Modern desalination relies on reverse osmosis: seawater is forced through semi-permeable membranes with pores small enough to block salt while letting water through. The pressure needed is immense — roughly 55 to 80 atmospheres, about seventy times normal air pressure.

The process yields two streams: clean freshwater and a concentrated brine with about twice the normal salt level. Managing that brine is one of desalination's biggest environmental challenges.