Basics of Environmental Science

Physical Resources / 101

In other words, what is being measured is chlorinity.

Regardless of its salinity, or chlorinity, the composition
of sea water is fairly constant (Table 3.1). Some of the
‘other’ (listed at the foot of Table 3.1) is of commercial
importance, actually or potentially. It contains about 3
parts per million of uranium, for example, and about
0.003 per cent of all water, including sea water, is
deuterium oxide, or ‘heavy water’, used as a moderator
in the Candu (Canadian deuterium-uranium) fission
reactors and, in years to come, as a fuel in fusion reactors.
Interpreted ionically, the percentage composition of sea
water is shown in Table 3.2. Fresh water has a much
more variable composition, but one dominated by
carbonates (79.9 per cent) and sulphates (13.2 per cent),
with chlorides contributing only 6.9 per cent.

Removing the dissolved salts from sea water leaves a
highly concentrated brine. Those salts for which
industrial markets can be found can be extracted and
sold. Common salt, metallic magnesium, magnesium
compounds, and bromine are obtained in this way.
Indeed, nearly 30 per cent of the world supply of salt is
obtained by evaporating sea water. In this process,
calcium sulphate and calcium carbonate precipitate first;
when they have been removed the brine is moved to
another pond, where salt crystallizes. The remaining
brine, called ‘bitterns’, is removed, fresh, concentrated brine is added, and this is repeated until the
layer of crystalline salt is thick enough to be harvested. Bromine can then be extracted from the
bitterns. Where no market for by-products can be found, however, disposal of the brine is difficult,
and for every 30000 litres of fresh water produced by desalination, 1 tonne of salts remains.

Water may be separated from its dissolved salts by distillation, freezing, electrolysis, or reverse
osmosis. Distillation is the most widely used method. In low latitudes, the Sun may supply enough
energy to evaporate sea water. The evaporate is then condensed and after several cycles of evaporation
and condensation the water is sufficiently pure to be fed into the public supply. More usually, however,
energy must be provided. Several distillation methods are used. Figure 3.6 illustrates multistage
flash evaporation, which is one of the most efficient. Incoming sea water is heated under pressure, to
prevent it from boiling, then released into a chamber where pressure is lower. It boils instantly
(‘flash boiling’) and the vapour rises, to condense on the pipe carrying cold, incoming sea water. The
latent heat of condensation warms the incoming water, reducing the amount of heating required. The
condensate is collected and removed and the remaining brine fed to the next chamber where the
process is repeated.

Ice contains little salt and so freezing sea water purifies it. In this technique, the sea water is chilled
almost to its freezing temperature, then either sprayed into a partly evacuated chamber or mixed with
a volatile hydrocarbon, such as butane, and poured into a chamber. The low pressure, or high volatility
of the hydrocarbon, causes immediate evaporation of the hydrocarbon or some of the water and the
chilling caused by the latent heat of evaporation causes some of the remaining water to freeze. The
slurry of ice and brine is then pumped into another chamber, fresh water is added to separate ice
from brine, and the fresh water is removed.

Table 3.1 Composition of sea water

Table 3.2 Ions in sea water