Reverse osmosis alone typically removes 97–99% of dissolved salts, which sounds thorough — until the application in question requires resistivity in the double-digit megohm range, where even trace remaining ions matter. That's the gap electrodeionization (EDI) is designed to close, and it's the core mechanism behind how systems like Molewater's ultrapure water system reach consistent 10–18 MΩ·cm output.

EDI works by combining ion-exchange resin with an electrical field and ion-selective membranes. As water passes through the resin bed, an applied voltage continuously pulls remaining ions out of the water and through the membranes into a concentrate stream, while the resin itself is continuously regenerated by the electrical current rather than by periodic acid or alkali dosing. This is the key operational difference from traditional ion-exchange systems: conventional resin beds need to be taken offline and chemically regenerated once they're exhausted, creating both downtime and a need for chemical storage and handling. EDI regenerates itself in real time, which is why it can run continuously without production interruptions.

In the Molewater ultrapure water system, this EDI stage sits downstream of a double-stage RO process, which handles the bulk ion and organic removal before EDI performs the final polishing step. The result is water in the 10–18 MΩ·cm resistivity range, produced without acid or alkali regeneration chemicals and without the sewage discharge associated with chemical regeneration cycles.

Stability matters as much as the peak resistivity number itself. A system that occasionally spikes to high resistivity but drifts significantly between cycles is less useful for continuous manufacturing than one that holds a narrower, more predictable range. Because EDI doesn't require cyclical regeneration downtime, resistivity output tends to stay steadier across long production runs — a meaningful difference for facilities running continuous processes rather than batch operations.

The system also includes automated multi-way valve flushing and backwashing on the pretreatment stage, automatic RO membrane flushing, and continuous online monitoring of pressure, flow, and conductivity/resistivity — all managed through PLC and HMI touchscreen control. For facilities evaluating ultrapure water technology, understanding why EDI achieves more stable resistivity than chemical regeneration methods is often the deciding factor between purification approaches, not just the headline purity number.