CDI-650Continuous Electrodeionisation (CDI / EDI)

How it works

1. Resin that self-regenerates — An EDI cell is packed with mixed-bed resin between an anion and a cation membrane. Ions are exchanged onto the resin exactly as in IX — but a DC field then drags them off the resin, through the membranes, and into an adjacent concentrate channel. No brine, no regeneration outage.
2. Water splitting — At the resin-to-resin contact points the field splits water into H⁺ and OH⁻. Those are the regenerant: they continuously strip the resin back to its free form. The regeneration chemistry is generated in situ from the water itself.
3. The current balance — Current must at least match the ionic load — one Faraday per equivalent, about 26.8 amp-hours. Too little and ions break through. Too much and you drive polarisation: excess water splitting, pH excursions and hardness scaling on the concentrate side.
4. Why RO comes first — EDI needs a feed already below about 40 µS/cm and essentially free of hardness — which means RO permeate. Feed CO₂ matters more than people expect: it is not rejected by RO, ionises inside the stack, and consumes current that should be removing salt.
5. What governs it — Ionic load sets the current; current sets the product quality and the power; excess current sets the scaling risk. Wind CO₂ up and watch the current requirement climb — degassing upstream is often cheaper than a bigger stack.
6. The thick cell — A deep resin compartment holds more resin per membrane area, so the stack is more forgiving of what arrives: hardness that would scale a thin cell has somewhere to go. It is the sensible choice behind a single-pass RO, where the feed is good but not guaranteed. The ion path is long, so it needs the most voltage of the three.

Design parameters

The panel opposite runs the same correlations as the CDI-650 design sheet, so the animation and the calculator cannot disagree. Every parameter below is live — drag one and the picture responds.

Open the full CDI-650 design sheet for the other ten inputs, all thirty-one results with their envelopes, the six equations printed beside the numbers they produce, and a calculation record you can export and put in a design file.