of severe pollution, but insufficient to cause complete deoxygenation. A shows how the oxygen level drops and the B.O.D. rises just below the outfall; farther down this process is reversed until the oxygen level is fully restored. B shows the parallel changes in the chemical constitution of the water. C and D show how the micro-organisms and the larger animals fare. The recovery of the “clean water fauna” will depend on recolonisation, if, as appears in this figure, it is totally eliminated just below the outfall. In some cases the purified river will still be richer in nutrients than above the point where the effluent entered, and the level of the clean water fauna may be actually enhanced. This indicates how moderate pollution, from a small village, for instance, may have little permanent harmful effect. With the growing population of Britain, however, it seems that other methods of disposal than the rivers will always have to be used if the water is to be kept safe for wild life; if it is only to be drunk by man such high standards are not required!
So far we have mainly considered pollution due to organic waste, and consisting of substances which in themselves, and in small quantities, are harmless or even beneficial to life. Many effluents particularly from industry contain toxic substances. Some of these, including phenols and thiocyanates, are usually broken down by bacteria, particularly if they are mixed and diluted with ordinary sewage which, of course, promotes rich bacterial growth. Many, but by no means all, toxic organic substances are affected in this way, but metallic poisons generally pass through filter beds without loss of toxicity. Some metals are remarkably toxic to certain forms of life. Thus copper is used to keep ponds free from algae, when 0·5 parts per million is often effective. Fish survive just over 1 p.p.m., and 2 p.p.m. is tolerated in human drinking water. Zinc affects certain invertebrates at widely different concentrations, some snails are killed by 0·3 p.p.m., some insects survive 500 p.p.m. Much more work is necessary on the long-term effects of metals at low concentrations, not only on fishes and invertebrates, but on man. Recent findings on the effects of low doses of lead on man are disquieting and more harm may be being done to other forms of life than is usually recognised.
One group of organic pollutants which has received special study is the synthetic detergents. They illustrate how serious damage can be done to amenities and wild life by the unexpected persistence of substances not originally expected to be harmful. The oldest known detergents, the soaps, are made from alkaline salts and certain (weak) fatty acids. The soap which went down the drain was broken down or precipitated in the sewage works and was never thought of again; it did little or no harm. Soaps have the disadvantage that they are relatively insoluble in hard water. Since about 1918 a series of synthetic chemicals has been developed which does not have this disadvantage. Various different chemicals have been successfully used as detergents. The housewife, in her home, has no complaint, and has even come to accept the illusion that they make her washing “whiter than white” when optical whiteners (which have not been shown to have any biological disadvantage nor to make the removal of dirt from clothes more efficient) are included. The trouble has arisen in recent years from the strikingly obvious effect of running the sewage effluent into rivers. As soon as a river has run over even the lowest weir, causing a small amount of turbulence, the detergent has produced enormous quantities of persistent foam which has sometimes caused trouble by blowing in large lumps the size of footballs into crowded streets. The apt name of “detergent swans” has been applied to these aggregations. One interesting point about their occurrence should be noted. The foaming is often worst a long way below the point where the sewage works discharges its effluent into a river. This is because foaming is least in dirty water. It is not until some degree of self-purification of the river has taken place that the maximum foam-production is possible.
Fig. 4 The effects of an organic effluent on a river below the outfall. A and B physical and chemical changes, C changes in micro-organisms, D changes in larger animals. (From H. B. Hynes.)
The aesthetic damage by detergent foam is obvious. Its biological effects are less easy to determine. Foam blowing from sewage works has been shown to carry pathogenic bacteria and worm eggs, and so is a hazard to human health. Some rivers contain intermittently as much as ten parts per million of detergent without apparently doing a great deal of harm to the flora and fauna, or to the humans who use the river as their water supply. However, these are usually rivers which have to start with a fair degree of pollution and a sparse fauna and flora. It is known that as little as 0·1 p.p.m. of detergent almost halves the rate at which a river takes up oxygen, and so small residues greatly slow down self-purification. Sensitive fish, like trout, are affected by concentrations as low as one part per million, and show symptoms similar to asphyxia. It seems likely that even a very small amount of detergent in a clean upland stream would have a severe biological effect on the sensitive plant and animal life; contamination of such streams from upland farms and cottages must occur.
The economic effect of detergents in sewage works is serious. These substances reduce the efficiency of the filter beds, which must be extended considerably if the effluent is to be maintained at a given standard of purity. Where this fact has not been realised, strongly polluted effluents have sometimes been accidentally discharged into rivers. The reason detergents are persistent, and the foam such a nuisance, is that the molecules are very stable, and that they are not broken down quickly in sewage filters or in the rivers into which the sewage effluent is run. The most troublesome detergents include substances like sodium tetrapropylene benzene sulphonate (TBS); it has a molecule with many branches in its carbon chain (Fig. 5) and this is associated with its resistance to bacteria. Other substances which act as anti-foaming agents, including kerosene, have been added to effluents to prevent foaming; unfortunately they do not remove the detergent, only mask its presence, and may actually aggravate the pollution. Recently entirely new chemicals, as efficient in washing clothes, but much more easily broken down by bacteria, have been introduced. These have straight carbon chains and include Dobane JN sulphonate, and finally sodium alkane sulphonate (SAS) which is at least 99 per cent broken down as it passes through the sewage works. The breakdown products are, as far as is known, non-poisonous. Already some countries, including Western Germany, have made the sale of the so-called “hard” detergents, i.e. those with branched-chain molecules which are so stable, illegal, and some progress with their replacement by the “soft,” straight-chain substances has been made in Britain. It therefore seems possible that in a few years this form of pollution may have disappeared. However, there are many useful lessons to be learned from this subject. Detergent pollution might not have been noticed but for the appearance of “swans.” Other cases of pollution may go undetected when new substances are used, for instance in industry, so constant monitoring of effluents and the rivers into which they run is obviously necessary.
Other very stable substances are found polluting water, but with more serious results than those resulting from the presence of the relatively non-poisonous detergents. The most serious effects have resulted from the presence of the chlorinated hydrocarbon insecticides, which are both persistent and poisonous to most forms of life, but particularly to fish and aquatic insects. This subject is discussed in detail below (see here), when consideration is given to the whole question of environmental pollution by pesticides.
Many rivers to-day are altered by industry, particularly by electric power stations, by having the temperature of the water raised. Where this warming accompanies organic pollution, the effects are greatly increased, as the oxygen level is lowered while the rate of metabolism of the bacteria and other forms of life is increased. In some cases different animals and plants, more adapted to warm conditions, have colonised regions where heated effluents are discharged. This heating of rivers and lakes is something which is likely to increase with growing industrialisation, and it needs to be watched from many points of view, including that of the preservation of wild life.
Rivers may also be cooled, if heat pumps are introduced to warm our cities. These installations remove the heat energy from the water, and pilot plants have reduced the temperature by two or three degrees. This will probably slow down many biological processes, but it is unlikely to have a very great effect on the composition of the flora and fauna.
It is difficult to foresee just what will happen to the lakes, streams and rivers of Britain in the future. So long as sewage