The effect of free chlorine on membranes

In reverse osmosis (RO) and nanofiltration (NF) systems, free chlorine management is one of the most important factors for long-term and stable membrane operation.

In practice, many serious membrane failures are not caused by prolonged chlorine exposure, but by short, often unnoticed chlorine peaks. These peaks may be low in concentration, but they have a very strong destructive effect.

In most water treatment systems, chlorine is unavoidable because it is used for biological control. Unfortunately, the same chlorine can cause irreversible damage to the membrane surface if it reaches reverse osmosis and nanofiltration systems without adequate protection.

Why Does Free Chlorine Damage Membranes?

Most commercially available RO membranes are made of aromatic polyamide. This structure provides high salt rejection, but at the same time makes the membrane extremely sensitive to oxidants, especially free chlorine.

Free chlorine attacks the polyamide structure through two main mechanisms:

Chlorolysis: Oxidative Degradation

Free chlorine destroys the amide bonds in the polymer chain. As a result, the following effects are observed:

a sharp drop in salt rejection;

increased permeability to organic compounds;

loss of mechanical and chemical stability of the membrane.

Chlorination of Aromatic Rings

Chlorination of the aromatic structure changes the surface charge of the membrane. This leads to:

reduced rejection of ions such as Ca²⁺, Mg²⁺, and SiO₂;

changes in selectivity;

a higher risk of uncontrolled fouling and deposit accumulation on the membrane surface, leading to reduced productivity.

How to Recognize Chlorine Damage in a Membrane

Chlorine degradation has several very characteristic signs:

a sudden and permanent increase in permeate conductivity;

a drop in salt rejection without a corresponding increase in ΔP;

unusually high permeate flow — an effect commonly known as a “damaged” or “breached” membrane;

no improvement after chemical membrane cleaning;

one membrane pressure vessel showing worse parameters than the others.

Chlorine damage is irreversible and cannot be removed by chemical cleaning, no matter how aggressive the cleaning procedure is.

Where Does Chlorine Enter the System “Unnoticed”?

In real operation, chlorine damage is most often the result of operational mistakes rather than poor design:

sudden chlorine breakthrough occurs when activated carbon becomes exhausted;

chlorinated mains water enters through backwash lines;

incorrect CIP procedures are used, such as hypochlorite at low pH;

leaks or defects occur in dosing pumps;

automation errors cause momentary chlorine peaks.

The most common and most dangerous scenario is the lack of control over the service life of activated carbon. When the carbon becomes saturated, chlorine passes directly to the RO system, and the damage is often detected too late.

Factors That Accelerate Chlorine Degradation

The oxidative effect of free chlorine becomes significantly more aggressive under the following conditions:

high temperature — for every +10°C, the reaction rate approximately doubles;

low pH — below pH 6, HOCl dominates, which is the most aggressive form of chlorine;

high ORP — the higher the oxidation-reduction potential, the faster the polyamide degrades.

For this reason, manufacturers do not limit their requirements only to free chlorine being equal to 0 ppm. They also define the following values as mandatory operating conditions:

ORP < 300 mV;

pH in the range of 6–8.

Five Key Practices for Membrane Protection

SO₂ / sodium metabisulfite (SMBS) dosing — a dose of 1–3 ppm SMBS before RO neutralizes chlorine almost instantly, converting it into sulfate.

Monitoring the service life of activated carbon — when the final capacity is reached, chlorine breakthrough is not gradual but sudden.

Strict discipline during CIP procedures — hypochlorite must never be used for RO membranes, even indirectly. An acidic environment combined with hypochlorite causes strong chlorination.

Control of flushing and bypass lines — mixing chlorinated water with RO feed through flushing lines is one of the most common real-world operational mistakes.

Regular free chlorine testing — operators must confirm 0 ppm free chlorine during every shift using a Hach/DPD test. Not “occasionally,” but every time.

Conclusion

An RO membrane is designed to operate for 3–5 years, but a single chlorine-related failure can reduce this lifetime to just a few weeks. The damage is irreversible, operating costs increase, salt rejection drops, and premature replacement becomes unavoidable.

For this reason, proper free chlorine management is the foundation of every membrane protection strategy. From system design to daily operation, this parameter allows no compromise and is often where the difference lies between stable performance and costly shutdowns.