RO Plant Output Dropping? Diagnose It From Your Own Readings
Why an industrial RO plant's output falls and how to find the cause from your own log sheet: temperature, pressure, pre-treatment, fouling, scaling or membrane damage.
By Nishu Enterprises engineering team
Published · 7 min read
Your RO gave 1,000 litres an hour last year; now it gives 700. Before replacing membranes or calling for a new plant, look at your own readings. The pattern of pressures, flows and TDS usually points straight at the cause, and many of the causes are cheap to fix.
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First, rule out the water temperature
RO membranes produce less water when the feed is colder: roughly 3% less for every degree Celsius drop. A plant rated at 25 °C can lose a noticeable share of its output in a cool winter morning without anything being wrong. Compare readings taken at similar temperatures, or ask for normalised figures that correct for temperature and pressure.
Read the pattern
| What you see | Likely cause | What to check |
|---|---|---|
| Low output, high pressure drop across the first stage | Particles, silt or biological fouling on the lead membranes | Cartridge filter condition, sand filter backwash, SDI of the feed |
| Low output, high pressure drop across the last stage | Scaling on the tail membranes | Antiscalant dosing, recovery setting, feed hardness |
| Low output, pressures normal | Cold feed water, low pump pressure or organic fouling | Feed temperature, pump discharge pressure, pump condition |
| Low output and rising permeate TDS | Scaling or heavy fouling | Last-stage pressure drop, antiscalant, cleaning history |
| High output and rising permeate TDS | Membrane damage, often from chlorine, or a leaking O-ring or seal | Chlorine in the feed, SMBS dosing, stage-by-stage conductivity |
| Cartridge filters choking within days | Pre-treatment is not coping | Sand and carbon filter backwash, feed turbidity, source changes |
Rising permeate TDS with rising output almost always means the membranes have been damaged, most often by chlorine. Cleaning will not fix it, so check the dechlorination before replacing membranes, or the new ones will fail the same way.
Check the simple things first
- Feed water temperature today compared with when the plant was commissioned.
- High-pressure pump discharge pressure against its design value.
- Pressure drop across the cartridge filter; change cartridges if it has risen.
- Antiscalant and SMBS dosing pumps actually running, with chemical in the tank.
- Sand and carbon filters backwashed on schedule.
- Any change in the water source: new borewell, tanker supply or season.
When the membranes need cleaning
Membrane makers commonly advise cleaning when, compared with the plant's own baseline, normalised permeate flow has fallen by about 10%, salt passage has risen by 5 to 10%, or the pressure drop across a stage has risen by about 15%. Scale is removed with an acid clean, organic and biological fouling with an alkaline clean. The cleaning chemicals, concentrations and temperatures should follow the membrane maker's guide.
Clean early. Fouling left too long may not come off, and the membranes then have to be replaced well before their time.
Keep a log sheet
None of this works without readings. Record every shift: feed, permeate and reject flow; pressures before and after the cartridge filter, at the pump and across each stage; feed and permeate TDS; feed temperature; and any alarms. Those numbers are what turn "the plant is giving less water" into a diagnosis.
If you already keep a log sheet, send us a photo of the last few weeks with your enquiry. The trend usually shows whether the plant needs cleaning, a pre-treatment repair or new membranes.
Next step
Need this specified for your site?
Send us your water analysis and daily requirement and we will size the plant before quoting.
