Читать книгу Pesticides and Pollution - Kenneth Mellanby - Страница 8

As was seen in the last chapter, air pollution has proved difficult to study, and many conflicting results have been obtained. Water pollution seems to have provided a more satisfactory topic for investigation. This is not because the subject is simpler; in fact in some ways it is even more complicated. Air almost always contains sufficient oxygen to sustain life, and “pollution” only means adding a lesser or greater amount of some foreign substance to an otherwise wholesome atmosphere. Water, on the other hand, may be greatly depleted of oxygen so that it cannot sustain most kinds of life, or it may have various substances added, so that animals and plants are poisoned. Some waters have both reduced amounts of oxygen and appreciable amounts of poisons in solution; the inter-relations between these factors, and their effects on aquatic animals and plants, may be very complicated. The advantage of water over air, from the point of view of the research worker, is probably that it forms a definite and restricted environment, from which animals cannot easily move. It is therefore possible to study the long-term effects of pollution under fairly constant conditions, and there is no difficulty in demonstrating the serious effects which pollution can produce. It may also be suggested that water is a marketable commodity of considerable economic value, while air is, theoretically at least, “free.” So although fresh water covers under one per cent of the surface area of Britain, there are probably more scientists studying its pollution than there are investigating air which covers one hundred pet cent of the globe, land and water alike.

Several excellent books on water pollution, and books on freshwater dealing authoritatively on aspects of the subject, have been published in recent years. These include The Biology of Polluted Water by H. B. N. Hynes, Fish and River Pollution by J. R. Erichsen Jones, the New Naturalist Life in Lakes and Rivers by T. T. Macan and E. B. Worthington, and Freshwater Ecology by T. T. Macan. The existence of this extensive and easily obtainable literature has enabled me to make this chapter much shorter than would otherwise be desirable, and to deal with the effects of pollution in a rather different way than would have otherwise been possible.

Man’s requirements regarding water are different from those of “wild life” generally. Man demands what he describes as “pure” water; what he really means is “safe” water. This must contain only a minimum amount of salts, and must be free from those bacteria, protozoa and arthropods which might develop in his body and cause disease. Man thus deliberately interrupts the life-cycle of many other forms of life by the various methods of purification which he uses. He is less concerned with the oxygen content of the water than are the fish and insects which live in it. These efforts to produce a pure water supply for city dwellers can even be thought of as a form of “pollution,” in that water catchments alter, and sometimes sterilise, large areas of the countryside. The conflict between Manchester Corporation and many naturalists and others over the fate of much of the English Lake District illustrates this point.

On the other hand, water which, for public health reasons, is considered to be “grossly contaminated” by sewage, may still be, from the biological point of view, a healthy and desirable environment for many animals. But by the deliberate discharge of his domestic and industrial wastes man most greatly affects streams and lakes, and so alters the whole composition of their flora and fauna.

Natural waters may not only be “impure” from man’s point of view because of the parasites they harbour; they may contain many substances, even poisons, without any human intervention. Quite high concentrations, sufficient to poison some fish and many insects, of lead and copper are found in waters which percolate through strata rich in these metals. Streams running through forests, particularly pine forests, may be contaminated with large amounts of organic matter, and the results may be quite similar to those arising from domestic pollution. As a rule a special flora and fauna is found, consisting of plants and animals adapted to such conditions, in these impure waters. Human pollution usually happens so quickly that impoverishment occurs, often without time to allow the introduction of many of these special types of organism.

Primitive man did not seriously harm the aquatic environment. He often lived beside rivers and lakes, and his waste products must have entered the water, but in insufficient quantities to have adverse effects on the flora and fauna. In fact excrement entering the water in this way no doubt contributed to its nutritive value, and the substances it contained entered into the normal cycles. In some of the less developed and less densely populated areas of tropical Africa we can see a similar situation to-day. The streams and ponds are full of healthy fish; the human beings have a rich internal fauna of parasite worms which pass part of their lives in the water, inside small crustaceans or fish. Man in this way contributes to the richness of wild life in his environment.

When man came to live in towns and cities, however, his increasing numbers had a very different effect. Sewage continued to be poured into the rivers, but the quantities were so great that most unpleasant results were obtained. By the middle of the nineteenth century the Thames, and many other major British rivers, had become open sewers. There are many accounts in the literature. I myself like the account of the Reverend Benjamin Armstrong, from his diary:

“July 10th, 1855. Took the children by boat from Vauxhall Bridge to show them the great buildings. Fortunately the Queen and Royal Princes drove by. The ride on the water was refreshing except for the stench. What a pity that this noble river should be made a common sewer.”

