Stream of purified water pouring into a clear drinking glass against a dark background

What Is Reverse Osmosis and How Does It Work?

Reverse osmosis (RO) is the most thorough drinking-water technology you can install at home — it forces water through a membrane with pores so small that dissolved contaminants like lead, arsenic, PFAS, and nitrates are left behind. Here’s how it actually works, what it removes, and how to tell if it’s the right fit for your tap.

The basic idea: a filter for things you can’t see

A standard carbon filter grabs chlorine, odors, and some chemicals. Reverse osmosis goes much further. Household pressure pushes water through a semi-permeable membrane with pores around 0.0001 micron — roughly a million times smaller than a human hair. Water molecules pass; dissolved solids, heavy metals, and most microorganisms don’t. The rejected contaminants are flushed away in a small waste stream, and the purified water is stored or dispensed on demand.

What happens at each stage

Most under-sink RO systems run water through three to five (sometimes as many as twelve) stages:

  1. Sediment pre-filter — catches rust, sand, and silt so they never reach the membrane.
  2. Carbon pre-filter — removes chlorine and chloramine, which would otherwise damage the membrane.
  3. RO membrane — the heart of the system; rejects dissolved solids such as lead, arsenic, fluoride, nitrates, and PFAS.
  4. Carbon post-filter (“polishing” filter) — a final taste-and-odor pass before the faucet.
  5. Optional remineralization — adds back trace calcium and magnesium for taste and a balanced pH.

What RO removes

Independent testing organizations certify RO systems against NSF/ANSI Standard 58, which covers total dissolved solids reduction, plus specific claims like lead, arsenic, and hexavalent chromium (NSF). A properly maintained RO system typically reduces:

  • Lead, arsenic, chromium, mercury, cadmium, and other heavy metals
  • Nitrates and nitrites (a key concern for private wells)
  • Fluoride, sodium, and total dissolved solids
  • PFAS (“forever chemicals”) — the EPA lists reverse osmosis among the most effective in-home treatment options (EPA)
  • Many bacteria, viruses, and cysts (as a polishing barrier — not a substitute for disinfecting an unsafe well)

Tank vs. tankless, under-sink vs. countertop

Traditional systems store purified water in a small pressurized tank under the sink. Newer tankless systems purify on demand at higher flow rates (measured in gallons per day, or GPD) and free up cabinet space. If you rent or can’t modify plumbing, countertop RO systems sit next to the sink and require no installation at all, while under-sink RO systems deliver filtered water from a dedicated faucet and stay out of sight. You can browse every RO form factor in our reverse osmosis collection.

The trade-offs, honestly

  • Waste water: RO sends a concentrate stream to the drain. Older systems wasted 3–4 gallons per purified gallon; efficient modern units run closer to 1:1 or better.
  • Flow: RO purifies drinking and cooking water at one point of use — it is not a whole-house solution (for that, see whole-home filtration).
  • Maintenance: pre- and post-filters change every 6–12 months; membranes every 2–5 years depending on your water.
  • Minerals: RO removes beneficial minerals along with contaminants — a remineralization stage restores taste if you miss it.

Is RO right for your water?

If your utility’s water quality report (or a well test) shows lead, arsenic, nitrates, PFAS, or simply high total dissolved solids, RO is the most complete single answer available for drinking water. Not sure what’s in your tap? Start with a free water analysis or take our 60-second Find Your System quiz and we’ll point you to the right technology — even if that’s not RO.

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