said that the water of the Thames when it reaches London has been drunk and passed through different sewage works at least five times. The result is, from the point of view of man, very much an improvement on the conditions which obtained a hundred years ago. There are, however, disadvantages in treated sewage effluent as compared with moderate doses of raw sewage from the point of view of some forms of life. Raw sewage is oxidised rapidly, but its breakdown products become gradually available over a period of several days or even longer. In even the most slowly moving river this means that they are diluted and spread over a considerable area. In treated effluent many salts, not themselves poisonous, are present, and are immediately available as sources of nutrition for plants including algae. Thus a “clean” sewage effluent can have a more rapid, and in some ways more undesirable, effect on the vegetation than the same amount of untreated sewage. This emphasises the conflict that may arise between the needs of human hygiene and the preservation of natural conditions in streams.
Severe organic pollution with complete deoxygenation of the water is an obvious and undesirable condition. Life is not completely absent. Many bacteria, some producing poisonous or unpleasant gases like hydrogen sulphide, abound. Some of the insects which actually breathe air at the water surface, such as the rat-tailed maggot Eristalis, are quite common, but insects which remain totally submerged and all fish are absent. This condition obtains in much of the Thames estuary, notwithstanding the marked improvement that has taken place in recent years.
In many rivers and streams, organic pollution is intermittent. At times it is severe, and complete or almost complete deoxygenation occurs. At other times the water is comparatively pure and oxygen is present. Such severe pollution will kill all the fish, many of the plants and most of the insects and other invertebrates. Recolonisation when it ceases occurs, but which animals and plants reappear depends on many factors. After severe pollution of a stretch of a river, the remainder of which is unaffected, recolonisation is rapid. Careful sampling has shown that some species of “coarse” fish come back even when the oxygen tension is still quite low. Fish have the obvious advantage of being quick moving and able to progress against all but the fastest currents. Many species of invertebrates cannot move so fast, and plants are dependent on water and air currents, animals and other factors for their distribution. Ecologists can quickly recognise a river which is recovering from a period of pollution.
The usual effect of organic pollution is partial, rather than complete, deoxygenation. This is a very complicated subject and for details readers should refer to the books already mentioned by Hynes and Erichsen Jones, and to the excellent work which is constantly coming from the Water Pollution Research Laboratory. It is important to remember that unless organic pollution is very severe, most bodies of water exhibit self-purification to a greater or lesser extent. Fig. 4 shows the changes in a river below an organic effluent outfall. This illustrates a case 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