Practically every other river was treated similarly. Even the Cam flowing through the Backs at Cambridge was in this way abused, as is illustrated by the (perhaps apocryphal) story of Queen Victoria’s conversation with the Master of Trinity when she looked over the bridge. “What,” she asked, “are all those pieces of paper in the water?” The Master promptly replied, “Those, Your Majesty, are notices saying that bathing is forbidden.”

The results of all this untreated, or “raw,” sewage, vary greatly, depending on the volume of water and the amount of organic matter. As indicated above, small amounts of raw sewage may be actually beneficial to most forms of aquatic life. To-day in some rivers, including the Bedfordshire Ouse, the comparatively small number of boats present discharge the contents of their water closets straight into the water. This does not cause noticeable offence. In some parts of the Norfolk Broads it does, for there are many boats producing much more sewage and this is dangerous. In really crowded rivers, such as the Thames, such disposal methods are not allowed.

Sewage, in quantities which are large enough to have a biological effect, acts in different ways depending on the temperature, the nature of the water and various other factors. The most important biological effect arises from its breakdown by bacteria; this requires oxygen, and as a result the water tends to become deoxygenated, and so less suitable to support most other forms of life. Almost all pollution of water with organic matter, be it sewage, effluents from factories (particularly food factories and dairies) or sawdust and similar wood waste, has this sort of effect. Organic pollution is usually measured by the “biochemical oxygen demand test” (B.O.D.). Experience has confirmed the value of this test, in which a sample of contaminated water is incubated, in the dark, at 20°C. for five days in a closed container containing a known amount of oxygen in solution; the amount of oxygen taken up by the sample is a measure of its B.O.D. Where this is high, and where the diluting water is not present in large amounts, trouble is likely to occur.

It is not generally realised how little oxygen is present, dissolved, in any sample even of “pure” water. A litre of water, at 5°C., in free contact with the atmosphere, only contains about 9 cc. of oxygen, weighing 13 mgs. As the temperature rises the oxygen content falls, so that at 20°C. it is only about two-thirds the level at 5°C. As the rate of metabolism of cold-blooded animals may treble with such a rise in temperature, an oxygen shortage is easily produced. Air, even polluted air, is a much richer source of oxygen. A litre of air contains about 210 cc. of oxygen, weighing approximately 300 mgs., i.e. over twenty times as much as is found in the same volume of well-oxygenated water. This may help to explain why some chemicals are toxic in very low doses when dissolved in water; an aquatic animal to breathe must make intimate contact with an immensely large volume of water in order to obtain enough oxygen.

Oxygen reaches the water in two main ways. First, it dissolves at the surface from the atmosphere. Still water takes up oxygen slowly, turbulent water rushing over falls takes it up much more rapidly, for this often submerges bubbles which act as does bubbling air through a domestic aquarium. This type of solution will rarely raise the oxygen level above saturation. The second source of oxygen in water is from photosynthesis. Where there are many green plants present, during the hours of daylight the water may often become supersaturated with oxygen. Unfortunately after dark photosynthesis stops and the plants continue to respire and so actually reduce the amount of oxygen in solution. Therefore during a twenty-four-hour period some waters have a range of oxygen levels which varies enormously, from practically nil around dawn to a very high volume in the early afternoon. Many animals are adapted to life under these conditions. Some biologists have not realised that they exist, and have given too much importance to single measurements of oxygen level in samples of water, not realising that in a few hours far more or far less of the gas may be available.

The capacity of organic pollution to deoxygenate water is enormous. The sewage produced by a single human being gives rise to a daily oxygen demand of 115 gms. (¹/₄ lb.). This represents the total amount of oxygen dissolved in 10,000 litres (over 2,000 gallons) if the water is saturated. In most rivers where sewage is discharged the water, before contamination, is usually far from saturation, so an even greater volume may be affected. Some industrial wastes have much greater effects. For instance it has been calculated that the oxygen demand created by the manufacture of a ton of strawboard corresponds to the sewage output of 1,690 persons, so it could deoxygenate some 17,000,000 litres (nearly 4,000,000 gallons) of oxygen-saturated water daily. These figures are somewhat academic, as they do not allow for the considerable amount of oxygen which dissolves into moving water from the atmosphere. Were it not for this important factor almost any river contaminated with any appreciable amount of organic matter would remain completely deoxygenated; deep lakes, with little water movement, become “purified” much more slowly, and severe pollution can have permanent effects.

