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The short answer
What is reverse osmosis? It’s a treatment method that uses pressure to force water through a semi-permeable membrane. Water molecules pass through; most dissolved salts, metals, and other substances don’t, and they’re flushed away in a reject stream. Home systems pair the membrane with sediment and carbon filters and a storage tank. RO can reduce dissolved solids, and models certified for them reduce lead, fluoride, arsenic, nitrate, and PFAS. The trade-offs are wastewater, slow production, and the loss of minerals.
How does reverse osmosis work?
In ordinary osmosis, water moves through a membrane from the less concentrated side to the more concentrated side, evening the two out. Reverse osmosis pushes the other way: pressure on the tap-water side forces water through the membrane and leaves dissolved substances behind. In most homes, the water line’s own pressure does the work; tankless and countertop units add a pump.
The membrane separates substances depending on their molecular weight, and water flowing across its surface carries rejected material to the drain. That’s why RO always makes some wastewater.
What are the stages of a reverse osmosis system?
| Stage | What it does |
|---|---|
| Sediment pre-filter | Traps sand, silt, and rust that would clog the membrane (UMass Extension) |
| Carbon pre-filter | Reduces chlorine and organics. Membrane makers warn that chlorine damages thin-film membranes (Clack) |
| RO membrane | The core: rejects most dissolved substances |
| Storage tank | RO is slow, about 2 to 7 hours per gallon depending on the unit, so a tank holds a supply (UMass Extension) |
| Post-filter | A carbon polishing filter for taste; some systems add a remineralizing stage |
What does reverse osmosis remove?
The EPA says RO removes many inorganics, dissolved solids, and radionuclides. UMass Extension lists nitrate, lead, fluoride, sodium, and manganese among them. Michigan’s health officials say RO can reduce PFAS. But removal varies by model, so check the certification: NSF/ANSI 58 requires only TDS reduction, and every other claim, from lead to fluoride, is optional (NSF).
- Not removed: dissolved gases and most volatile organic chemicals pass through the membrane, which is why systems include carbon (UMass Extension).
- Microbes: the CDC says RO filters remove parasites, bacteria, and viruses, but UMass Extension says RO alone is not recommended to treat bacteria. For unsafe water, use disinfection such as UV.
- Minerals: the membrane rejects calcium and magnesium along with contaminants (NSF).
How much water does reverse osmosis waste?
A typical under-sink RO unit sends five gallons or more down the drain for every gallon it makes, and some up to 10, according to EPA WaterSense. Systems with the WaterSense label send 2.3 gallons or less. NSF/ANSI 58 data sheets give two numbers to compare:
- Efficiency rating: the share of water entering the system that you get as treated water in typical daily use (example data sheet). This is the realistic figure.
- Recovery rating: the same share measured at the membrane with the tank bypassed, so it’s always higher.
Does RO water need minerals added back?
RO water is low in minerals and, the WHO says, poorly buffered, which can make it taste flat. The WHO calls remineralizing desalinated water with calcium and magnesium “more desirable”, but the evidence that low-mineral water harms health remains controversial. Food is the main source of these minerals for most people. If you want minerals back, some systems include a remineralizing stage; see how to remineralize water.
Which kind of RO system fits your home?
Under-sink with tank
The classic setup. Lowest price per system; needs cabinet space for the tank. See APEC RO-90.
Tankless under-sink
A pump and larger membrane make water on demand. Smaller footprint; needs an outlet. See Waterdrop G3P800.
Countertop
No plumbing: fill a reservoir and the unit filters it. See AquaTru Carafe.
| # | System | Score | Certified by | Lead | PFAS | Fluoride | Running cost | Price |
|---|---|---|---|---|---|---|---|---|
| 01 | AquaTru CarafeCountertop, no plumbingCountertop RO | 4.6 | IAPMO R&T | Certified | Certified | Certified | $0.33/gal | $356.25on Amazon › |
| 02 | Waterdrop G3P800Tankless under-sinkUnder-sink RO | 4.3 | IAPMO R&T | Certified | Certified | Certified | About $160/yr | $849.00on Amazon › |
| 03 | APEC RO-90Under-sink with tankUnder-sink RO | 4.2 | WQA | Certified | No | Certified | $60 to $75/yr | $219.16on Amazon › |
| 04 | Aquasana SmartFlow ROUnder-sink with remineralizerUnder-sink RO | 4.1 | WQA | Certified | Certified | Certified | $0.49+/gal | $239.99on Amazon › |
| 05 | iSpring RCC7AK-BNBudget under-sink (BN or BLK model)Under-sink RO | 3.9 | NSF International | Certified | Claim | Certified | $100 to $140/yr | $234.99on Amazon › |
Certified: Certified by an independent body (named in each pick). Lab test: Tested by a named independent lab, not certified. Claim: The maker's claim, not certified. No: No claim. PFAS means PFOA/PFOS, or Total PFAS where the pick says so.
More picks are in our guide to the best reverse osmosis systems.
Common questions
What is reverse osmosis in simple terms?
Pressure pushes water through a membrane so fine that most dissolved substances can’t follow; they’re flushed away in a reject stream (EPA).
Is reverse osmosis water safe to drink?
Yes. It’s low in minerals, which the WHO says makes it poorly buffered, and some people add minerals back for taste.
Does reverse osmosis remove fluoride?
RO can, and models certified for it under NSF/ANSI 58 list fluoride as a claim. Check the listing, since fluoride is optional under the standard.
Does reverse osmosis kill bacteria?
The CDC says RO filters remove bacteria and viruses, but UMass Extension says RO alone isn’t recommended for bacteria (UMass). Use UV or follow your health department for unsafe water.
How often do RO filters need changing?
Pre-filters typically every 6 to 12 months and the membrane every 2 to 3 years, per makers such as iSpring. See how often to change a water filter.
Sources
- EPA: Overview of drinking water treatment technologies
- EPA WaterSense: Point-of-use reverse osmosis systems
- UMass Extension: Reverse osmosis treatment of drinking water supplies
- NSF International: NSF/ANSI 58 reverse osmosis systems
- NSF International: How reverse osmosis works
- California State Water Board: Example NSF/ANSI 58 performance data sheet
- CDC: Water treatment options when hiking, camping, or traveling
- Michigan EGLE: Home filters and filtration systems
- WHO: Safe drinking-water from desalination (2011)
- Clack: Microline T.F.C.-435 specification sheet
- IAPMO R&T product listing directory
- NSF International listings
- WQA certified product listings
Prices as of October 14, 2026. Product prices and availability are accurate as of the date/time indicated and are subject to change. As an Amazon Associate we earn from qualifying purchases.