Reverse osmosis works by pushing water through a semipermeable membrane in the opposite direction to natural osmotic flow. Mains pressure supplies the push, water crosses to the clean side, and the dissolved material that cannot follow is flushed away down a drain line. Everything else in the cabinet exists to protect that membrane or to make its output usable.
Stage one and two: the prefilters
Water first meets a sediment cartridge, which takes out grit, rust flakes and particles that would otherwise scour or block the membrane surface. Next comes carbon, which removes chlorine and chloramine. That stage is not optional cosmetics: the thin film composite material used in domestic membranes is damaged by chlorine, so a neglected carbon prefilter is the most common way people destroy the expensive part of their own system without noticing.
The membrane itself
The membrane is a long sheet wound into a spiral around a central tube. Pressurised water enters at one end, part of it crosses the membrane and travels down the center tube as product water, and the rest sweeps along the surface carrying rejected material with it and exits as the drain stream. That constant cross flow is what stops the membrane blinding over. Performance depends on incoming pressure, temperature and how loaded the feed water is.
The tank, the faucet and the last stage
Product water trickles out far too slowly to serve a tap directly, so it accumulates in a small pressure tank with an air-charged bladder. Open the dedicated faucet and that stored pressure pushes water out at a usable rate. On the way, a final polishing carbon cartridge removes any taste picked up in the tank, and some systems add a remineralisation cartridge here to put a measured mineral content back for taste.
What keeps it working
Change the prefilters on schedule, because they exist to protect the membrane and a lapsed carbon stage is expensive. Check the tank's air charge occasionally, since a flat bladder shows up as a trickle at the faucet that people wrongly blame on the membrane. Compare a reading from the feed water and the product water now and then, and when the gap between them narrows, the membrane is on its way out. Our reverse osmosis buying guide covers stage counts and sizing.
Questions people ask about reverse osmosis how it works
Does reverse osmosis need electricity?
A standard under-sink system runs on mains water pressure alone. A booster pump is added only where incoming pressure is too low.
Why is there a drain line?
Because rejected material has to be swept off the membrane surface continuously, otherwise it would blind over and the system would stop producing.
Why is the flow from the faucet slow?
Delivery comes from the storage tank, so a slow trickle usually means an empty tank, a flat bladder charge, or clogged prefilters upstream.