Both systems produce pure water by removing the ions left in the water; the difference lies in how the resin is regenerated and how stable the product water is. EDI (electrodeionisation) is an advanced water treatment technology that produces ultrapure water by continuously removing the ions dissolved in water with the help of electrical energy and without chemicals. DI (deionisation), on the other hand, retains the ions with anion and cation resins; when the resin is exhausted, it is regenerated with chemicals. The short answer: DI for small or intermittent needs; reverse osmosis + EDI for facilities that need continuous, chemical-free pure water.
Comparison
| Deionisation (DI, anion-cation) | Electrodeionisation (EDI) | |
|---|---|---|
| Operating principle | Cation and anion resins retain the ions | Resin, ion-selective membranes and direct current work together |
| Regeneration | With acid and caustic, at intervals | Continuous and instant with direct current (DC) electricity; no chemicals are used |
| Feed water | Pretreated water; reverse osmosis ahead of it if conductivity is high | Reverse osmosis product water only; hardness and conductivity must be low |
| Product water | Product conductivity is not stable and varies; ≤1 µS/cm. A mixed-bed system may be needed afterwards for lower values | Product conductivity is stable; ≤0.05–0.01 µS/cm |
| Operation | Regeneration requires downtime or a standby column; chemical waste is neutralised | Runs without interruption; no chemical tank or waste |
| Investment | Low | High |
| Operating cost | Depends on acid, caustic and resin consumption | Electricity and module maintenance |
| Typical use | Laboratories, small-capacity pure water, boiler and process water | Pharmaceutical and cosmetics production, electronics, high-pressure boiler feed water |
Which one when?
- Low capacity, intermittent use → DI or a mixed-bed column; a quick solution with low investment.
- Continuously operating, high-capacity pure water line → reverse osmosis + EDI; regeneration downtime and the chemical load disappear.
- If stable conductivity below 0.05 µS/cm is required → reverse osmosis + EDI; an anion-cation system alone cannot hold this value steady.
- If you do not want to store acid and caustic at the facility → EDI.
- If the conductivity of the raw water is high → reverse osmosis before DI; otherwise the resin needs regenerating very frequently.
Using them together
In pure water lines, DI and EDI are usually installed after reverse osmosis as the final stage: reverse osmosis removes most of the salts, and DI or EDI cleans up the remaining ions. Which one is suitable is determined by water analysis according to the target conductivity and daily capacity. You can use our RO calculator for the capacity calculation and request a quote together with your water analysis.
