Reverse Osmosis Systems: A Complete Guide
Reverse osmosis removes more contaminants than any other common home method. That strength comes with trade-offs in water waste, mineral removal, and cost, so it pays to know whether you need it.
Reverse osmosis, or RO, is the most thorough water treatment most households can install. It is the technology behind much of the bottled water on shelves and behind home systems that tackle the hardest contaminants. If your water has lead, arsenic, nitrate, or PFAS, reverse osmosis is usually on the short list of solutions.
How reverse osmosis works
RO pushes water under pressure through a semipermeable membrane with pores so small that water molecules pass while most dissolved contaminants, salts, and particles are left behind and flushed away as waste. A typical under-sink system pairs the membrane with carbon pre- and post-filters: carbon protects the membrane and removes chlorine, and the membrane does the heavy lifting on dissolved solids. Treated water collects in a small tank for use at a dedicated faucet.
What it removes
Reverse osmosis removes the broadest range of contaminants of any common home method: lead, arsenic, nitrate, fluoride, PFAS, many dissolved metals and salts, and, with the carbon stages, chlorine and many organic chemicals. This breadth is why it is recommended for so many of the contaminants we cover, from arsenic to nitrate to PFAS.
The trade-offs
- Water use: RO sends some water down the drain as concentrate. Modern systems are far more efficient than older ones, but there is still waste.
- Removes minerals: RO strips beneficial minerals like calcium and magnesium along with contaminants, leaving flatter-tasting water. Some systems add a remineralization stage.
- Slower and tanked: water is produced gradually into a storage tank rather than instantly.
- Cost and maintenance: higher upfront cost than a pitcher, plus periodic filter and membrane changes.
Do you need one
You probably do not need reverse osmosis if your only goal is better taste or chlorine removal; a carbon filter does that more simply. RO earns its place when a test or report shows a contaminant that carbon cannot handle, such as arsenic, nitrate, or high dissolved solids, or when you want the broadest protection at one tap. The decision should follow a water test, not a sales pitch.
What to look for
Choose a system certified to NSF/ANSI Standard 58 for reverse osmosis, and confirm certification for the specific contaminants you care about. Consider efficiency (the ratio of treated to wasted water), whether it includes remineralization, the footprint under your sink, and the cost and schedule of replacement filters. For where RO fits among other options, see our home water filters guide.