Reverse Osmosis: Pushing Water Uphill Through a Membrane

The solar still and the alembic both separate salt from water by evaporating the water — and both pay the latent heat of vaporisation to do it, about 2,400 kilojoules for every kilogram. That energy bill is why distillation stayed expensive for three thousand years. Reverse osmosis does the same separation without a phase change. Osmosis drives fresh water INTO brine through a membrane that passes water and rejects salt; push back harder than that osmotic pressure and the flow reverses, leaving the salt behind. The idea was old, but it was useless until Sidney Loeb and Srinivasa Sourirajan at UCLA made a cellulose acetate film in 1959 with a dense skin a fraction of a micrometre thick on a porous backing — thin enough to pass useful water, sound enough to hold back the salt (US 3,133,132). Every desalination plant since descends from that film.