Why RO Membranes Foul: Four Fouling Types, Pretreatment Design and CIP Procedure

After some months of operation the log sheet starts telling a story: product flow drifting down, inter-stage differential pressure creeping up, product conductivity rising. All three usually point to the same cause — fouled membranes. Around 80% of RO system failures originate in membrane fouling and pretreatment.

1. Identify the fouling type first

Scaling (inorganic salt)

Calcium carbonate is most common, followed by calcium sulphate, barium sulphate and silica. Symptoms: clearly reduced flow, rising differential pressure, heavier elements. Autopsy shows hard white deposits. The higher the recovery, the closer the concentrate is to saturation and the greater the scaling risk.

Organic fouling

Humic acids, oils and surfactants from the feed adsorb onto the membrane. Symptoms: falling flow, slight loss of rejection, slippery feel on the membrane surface. Near-shore harbours and areas with vessel discharge are prime candidates.

Colloidal fouling

Clay, iron oxide, silica colloids and humic particles. Symptoms: rapid differential-pressure rise, persistently high SDI. Keep feed SDI15 below 5, ideally below 3.

Biological fouling

Biofilm formed by bacteria growing on the membrane, usually accompanied by organic fouling. Symptoms: differential pressure rising day by day and quick recurrence after cleaning. High risk near aquaculture and sewage outfalls.

2. Pretreatment must match the foulant

3. When to clean

Do not clean by feel. Use three hard triggers:

• normalised product flow down 10–15% from baseline;
• normalised salt rejection down, or product conductivity up, by more than 10%;
• inter-stage differential pressure up 15% (or above 0.7 bar absolute).

Any one of the three means clean now. Delay and the foulant compacts into irreversible damage. A normal interval is 3–12 months; if you are cleaning monthly, pretreatment has a design fault and must be fixed rather than cleaned more often.

4. Standard CIP sequence

1. Low-pressure flush with permeate to displace concentrate, 10–15 minutes.
2. Prepare the solution: 1–2% citric acid (pH 2–3) for carbonate and sulphate scale; 0.1% NaOH with surfactant (pH 11–12) for organic and biological fouling; silica needs a dedicated cleaner — caustic is largely ineffective on silica.
3. Circulate at 40–60 L/min per element for 30–60 minutes, holding 25–35 °C (never above 40 °C, which damages the membrane).
4. Soak 1–2 hours for heavy fouling, stage by stage if needed.
5. Rinse with permeate until the outlet is neutral and foam-free.
6. Performance test — record flow, rejection and differential pressure and compare with the pre-clean baseline.

Never allow free chlorine in the cleaning solution; never mix acid and caustic without a full intermediate rinse; and treat first and second stages separately when their fouling differs.

5. Standstill protection — where many operators get caught

Short shutdowns (under 15 days): low-pressure flush every 1–2 days when water temperature is above 20 °C, every 3 days below that, to prevent biological growth.

Longer shutdowns (over 15 days): fill the elements with preservative solution — typically 1% food-grade sodium bisulphite or a proprietary membrane preservative — then close all valves. If a membrane dries out, the flux loss is permanent.

Shenzhen Duruofei Technology Co., Ltd. supplies a cleaning and layup procedure specific to each model, and configures pretreatment against the customer's actual water analysis. Membranes are the most expensive consumable in the plant; protecting them is how you keep water cost locked low.