What Irreversible Damage Can Residual Chlorine and Oxidants Cause to RO Membranes? (Expert Industrial Guide)| Insights by AQUALITEK

Introduction

Reverse osmosis (RO) membranes—especially polyamide thin-film composite (TFC) membranes—are highly sensitive to oxidizing substances such as free chlorine, hypochlorite, permanganate, ozone, and other strong oxidants. While these chemicals are commonly used in pretreatment for disinfection, any residual amount reaching the RO system can cause permanent and irreversible membrane damage.

This Best-class article details how oxidants damage membranes, what irreversible changes occur, how to recognize the symptoms early, and how to prevent catastrophic degradation.

1. Why RO Membranes Are Extremely Sensitive to Chlorine and Oxidants

Most industrial RO membranes are made of polyamide (PA).
PA has excellent desalination capability but poor oxidation resistance.

Oxidants break the chemical bonds within the membrane’s polyamide layer, causing:

•Structural changes

•Surface degradation

•Permanent loss of selectivity

Even 0.1–0.3 mg/L free chlorine can cause measurable damage.
Higher concentrations cause rapid destruction in minutes.

2. Irreversible Damage Caused by Residual Chlorine & Oxidants

Below are the main forms of irreversible damage that occur once chlorine reaches the RO membrane.

(1) Destruction of the Polyamide Active Layer

Residual chlorine oxidizes the PA layer, breaking amide bonds.

Results:

•Active layer thins

•Selectivity collapses

•Salt rejection drops permanently

This is the most common irreversible failure mode.

(2) Sharp Increase in Permeate Conductivity (Permanent)

Once the active layer is chemically destroyed:

•Conductivity rises drastically

•Permeability increases

•Rejection cannot recover after flushing or cleaning

This performance drop cannot be restored by any cleaning method.

(3) Increased Water Flux (Abnormally High Permeability)

While fouling usually decreases flux, chlorine attack causes the opposite:

•Water flux suddenly increases

•But quality becomes poor

•Indicates structural deterioration

This is a typical sign of oxidation damage.

(4) Loss of Membrane Integrity and Mechanical Strength

Oxidation weakens the membrane polymer matrix:

•Cracking

•Brittleness

•Surface pitting

•Enlarged pores

•Separation of layers

The membrane becomes structurally unstable.

(5) Accelerated Biofouling After Partial Oxidation

Paradoxically, oxidized PA surfaces become:

•More hydrophilic

•More adsorptive

•More prone to microbial attachment

Once damaged by chlorine, the membrane biofouls faster—even after chemicals are removed.

(6) Glue-Line and Brine Seal Degradation

Oxidants can also affect bonding resins and seals:

•Glue-line weakening

•Brine seal hardening

•Increased bypass or leakage

This results in uneven flow and further loss of rejection.

3. Early Warning Signs of Oxidant Damage

Operators should look for these symptoms:

(1) Conductivity of permeate suddenly increases

Indicates loss of rejection capacity.

(2) Water flux increases unusually

Opposite of fouling; suggests chemical attack.

(3) Irrecoverable performance drop after cleaning

Cleaning cannot repair oxidized membranes.

(4) Multiple elements fail simultaneously

Chlorine damage affects all elements in the same stage.

4. How to Prevent Oxidant Damage in RO Systems

(1) Strict Dechlorination Before RO

Use:

•Sodium bisulfite (SBS)

•Activated carbon

•Catalytic carbon

Keep free chlorine < 0.05 mg/L (often target: ND).

(2) Use Online ORP Monitoring

ORP 200–300 mV is typical for dechlorinated water.

(3) Prevent Overdosing of Oxidant in Pretreatment

Especially with:

•Chlorine

•Sodium hypochlorite

•Ozone

•Permanganate

(4) Add SBS before any possible chlorine source

Place SBS injection immediately after chlorination and ahead of multimedia filters.

(5) Ensure SBS dosing system is stable

Check:

•Pump pulsation

•Stock solution concentration

•Tank mixing

•Flow-paced dosing

(6) Avoid bio-shock chlorination without isolating RO

Always bypass RO during high-chlorine treatments.

5. What to Do if Oxidant Damage Is Confirmed

Unfortunately, oxidation damage is irreversible.

Once confirmed:

•Replace the affected membrane elements

•Inspect pretreatment dosing system

•Adjust ORP/free chlorine monitoring

•Modify process control logic to prevent recurrence

There is no chemical cleaning method that can restore oxidized membranes.

Conclusion

Residual chlorine and other oxidants are among the most dangerous contaminants for RO membranes. Even small amounts can cause irreversible chemical degradation, leading to permanent loss of salt rejection, abnormal flux, structural breakdown, and accelerated biofouling. Proper dechlorination, ORP monitoring, and dosing control are essential to protecting the membrane and ensuring long-term RO system performance.