If no pressure were applied to the salt water solution, osmosis would transfer water into the solution and gradually dilute it, but when a pressure greater than the osmotic pressure is applied on the solution, the osmosis is driven in the opposite direction and pure water is squeezed out of the sea water. The pressure required for this is quite high and one type of membrane able to withstand the stress is a film of cellulose acetate placed over a suitable supporting structure. These days many countries use desalination plants to meet their water requirement.
