Multiple Choice, Multiple Answers

Desalination, the removal of salt from seawater, now supplies drinking water to hundreds of millions of people, particularly in the Middle East, North Africa and parts of Australia and the United States. Older plants worked by boiling seawater and collecting the steam, a method that requires large amounts of heat. Most new plants instead use reverse osmosis, in which seawater is forced at high pressure through membranes that let water molecules pass but hold back salt. Improvements in membranes, and in devices that recover energy from the high-pressure waste stream, have cut the energy needed per cubic metre by more than half since the 1990s. Even so, desalinated water usually costs more than water from rivers or underground sources, and the energy it uses can add to carbon emissions unless it comes from renewable sources. A further concern is brine, the very salty water left over after treatment. For every litre of fresh water produced, a typical plant discharges roughly one and a half litres of brine, often back into the sea. If it is not diluted properly, this heavy, salty water can sink to the seabed and harm the organisms living there. For these reasons, many water experts see desalination as a valuable supplement for dry coastal cities rather than a replacement for conservation, water recycling and better management of existing supplies.