Glossary · Treatment

What Is Reverse Osmosis? How an RO System Cleans Water

Reverse osmosis (RO)

A treatment process that pushes water through a very fine membrane under pressure, leaving most dissolved salts, metals and many other contaminants behind.

  1. Pre-filterSediment filter: catches sand and silt that would foul the membrane.
  2. RO membranePressure pushes water through; most dissolved contaminants stay behind. Concentrate to the drain: the water that carries away what the membrane held back.
  3. Storage tankThe membrane is slow, so treated water waits here until you open the tap.
  4. Post-filterUsually carbon: polishes taste and catches what the membrane missed.
  5. Remineralization (optional)Adds some minerals back after the membrane, mainly for taste.
  6. RO faucetIts own small faucet at the sink.
A typical under-sink RO system: the membrane does the separating, and part of the water carries what it holds back to the drain.

At a glance

Certified under
NSF/ANSI 58, which requires at least 75% TDS reduction
Membrane pore size
About 0.0001 micron (CDC)
Water to the drain
5+ gallons per gallon made (typical); 2.3 or less (WaterSense labeled)
Weak spots
Dissolved gases and most volatile organic chemicals; carbon stages handle those
Best used
At one tap for drinking and cooking, not for the whole house

Reverse osmosis is the heavy-duty option at a single tap: one system can reduce lead, arsenic, nitrate, fluoride and PFAS together. It also sends water down the drain and costs more to run than a simple filter, so it’s worth knowing when you actually need one.

What does reverse osmosis remove?

The CDC says RO systems remove some chemicals, including lead, copper, chromium, chloride and sodium, and may reduce arsenic, fluoride, radium, sulfate, nitrate and others. The membrane also holds back parasites, bacteria and viruses. The Minnesota Department of Health says RO removes a wider variety of contaminants than many other home options.

It has gaps, too. The Connecticut Department of Public Health lists dissolved gases and most volatile organic chemicals as things RO isn’t effective against, which is why systems pair the membrane with carbon filters. It also says RO units alone aren’t recommended for treating bacteria in a well.

What counts is the exact model’s certification. NSF/ANSI 58, which the EPA calls the primary US standard for RO, requires every certified system to cut total dissolved solids by at least 75 percent. Lead, nitrate, PFAS and the rest are optional claims that must appear on that model’s listing.

How much water does a reverse osmosis system waste?

A typical under-sink system sends five gallons or more to the drain for every gallon it makes, and inefficient units up to 10, according to the EPA’s WaterSense program. Systems that earn the WaterSense label send 2.3 gallons or less. Our guide to how reverse osmosis works explains the efficiency ratings and how to compare them.

Do you need a reverse osmosis system?

Often not. The CDC’s advice is to test your water first, and it notes you may not need any treatment if your water has no harmful chemicals or germs. The EPA’s WaterSense page adds that in some cases, methods that use little or no water, such as filtration, are enough. RO makes the most sense when a test shows something it’s certified to reduce and a simpler filter isn’t, such as nitrate in a well.

It also isn’t meant for the whole house. Connecticut calls whole-house RO impractical because of the water it wastes, and the EPA notes that uses such as flushing, laundry and bathing don’t need RO-quality water. When you’re ready to compare models, our reverse osmosis comparison checks each claim against the certifier’s listing.

Published by the Checked Water editorial team

Sources

  1. About Home Water Treatment Systems (Clinical guideline, Centers for Disease Control and Prevention, 2024)
  2. WaterSense: Point-of-Use Reverse Osmosis Systems (Clinical guideline, US EPA, 2026)
  3. WaterSense Specification for Point-of-Use Reverse Osmosis Systems, Supporting Statement (Clinical guideline, US EPA, 2024)
  4. Publication No. 21: Reverse Osmosis Treatment of Private Drinking Water Systems (Clinical guideline, Connecticut Department of Public Health, 2009)
  5. Home Water Treatment (fact sheet, revised March 2024) (Clinical guideline, Minnesota Department of Health, 2024)

Where Reverse osmosis (RO) comes up