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
