What Is Electrodeionization (EDI)?

Electrodeionization (EDI) is a process for further treating already partially desalinated water – typically the permeate from a reverse osmosis system – into ultrapure water. It combines the principles of ion exchange with an electric field, and does so entirely without the use of regeneration chemicals.

How Does EDI Work?

At the core of an EDI system are chambers filled with ion exchange resin and bounded by ion exchange membranes. When a DC electric field is applied, the ions remaining in the water migrate through the resin toward the respective electrodes and are carried off through the membranes into a separate concentrate stream. The ion exchange resin is continuously regenerated by the electric current – an advantage over conventional ion exchangers, which must be regenerated at fixed intervals with acid or caustic.

Why EDI After Reverse Osmosis?

Reverse osmosis alone already removes 95–99 % of the dissolved salts from the raw water. For applications that require even higher purity – such as semiconductor manufacturing, pharmaceutical production, or research laboratories – an EDI stage is therefore often connected downstream of reverse osmosis. The result is continuously available water with a conductivity below 0.2 µS/cm, entirely without the chemical use required by conventional mixed-bed ion exchangers.

Advantages at a Glance

EDI systems run continuously, require no regeneration chemicals, produce no acidic or alkaline wastewater, and deliver consistently high water quality without the typical quality fluctuations of conventional ion exchangers as their resin nears exhaustion. We use this combination of reverse osmosis and EDI at IEM in many of our multi-stage ultrapure water systems.