Industrial RO Troubleshooting Guide: Low Flow, High TDS and Pressure Problems
Quick answer: what should you check first when RO performance drops?
Start with verified operating data before replacing membranes. Confirm instrument calibration, feed pressure, permeate pressure, feed/permeate/reject flows, conductivity, temperature and recovery, then compare normalized performance with a stable baseline.
Problem 1: low permeate flow
Low flow can be caused by lower feed temperature, inadequate net driving pressure, fouling, scaling, blocked pretreatment, pump problems, incorrect recovery or membrane damage. Check pressure and temperature before assuming the membrane is exhausted.
Problem 2: high permeate TDS or conductivity
High product-water conductivity can result from membrane damage, seal leakage, excessive recovery, inadequate pressure, poor membrane condition, instrumentation error or changes in feed-water composition. Carbon dioxide can also pass through RO and influence permeate chemistry.
Problem 3: rising pressure drop
A rising differential pressure across pressure vessels often points toward feed-channel fouling, suspended solids, biological growth, scale or debris. Sudden changes may also indicate a valve, piping or instrumentation problem.
Problem 4: unstable recovery
Recovery depends on reliable flow measurements and proper valve/pump operation. Verify that feed flow is approximately equal to permeate plus concentrate flow and check whether the plant is being operated near its design condition.
What chlorine or oxidant damage can look like
Many polyamide RO membranes are sensitive to oxidants. If chlorine or another oxidant reaches the membrane beyond the supplier’s allowed exposure, salt rejection can deteriorate even when pressure drop looks normal.
- Confirm the disinfectant being used upstream.
- Check free/total chlorine or the relevant oxidant parameter at the membrane feed.
- Verify activated-carbon or dechlorination performance where used.
- Review chemical dosing, breakthrough events and shutdown/startup history.
- Do not diagnose oxidant damage from high permeate conductivity alone; first check seals, pressure, recovery and instrumentation.
Once chemical damage has occurred, cleaning may not restore membrane rejection. Follow the membrane manufacturer’s guidance before deciding on replacement.
Practical troubleshooting sequence
- Confirm all flow, pressure and conductivity instruments are reading correctly.
- Record feed temperature and water analysis changes.
- Compare actual and normalized permeate flow, salt rejection and pressure drop with the baseline.
- Inspect pretreatment: filters, softener, antiscalant dosing, SDI/turbidity and chlorine control where applicable.
- Check pump condition, valves and pressure losses.
- Review recovery and concentrate flow against design.
- Identify the likely foulant before selecting a cleaning chemical.
- Only then consider membrane integrity testing or replacement.
Frequently asked questions
Does low RO flow always mean the membrane is blocked?
No. Temperature, pressure, pretreatment, pump condition and operating recovery can also reduce flow.
Does high permeate TDS always mean the membrane has failed?
No. Check instrumentation, seals, pressure, recovery and feed-water changes before concluding that the membrane is damaged.
Why is normalized data important?
Raw flow and rejection change with temperature, pressure and feed-water conditions. Normalization makes trend comparisons more meaningful.
Reference
DuPont’s current FilmTec RO/NF Technical Manual identifies loss of salt rejection, loss of permeate flow and increased pressure drop as common troubleshooting problems and recommends evaluating normalized plant data and calibrated instruments before corrective action. See the FilmTec RO/NF Technical Manual.
Also see ABHIRO’s guides on RO pressure drop, RO recovery and CIP cleaning.
