Drinking the Sea
Passage 1 · Environmental technology / Engineering
Across much of the world, the supply of fresh water is failing to keep pace with the demands of growing cities and farms, and in the driest regions the shortfall is increasingly met by turning to the one source that never runs dry: the sea. Removing the salt from seawater, however, is not easy, for salt dissolves into water at the molecular level and cannot simply be strained out. For most of the twentieth century the task was done by distillation, boiling seawater and condensing the pure vapour, a method that works but swallows enormous quantities of energy. Today the great majority of new plants use a quite different approach, known as reverse osmosis, which now accounts for most of the world's desalinated water.
A reverse osmosis plant begins by drawing seawater in through an intake pipe set out from the shore. Before the water can be desalinated it must be cleaned thoroughly, because anything left in it will quickly clog the delicate machinery downstream. The incoming water is first passed through screens that catch seaweed, fish and other large debris. It then goes through a stage known as pre-treatment, in which finer particles of sand, silt and organic matter are removed by filtration, and chemicals are added to discourage the growth of the microorganisms that would otherwise breed on the membranes.
Osmosis is the natural tendency of water to move across a partial barrier from a weaker solution towards a stronger one, and in seawater it would pull fresh water towards the salt. Reverse osmosis, as the name suggests, turns this around. A powerful pump raises the seawater to a very high pressure and drives it against a semi-permeable membrane.
