A reverse osmosis system pushes household water through a semi-permeable membrane so tightly woven that most dissolved solids cannot follow the water through. What comes out of the little faucet is water with the great majority of its dissolved mineral, metal and salt content stripped out; what those contaminants were carried in goes down the drain.
The membrane does the real work
Line pressure presses water against a rolled sheet of thin-film composite membrane. Water molecules pass; most dissolved ions, being larger and electrically charged, are rejected and swept away in a continuous drain stream. That rejection is why reverse osmosis is the household answer for dissolved problems such as sodium, nitrate, arsenic and total dissolved solids, where a carbon filter alone does nothing. It is a separation, not an absorption, so the membrane does not fill up and stop the way a cartridge does.
The cartridges around it protect it
A membrane on its own would foul in weeks, so every system wraps it in stages. A sediment cartridge takes out grit and rust so the membrane face stays clear. A carbon block takes out chlorine, which chemically attacks thin-film membrane material and will destroy it. After the storage tank, a polishing carbon filter takes the flat, stored taste off the water on its way to the faucet. Those cartridges are consumable; the membrane behind them lasts far longer because they are there.
It stores water because it makes water slowly
A membrane produces a trickle, not a stream, so the system parks its output in a small pressurized tank under the sink. The tank is what lets you fill a pitcher at a sensible rate. When the tank empties, refill takes time, which is why an under-sink unit feeds a dedicated drinking faucet and sometimes the refrigerator line, and never the whole house. Whole-house reverse osmosis exists but needs a large atmospheric tank and a repressurization pump.
What it does not do
Reverse osmosis does not soften the water going to your shower, boiler or washing machine, because it treats one tap. Hardness will keep scaling your fixtures unless a softener handles the whole incoming line. It also does not reliably deal with bacteria on its own, which is a job for ultraviolet disinfection or chlorination, and it removes beneficial calcium and magnesium along with everything else. Match the system to what your water report actually shows: our well water testing guide walks through getting that report first.
Questions people ask about what does a reverse osmosis system do
Does a reverse osmosis system need electricity?
A standard under-sink unit runs on household water pressure alone. Only systems with a booster pump, a permeate pump variant or a tankless design need a power outlet nearby.
Why does my reverse osmosis system send water down the drain?
The drain line carries away the concentrated stream that keeps the membrane surface swept clean. Without it the membrane would blind over quickly and stop producing.
Can reverse osmosis replace a water softener?
No. It treats one faucet, so scale keeps forming in your water heater, shower and appliances. Softening is a whole-house job handled ahead of the reverse osmosis unit.