A simplified, animated cut-away of a reverse osmosis membrane, not to scale. Blue dots are water and amber hollow diamonds are dissolved salts. At low pressure, water moves through the membrane towards the salty side. At the osmotic pressure, the two pushes balance and the flow stops. At higher pressure, the flow turns round: water passes through the membrane to the clean side, and most salts stay behind and are carried away.
- Feed water, under pressure
- RO membrane
- Clean water passes through
- Salts held back, carried away
- Salty side
- Fresher side
- Water
- Dissolved salts
How reverse osmosis works
A very fine membrane holds back most dissolved salts while water is pushed through it under pressure.
Reverse the flow
- 1Left alone, water moves towards the salty side.
- 2Push hard enough and the flow turns round. That is reverse osmosis.
- 3Water passes. Most salts don’t.
- Low
- Osmotic pressure
- High
- Water
- Dissolved salts
Simplified
What each stage of a purifier does
In an under-sink purifier, the membrane works with other stages. Stages differ by model.
Sediment
Catches sand, rust and silt, so the finer stages last longer.
Carbon
Reduces chlorine, odours and off-tastes.
RO membrane
A very fine membrane that holds back most dissolved salts. What it holds back leaves through the drain line.
Final stage
A last polishing stage for taste, just before the tap.
What RO does not do
It holds back most dissolved salts, not all.
A purifier treats the water at your tap, not the tank or the pipes.