Drinking the sea
AAround ninety-seven per cent of the Earth’s water is too salty to drink. For coastal regions facing drought, turning seawater into fresh water has long seemed an obvious solution. The earliest large plants used heat, boiling seawater and collecting the steam, a process that consumed huge amounts of fuel. Today most new plants rely instead on reverse osmosis, in which seawater is pushed at high pressure through membranes that allow water molecules through but hold back salt.
BReverse osmosis has cut the energy needed to produce a cubic metre of drinking water by roughly three-quarters since the 1970s. Even so, desalinated water typically costs two to three times as much as water from rivers or underground sources, and the plants must be built close to the sea and to a reliable electricity supply. For wealthy, water-scarce states on the Arabian Peninsula, this cost is acceptable; for many poorer inland regions it is not.
CThe process also leaves behind a concentrated salty liquid known as brine, roughly one and a half litres for every litre of fresh water produced. When brine is released back into the sea close to the shore, it can sink and spread along the sea floor, lowering oxygen levels and harming organisms that cannot move away. Engineers are testing diffusers that mix the brine quickly with surrounding water, as well as methods that extract useful minerals such as magnesium from it.
DSome analysts argue that the attention given to desalination distracts from cheaper options. Repairing leaking pipes, which in some cities lose a third of treated water before it reaches consumers, and recycling wastewater for agriculture can often provide more water per dollar. Desalination, they suggest, should be the last resort rather than the first.