Reverse osmosis is a water-treatment process that uses pressure to move water through a semipermeable membrane. The membrane allows water molecules to pass more readily than many dissolved substances, producing a treated stream and a concentrate stream.
Why does RO use pressure?
Natural osmosis tends to move water across a membrane toward a more concentrated solution. Reverse osmosis applies pressure to drive water in the opposite direction.
RO is usually part of a system
Household units commonly combine sediment and carbon stages with the RO membrane, followed by post-treatment. The exact configuration varies.
What about minerals?
Because RO can reduce many dissolved minerals, some products include a remineralisation stage after the membrane. Whether that is desirable depends on the system and the user's objectives.
What about waste water?
RO creates a reject stream containing a higher concentration of material removed from the feed water. Efficiency varies by system, pressure, temperature and other conditions.