Reverse osmosis is a way of treating water by pushing it through a membrane to remove contaminants. The EPA’s WaterSense program defines it as the process by which pressure forces water through a semi-permeable membrane. The rest of an RO system is built around that membrane: filters before and after it, a faucet for its water and a drain line for what it rejects.
- 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
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What is reverse osmosis?
In an RO system, pressure on the incoming side forces water through the membrane, and what the membrane holds back is carried away in a separate stream. Two streams come out. The EPA calls the treated water permeate, and the reject water concentrate or brine.
At home, RO is used in what the EPA calls point-of-use systems: systems connected to a single fixture, such as under the kitchen sink. Larger whole-house systems exist too; they are a bigger installation that an installer sizes after a water test. See whole-house reverse osmosis.
The stages of a home RO system
The EPA describes RO systems as typically a three, four or five stage filtration process. The five-stage layout drawn on this site is one arrangement; systems differ in how many stages they have and what each holds, so read the maker’s description of yours.
| Stage | Its job |
|---|---|
| Sediment pre-filter | Catches grit and particles before they reach the membrane. |
| Carbon pre-filter | A pre-filter stage before the membrane. |
| Second pre-filter | A further pre-filter stage on systems that have one. |
| RO membrane | Pressure forces water through it; the rest goes to the drain. |
| Post filter | The last stage before the faucet. |
Treated water goes to a separate faucet on the sink, through a storage tank on systems that have one. The concentrate goes to the drain through its own line.
What reverse osmosis can remove
The EPA says RO systems can potentially remove water contaminants such as lead, volatile organic compounds (VOCs), PFAS, arsenic, bacteria and viruses. The word potentially matters. What a particular system removes depends on its design and its certification. NSF/ANSI 58 is the standard for reverse osmosis drinking water treatment systems, and the EPA says to look for certification under NSF/ANSI 53 or NSF/ANSI 58 for PFAS reduction. The Texas Commission on Environmental Quality advises choosing a unit certified by NSF, Underwriter’s Laboratory or the Water Quality Association to remove the specific contaminant you are concerned about.
For PFAS in particular, see does reverse osmosis remove PFAS.
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Why RO sends water down the drain
What the membrane holds back does not stay in the system: the reject stream carries it to the drain. The EPA says a typical point-of-use RO system generates five gallons or more of reject water for every gallon of treated water, and some inefficient units generate up to 10. A WaterSense labeled system must send 2.3 gallons or less down the drain per gallon treated. More in reverse osmosis waste water.
Why pressure and upkeep matter
Because the process runs on pressure, a pro may ask about the water pressure feeding the system before an install. The EPA notes that RO systems require changing filter cartridges and the RO membrane, so follow the maker’s schedule for each. See RO maintenance and, if something is already wrong, RO repair.
What to ask before buying
- Which contaminants is the system certified to reduce, and under which standard?
- Is it WaterSense labeled?
- How many stages does it have, and what is in each?
- Which filters and membrane does it take, and on what schedule?
If you are on a private well, test it before choosing any treatment: the EPA advises a yearly test with a certified laboratory. See reverse osmosis for well water.
Ready to have one fitted? Read RO system installation.
Questions people ask
What is reverse osmosis in simple terms?
Water is pushed by pressure through a semi-permeable membrane. Treated water comes out one side; the rest, carrying what the membrane held back, goes down the drain.
Why is it called reverse osmosis?
The name refers to osmosis, the natural movement of water through a membrane. An RO system uses pressure to drive water through the membrane instead.
Does RO remove everything?
No system removes everything. The EPA says RO can potentially remove contaminants such as lead, VOCs, PFAS, arsenic, bacteria and viruses. Check what a system is certified for.