What is reverse osmosis and how does it work?

Reverse osmosis applies pressure greater than osmotic pressure to force water through a semi-permeable membrane, leaving dissolved salts behind. Normal osmosis naturally moves water from low to high solute concentration; RO reverses this by external pressure. It is widely used for seawater desalination, producing ultrapure water for electronics, and wastewater treatment. Typical operating pressures range from 15-80 bar.