RO membrane troubleshooting: reading salt rejection, output and pressure drop
One table that links changes in salt rejection, output and pressure drop to their causes and fixes.
Updated
How to use the table
In operation, the way salt rejection, output and pressure drop change together points to the cause of a fault. Find the closest row below and apply the fix.
| Cause | Salt rejection | Output | Pressure drop | Fix |
|---|---|---|---|---|
| Oxidation damage (chlorine, ozone) | Down | Slightly up | No change | Replace the element |
| Leaking membrane (product back-pressure, abrasion) | Down | Slightly up | No change | Replace the element; for abrasion also replace the cartridge filter |
| Leaking O-ring (poor installation) | Down | Slightly up | No change | Replace the O-ring |
| Leaking product tube (damaged during installation) | Down | Slightly up | No change | Replace the element |
| Scaling (poor scale control) | Slightly down | Down | Slightly up | Clean; improve scale control |
| Colloidal fouling (poor pretreatment) | Slightly down | Down | Slightly up | Clean; improve pretreatment |
| Biofouling (microbes in the feed, poor pretreatment) | No change | Slightly down | No change | Clean and disinfect; improve pretreatment |
| Organic fouling (oil, cationic polyelectrolytes) | No change | Down | No change | Clean; improve pretreatment |
| Compaction (water hammer) | Up | Down | No change | Replace the element |
Five factors that affect performance
- Feed pressure: output rises roughly in line with pressure; salt rejection rises too, then levels off.
- Temperature: output rises roughly in line with temperature, but salt rejection falls.
- Feed concentration: higher concentration raises osmotic pressure, so output and salt rejection both fall.
- Recovery: at constant pressure, higher recovery lowers output and salt rejection; the maximum recovery depends on how readily sparingly soluble salts such as calcium carbonate, calcium sulfate and silica scale.
- pH: keep it close to neutral; too high or too low ages the membrane faster and lowers salt rejection.
Good water starts with a CONGEXIN membrane
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