RO membrane cleaning procedure and chemicals
When to clean
Compare today's readings with the readings when the membranes were new or freshly cleaned, corrected for temperature and pressure. Clean when any one of these happens:
- Normalised permeate flow has dropped 10 to 15 percent
- Normalised salt passage has risen 5 to 10 percent (permeate TDS going up on your TDS meter)
- Pressure drop across a stage has risen 15 percent
Cleaning early works better. A membrane that has lost 30 percent of its flow is much harder to recover.
Which chemical for which foulant
| Foulant | Signs | Cleaning |
|---|---|---|
| Calcium carbonate scale | Last stage worst, salt passage up | Low pH: citric acid 2% or HCl to pH 2 |
| Iron and metal oxides | Brown deposit, first stage | Low pH: citric acid 2% or sodium hydrosulphite |
| Organics and biofilm | First stage, pressure drop up, slime | High pH: NaOH to pH 11–12, with EDTA or SDS detergent |
| Silt and colloids | First stage, pressure drop up | High pH clean first, then low pH |
When the foulant is unknown, clean at high pH first and then at low pH. Cleaning at low pH first can set organic fouling harder.
CIP procedure, step by step
- Mix the cleaning solution in the CIP tank with RO permeate, not raw water. Heat it if the membrane allows (often up to 35 to 40 °C).
- Low-flow pump: push the solution into the vessels at low pressure and low flow to displace the feed water. Send the first displaced water to drain.
- Recirculate for 30 to 60 minutes. Check pH and adjust; if the solution changes colour quickly, drain it and mix fresh.
- Soak for 1 to 8 hours for heavy fouling.
- High-flow recirculation for 30 to 60 minutes to flush loosened deposits.
- Rinse with RO permeate until the outlet pH matches the inlet.
- Restart the RO, send the first permeate to drain, and record new baseline readings.
Prevent the next clean
- Keep SDI below 3 with good pre-treatment. See our SDI test procedure.
- Dose antiscalant with a chemical dosing pump, sized to your water analysis and recovery.
- Change the 5 micron cartridge filter on time: see when to replace a filter cartridge.
If membranes no longer recover after cleaning, we supply replacement RO membranes, pressure vessels and adaptor kits.
FAQs
How often should RO membranes be cleaned?
Typically every 3 to 12 months, depending on feed water and pre-treatment. Clean when flow drops 10 to 15 percent or salt passage rises 5 to 10 percent.
Which chemical is used for RO membrane cleaning?
Citric acid or hydrochloric acid at low pH for scale and iron, and caustic soda at high pH with a detergent such as EDTA or SDS for organics and biofouling.
Can I clean an RO membrane with raw water?
No. Mix and rinse with RO permeate so the hardness and chlorine in raw water do not re-foul or damage the membrane.