Reverse osmosis is filtration by pressure through a membrane so fine that most dissolved substances cannot pass it. Household pressure pushes water through the membrane, the water goes one way, and the dissolved solids are swept the other way down a drain line. It is the most thorough treatment most homes can buy at a single tap.
How the membrane works
Ordinary osmosis moves water across a membrane from the less concentrated side toward the more concentrated one. Reverse osmosis does what the name says: it applies pressure to the concentrated side and drives water back the other way, leaving the dissolved material behind. The membrane is a tightly wound sheet with openings small enough to block most dissolved salts, metals and larger molecules while letting water molecules through. Because the rejected material would otherwise build up against the membrane face and blind it, a continuous stream of water sweeps that surface and carries the concentrate to drain. That sweep is why RO produces a drain flow, and why the ratio of treated water to drain water is a real design characteristic rather than a fault.
What a household RO system is made of
A point-of-use system is a set of stages, not just a membrane. Sediment prefiltration protects the membrane from grit. Carbon prefiltration removes chlorine, which is important because chlorine damages the common membrane material. The membrane does the separation. A small pressurized storage tank holds treated water, because production is slow and a tap needs it instantly. A postfilter polishes taste on the way out, and many systems add a remineralizing cartridge for the flat taste that very pure water has. An automatic shutoff stops production when the tank is full. Manufacturers publish these systems in capacity steps with rated daily production, which is what you match against how much the household actually drinks.
What it removes, and what it does not
RO reduces a very wide range of dissolved contaminants, which is why it is the standard recommendation when a test comes back with a dissolved problem at the kitchen tap. The EPA's 2024 drinking water rule set maximum contaminant levels for several PFAS compounds, at 4.0 parts per trillion for PFOA and PFOS and 10 parts per trillion for PFHxS, PFNA and GenX, with May 2026 proposals keeping the PFOA and PFOS limits alongside a compliance option to 2031 while proposing to rescind the others. Regulatory attention like that is much of why household interest in RO has grown. What RO does not do is disinfect reliably against a compromised supply, and it does not soften the whole house: it treats one tap, at low flow, so scale, showers and laundry are untouched.
Is it the right purchase
RO earns its place when you have a confirmed dissolved contaminant that cheaper methods do not address, or when you simply want the best tasting drinking water in the house. It is a poor answer to hardness across the home, since a softener does that job at whole-house flow, and a poor answer to sediment or chlorine alone, which a carbon or sediment stage handles at a fraction of the cost and with no drain water. It needs an under-sink drain connection, cabinet space for the tank, adequate incoming pressure and a modest maintenance habit. Test first, decide what actually needs removing, then choose, and read our under-sink RO comparison before committing to a format.
Questions people ask about whats reverse osmosis
Does reverse osmosis remove bacteria?
The membrane rejects a great deal, but RO is not certified as a disinfection method for a microbiologically unsafe supply. Bacteria are a job for UV or chlorination.
Why does my RO system send water down the drain?
The drain stream sweeps rejected dissolved solids off the membrane face. Without it the membrane would foul quickly, so the flow is by design rather than a leak.
Does reverse osmosis soften water?
It removes hardness minerals from the water it treats, but only at one tap and at low flow, so it does nothing for scale in the water heater, showers or laundry.
How long does an RO membrane last?
It depends on feed water quality and how well the prefilters are maintained. Chlorine damage and neglected prefilters are the two things that end membranes early.