Where CDI came from

Capacitive deionisation sounds like something invented for a marketing deck. It was not. It comes from industrial water treatment, where the economics of pushing water through membranes under pressure stop working at scale.

The desalination problem

Removing salt from seawater by reverse osmosis works, but it is energy-hungry and wasteful — you are using pressure to force water through a barrier while everything you removed builds up against it.

CDI approaches it from the other side. Rather than pushing water through a barrier, it pulls the dissolved ions out of the water electrically and holds them on a surface.

How the cycle works

• Charge. Water passes between two carbon electrodes at low voltage. Dissolved ions are drawn to the oppositely charged surface and held there by electro-adsorption.
• Discharge. Polarity reverses. The captured ions release into a reject stream and flush away.
• Regenerate. The electrodes are clean and ready. No membrane has been consumed.

Why it took so long to reach a kitchen

Electrode materials, cell miniaturisation and control logic all had to improve before the technology could sit on a counter rather than in a plant. That is the engineering work, and it is why the category word is new even though the science is not.

What it changes in practice

Two things, mainly. Because it is not fighting pressure against a barrier, recovery is higher — 54.6–72.7% in the IAPMO series against 47.4–52.3% for the competitor RO. And because the electrodes regenerate rather than clog, there is no membrane on a replacement schedule.