Reverse osmosis is named for what it reverses. Left alone, water moves across a semi permeable membrane from the less concentrated side to the more concentrated one, which is osmosis. Apply enough pressure to the concentrated side and the flow reverses: water is pushed through the membrane and leaves the dissolved material behind. Everything else in a domestic system exists to make that possible and usable.
Stage one and two: protecting the membrane
Feed water enters a sediment prefilter, which removes particles that would score the membrane surface and block its feed channels. It then passes a carbon prefilter, whose job is to remove chlorine, because chlorine degrades thin film composite membrane material permanently. Neither stage is optional and neither is really about water quality for the household: they exist so that the expensive component behind them survives. Their replacement intervals are set on a calendar because their exhaustion is invisible.
Stage three: the separation itself
At the membrane, line pressure pushes water against a spirally wound sheet. Some water works its way through to the permeate side and heads for the storage tank. The rest keeps flowing across the surface, carrying away the dissolved material that did not pass, and leaves as the reject stream to drain. That cross flow is what stops rejected material caking on the sheet, which is why restricting the drain line to save water fouls the membrane quickly. Higher feed pressure means more production; colder water means less.
Storage, and why a tank is needed
The membrane produces slowly relative to how fast anyone wants to fill a glass, so treated water accumulates in a small pressure tank with an air charge on one side of an internal bladder. Opening the faucet lets that air pressure push the stored water out at a usable rate. A tank that has lost its air charge is the most common cause of a system that trickles, and it is frequently misdiagnosed as a failing membrane. The tank also means the system produces in bursts rather than continuously.
Final polish, and proving it works
Water leaving the tank passes a final carbon filter on the way to the faucet, removing any taste picked up in storage, and on many systems a remineralization cartridge follows to restore a little mineral content and take the flat edge off the taste. To confirm the whole process is working, compare a dissolved solids reading of the feed with one from the membrane output taken before the tank: a wide gap means the membrane is doing its job. Our whole house RO guide covers the same process at building scale.
Questions people ask about reverse osmosis process
Why does reverse osmosis send water to drain?
The cross flow carries rejected dissolved material off the membrane surface. Without it the sheet fouls quickly, so the reject stream is functional, not waste.
Why is a storage tank needed?
The membrane produces slowly, so treated water is accumulated in a pressure tank and delivered from there at a usable rate when the faucet opens.
What does pressure have to do with it?
Pressure is what drives water through the membrane against osmosis. Low line pressure means slow production and a tank that never quite fills.
Why is there a carbon filter both before and after?
The one before protects the membrane from chlorine. The one after polishes taste picked up during storage in the tank.