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"Self-purification” is the natural process or combination of natural agencies which tends to lessen the impurities of waters which have become polluted. Self-purification also means the result of such processes. The term is most often applied to streams, because it is in them that the effects are most conspicuous. It is just as logical to speak of the self-purification of lakes and reservoirs, for the natural forces involved operate in standing waters as well as in running waters, though in different degrees of magnitude. These forces are biological, chemical and physical. Each may be considered by itself and its influence studied and evaluated, but they are so closely interrelated that it is better to begin by considering the problem as a complex one of bio-chemistry or bio-physics. And inasmuch as self-purification represents a recovery from the effects of pollution it is well to consider first natural conditions, then conditions resulting from pollution, and finally the processes which tend to restore natural conditions.

Natural conditions

A natural stream of clear water flowing with moderate velocity is full of animal and vegetable life. There are organisms in the water, on the surface, on the shore, on the bottom and under the bottom. Among these organisms there are many antagonisms. The larger animals eat the smaller ones; muskrats and otter eat fish, fish eat crustacea, crustacea eat protozoa, and protozoa eat algae and bacteria. It is a continual struggle for existence in which the victory is continually shifting between one class and another. There are also relations which are mutually beneficial, especially between plants and animals. Animals, in the process of respiration, take in oxygen and give off carbonic acid. Plants in darkness do the same, but in sunlight those which contain chlorophyll take in carbonic acid and give off oxygen. This process is known as photosynthesis. By it the element carbon is taken from the inorganic state and built up into sugar and starch and plant protoplasm. In natural conditions the two processes are complementary and so well balanced that dissolved oxygen in the water does not become deficient, nor carbonic acid become excessive. There is another factor involved in this balance. Both animals and plants have excretory products and in due course die. The dead organic matter resulting from excretion and death decomposes as a result of the action of bacteria which utilize it as a food. This process also results in the liberation of carbonic acid. If dissolved oxygen is present in the water, the bacteria utilize it, thus tending to reduce its amount. After the dissolved oxygen has been exhausted, as it often is in water overcharged with dead organic matter, certain sorts of bacteria (the so-called anaerobic bacteria) are still able to live as they have the power of getting the oxygen which they need from nitrates, from ferric iron compounds, and even from organic matter itself. This process of "putrefaction” gives rise to other gases than carbonic acid, such as methane, sulphuretted hydrogen, and gases which have offensive odors.

Under normal conditions the photo-synthetic processes so balance those of respiration and decomposition that some oxygen remains dissolved in the water and putrefaction does not occur. Under these conditions the water does not have a foul appearance or give forth foul odors. This statement, of course, does not pertain to the lower or stagnant layers of standing bodies of water. A stream which has oxygen dissolved in the water, no matter how small in amount, is like the man who has a balance in his bank account; a stream which has its dissolved oxygen used up by reason of excessive animal life or decomposition is like the man who has overdrawn his bank account. A prudent man does not allow his account to approach zero, but maintains a safe margin on deposit; and in the same way it is unwise to allow the oxygen in a stream to approach too near the zero point.

Some streams are turbid, notably those in the south and middle states, which are below the limits of the glacial deposits. Turbidity prevents the sun's rays from penetrating far into the water, and consequently the amount of plant life is reduced and oxygen production by photo-synthesis is lessened. Some streams run slowly and there is little opportunity for oxygen absorpt on at the surface; other streams flow rapidly with a rippled surface or are broken into drops or spray, in which cases oxygen absorption from the air may be large. Temperature is an important factor, because the solubility of oxygen and carbonic acid decreases as the temperature rises, while most of the life activities are greater in warm water. Hence it is usually in summer that crises affecting the oxygen balance are likely to occur.

Pollution Let it be supposed, now, that sewage, or some similar polluting substance, is discharged into a natural stream in which plant and animal organisms are living the antagonistic and mutually dependent life above outlined. A whole train of events is put in action. The sewage makes the water turbid: sunlight is shut out; photo-synthesis, with its oxygen production, is lessened; green plants die and, dying, decompose; bacteria increase; as well as protozoa, crustacea, and other organisms. Meantime the stream is flowing, so that these events do not occur at one place, but in a series of zones below the point of pollution. If the sewage is discharged in large enough quantities, all of the oxygen will disappear and most of the higher animal forms as well as the green plants will die, leaving the field to certain fungi and low animal forms capable of living with almost no oxygen. At the end even these may die leaving bacteria alone in anaerobic supremacy.

Thus by successive additions of polluting substances a stream may be degraded, that is, its biological activities reduced to a lower plane of life. Degradation may be accompanied by a foul appearance due to the polluting substances themselves or to the products of the struggle for life. With putrefaction occurring it is common for bubbles of gas to rise to the surface and for foul odors to be emitted. A blackening of the appearance of the water is partly a result and partly a cause of the putrefying condition.

Sometimes degradation begins immediately below the point of pollution; sometimes it is a progressive and cumulative process. It depends upon the intensity and character of the pollution, the temperature of the water, rate of flow, depth of water, and other physical and chemical factors.

Sell-purification If a stream thus degraded is followed downstream, its condition may be seen to improve. It tends to recover itself and to regain

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