RO Membrane Cleaning Guide
An RO system that has lost output is almost always fouled, and cleaning-in-place (CIP) is the fix. Get it right and flux and rejection come back and the membranes run longer. Get it wrong and you either burn through chemicals or damage the elements. Here is how to do it properly.
When to clean
Judge it on normalised figures rather than the raw gauges, because a change in feed temperature or pressure can mask fouling or imitate it. Three things tell you to clean now. Normalised permeate flow off by about 10%. Differential pressure across the elements up by 10–15%. Salt passage rising, with rejection down 5–10%. Any single one is enough. Some sites also clean to a calendar. What you shouldn't do is delay, because fouling left on the membrane too long sets hard and won't lift, and then you're replacing elements instead of cleaning them.
Acid cleaning
Acid, at low pH, deals with the inorganics: carbonate scale, and iron and manganese oxides. Usually that's citric, hydrochloric or sulphamic acid taken to roughly pH 2–3. Keep it well away from organic or biological fouling. Used on an organic layer it can fix the foulant onto the film and set you back. Treat it as a scale-and-metals tool.
Alkaline cleaning
Alkaline, at high pH, is for organics, biofilm, silica and colloids. Usually caustic soda near pH 11–12, often with a surfactant or detergent to release the organics. This is the clean most plants run most, because organic and biological fouling turn up everywhere. A bit of warmth helps it along, provided you stay below the membrane's temperature limit, commonly around 35°C.
Cleaning sequence
With both organics and scale on the membrane, the order counts. The general manufacturer advice is alkaline first, then acid, with a plain-water flush in between, so the organics and biofilm are cleared before the acid can set them in place. Heavy carbonate scale is the exception. When that's the main problem, go acid first, or acid on its own.
Each step runs the same basic cycle:
- Make the solution up with RO permeate, not feed.
- Introduce it gently to displace the water already in the system.
- Recirculate it through the elements.
- Soak it, from thirty minutes up to a few hours, and overnight for the stubborn cases, giving it a recirculation now and then.
- Raise the flow to sweep the loosened foulant out.
- Rinse through with permeate before returning to service or changing chemistry.
Work the array stage by stage, so first-stage fouling doesn't end up pushed into the second.
Common mistakes
Top of the list is leaving it too late, when nothing shifts the fouling anymore. After that come the everyday errors. Mixing the clean with feed water rather than permeate. Taking pH or temperature over the membrane's rating. Putting the wrong chemistry on the foulant, acid onto an organic film being the classic one. Missing the flush between the acid and alkaline steps, so the two cancel out and drop solids inside the elements. Doing the whole array in one go instead of stage by stage. Cleaning without knowing what the foulant even is. And letting an oxidiser in, chlorine above all, which oxidises a polyamide membrane and shortens its life. Flow rate has a window too: too much and you can damage elements, too little and the clean does almost nothing.
CIP optimization
The best cleaning is early and aimed at a known target. Identify the foulant before you start, from the normalised trends, a feed water analysis, and a membrane autopsy if it comes to that, and you'll reach for the right chemistry straight away instead of guessing. After that you're tuning for the foulant: temperature, pH, soak time, recirculation time. Run the vessel flow the maker specifies, clean stage by stage, and rinse between steps.
Then watch the frequency. Cleaning that creeps up month on month is telling you the real fault sits in pretreatment or antiscalant dosing, so fix it there rather than cleaning more often. Log every clean and the pattern shows itself.
Explore CIP chemicals
A good clean begins with chemistry matched to your membranes and your fouling. Membrania stocks acid and alkaline CIP chemicals, membrane cleaners and antiscalants, and can advise on method and dosing. Browse the CIP chemicals range, or tell us how your system is behaving and we'll help you set up the right protocol.