Page 63 - Studio International - May 1974

a large midge larvae population upon which the
trout fed.
Organic pollution, then, may be regarded as a
natural response by the river to changed
nutrient conditions. A river which is capable of
reacting in this way to an increased organic load
may be regarded as being ecologically healthy,
although some of the consequences may
adversely affect the uses of the river.
The problem of preventing organic pollution
of our rivers is largely an economic one. It is
technologically feasible and economically
acceptable to treat our organic wastes by
biological means, as on a sewage works, and to
reduce the organic concentration in the
effluents so as not to induce adverse oxygen
conditions in the receiving water. The problem
of plant nutrients in effluents, which we will
consider later, is a more difficult problem.
Contrary to popular belief sewage works are
not designed to remove pathogenic (disease
causing) organisms. Although a very high
proportion of such organisms are removed in
the treatment processes, because of the large
number in the sewage from different sources,
appreciable numbers are discharged to the
rivers in effluents. In some countries these are
reduced by chlorination of the effluent. This,
however, may affect the toxicity of the effluent
to the detriment of river life. Our water supplies
are safeguarded by the the treatment of the
waters at the water works after abstraction from
the rivers. Anglers may be assured that, in the
design of sewage treatment works and in the
tests to assess their efficiency, more concern is
shown for the welfare of fish than of man.
Most aquatic organisms, including most fish, (Top) Sewage as an imbalanced diet for bacteria, However, by protecting the natural waters in
resulting in poor removal of nitrogen and the interests of fish and other aquatic organisms,
rely on the oxygen dissolved in the water for
phosphorus in sewage treatment
their respiratory requirements. When, in man's interest and welfare are best served.
organically polluted rivers, this is depleted by (Bottom) Self-purification of river below an organic The public health hazards connected with
discharge — showing succession of river bed
the activity of the large population of recreational activities, especially bathing, in
communities
micro-organisms, the fish and other aquatic rivers and coastal waters receiving sewage
organisms are seriously affected. Aquatic effluents, is controversial. The evidence
animals differ in their tolerance to degrees of individual organisms to the degree of oxygen available suggests that in this country, in
oxygen depletion; stone-flies and most may-flies, depletion. In the final stages of recovery when rivers containing diluted oxidized sewage
for example, require well aerated waters, whilst freshwater shrimps, may-flies and caddis have effluents, the risk is minimal however great
the water-hog louse can withstand considerable become reestablished, fish are usually present. the theoretical potential danger. The risk is
deoxygenation. Some organisms have been Some of the micro-organisms encouraged by greatest where excreta from infected individuals
found to survive longer at low oxygen levels and organic discharges may take the form of is discharged not to a municipal sewage works
to be adversely affected by high ones. Some filamentous growths on the river bed and but to a stream via a private cess pit or septic
animals are able to obtain atmospheric oxygen banks; these growths are commonly referred to tank. There is also evidence that due to
and are thereby independent of the oxygen as 'sewage fungus' although strictly they are increased drug therapy some previously
concentration in the water. The larvae of the rarely fungi. By collecting solid matter, they harmless bacteria which are common in the
hoverfly, the rat-tailed maggot, has a change the physical nature of the stream bed and waters may be assuming a more pathogenic role.
telescopic respiratory tube which penetrates thus secondarily affect the bottom dwelling In other countries infected stages of parasitic
the surface of the water to enable it to obtain animals. Long streamers of sewage fungus very worms present in the water may constitute
its air supply. These organisms, tolerant of low often occur below industrial organic discharges, serious public health hazards (e.g. bilharzia).
oxygen, feed on the increased organic matter such as that from paper mills. Growths may Another form of nutrient imbalance occurs
in the polluted river and in the absence of occur in very low concentrations of organic when excess plant nutrients such as nitrates and
competition from the other organisms, matter and the removal of the matter to prevent phosphates enter rivers. Although in low
suppressed by the low oxygen, they may develop such growths can be very costly. The sewage concentrations these increase the productivity
huge populations. Thus deoxygenation has a fungus itself is not detrimental to fish except of the river, in higher concentrations excessive
differential effect on the fauna. when in large amounts it decays and causes productivity takes the form of objectionable
Associated with the successive stages of deoxygenation. In experimental work to growths of algae such as cladophora (blanket-
recovery of the river by self-purification, increase trout production in upland streams, weed). Excessive growth of this alga has caused
different river communities become sugar solutions were added to encourage serious problems in some chalk streams. They
established, determined by the tolerance of sewage fungus growth, which in turn supported suppress the populations of aquatic insects upon
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