Yes. A household reverse osmosis (RO) system sends some incoming water down the drain as reject water while it produces treated water. For U.S. point-of-use systems, the EPA says a typical unit may discharge at least 5 gallons for every gallon treated; some inefficient units may discharge as many as 10. An EPA WaterSense-labeled system is limited to 2.3 gallons or less of reject water per gallon treated. These are category-level figures, not a promise about every model or installation.
How much water does a household RO system waste?
RO produces two streams: permeate, the treated water collected for drinking or cooking, and concentrate—also called reject water or brine—which carries substances retained by the membrane. Water passes through a semipermeable membrane under pressure. The permeate is the treatment stream; the concentrate is the remaining stream, usually sent to a drain in a household installation.
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| U.S. point-of-use system category | Reject water per gallon treated | What the figure means |
|---|---|---|
| Typical system | 5 gallons or more | EPA consumer guidance describes this as a typical level, not a model-specific guarantee. |
| Some inefficient systems | Up to 10 gallons | EPA says some units can reach this amount. |
| WaterSense-labeled system | 2.3 gallons or less | The label requires a minimum efficiency rating of 30% under the EPA specification. |
To put the ratios in everyday terms, producing 10 gallons of treated water at the typical-system figure would mean at least 50 gallons of reject water; at the upper figure reported for some inefficient units, it could mean 100 gallons. A WaterSense-labeled system would send no more than about 23 gallons down the drain for those same 10 gallons of treated water, based on EPA’s rounded per-gallon comparison. These calculations illustrate the ratios; they are not predictions for a particular household.
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These comparisons apply to residential point-of-use RO, such as a drinking-water system installed under a kitchen sink. Commercial, industrial, and desalination systems have different operating conditions and should not be assumed to match household ratios.
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How to interpret an RO efficiency rating
Efficiency is the share of water entering the complete system that becomes treated water. The balance is rejected. For example, 30% efficiency means about 30 gallons of treated water for every 100 gallons entering the system; roughly 70 gallons are reject water. EPA’s product-search page explains this relationship.
That 70 gallons is not the same as saying 70 gallons are wasted for every 30 gallons treated. The latter comparison is about 2.33 gallons rejected per gallon treated. The difference is the denominator: efficiency compares treated water with all water entering the system, while a reject-to-treated ratio compares the rejected stream directly with the finished-water output.
To convert an efficiency rating into reject gallons per gallon treated, use: (1 ÷ efficiency) − 1, with efficiency written as a decimal. At 30% efficiency, the calculation is (1 ÷ 0.30) − 1 = about 2.33 gallons rejected per gallon treated. EPA rounds this to 2.3 gallons in its consumer guidance. The formula can help compare ratings expressed as percentages, but it does not replace checking the product’s test conditions or flushing behavior.
Efficiency and recovery are not the same
Under NSF/ANSI 58 testing, recovery is the percentage of membrane influent that becomes treated water in test conditions without storage-tank backpressure. Efficiency measures the share of water entering the system that becomes treated water under conditions intended to approximate typical daily use. For a tank system, backpressure lowers efficiency relative to recovery, so EPA describes efficiency as the more conservative comparison.
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Do not compare a tank system’s recovery percentage directly with another system’s efficiency percentage. They describe different test conditions, so a higher recovery number is not automatically evidence that one tank system will use less water in daily operation than another. Look for the efficiency figure when assessing a tank system’s likely water use, and read the test conditions stated by the manufacturer.
When comparing products, look for a clearly identified efficiency rating or waste-to-treated-water figure rather than relying on a vague claim that a system is “efficient.” EPA’s product-search information connects the efficiency rating to the reject-water ratio. If the rating is given only as a percentage, the conversion above makes the relationship easier to understand.
What the annual difference could look like
In a 2024 illustrative EPA comparison assuming a household uses about 1,000 gallons of treated water per year, a system operating at 15% average efficiency would produce 5,667 gallons of reject water annually. At 30% efficiency, the modeled reject-water amount is 2,333 gallons per year. EPA characterizes the difference as a 59% average reduction in reject water. This is a modeled example, not a universal household measurement: actual treated-water demand and system performance vary.
The figures also show why an efficiency rating can matter over time. In the EPA example, doubling efficiency from 15% to 30% does not cut reject water in half exactly: the modeled annual amount falls by 3,334 gallons, or 59%. Those annual totals depend on the example’s 1,000 gallons of treated water and specified efficiency assumptions; a household that makes a different amount of RO water will have a different total.
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EPA also estimates that choosing a WaterSense-labeled model instead of a typical point-of-use RO system can save more than 3,100 gallons of water per year on average. That is a program estimate, not a guaranteed saving for a particular home. Your result depends in part on how much treated water you use and the performance of the system being replaced.
Why the amount varies from system to system
- Tank backpressure: It affects efficiency during conditions intended to reflect daily use, which is why recovery alone may overstate practical performance for a tank system. EPA treats efficiency as the more conservative measure for tank systems.
