Page 62 - Studio International - May 1974

and the resultant fish mortality in otherwise
unpolluted rivers, which hit the headlines. This
we may consider as a case of acute toxicity.
In many of our rivers such discharges may go
unnoticed, their fish populations being
suppressed or exterminated by continuous low
level concentrations of toxic materials. Such
pollution may be regarded as chronic toxicity.
Many of our fish-less industrial rivers are in
this condition. In contrast with the publicity
given to the occasional case of acute toxic
pollution, the decline and eventual death of fish
in our industrial rivers which occurred earlier
in the century, evoked little protest. In those
days jobs and food were probably of more
concern than the environment.
The control of toxic pollution is difficult
because of the wide range of toxic substances
that may be discharged to rivers, either directly
from factory effluents or via sewage works.
The toxicity of an effluent may be assessed by
toxicity tests using fish as test animals. These
tests, however, are usually of relatively short
duration and the results are more applicable to
the control of acute toxicity. Chronic effects are
more difficult to assess. To maintain a
population in the river fish must not only be
capable of surviving but also be able to feed,
grow and reproduce successfully. Sub-lethal
concentrations of chemicals may therefore
suppress or eliminate a fish population without
causing fish mortality.
Many of the toxic metals which enter our
rivers do so via sewage works effluents. From
a variety of industrial processes toxic materials
are discharged to the sewers and hence to the
sewage works. The percentage of metal removed
in sewage works treatment differs for different
metals : about 90 per cent of the zinc, 75 per cent
of the copper, less than 5o per cent of the
chromium and about 25 per cent of nickel for
instance. This is not due to faulty operation of
the sewage treatment works which are designed
primarily for the oxidation of organic matter.
To reduce the concentration of metals entering
rivers therefore, control of the amounts
entering the sewers is necessary and pre-
treatment of the industrial effluents before
discharging to the sewers may be required.
Biotic Imbalances. Many of man's personal,
domestic, and industrial wastes are of an
organic nature. Sewage, wastes from food
processing factories, paper and textile
manufacture and many chemical manufacturing The arteries and veins of water supply and activity of algae which become re-established
processes are examples. The discharge of such reclamation in the recovery zone. This phenomenon is
wastes either untreated or partly treated, has a the presence of ample food supply in the form known as 'self-purification' of the river and the
similar effect on the ecology of the river — of organic matter, this oxygen demand may associated oxygen depletion as the 'oxygen
commonly termed 'organic pollution'. In the deplete the amount of oxygen present in the sag'. To the ecologist this is an interesting
river the organic matter is broken down or water. Because of the current the depletion of example of the homeostatic (self-regulating)
oxidized by numerous micro-organisms which oxygen occurs some distance downstream of the properties of an ecosystem. Similar processes
feed on it. Although the activity of these effluent discharge. Farther downstream the of biological purification occur in the sewage
micro-organisms eventually brings about a reduction in the organic concentration due to treatment works, where under controlled
reduction in the concentration of organic the activity of the micro-organisms, permits a conditions, the same organisms reduce the
matter and stabilizes it, the oxygen they use gradual increase in the oxygen due to reduced organic concentration, as the sewage percolates
for their respiratory requirements is taken microbial activity as nutrients become limiting. through a biological filter some six feet in depth
from that dissolved in the water. Because of The aeration is brought about by physical compared with the self-purification in a river
their rapid growth and dense populations in surface aeration and by the photosynthetic over distances measured in miles.
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