2 Chapter 6 • organising elements Organising elements

34
Given the fact that so many countries do not have enough fresh water
suitable for drinking, it is little wonder that many governments,
including those in Australia, have decided to establish desalination
plants to extract water from our oceans.
They are two main methods used in desalination plants. One is
based on distillation. In this case, sea water is drawn into the plant
and boiled. The water evaporates, leaving all the salts behind. The
evaporated water is then cooled so it condenses; it is now pure water.
This is the method used in a number of Middle Eastern countries,
including Saudi Arabia and the United Arab Emirates. All the
pumping and heating consumes a huge amount of electricity, although
some of the cost is offset by some of the valuable minerals extracted as
by-products from the water.
An alternative method is called reverse osmosis. This is the
method used in Australian desalination plants such as the Kwinana
desalination plant in Western Australia, which provides 17% of Perth’s
drinking water. In this process, sea water is passed through membranes
at high pressure, which removes a large proportion of the dissolved
salts. The waste water from the plant, which will have a very high salt
content, is pumped back into the ocean, further out to sea.
Fig 6.41 Desalination plants in Australia are providing
more and more of our drinking water.
Chapter 6 • organising elements
1 Given the properties of ionic
compounds, such as sodium
chloride, and molecular compounds,
such as water, suggest why the water
evaporates away leaving the salt
behind when the sea water is heated
during distillation.
2 Given the properties of metals, what
might be a problem with all the metal
parts within a desalination plant?
3 Suggest reasons why distillation
methods are commonly used
for desalination plants in oil-rich
countries such as Saudi Arabia,
whereas reverse osmosis is
the method used in Australian
desalination plants.
4 Conduct research into existing or
planned desalination plants in your
State or Territory and find out:
a the reasons the plants have been
sited where they are
b the potential environmental impact
of the desalination plant.
UNCORRECTED PAGE PROOFS
CAS_SB10_TXT_06_1pp.indd 34
11/11/11 4:58 PM