Every RO sends some water to the drain – that is how it flushes salts off the membrane. But if the drain runs non-stop or far more than usual, something is wrong. Here is what is normal and what to check.
How much waste water is normal?
Most domestic RO purifiers produce roughly 1 litre of pure water for every 2–4 litres sent to drain, depending on input TDS, pressure and the membrane. High-TDS water means more waste. What matters is a sudden change from your normal.
Causes of too much waste water
| Symptom | Likely cause | Fix |
|---|---|---|
| Drain runs even when the tank is full | Solenoid valve not closing, or float/HP switch fault | Test the solenoid valve and float valve |
| Drain flow is much stronger than before | Flow restrictor missing, wrong size or damaged | Fit the correct flow restrictor – e.g. FR 300 or FR 450 |
| Very little pure water, lots of drain | Worn or choked membrane | Replace the RO membrane |
| Waste increased after a part change | Membrane GPD changed without changing FR | Match the FR to the new membrane |
| Slow filling, long running time | Low pressure: choked pre-filters or weak pump | Change pre-filters; check the pump |
Do not just block the drain
Some people tighten or block the drain line to “save water”. This makes salts build up on the membrane and it fails much sooner. Use the correct flow restrictor instead.
Use the waste water
RO reject water is fine for mopping, washing vehicles and flushing toilets. See how to reuse RO waste water.
Frequently asked questions
What is the right flow restrictor for an 80 GPD membrane?
FR 400–450 is common for 80–100 GPD and FR 300 for 75 GPD. Check your purifier manual.
Why does my RO drain water when it is switched off?
The solenoid valve may be stuck open, or there is back-flow from the tank. Test the SV.
Can RO waste water be reduced to zero?
No. Some reject flow is needed to keep the membrane clean.
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