How Does EDI Work?

How Does EDI Work?

Electrodeionization (EDI) is a continuous, chemical-free water purification technology that removes ions from water using electricity, ion exchange resins, and specialized membranes. It is widely used in industries requiring ultrapure water, such as pharmaceuticals, power generation, semiconductors, and laboratories.

The EDI Process – Ion Movement (Upflow Configuration)

In an EDI module, feed water (typically RO permeate) flows through alternating Product chambers and Concentrate chambers. An DC voltage field is applied across the chambers, driving ions out of the feedwater and into the concentrate streams. The resins are an ion diffusion conduit, not ion absorbents, and never need regeneration with chemicals (acid or caustic).

Here’s how it works step by step:

Feed Water Enters Product Chamber

RO-purified water enters the product chambers, which are filled with ion exchange resins (cation and anion resins). These resins initially adsorb and capture ions from the water:

Cations (+): Na⁺, Ca²⁺, Mg²⁺, K⁺

Anions (−): Cl⁻, HCO₃⁻, H₂SiO₄⁻ (silica)

DC Electric Field

A DC voltage is applied constantly between the Cathode (−) on one side and the Anode (+) on the other. This creates an electric field across the module.

Ion Migration Through Membranes

Cations move toward the cathode (−) and pass through Cation Exchange Membranes (CEM).

Anions move toward the anode (+) and pass through Anion Exchange Membranes (AEM).

The ions are selectively transported from the Product chambers into the adjacent Concentrate chambers.

Ions Are Contained and Removed Continuously

Once in the concentrate chambers, the ions are carried away in the small (5-10%) reject (concentrate) stream. Membranes prevent the ions from migrating back into the product water.

Continuous Resin Regeneration (Water Splitting)

Inside the polishing section of the product chambers, electrochemistry splits water molecules into H⁺ and OH⁻ ions:

OH⁻ ions continuously regenerate the anion resin surface.

H⁺ ions, continuously regenerate the cation resin surface.

This continuous regeneration keeps the resins active and fresh indefinitely, allowing non-stop operation without downtime for chemical regeneration.

Visual Summary of Ion Movement (see Diagram)

• Product chambers (light blue): mixed resin beds where ions are removed → RO water is purified.
• Concentrate chambers (light green): collect and remove the concentrated ions.
• CEM (yellow): Allows only cations to pass.
• AEM (blue): Allows only anions to pass.

Upflow Design

The upflow design shows water and ions moving upward through the vertical chambers. Upflow is ideal as it enables any gas to flow out naturally. Downflow may trap and collect bubbles which interfere with the ion removal process.

Key Electrochemical Reactions at the EDI Electrodes

Anode side: (O₂ is generated, pH drops)

2 H₂O → O₂ + 4 H⁺ + 4e⁻ (favored if done properly)

2 Cl⁻ → Cl₂ + 2 e⁻ (disfavored to prevent premature EDI failure)

Cathode side (H₂ and Cl₂ are generated, pH increases)

2 H₂O + 2e⁻ → H₂ + 2 OH⁻

Why EDI is Highly Effective

• Produces ultrapure water with very low conductivity (often < 0.1 µS/cm).
• No hazardous chemicals required for regeneration.
• Continuous operation with minimal maintenance.
• Environmentally friendly and cost-effective over time.

Key Takeaway:

Ions are continuously removed from the product water by the electric field and captured by the resins. The resins are simultaneously and continuously regenerated by H⁺ and OH⁻ ions produced from water splitting — making EDI a highly efficient, self-regenerating purification process.

See more at Electrodeionization.

View SnowPure’s Electropure EDI products.

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