New Scientist 2018 sep

8 September 2018 | NewScientist | 33

That is where desalination comes in, and
there are plenty of ways to do it. For example,
you can evaporate water from one spot and
condense it in another, leaving salt and
impurities behind. But the most energy-
efficient method is reverse osmosis.
Imagine a strong salt solution and a weak
one, separated by a membrane that allows
only water through. In this situation, the
water will flow naturally from the weak
solution to the strong, evening out their
concentrations in a process called osmosis.
Do the reverse, forcing ocean water at high
pressure through a salt-excluding membrane
in the opposite direction, and you are well
on the way to making drinking water.
This approach is becoming more
common. In 2005, desalination produced

about 40 billion litres of water daily, according

to the International Desalination Association.

By 2015 that had increased to 87 billion litres,

produced by nearly 1900 plants around the

world. The vast majority of those are in the

dry countries around the Persian Gulf, but

the technology is on the up elsewhere too.

The Australian city of Adelaide gets roughly

half of its water from a huge desalination

plant. California already has a few plants

and is spending more than $30 million on

eight new ones.

Great news. Except what about all that brine

left behind? It is about twice as salty as the

starting water, depending on the desalination

technique used, and most plants dispose of it

by pumping it back out to sea. That is a source

of concern for researchers, including

Childress, who is based at the University

of Southern California.

The concentration of salts in the sea

varies. On average it is 35 parts per thousand,

meaning that for every 1000 grams of

seawater, about 35 grams is salt. Once salt

levels exceed what marine plants and animals

are used to, there is a danger that their cells

might cease to work properly.

The evidence so far for what happens to

life around brine outflows from desalination

plants is mixed. In 2012, a study for the

California State Water Resources Control

Board looked at how animals responded to

increased salt concentrations in lab >

For coastal communities, the ocean is

a tempting source of water

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