Why EDI Is Installed After RO in Ultrapure Water Systems| Insights by AQUALITEK

Introduction

Industries such as semiconductors, LCD fabrication, biotechnology, and pharmaceuticals require ultrapure water (UPW) with resistivity approaching 18.2 MΩ·cm and total ion content below 1 μg/L.

Achieving this level of purity is impossible through reverse osmosis (RO) alone. While RO effectively removes over 95–99% of dissolved salts, trace ions, silica, and carbon dioxide can remain.
To reach ultrapure specifications, RO is followed by EDI (Electrodeionization) — a continuous, chemical-free polishing step.

The Role of RO in Ultrapure Water Systems

Reverse Osmosis (RO) uses a semipermeable membrane to reject most dissolved ions, organics, and microorganisms.
In the UPW process, RO typically serves as the primary desalination stage, producing deionized feed water for the EDI unit.

RO Key Capabilities:

•Removes 95–99% of ions and salts

•Reduces TOC (Total Organic Carbon)

•Eliminates microorganisms and colloids

•Lowers conductivity to ~10–20 µS/cm

However, RO cannot completely remove:

•Weakly ionized species (e.g., CO₂, silica)

•Trace ions that pass through membranes

•Non-ionic organics

That’s where EDI comes in.

What Is EDI (Electrodeionization)?

Electrodeionization (EDI) is a hybrid technology combining ion exchange resins and ion-selective membranes, driven by a DC electric field.

It continuously removes remaining ions from RO permeate and regenerates the ion exchange resins electrically, without chemical regenerants such as acid or caustic.

Core Principle:
Under the electric field, cations migrate toward the cathode through cation-exchange membranes, and anions migrate toward the anode through anion-exchange membranes — leaving ultrapure water in the product stream.

Why EDI Is Installed After RO

The success of an EDI unit heavily depends on its feed water quality. It cannot tolerate high TDS, hardness, or organic load.

Therefore, RO must be placed before EDI to protect the module and ensure stable performance.

In all these sectors, RO ensures bulk desalination, while EDI polishes the product to meet ultrapure specifications.

Maintenance and Monitoring Tips

To maintain consistent performance:

•Ensure RO permeate conductivity ≤ 20 µS/cm before EDI

•Periodically check EDI voltage and current (to detect scaling or channeling)

•Maintain feedwater free of chlorine and organics

•Implement automatic CIP if scaling potential increases

•Monitor product resistivity and silica levels for quality assurance

Conclusion

The EDI unit’s placement after the RO system is essential for achieving ultrapure water standards in electronics and pharmaceutical industries.
RO provides high salt rejection and organic reduction, while EDI eliminates the final traces of ions and continuously regenerates resins without chemicals.

Together, RO + EDI delivers a sustainable, low-maintenance, and environmentally friendly ultrapure water solution — the gold standard for high-tech manufacturing and pharmaceutical production.