5.1 Organic Wastes
The commonest type of water pollution is by organic matter such as sewage. This
has the effect of stimulating bacterial and fungal growth and these processes
absorb oxygen and so de-oxygenate the water. Today, most domestic sewage is
treated before discharge to inland waters but this is not universally the case.
Most of the organic matter is removed, mainly by sedimentation and the action of
aerobic micro-organisms. Unfortunately, in many cities the population bas out-
grown the capacity of the sewage works and so tbe treatment is incomplete. Some
raw sewage is also discbarged; however, the situation is generally improving.
Domestic sewage is seldom acutely poisonous; its harmful effects are thc
encouragement of the wrong organisms in water enriched by sewage effluent.
Sewage fungus is the collective name used to accommodate a whole range of micro-
organisms, which in polluted waters form very characteristic communities. These
are normally readily recognisable by their white cottony appearance. It becomes
prominent and often forms enormous growths under conditions of enhanced nutrient
status. Sewage fungus communities may often blanket the river bed for several
hundred metres below the effluent with a greyish fur. This very effectively
smothers other living organisms. The masses are susceptible to breaking up and
they can often he found drifting downstream some distance from their source.
Carbohydrates or sugars discharged from processing industries may lead to the
formation of sewage fungus. Organic pollution also is produced downstream of
wood pulp industries.
Many studies have been reported describing the monitoring of sewage discharge into
streams and estuaries, by Klooster and Scherz (197^), Piech and Walker (1972),
Scherz (1971) and Strandberg (1967). The main techniques have been the use of
colour and colour infra-red aerial photography and thermal infra-red imagery.
Colour photography has proved useful because of its water penetrating ability anc
hence it shows up markedly any discolouration and discharge structures. False
colour tends to highlight any algae development along banks resulting from sewage
discharge. As the sewage is usually at a different temperature to the river's
water, at least at the point of contact, the dispersal pattern can be monitored
for some way from the source.
3.2 Eutrophication
Nutrient salts are deliberately added to rivers in sewage effluents and accidentally
when farm animal wastes are not properly disposed. Modern arable farming also
contributes by adding nitrates and phosphates to water. Most of the increase in
levels of nitrogen comes from arable land caused largely by the use of varieties
of cereal which respond to large amounts of chemical fertilisers. Unfortunately,
only about half the nitrogen applied to the land is taken up by the crop; the
rest is lost and some contributes to eutrophication. Little phosphate, cven when
liberally applied in chemical fertilisers, is lost as it is usually bound firmly in
the soil. However, phosphate ievels have increased spectacularly since about 1952
and this rise is believed to come mainly from detergents.
Eutrophication affects the vegetation of running water, but its greatest effects
are seen when this water is impounded in a reservoir or where the river runs into
a lake. Tbe most obvious resuit is an algal bloom. Many species of elgae are
involved which turn the water into something like a 'pea soup! or the whole surface
is covered with a mat of blanket weed. They, of course, liherate oxygen, but with
blanket weed this occurs at the surface and much is lost, while the lower layers of
the water are in shadow aud plants cannot flourish. Finally, the algae die and
decompose removing the oxygen from ib» water. In addition, blooms of blue-green
algae may liberate toxins.
The main remote sensor used to identify eutrophic conditions is colour infra-red
photography which can readily locate floating algal masses by its high infra-red
reflectance.