Why Does RO Permeate Conductivity Temporarily Increase After Membrane Cleaning?| Insights by AQUALITEK

Introduction

During routine operation, RO permeate conductivity is one of the most critical indicators of membrane performance.
However, many operators observe a puzzling phenomenon:

Immediately after chemical cleaning, permeate conductivity increases instead of improving.

This often raises concerns about membrane damage or cleaning failure.
In reality, a short-term conductivity increase after cleaning is usually normal—but only under specific conditions.

This article explains why this happens, how long it should last, and when it becomes a warning sign.

1. Residual Cleaning Chemicals Inside the Membrane

Primary and Most Common Reason

After CIP (Clean-In-Place), small amounts of cleaning agents may remain:

•Inside membrane pores

•Between membrane leaves

•In the permeate channel

Many cleaning chemicals contain:

•Sodium salts

•Chelating agents

•Acids or alkalis with high ionic strength

These dissolved ions temporarily increase permeate conductivity.

✅ This effect disappears once thorough flushing is completed.

2. Temporary Change in Membrane Surface Characteristics

Clean Membranes Are More “Open” Initially

Before cleaning, fouling layers partially block:

•Active membrane surface

•Water transport pathways

After cleaning:

•Fouling is removed

•Effective membrane area increases

•Salt passage may rise briefly

This does not mean membrane rejection has permanently decreased.
It reflects a transitional stabilization period.

3. Osmotic and Concentration Equilibrium Adjustment

System Needs Time to Rebalance

After cleaning:

•Salt distribution inside the membrane module is disrupted

•Permeate and concentrate channels require re-equilibration

During this short phase:

•Salt diffusion temporarily increases

•Conductivity readings appear higher

This effect typically stabilizes within:

•30 minutes to several hours (industrial systems)

4. Insufficient Post-Cleaning Flushing

Operational, Not Membrane-Related

If flushing time or flow rate is insufficient:

•Cleaning solution mixes with permeate

•Conductivity remains elevated

Common mistakes include:

•Rushing system restart

•Connecting product water tank too early

•Low flushing flow velocity

This is one of the most avoidable causes.

5. Temperature Effects After Cleaning

Cleaning is often performed at:

•Higher-than-normal water temperatures

Higher temperature results in:

•Increased ion mobility

•Higher conductivity readings

Once temperature returns to normal operating range, conductivity typically drops accordingly.

How Long Is a “Normal” Conductivity Increase?

If conductivity does not trend downward, further investigation is required.

When Should You Be Concerned?

 Investigate immediately if:

•Conductivity remains high after extended flushing

•Salt rejection is permanently reduced

•Differential pressure is abnormal

•Cleaning involved incompatible chemicals

Possible causes then include:

•Membrane oxidation damage

•Incorrect cleaning pH

•Overexposure to cleaning agents

Best Practices to Minimize Conductivity Spikes

•Flush with RO permeate or qualified feed water

•Monitor pH and conductivity simultaneously

•Delay connection to product water tank

•Resume operation gradually

•Record baseline conductivity after stabilization

Conclusion

A temporary increase in RO permeate conductivity after membrane cleaning is normal and expected in most systems.

It is usually caused by:

•Residual cleaning chemicals

•Temporary membrane surface changes

•Incomplete flushing

•Temperature effects

The key is not the spike itself—but how quickly conductivity stabilizes.

Proper post-cleaning procedures ensure membrane performance improves, not degrades.