Why EDI Is Installed After RO in Ultrapure Water Systems| Insights by AQUALITEK
In ultrapure water production for electronics and pharmaceutical manufacturing, the combination of RO (Reverse Osmosis) and EDI (Electrodeionization) is the industry standard. This article explains why EDI units are typically installed after RO systems, how they complement each other, and what advantages this configuration offers for high-purity applications.
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.
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Reason |
Explanation |
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1. Protects EDI from scaling and fouling |
RO removes calcium, magnesium, silica, and organics that could damage EDI membranes. |
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2. Ensures low conductivity feed |
EDI requires feedwater ≤ 20 µS/cm for efficient ion removal. RO achieves this precondition. |
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3. Improves ion exchange efficiency |
With lower ion load, the EDI resin operates efficiently and lasts longer. |
|
4. Enables continuous regeneration |
Clean RO permeate allows EDI to self-regenerate via electric current, without chemical regeneration. |
|
5. Reduces overall operating cost |
Using RO + EDI minimizes chemical use, waste generation, and maintenance downtime. |
RO + EDI: The Standard Configuration for Ultrapure Water
In modern ultrapure water systems, the RO + EDI combination has replaced traditional RO + Mixed Bed Ion Exchange systems.
Typical Flow Path:
Pre-treatment → RO (1st Pass) → RO (2nd Pass) → EDI → Polishing Filter → Storage & Distribution
Advantages of RO + EDI:
•Continuous, chemical-free operation
•Stable product resistivity: 15–18.2 MΩ·cm
•Lower operating and maintenance cost
•No acid/alkali handling risk
•Compact footprint suitable for cleanroom environments
Application Scenarios
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Industry |
Typical Application |
Water Quality Requirement |
|
Semiconductor |
Wafer cleaning, photolithography |
18.2 MΩ·cm, TOC < 5 ppb |
|
Pharmaceutical |
Purified water (PW), WFI (Water for Injection) |
USP/EP compliant, low TOC |
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Power Plant |
Boiler feed water, turbine wash |
< 0.1 µS/cm |
|
Laboratory |
Analytical reagent water |
ASTM Type I/II water |
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.
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