Reverse osmosis, explained

How it works

Reverse osmosis pushes water under pressure through a semipermeable membrane whose pores are small enough to pass water molecules while holding back most dissolved contaminants. Those rejected contaminants are flushed away in a small stream of wastewater, and the purified water collects for use. Most home systems pair the membrane with carbon stages before and after, so the unit also handles chlorine and taste.

What it removes — and what it doesn't

RO is unusually broad: it reduces dissolved solids and many of the contaminants in this hub, including lead, arsenic (especially the pentavalent form), nitrate, fluoride, and PFAS. That breadth is why it anchors so many certified point-of-use recommendations. What it is not: a disinfection method. RO is not a substitute for UV when the concern is bacteria — those are different jobs.

The trade-offs worth knowing

Three honest caveats. First, RO produces some wastewater; efficient modern systems have improved this, but it isn't zero. Second, RO removes beneficial minerals along with contaminants, so many units add a remineralization stage for taste and balance. Third, RO delivers water at a modest flow — older designs use a storage tank, while tankless designs trade the tank for a higher-output membrane. None of these are dealbreakers; they're just the reasons RO is typically installed at one tap rather than the whole house.

For the standard that governs RO performance claims, see NSF/ANSI 58; for whether RO belongs at the tap or the whole house, see point-of-use vs. whole-house.