RO recovery is the share of feed water that becomes treated product water within a stated measurement boundary. The boundary matters as much as the percentage. A membrane recovery figure and a whole-system efficiency rating may describe different operating conditions.
Read the three water streams
- Feed: water entering the measured process.
- Product or permeate: water passing through the membrane for use.
- Reject or concentrate: the remaining stream sent away from that process.
For a simple steady-state balance with no other outlets or stored-volume change:
Recovery (%) = product volume ÷ feed volume × 100
If four gallons enter and one gallon becomes product, recovery is 25%; three gallons remain as reject. If four gallons enter and three become product, recovery is 75%; one remains as reject. These are arithmetic examples, not recommended operating settings.
Why two advertised percentages may not be comparable
A storage tank filling against increasing pressure does not behave like an unrestricted membrane outlet. Automatic flush cycles also use water. A figure obtained during favorable continuous operation may therefore differ from what reaches the user over the system’s complete cycle.
Before comparing offers, obtain these five fields:
| Field | Question for the data sheet |
|---|---|
| Rating name | Is this membrane recovery or system efficiency? |
| System boundary | Does it include the supplied tank, pump and controls? |
| Operating conditions | What pressure, temperature and feed composition were used? |
| Flush treatment | Are automatic flush volumes included? |
| Test method | Which documented procedure supports the number? |
The EPA WaterSense specification distinguishes tank-system efficiency testing from recovery testing for systems without tanks and addresses automatic flushing. Use the same rating basis when comparing products. Certification for efficiency does not replace checking your required contaminant-reduction claim.
Translate percentage into a ratio without reversing it
For the same simple water balance, with recovery expressed as a decimal:
Waste-to-product ratio = (1 − recovery) ÷ recovery
At 25%, that is 0.75 ÷ 0.25 = 3:1 waste to product. At 75%, it is 0.25 ÷ 0.75, approximately 0.33:1 waste to product. A label saying only “3:1” is incomplete: ask which stream is first. Our waste-water ratio guide turns a confirmed ratio into household volumes and costs.
What a homeowner should change
Choose a system whose documented performance fits your water and demand. Do not increase recovery by restricting the reject line or changing an unapproved flow restrictor. The reject stream is part of membrane operation; a higher number is not automatically a safe upgrade to an existing unit.
If water use changes unexpectedly, compare normal operating conditions and consult the system’s troubleshooting procedure. A running drain can have several causes, including normal filling or flushing. Avoid disconnecting pressurized or sanitary tubing just to produce a DIY efficiency measurement.
This explanation uses water-balance arithmetic and published test distinctions. It does not establish a universal ideal recovery band or predict performance at your particular sink.
Convert a stated waste-water ratio into annual water use with the worked ratio example .