Every reverse osmosis system sends part of its water down the drain. That is not a fault: it is how RO works. How much goes down the drain varies a great deal between systems, and the EPA has published the figures that let you compare them.
- 01SedimentCatches sand, rust and grit
- 02CarbonReduces chlorine taste and odor
- 03Carbon blockA second, finer carbon pass
- 04MembraneWater pushed through the RO membrane
- 05Post filterPolishes the taste at the faucet
Reverse Osmosis Company is an independent referral service, not an installer or plumber. When you send the form, your details go to an independent local water treatment pro who can contact you and prepare a quote.
Why RO makes waste water
The EPA’s WaterSense program describes reverse osmosis as the process by which pressure forces water through a semi-permeable membrane. The water that passes through is the treated water, called permeate. What the membrane holds back has to be carried away, so a second stream of water carries it to the drain. The EPA calls that stream reject water, concentrate or brine.
How much water goes to the drain
| System | Reject water per gallon of treated water | Source |
|---|---|---|
| A typical point-of-use RO system | Five gallons or more | EPA WaterSense |
| Some inefficient units | Up to 10 gallons | EPA WaterSense |
| A WaterSense labeled RO system | 2.3 gallons or less | EPA WaterSense |
In the EPA’s words, a typical point-of-use RO system will generate five gallons or more of reject water for every gallon of treated water produced, and some inefficient units will generate up to 10 gallons. To earn the WaterSense label, a unit must send 2.3 gallons or less down the drain per gallon treated. Two systems that fit under the same sink can differ widely in the water they send away.
What to ask before you buy
- Is the system WaterSense labeled?
- If not, how many gallons does it send to the drain per gallon treated, according to its maker?
- Is that figure tested to a standard, and where is it published?
- Does the system’s upkeep affect how much it sends to the drain?
If you are replacing an older system that seems to run to the drain constantly, that may be a fault rather than a design. See reverse osmosis system repair.
Want a quote for a system that uses less water? Send the form and an independent local RO pro can contact you.
The drain connection itself
Because the concentrate has to reach the drain, every RO install includes a drain connection, tied into the sink’s drain line. Many cities require a plumbing permit for a new RO drain connection, and the rules are local: ask the pro whether yours needs one and who pulls it. See RO system installation.
Can the reject water be used?
People ask whether RO reject water can be collected for watering plants or other uses. It carries what the membrane held back, and rules on plumbing changes vary by city. Ask the pro before changing how the drain line runs.
Whole-house systems
The EPA figures above are for point-of-use systems, the kind under a kitchen sink. A whole-house RO system is a bigger installation that the installer sizes after a water test, and it treats far more water. Ask the pro for the expected drain volume for your household before you decide. See whole-house reverse osmosis.
Why the WaterSense label matters
The EPA says all products bearing the WaterSense label are independently certified to meet its criteria for efficiency and performance. For point-of-use RO, the line is clear: 2.3 gallons or less down the drain per gallon treated. Against the EPA’s figure for a typical system, five gallons or more, that is a large difference for the same glass of water. If two quotes are otherwise close, the drain figure is a fair way to choose between them.
The EPA is also frank about RO in general. Because RO uses a lot of water, WaterSense does not intend to promote RO systems for every use, and the EPA notes that in some cases other treatment methods that use little or no water, such as filtration, are enough. A straight quote considers those too.
Upkeep and the drain
An RO system that runs to the drain long after it should have stopped may have a fault, not just a poor design. Keep the filters and membrane on the maker’s schedule, and if the drain never seems to stop, ask a pro to look. See RO maintenance.
For how the whole system fits together, read how reverse osmosis works.
Questions people ask
How much water does reverse osmosis waste?
The EPA says a typical point-of-use system sends five gallons or more to the drain per gallon treated, some up to 10. WaterSense labeled systems send 2.3 gallons or less.
Why does RO waste water?
The membrane holds back dissolved material, and a stream of water carries it to the drain.
How do I find an RO system that wastes less?
Look for the WaterSense label, or ask the maker’s figure for gallons to the drain per gallon treated.