Multiple Choice, Multiple Answers

For countries with long coastlines and little rain, turning seawater into drinking water is an obvious attraction. Desalination plants now supply a large share of the water used in several Middle Eastern countries, and cities in Australia, Spain and California have built plants to protect themselves against drought. Most modern plants use reverse osmosis, in which seawater is pushed at high pressure through membranes that allow water molecules to pass but hold back salt. Improvements in membranes and in devices that recover energy from the pressurised water have made the process considerably cheaper than it was a few decades ago. Even so, desalinated water usually costs more than water taken from rivers or underground, mainly because pushing water through membranes requires a great deal of electricity. Where that electricity comes from fossil fuels, desalination adds to greenhouse gas emissions, which is one reason some new plants are paired with solar or wind power. A second problem is what to do with the salt. For every litre of fresh water produced, a plant typically discharges more than a litre of brine, a concentrated salt solution that may also contain chemicals used to clean the membranes. Released carelessly, this heavy brine can sink and spread along the seabed, harming organisms that cannot tolerate high salinity. Careful design of outfalls, which mix the brine with the surrounding water, can greatly reduce this damage. For these reasons, many water planners regard desalination as an insurance policy rather than a first choice. Reducing leaks, reusing treated wastewater and encouraging households to use less often provide water more cheaply, while a desalination plant guarantees supply when droughts make other sources unreliable.