There is little doubt that the Thames, formerly an excellent salmon river, reached a peak of pollution, and complete deoxygenation, during the nineteenth century. It was almost entirely due to untreated sewage produced by the human population that this disgusting condition was produced. This is not surprising. The flow of the river may be as low as 200,000,000 gallons a day. The water entering the London area is already depleted of oxygen, and as it is slow-moving only relatively small amounts of further oxygen go into solution. The sewage from a population of 100,000 people would, if the water were originally saturated and if no oxygen were added (and these two factors tend to cancel out), produce complete deoxygenation. It is no wonder that much of the sewage remained undecomposed for days, carried backwards and forwards through the city by the ebbing and flowing tide. Notwithstanding the increased population of to-day the situation, through improved methods of sewage treatment, is in fact considerably improved, at least from the aesthetic, and hygienic, point of view, but the water is still frequently completely or almost completely devoid of oxygen and the fauna and flora are of the kind resistant to such conditions. Pollution is now due not only to (treated) sewage effluent, but also to a great deal of industrial waste, which presents many problems mentioned below.

Many methods have been suggested for dealing with sewage. Ideally it should be returned to the land as fertiliser; if all the salts which we pour down the drains and, eventually, into the sea could be recovered, they would replace the greater part of our imports of chemical fertilisers and might replace them in a more desirable form. Various methods of composting sewage have been devised, and successfully adopted in a few places. In China agriculture in many areas depends on the use of human excreta as manure. The main difficulty is that unless carefully done, the composting process may not kill parasitic worms and other pathogenic organisms, and the compost may be a danger to health. Nevertheless I think that eventually these problems may be solved to the benefit of our rivers and our agriculture.

At one time “sewage farms” were commonly developed. The raw effluent was run into channels and allowed to percolate into the ground. Excellent vegetables were grown on ridges between the channels. Where large areas of well drained soil were available, with no rapid percolation into the water supplies, this was a reasonably safe method, and the material was broken down by the soil bacteria in a fairly short time. An optimum addition of sewage gave maximum fertility and no serious pollution, though parasitic worms and pathogenic bacteria often fouled the vegetables which therefore needed careful cooking. However, there is an upper limit to the amount of material which can be treated in any area as over-treatment overwhelms the bacterial fauna and disgusting conditions result. As suitable ground is becoming less easily available, this method has been largely abandoned.

To-day most urban waste is dealt with before being discharged into rivers, though quite a lot of raw sewage is still run directly into the sea and into tidal estuaries. This latter procedure has in recent years been the subject of much justifiable criticism, as it has been a cause of severe health hazards as well as aesthetic unpleasantness; nevertheless it has probably contributed to the richness of the flora and fauna on the shore near to several popular seaside resorts. The usual methods of sewage treatment depend essentially on oxidation by aerobic organisms. The most widely used system includes filtration through trickling filters, which are the circular structures seen in most sewage works. They are made of clinker or broken stones, and the fluid trickles slowly through them, leaving the interstices full of air. It takes some months for a filter bed to reach its maximum efficiency. It becomes covered with many different micro-organisms which feed on, and so remove, most of the organic matter. The filter is prevented from quickly becoming clogged by their growth because insect larvae and worms also develop in large numbers and feed on the micro-organisms. Another system of sewage treatment is the active sludge process. In this the sewage is run into tanks. These are inoculated with the sludge from a previous batch (to make sure the correct micro-organisms are present) and the whole is kept stirred to ensure aeration. The organic matter is broken down as in the filters. A clear effluent, and “sewage sludge” which is dried and may be sold as a fertiliser, is produced.

These methods of sewage treatment, supplemented by filtration through sand in some cases, are remarkably successful. The greater part of the flow of some of our rivers is in fact treated sewage effluent. It is sometimes 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 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 and industrial effluents are discharged into our rivers, these will cease to have their “natural” fauna and flora, even if the more unpleasant symptoms of pollution are no longer tolerated. If no effluent were to be discharged, under present conditions, many of our river beds would be dry for most of the year. The policy of drawing water supplies from the lowest reaches of the rivers instead of from the cleaner streams above the towns is one which appeals to the conservationist more than it did to most water engineers in the past, though changes in policy now bring these different organisations closer together. Certain river authorities now wish to use their upland reservoirs for storage only, discharging them into the rivers which act as channels to bring the water to the lowland towns. This means that the catchment areas can be used for recreation and agriculture, as pollution is less important when the water, abstracted further down, must be purified anyhow.

Fig. 5 Chemical structure of “hard” and “soft” detergents.

A: TBS (hard)

B: Dobane PT (hard)

C: Dobane JN sulphonate (soft)

D: Sodium alkane sulphonate (soft)

If sea-water can be economically desalinated, the pressure on our fresh-water supplies will be eased, and further improvement of their purity will be possible. It may be difficult fully to restore conditions in lakes which have become polluted, and every effort should be made to prevent the discharge of even the “cleanest” effluent into such places. So far comparatively few species of animals or plants have been totally exterminated from fresh water in Britain, and streams which have been cleaned up have usually been recolonised with the appropriate forms. This may not happen in the future unless every effort is made to render our fresh waters not only safe but also pure.