Why RO sends water down the drain
An RO membrane holds back dissolved salts and metals. Something has to carry them away, or the membrane clogs. That job falls to a steady stream of reject water, also called concentrate or brine, which flows to your drain.
The share of incoming water that ends up in your glass is the system's efficiency rating. EPA says typical point-of-use RO systems run at 10 to 20 percent efficiency. In plain terms:
- At 20 percent, you get 1 gallon of treated water and 4 gallons of reject.
- At 10 percent, you get 1 gallon and 9 gallons of reject.
- At 30 percent, the WaterSense minimum, you get 1 gallon and about 2.3 gallons of reject.
To turn any efficiency rating into a waste ratio, divide 100 by the rating and subtract 1.
What the EPA WaterSense label requires
EPA finalized its WaterSense specification for point-of-use RO systems in November 2024. It covers under-sink, countertop and faucet-mounted units that treat drinking and cooking water. Whole-house RO is not covered.
To earn the label, a system must:
- Reach at least 30 percent efficiency, tested under NSF/ANSI 58 procedures.
- Be certified to NSF/ANSI 58, the RO performance standard.
- Have an automatic shut-off that stops reject water when the system isn't making water.
- Reduce total dissolved solids by at least 75 percent, including over a membrane-life test.
- State its efficiency rating and waste ratio in its product literature.
Systems with more than one flush setting must meet 30 percent at every setting. If a system needs an add-on, such as a pump, to qualify, it must come in the box.
The label is about water use. It doesn't mean the system removes more contaminants. Check the individual NSF/ANSI 58 claims for that.
What the waste means for a household
EPA assumes each person uses about 1 gallon of RO water a day for drinking and cooking. For an average household of 2.6 people, that's roughly 950 gallons of treated water a year.
At 15 percent efficiency, which EPA uses as a typical system, that household sends about 5,400 gallons a year down the drain. A 30 percent system cuts that to about 2,220 gallons.
EPA estimates a WaterSense-labeled system saves a household more than 3,100 gallons a year. Over the system's life, that's about 47,000 gallons. Your actual savings depend on how much RO water you use and your local water and sewer rates.
| Efficiency rating | Gallons to drain per gallon treated | Drain water per year (2.6-person home) |
|---|---|---|
| 10% | 9 | About 8,580 gallons |
| 15% (EPA's typical system) | 5.7 | About 5,400 gallons |
| 20% | 4 | About 3,810 gallons |
| 30% (WaterSense minimum) | 2.3 | About 2,220 gallons |
| 50% | 1 | About 950 gallons |
Why some systems waste more than their rating
A rating is a lab number. Real installations can do worse for several reasons:
- No working shut-off valve. Without one, reject water can run around the clock. Virginia Tech's extension service notes RO units can be wasteful unless equipped with an automatic shut-off.
- Tank back pressure. As the storage tank fills, it pushes back and slows treated-water production while reject water keeps flowing.
- Low inlet pressure. RO works best at higher water pressure. Low pressure means slower output and more waste per gallon.
- Fouled pre-filters or membrane. Sediment, iron and chlorine shorten membrane life and cut efficiency.
- Tankless flush cycles. Some tankless units flush periodically. The NSF/ANSI 58 test doesn't count that flush water, so the rating can understate total use.
How to cut RO water waste
- Buy a WaterSense-labeled system. It's the simplest way to guarantee 30 percent efficiency or better.
- Listen to your drain. If water trickles to the drain when the tank is full and no one is using the faucet, the shut-off valve may be failing.
- Consider a permeate pump. This non-electric add-on reduces tank back pressure on many tank-style systems.
- Keep up with pre-filters. Clean pre-filters protect the membrane and keep efficiency close to its rating.
- Treat only what you drink. Use RO at the kitchen tap for drinking and cooking, not for the whole house.
Also ask whether you need RO at all. EPA notes that for many needs, filtration that uses little or no water is enough. A certified carbon filter handles chlorine, lead and many chemicals with zero waste.
Is the waste worth it?
It depends on what's in your water. RO is often the best home option for nitrate, arsenic, PFAS and high dissolved solids. If a test shows one of those, the drain water is a fair trade for safer drinking water. Just choose an efficient system and keep it maintained.
If your concern is chlorine taste or lead from old fixtures, a certified carbon filter usually does the job without any waste. See whether RO water is healthy and which filters remove lead to compare.