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.