When to replace an RO membrane — five clear signs
Short answer: A membrane needs replacing when permeate TDS keeps rising even after CIP cleaning. Falling flow on its own usually means fouling, which cleaning can reverse. Rising TDS that cleaning does not fix means the membrane itself has degraded.
Typical membrane life is 2 to 4 years, but that depends almost entirely on how well the pre-treatment and dosing are managed.
Record the baseline first
You cannot judge decline without knowing the starting point. When a membrane is new, record:
- Permeate flow (LPH)
- Reject flow (LPH)
- Feed pressure and permeate pressure
- Feed TDS and permeate TDS
- Feed water temperature
Sign 1: Permeate TDS rising
This is the decisive one. Calculate rejection:
| Rejection | Condition |
|---|---|
| 96 – 98% | New or healthy membrane |
| 92 – 96% | Ageing but usable |
| 88 – 92% | Nearing end of life |
| Below 88% | Replace |
Measure with a TDS or conductivity meter on both feed and permeate — you need both to calculate rejection at all.
Sign 2: Permeate flow falling
A drop of more than 15 percent from the baseline needs investigation. But note that low flow alone does not mean the membrane is finished — read it together with pressure.
Sign 3: Feed pressure rising
If the pump has to work harder to produce the same flow, something is restricting the membrane surface.
Reading the three together
This is what actually tells you the answer:
| Flow | Pressure | TDS | Diagnosis | Action |
|---|---|---|---|---|
| Down | Up | Normal | Scaling or fouling | CIP cleaning |
| Down | Down | Normal | Pre-filter choked or pump weak | Check upstream |
| Normal | Normal | Up | Membrane degraded | Replace |
| Up | Down | Up | Membrane or O-ring damaged | Inspect, likely replace |
| Down | Up | Up | Heavy fouling plus damage | Clean first, then judge |
Sign 4: No improvement after CIP
Carry out a proper CIP cleaning. If flow and rejection return, the membrane was simply fouled. If neither improves meaningfully, the membrane is finished.
See our CIP cleaning guide for the procedure.
Sign 5: Chlorine damage
Chlorine attacks the polyamide layer of a thin film membrane. The damage is permanent and irreversible.
The signature is distinctive — flow increases while TDS rises sharply. The membrane has effectively developed holes, so more water passes but it is not being purified.
Prevention: remove all chlorine before the membrane by dosing SMBS. See our SMBS page.
What shortens membrane life
| Cause | Effect | Prevention |
|---|---|---|
| Chlorine | Permanent damage | SMBS dosing |
| Scaling | Flow falls, hard to reverse | Antiscalant dosing |
| High turbidity or SDI | Fouling | Proper pre-filtration |
| Excessive recovery | Scaling | Keep recovery correct |
| Bacterial growth | Biofouling | Do not leave standing wet |
| Running without antiscalant | Rapid scaling | Level switch on dosing tank |
Monitoring that catches problems early
Fit these and record readings weekly:
- Rotameters on permeate and reject — see our flow meter placement guide
- Pressure gauges before and after the membrane
- TDS meters on feed and permeate
A weekly record of six numbers tells you months in advance that a membrane is declining, so replacement can be planned rather than forced.
Normalising for temperature
One thing that confuses people: membranes produce less water in cold weather. Permeate flow can fall 20 to 30 percent in winter with nothing wrong at all.
So always compare readings at a similar temperature, or you will replace a perfectly good membrane in January.
Planning replacement
- Replace all elements together in a multi-element vessel — mixing old and new gives poor results
- Keep a spare if the plant cannot stop
- Replace the O-rings and brine seal at the same time — these are cheap and a worn seal lets feed water bypass
- Record the new baseline immediately after fitting
For membrane options see our membrane range or send us your requirement.
Need a replacement membrane?
Send your plant capacity, feed TDS and current readings, and we will confirm whether replacement is needed and quote it.
💬 WhatsApp par poochhein 📞 9910646957Frequently asked questions
How do I know whether to clean or replace a membrane?
If flow has fallen and pressure has risen but TDS is normal, the membrane is fouled and cleaning should restore it. If TDS has risen while flow and pressure are normal, the membrane has degraded and needs replacing.
What rejection should a healthy membrane give?
A new membrane should give ninety six to ninety eight percent rejection. Below about eighty eight percent it should be replaced. You need TDS readings on both feed and permeate to calculate it.
How long does an RO membrane last?
Typically two to four years, but that depends almost entirely on pre-treatment and dosing. Chlorine, scaling and poor filtration all shorten life considerably.
Why does permeate flow drop in winter?
Membranes produce less water at lower temperature, and flow can fall twenty to thirty percent in cold weather with nothing actually wrong. Always compare readings taken at a similar temperature.
What happens if chlorine reaches the membrane?
It attacks the polyamide layer and the damage is permanent. The signature is flow increasing while TDS rises sharply, because the membrane has effectively developed holes. SMBS dosing prevents this.