Using RO Permeate as Cooling Water: Key Precautions You Must Take| Insights by AQUALITEK

Introduction

Reverse osmosis (RO) permeate is often viewed as an ideal cooling-water source because of its very low hardness, low TDS, and minimal scaling tendency. In theory, this allows higher cycles of concentration and improved heat-exchange efficiency.

However, ultra-low mineral content does not automatically mean “safe” cooling water. In fact, RO permeate can be chemically aggressive, biologically unstable, and damaging to carbon-steel and copper alloys if used without proper control.

This article outlines the most important precautions to consider before using RO permeate directly as cooling water in industrial applications.

1. Understand the Corrosive Nature of RO Permeate

Why RO Permeate Is Corrosive

RO permeate typicallyDS, alkalinity, and calcium hardness are extremely low, which results in:

•Very low buffering capacity

•Negative Langelier Saturation Index (LSI)

•High tendency to dissolve metals

Without adjustment, RO permeate can aggressively attack:

•Carbon steel piping

•Heat exchanger tubes

•Condenser shells

•Welded joints and fittings

Key Precaution

Do not use RO permeate directly without corrosion control.
pH adjustment, alkalinity addition, or corrosion inhibitors are mandatory.

2. Control pH and Alkalinity for Water Stability

Recommended Targets

Typical stabilization targets when RO permeate is used as cooling water:

•pH: 7.0–8.5 (depending on metallurgy)

•Alkalinity: 30–100 mg/L as CaCO₃

•Slightly positive or near-neutral LSI

Common Conditioning Methods

•Sodium hydroxide or lime dosing

•Blended bypass of raw or softened water

•Specialized corrosion inhibitor programs

Best practice:
Stabilize water chemistry before it enters the cooling loop.

3. Re-Evaluate Corrosion Inhibitor Programs

Traditional cooling-water treatment programs are often designed for harder water. When using RO permeate:

•Film-forming inhibitors may behave differently

•Over-dosing becomes easier due to low background ions

•Copper alloy protection requires special attention

Key Precaution

Work with a chemical supplier to redesign the inhibitor formulation specifically for low-TDS water.

4. Manage Biological Growth Risks

Why Biological Fouling Can Increase

RO permeate:

•Contains almost no residual disinfectant

•Warms quickly in cooling systems

•Can support rapid microbial colonization once nutrients are introduced

This creates ideal conditions for:

•Biofilm formation

•Microbiologically influenced corrosion (MIC)

•Slime fouling on heat transfer surfaces

Key Precautions

•Continuous or intermittent biocide dosing

•Maintain measurable oxidizing or non-oxidizing biocide residuals

•Regular microbiological monitoring (ATP, plate counts)

5. Monitor Conductivity and Cycles of Concentration

RO permeate starts with very low conductivity, which can be misleading.

Common Risks

•Excessive cycles of concentration

•Undetected contamination ingress

•Over-concentration of treatment chemicals

Best Practices

•Install high-accuracy conductivity sensors

•Set conservative alarm limits

•Adjust blowdown control logic specifically for RO permeate

6. Evaluate Materials Compatibility

Low-mineral water can be incompatible with certain materials.

Materials at Higher Risk

•Carbon steel

•Galvanized steel

•Yellow metals (brass, copper alloys)

Safer Alternatives

•Stainless steel (304/316)

•Certain plastics and coatings

•Properly lined carbon steel

Key Precaution:
Conduct a materials review before converting to RO permeate cooling water.

7. Plan for Emergency Bypass and Blending

Sudden loss of chemical dosing or control can cause rapid corrosion damage.

Recommended Safeguards

•Emergency blending with raw or softened water

•Automatic shutdown on pH or conductivity deviation

•Redundant chemical dosing systems

These measures prevent catastrophic failures during upsets.

8. Economic and Operational Considerations

While RO permeate can:

•Reduce scaling

•Increase heat-exchange efficiency

•Lower blowdown volumes

It may also:

•Increase chemical consumption

•Require tighter monitoring

•Raise corrosion-related maintenance costs if mismanaged

A total cost of ownership (TCO) analysis is strongly recommended.

Conclusion

Using RO permeate directly as cooling water is feasible and increasingly common—but only when water stability, corrosion control, biological management, and system materials are properly addressed.

The key takeaway is simple:

RO permeate is clean, but not inherently safe.

With the right conditioning and monitoring strategy, it can deliver excellent performance and long-term reliability in industrial cooling systems.