- Automatic flushing: Some tankless systems flush automatically. EPA has noted that the relevant recovery test may not account for all flush water, so a published test rating may not capture every operating detail.
- Actual use: The annual volume of treated water a household needs affects the total reject-water volume. A per-gallon ratio is useful for comparing systems, but annual gallons also depend on how much RO water is produced.
- Product and test conditions: A rating is tied to its test conditions. Product documentation can clarify the rating, production information, and whether flushing behavior is included.
When product documentation does not clearly explain flushing behavior or rating conditions, the label or rating should not be treated as a complete estimate of every gallon that will go to the drain. Ask what the published figure measures and whether automatic flush water is included, particularly for a tankless model.
How to choose or use an RO system with less waste
Compare the right numbers
- Check whether RO is needed for the water concern. EPA frames point-of-use RO as drinking- and cooking-water treatment. Confirm that the specific model has a verified reduction claim for the contaminant you want to address. A general description of a system as reverse osmosis does not establish every contaminant-reduction claim.
- Look for a WaterSense label and efficiency rating. If replacing a system, compare WaterSense-labeled models and their verified efficiency ratings. For a tank system, prioritize efficiency rather than relying on recovery alone. EPA’s WaterSense product search can help identify labeled systems and understand how the rating translates into reject water.
- Read the operating details. Check whether a tankless model flushes automatically and whether the published rating includes that water. Compare the manufacturer’s stated production information and test conditions rather than assuming every unit with the same broad technology has identical water use.
- Check treatment fit as well as water efficiency. A favorable reject-water ratio does not establish that a model reduces the contaminant you care about. Consider the system’s verified contaminant-reduction claim alongside its water-use rating before choosing.
- Follow maintenance instructions. Replace filters and membranes as directed and maintain the storage tank according to the manufacturer. EPA’s consumer report advises periodically draining a storage tank, about every two weeks. Proper use and maintenance matter to water quality and operation; do not substitute a generic schedule for model-specific instructions.
Use RO water where it is useful
EPA frames point-of-use RO as drinking- and cooking-water treatment. Avoid treating water for uses that do not require that level of treatment; limiting unnecessary production also limits reject water. The practical point is to match the treatment to the use rather than assume that all household water needs to pass through an RO system.
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A compatible retrofit kit may capture or redirect reject water for suitable non-consumption uses, such as laundry, watering plants, or mopping floors. EPA identifies these as possible reuse tasks. Confirm that the water is suitable for the intended use and that the kit is compatible with the installed system. Do not use redirected reject water for drinking or cooking.
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Reuse is an option, not a reason to assume the reject stream is suitable for every task. Check the intended use and system guidance first, and remember that capturing the stream does not change how much water the RO system sends through its reject outlet.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Where these figures apply
The ratios and EPA label criteria here concern U.S. residential point-of-use drinking-water RO systems. They are useful for comparing household systems, but they are not universal performance figures for every RO application. A system’s published rating, tank configuration, flushing behavior, and household use all matter when estimating its actual water use.
Frequently Asked Questions
How many gallons go down the drain for each gallon of RO water?
For U.S. household point-of-use systems, EPA says a typical system may send at least 5 gallons down the drain per gallon treated, and some inefficient units may send up to 10. A WaterSense-labeled system is limited to 2.3 gallons or less per gallon treated. These are category-level comparisons, not guarantees for an individual installation.
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What does a 30% RO efficiency rating mean?
It means about 30 gallons of treated water are produced for every 100 gallons entering the system, with about 70 gallons rejected. Expressed per gallon treated, that is roughly 2.33 gallons of reject water, which EPA rounds to 2.3.
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Is recovery the same as efficiency on an RO system?
No. Recovery is measured under test conditions without storage-tank backpressure. Efficiency measures the share of water entering the system that becomes treated water under conditions intended to approximate typical use. EPA says efficiency is the more conservative measure for tank systems.
Can I reuse RO reject water?
EPA says a compatible retrofit kit may capture or redirect reject water for suitable non-consumption tasks such as laundry, watering plants, or mopping. Confirm that the water is suitable for the intended task and that the kit works with your system. Do not use reject water for drinking or cooking.
Will a WaterSense-labeled system save my household exactly 3,100 gallons a year?
No. EPA estimates an average saving of more than 3,100 gallons per year compared with a typical point-of-use RO system, but that is not a household-specific guarantee. Actual savings depend on treated-water demand and the systems’ performance.
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Household RO systems do send reject water down the drain, but the amount varies widely: EPA’s comparison ranges from at least 5 gallons per gallon treated for a typical point-of-use system to no more than 2.3 for a WaterSense-labeled model. Compare efficiency—not just recovery for a tank system—check flushing and test details, and use RO for the drinking- and cooking-water needs it is meant to serve.
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