2020-04-02_Science_Illustrated

The salty oceans of the world are huge potential drinking water reservoirs for mankind,

but technologies for desalinating ocean water require too much energy. Scientists

have developed new methods that not only produce drinking water from ocean water

with higher energy efficiency, but also from bone-dry desert air, and from human faeces.

Drinking water to be derived

from waves... and faeces

E

ven from a distance of six billion

kilometres, Earth high levels of

water are made evident by the

planet’s deep-blue colour. In such

remote images, taken by the

Voyager probe on its way out of the Solar

System, Earth still renders as a blue dot. No less

than 71% of our planet is covered in water. If it

were all bottled, it would fill 2670 billion billion

half-litre bottles. But we couldn’t drink much

of it. Ninety-seven per cent of the world’s water

is bound in salty oceans.

Still, ocean water has become a vital

source of drinking water in many places in

the world, where desalination plants are

used to convert salt water into drinking

water. In the future, desalination could prove

to be an important part of the solution to the

general pressure on ground water supplies

throughout the world.

Desalination consumes energy

Why can’t we drink ocean water? After all,

our bodies need salt – but only in modest

qualities. Our bodies consist of

about 60% water, and

the human body

absorbs 5-6 grams

of salt a day, while

the kidneys secrete

the rest into our

urine, at a concen-

tration which is

less salty than sea

water, about 35g per

litre. So if you drink a glass of salt

water, getting rid of the excess would require

the body to put more water into urine than

there was water in the glass. Keep repeating

that process, and the result would be death

by dehydration.

No place in the world desalinates as

much water as Saudi Arabia. The nation’s 28

desalination plants supply 6.6 billion litres

of pure water a day, which is 22% of all the

desalinated water in the world. In Australia

Perth desalinates some 45 billion litres annu-

ally, 18% of the city’s consumption. In late

January Sydney residents received a bulletin

from Sydney Water announcing that its

desalination plant was at ‘full production’,

delivering 250 million litres a day, or 15% of

Sydney’s water needs.

This may sound like a lot, but globally

only about 1% the total fresh water require-

ment is currently covered by desalination

plants. One reason is the cost, and the major

consumption of electricity, the burning of oil

and gas required for the desalination

Even from the dry air of the

Mojave Desert (7% relative

humidity) water can be retrieved.

A porous material known as MOF

(metal-organic framework) is the

main ingredient of a harvester

developed by scientists from the

University of California. The

material’s tiny pores have such a

large surface area that 1g of MOF

corresponds to the area of a

soccer field. MOF binds water

molecules, and a 1kg unit can

extract 1.3 litres of water a day.

Porous metal

forces water out

of desert air

Filters bind

the water

2 Frames including MOF

are stacked inside

the device. Via a

process known as

adsorption, the air’s

water molecules are

absorbed by the

MOF material.

Fan sucks

in air

1 The water harvester

sucks in air by

means of a solar-

powered fan.

The device also

has a battery, so

the fan can be

active at night.

Heat forces water out

3 Heating elements heat the water of the material. Fans force

the water through a tube as vapour

before it is cooled and condenses.

The water is so pure that it can be

consumed without further treatment.

SOLUTION

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