Under Electric Field
  Once adsorbed, ions move directionally toward the ion exchange membranes due to the applied electric field:
  • Cations migrate toward the cathode and pass through cation-selective membranes.
  • Anions migrate toward the anode and pass through anion-selective membranes.
• Ion Transfer to Concentrate Chamber
  The ions pass through the selective membranes and accumulate in the concentrate chamber, creating a low-conductivity water stream in the dilute compartment.
• Water Splitting and Resin Regeneration
  At the resin–membrane interface, polarization occurs, inducing water splitting into H⁺ and OH⁻ ions.
  These ions continuously regenerate the resins:
  • H⁺ regenerates cation exchange resin
  • OH⁻ regenerates anion exchange resin

  This allows EDI to operate continuously without chemical regenerants.

• Ion exchange
• Ion migration
• Electrical (electrochemical) resin regeneration

These processes reinforce each other, enabling continuous, high-efficiency deionization.

Raw water conductivity is one of the most critical parameters affecting EDI performance and effluent quality.

When the influent conductivity is low (e.g., 21.5 μS/cm):

• Ion concentration is low, increasing the potential gradient across the resin and membranes.
• Water splitting intensifies, producing more H⁺ and OH⁻.
• Resin regeneration improves, maintaining strong ion exchange capacity.
• Effluent conductivity becomes extremely low (0.05–0.1 μS/cm).
• When current increases, both:
  • Hydrolysis increases
  • Electrical regeneration strengthens
  • Ion exchange reaches a steady efficiency

This leads to very stable and high-purity effluent water.

When influent conductivity is high:

• Concentration polarization is reduced.
• Hydrolysis becomes weaker.
• Resin regeneration becomes insufficient.
• The resin quickly saturates with ions, decreasing exchange efficiency.

For example, with 100 μS/cm influent water:

• As operating current increases from 0 to 5 A, effluent conductivity increases from 0.17 μS/cm to 0.5 μS/cm.
• Water quality worsens because:
  • Ion exchange dominates instead of water splitting
  • Resin saturates faster
  • Electrical regeneration is insufficient at high salinity

Regardless of influent salinity, multi-stage EDI systems (Stage 2 or Stage 5) maintain:

• Desalination rate ≈ 99%
• Effluent conductivity meeting ultrapure water standards

This is because later stages operate with progressively lower ion loads, allowing hydrolysis and electrical regeneration to occur efficiently.

Electrodeionization (EDI) is one of the most advanced and environmentally friendly technologies for producing ultrapure water. Its core advantage lies in the combination of ion exchange, electrical regeneration, and electrodialysis, enabling continuous removal of ions without chemical regenerants. Among all influencing factors, raw water conductivity is the most significant, as it directly affects hydrolysis strength, resin regeneration efficiency, and final water quality.

By optimizing raw water pretreatment and operating current, EDI systems can consistently produce high-purity water that meets demanding industrial and medical standards.

For advanced RO and pretreatment systems, explore CHIWATEC’s full product range:
👉 CHIWATEC Water Treatment Equipment & Solutions

EDI regenerates resin electrically without acid or alkali, enabling continuous operation with lower operating costs and zero chemical discharge.

Conductivity reflects ion concentration. Low conductivity improves hydrolysis and electrical resin regeneration, while high conductivity reduces efficiency and increases effluent conductivity.

Typically, RO (reverse osmosis) is needed before EDI to ensure low ionic load and prevent scaling and fouling.

Yes. Multi-stage EDI systems can achieve >99% desalination and produce water with conductivity as low as 0.055–0.1 μS/cm.

No.
At high conductivity, increasing current can worsen water quality because resin regeneration becomes insufficient relative to ion load.

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:

• The preparation of ultra-pure water treatment equipment in the electronics industry adopts bipolar reverse osmosis host plus EDI
• How to maintain EDI ultrapure water equipment?
• Advantages of EDI technology compared with hybrid ion exchange technology
• EDI ultrapure water equipment problems and solutions
• Performance comparison of mixed bed and EDI