RO recovery and reject water — how much is normal
Short answer: Recovery is the proportion of feed water that becomes permeate. On typical Indian borewell water it should be 60 to 70 percent. Pushing it higher to save water scales the membranes, and a set of membranes costs far more than the water saved.
Why recovery is limited
As permeate is removed, the remaining water becomes more concentrated. At 75 percent recovery, the reject stream is four times as concentrated as the feed.
At some point the salts in that concentrated stream exceed their solubility and crystallise — on the membrane surface, usually on the last element in the series.
Safe recovery by feed TDS
| Feed TDS | Recovery | Reject concentration factor |
|---|---|---|
| Below 500 | 70 – 75% | 3.3 – 4× |
| 500 – 1,000 | 65 – 70% | 2.9 – 3.3× |
| 1,000 – 2,000 | 60 – 65% | 2.5 – 2.9× |
| 2,000 – 4,000 | 50 – 60% | 2.0 – 2.5× |
| Above 4,000 | 40 – 50% | 1.7 – 2.0× |
Hardness matters as much as TDS. Very hard water needs lower recovery even at moderate TDS, unless it is softened first.
Measuring your actual recovery
- Measure permeate flow with a rotameter
- Measure reject flow with another rotameter
- Feed = permeate + reject
- Recovery = permeate ÷ feed × 100
Example: permeate 750 LPH, reject 400 LPH → feed 1,150 → recovery = 65 percent.
Rotameter placement is covered in our flow meter guide.
Adjusting recovery
Recovery is set by the reject valve. Closing it increases recovery, opening it reduces recovery.
What happens at excessive recovery
- Reject stream becomes more concentrated
- Salts exceed solubility on the last element
- Scale forms, flow falls, pressure rises
- Operator closes the reject valve further to compensate
- Scaling accelerates
- CIP cleaning recovers less each time
- Membranes replaced early
This cycle is common and entirely avoidable.
Minimum reject flow
There is also a minimum reject flow per element, required to sweep the membrane surface and carry concentrated water away. Falling below it causes scaling even if the overall recovery figure looks acceptable.
This is why on multi-element systems the reject flow at the last element matters most — it sees the most concentrated water with the least flow.
Using the reject water
Reject water is not sewage — it is simply concentrated feed water. Depending on its TDS it can be used for:
| Reject TDS | Possible uses |
|---|---|
| Below 2,000 | Gardening (salt tolerant plants), floor washing, toilet flushing |
| 2,000 – 4,000 | Floor washing, vehicle first rinse, cooling tower make-up (with treatment) |
| Above 4,000 | Limited use, mainly disposal |
How to raise recovery safely
If you genuinely need higher recovery, the correct approach is to change the water chemistry, not just close the valve:
- Soften the feed — removes the hardness that limits recovery. See our softener guide
- Improve antiscalant selection — a formulation matched to your specific water chemistry
- Acid dosing to lower feed pH, which increases carbonate solubility
- Two stage arrangement — reject from the first stage feeds a second stage
Each of these raises the safe recovery limit rather than simply exceeding it.
Monitoring
- Record recovery weekly — if it has changed without anyone adjusting the valve, something else has changed
- Watch reject TDS alongside permeate TDS
- Watch pressure drop across the membrane housing
A rising recovery figure that nobody set deliberately usually means permeate flow is falling — the first sign of fouling. See our membrane monitoring guide.
Optimise your recovery
Send your feed TDS, hardness and current flows, and we will advise on the safe recovery and how to improve it.
💬 WhatsApp par poochhein 📞 9910646957Frequently asked questions
What is RO recovery?
The proportion of feed water that becomes permeate, calculated as permeate flow divided by feed flow. On typical Indian borewell water it should be sixty to seventy percent.
Why can I not simply increase recovery to save water?
Because the reject stream becomes more concentrated as recovery rises, and at some point salts exceed their solubility and crystallise on the membrane. A set of membranes costs far more than the water saved.
What recovery is safe for my water?
It depends on feed TDS and hardness. Seventy to seventy five percent below five hundred TDS, sixty to sixty five percent between one and two thousand, and forty to fifty percent above four thousand.
Should I close the reject valve if permeate flow falls?
No. That simply raises recovery and accelerates the scaling that probably caused the flow drop. Diagnose the real cause instead, which is usually fouling or scaling.
Can the reject water be used?
Yes. Below two thousand TDS it can be used for gardening with salt tolerant plants, floor washing or toilet flushing. Recovering reject for secondary use is far better than raising recovery, because it saves the same water without risking the membranes.