New Scientist 2018 sep

32 | NewScientist | 8 September 2018

F

OR the time being, Cape Town has dodged

a bullet. After months of unrelenting

drought, the recent winter rains have

begun to refill its parched dams. That doesn’t

mean things are easy. City residents are still

limited to using 50 litres of water a day,

scarcely enough to half-fill a bath. But at least

so-called day zero, when the taps run dry and

residents have to wait in line to collect survival

rations of water, has been averted.

The South African city is an extreme

example, but it is far from the only place

facing a severe water shortage. To slake that

thirst, many cities are turning to the ocean,

a seemingly inexhaustible supply of water.

They are doing this through desalination,

a water purification technology that has been

around for decades. Cape Town is bringing a

couple of desalination plants online in a hurry

and many others are being built elsewhere. As

they spring up, however, attention is focusing

on what they leave behind: concentrated

brine, millions of litres of it a day.

Now scientists are sounding a note of

caution about the impacts of dumping all that

salt in the environment. “Increasing salinity

is one of the most important environmental

issues of the 21st century,” says engineer

Amy Childress. But smarter methods of

desalination are emerging and they have

benefits far beyond providing clean water.

Sao Paulo, Cairo, Beijing, Bangalore – the

list of cities with water shortages runs long

and touches every continent. Even London,

often thought of as a wet city, only gets about

600 millimetres of rain a year and will

probably have supply problems by 2025.

As populations grow, things are set to worsen.

In 2007, the UN found that about 1.6 billion

people lacked adequate infrastructure to

supply them with drinking water. By 2025,

the organisation expects 1.8 billion people,

almost a quarter of the world’s population,

to be living in areas where there is not

enough water to sustain them.

Ideally, we would meet demand by tapping

into stores of fresh water such as rain-filled

reservoirs and rivers, or perhaps groundwater

wells. But plenty of places don’t have

sufficient, easily accessible sources of

fresh water to support growing populations.

One option is to recycle waste water on a

mammoth scale. Another is to turn to salty

water, like the ocean or brackish lagoons.

The briny

deep

Ocean desalination would provide

drinking water for all – if only we knew

what to do with the waste salt.

Katherine Bourzac reports