A point-of-use reverse-osmosis system sends part of its feed water to the drain to carry rejected salts and contaminants away from the membrane. The amount varies widely by design, pressure, temperature, maintenance and water chemistry.
For every gallon of treated water, older or inefficient systems may discharge five gallons or more. Current WaterSense-labelled point-of-use systems must send no more than 2.3 gallons to drain per gallon of treated water under the programme’s test conditions.
Waste ratio and efficiency are not the same number
| Description | Equivalent efficiency | Water sent to drain |
|---|---|---|
| 1 gallon treated, 1 gallon drained | 50% | 1 gallon |
| 1 gallon treated, 2 gallons drained | 33% | 2 gallons |
| 1 gallon treated, 2.3 gallons drained | About 30% | 2.3 gallons |
| 1 gallon treated, 5 gallons drained | About 17% | 5 gallons |
Efficiency is the treated-water volume divided by total feed-water volume. It should not be confused with recovery claims measured under different conditions or contaminant-reduction performance.
What changes the drain flow?
- Low inlet pressure reduces production and can worsen the ratio.
- Cold water passes through a membrane more slowly than warm test water.
- High dissolved solids and scaling can reduce membrane performance.
- Clogged prefilters create pressure loss.
- A poorly charged storage tank can make the system run inefficiently.
- A damaged flow restrictor or automatic shut-off valve can cause continuous draining.
How to compare systems
Look for the exact model in the EPA’s WaterSense point-of-use RO guidance and certified product search. WaterSense-labelled systems meet efficiency and performance requirements, including a maximum drain-flow ratio under defined conditions.
NSF/ANSI 58 covers point-of-use reverse-osmosis systems and includes material safety, structural integrity and performance requirements. NSF’s standard overview explains the scope. Verify the exact model and reduction claims in a certifier’s listing.
Estimate household water use
If a household draws three gallons of RO water per day and the unit sends two gallons to drain for each gallon produced, total feed water is nine gallons: three treated and six drained. Actual use includes tank cycling, flushing and maintenance, so a meter gives a better result than a brochure ratio.
Do not compare point-of-use drinking-water systems directly with whole-house or commercial systems; their recovery, pretreatment and operational controls can be very different.
Reduce unnecessary discharge
- Choose a right-sized, independently certified efficient system.
- Confirm adequate feed pressure and use a manufacturer-approved pump if required.
- Replace prefilters and the membrane on evidence and schedule.
- Check the tank air charge and automatic shut-off operation.
- Investigate continuous drain noise after the storage tank is full.
- Treat hardness or fouling only when testing and the manufacturer indicate it is needed.
Drain water is more concentrated than the feed. Reuse may be possible for some non-potable purposes, but do not store it in a way that creates microbial or cross-connection risks, and do not use it on salt-sensitive plants without understanding the source water.
See our guide to choosing an RO system, our explanation of RO wastewater reuse and advice on remineralising RO water.
Bottom line
RO drain flow is not a fixed universal ratio. Compare certified efficiency for the exact model, then maintain pressure, filters, tank and shut-off components. A WaterSense-labelled unit provides a useful current benchmark, but real household performance still depends on installation and water conditions.
Does the waste water only run off when the outlet tap is opened or does it run off continually?
Hi Ian,
The water will run off only when the tap is opened.