Reverse osmosis explained

Osmosis is the movement of water from a less concentrated solution to a more concentrated solution through a partially permeable membrane. It’s what causes your fingers to swell up and wrinkle if you spend too long in the bath, or slugs to shrivel up if you cover them in salt. And, if you apply pressure, you can make the process happen in reverse.

Desalination plants have membranes covered in pores that measure less than 1/100,000th of the width of a human hair. When pressure is applied, water passes through, but salt is too big to pass, allowing freshwater to be squeezed from brine.