its former status. Sometimes its recovery is almost complete; more often it is only partial. If the stream flows slowly, the suspended matter tends to settle. This lets in the light; algae and higher aquatic plants grow again and oxygen is put into the water. If the stream flows rapidly, much oxygen is absorbed from the atmosphere. With dissolved oxygen again available, animal forms return and proceed to eat up bacteria and fragments of dead organic matter. Finally after a sufficient time or a sufficient distance of flow restoration becomes almost complete. It is the complex action of these various restorative processes which result in what is called self-purification. Partial views of self-purification If one considers a stream from the standpoint of any one factor alone, false impressions are likely to be obtained. Fish may disappear from a certain reach of a river and be found at lower points; this may indicate self-purification, or it may not. The color of the water or its turbidity may clear below a polluted zone, yet dissolved oxygen may still be lacking. Tests for dissolved oxygen may show a recovery and yet the bacteria in the water may still be high and the water far from clear. Most dangerous of all conceptions is that which assumes that the processes of self-purification eliminate pathogenic bacteria from water and that because a degraded water has restored itself to apparent normalcy therefore the safety of the water from the standpoint of infection has been restored. To a certain extent it may be true, but it is not necessarily true. Pathogenic bacteria, such as the typhoid bacillus, settle in water as other bacteria do; they are killed. by exposure to sunlight near the surface of water; they may be eaten by protozoa as other bacteria are eaten. But they are essentially parasitic and not saprophytic organisms; they do not live upon dead organic matter; they do not multiply in water; but they do remain alive some of them-for a considerable time. It was with` these thoughts in mind that British bacteriologists a generation ago set up the dogma that "There are no rivers in England long enough to purify themselves." If they had said that all the rivers reach the sea before pathogenic organisms present have had time to disappear it would have been a fairer statement and one which would not have cast aspersions upon "self-purification." Other sanitarians, thinking only of bacteria, have said that it is standing water rather than running water which purifies itself. This also is a statement made from a partial conception of the phenomenon. Self-purification should not be regarded as one solely of bacteriological significance. Devitalization of pathogenic bacteria The most important element in the elimination of pathogenic bacteria which have gained entrance to a body of water is time. Like all organisms, bacteria "have their day and cease to be." Give them time and put them in such an unfavorable environment as water, typhoid bacilli, cholera spirillae, and the like will die. Their longevity is influenced by various factors, the temperature of the water and its density; their condition, whether present as single individuals or lumped in masses; exposure to sunlight; sedimentation in quiet water; the existence of bacterivorous protozoa; and so on. It is well known that they live longer in winter than in summer. But almost always they tend to disappear in accordance with the "die away curve;" their rate of decrease remains fairly constant for a given set of conditions, this rate being in round numbers about 20 to 40 per cent per day. Thus an infected water "purifies itself" in the course of time. Distance of flow is a factor in so far as it influences time and gives opportunity for the devitalizing influences to work. Storage of water in a lake or reservoir is especially important in this respect. The actual time of storage of particular masses of water is often different from the "nominal storage," i.e., the volume of water stored divided by the rate of flow through the reservoir, because of short circuiting. Even in a very large reservoir the wind will sometimes drive surface water across it in less than a day. Surface currents under the ice often rapidly flow through a reservoir from inlet to outlet without mixing with the water beneath. Dilution As rivers flow they gain in volume by the addition of tributary streams or the inflow of ground water. Polluting liquids which enter a stream thus tend to be progressively diluted and the bacteria and other substances become dispersed through larger and larger volumes. The effectiveness of dilution depends not only upon the in creased volume of water, but upon conditions which produce speedy or slow mixing. A hot liquid entering a body of cold water near the surface tends to remain near the top because it is lighter, while if it enters near the bottom it tends to rise and, as it rises, to become mixed with the water. Sewage entering a river which is flowing in a straight course tends to follow in lines of parallel flow without cross-mixing, but if the river curves cross-mixing takes place. Meandering streams thus become well mixed even though their velocities are low. Changes in cross section, rapids, stones and other obstructions tend to induce mixing. Acquisitions of diluting water may bring with them dissolved oxygen or bacterivorous protozoa. Practically speaking, therefore, dilution and dispersion may be regarded as factors in the general process of self-purification. Sedimentation Gravity plays an important part in self-purification, as it causes suspended matter heavier than water to settle. It is counteracted by currents and by the micro-dynamic forces that act at the surface of the suspended particles. There is friction between the particles and the water which resists settling. The greater the specific surface, i.e., the ratio of the surface to the volume of a particle, the greater the friction and the slower the rate of settling. The hydraulic subsiding value of spherical particles having the specific gravity common to quartz sand (2.65) has been estimated to be as follows in pure still water at 50°F. It is obvious from these figures that water which contains particles finer than silt will not become clear by sedimentation even when the particles are relatively heavy and the water is perfectly still, unless abundant time be given. As the specific gravity of the particles decreases, the hydraulic subsiding value decreases and the time of settling increases. Clarification does occur in lakes and reservoirs and may occur in long, slowly moving rivers. Rapidly flowing streams do not clear themselves by sedimentation. The carrying power of water as measured by the velocities that will move solid particles on the stream bottom approaches the following values. Obstructions in a stream, such as rocks, and vegetation which reduce velocities or afford places where water is retained in eddies, favor sedimentation. Density of the water affects sedimentation. Suspended matter settles more slowly in salt water than in fresh water and more slowly in cold water than in warm water. It is increased viscosity more than increased density which is involved in the matter. The viscosity of water near the freezing point is nearly twice as great as that at 70°F; consequently settling velocities are much greater in summer than in winter. Suspended matter lighter than water tends to rise to the surface. Bacteria have about the same specific gravity as water. Organisms, which produce oxygen by photo-synthesis, or carbonic acid by respiration, more rapidly than these gases can be dissolved by the water, are often buoyed up by gas bubbles and carried to the surface. In lakes diurnal migration of organisms occurs from this cause, the distances traversed being many vertical feet. Aggregation and coagulation For various reasons suspended particles may become attached to each other to form aggregates of larger size which settle more readily than the individual particles. Some of the microscopic organisms have sticky gelatinous coatings, and this is also true of bacteria. Sweeping through the water, sticky particles may gather finer particles and thus the water may be clarified. Chemical reactions between organic or inorganic substances may occur and natural coagulants may be formed, as where alkaline waters and acid iron waters from different tributaries meet. Matter suspended in fresh water is coagulated on entering salt water and settles more rapidly. The coagulating processes are more potent and more frequent than is generally realized. Colloidal particles. oppositely charged with electricity tend to unite, lose their charges and settle. Aeration is sometimes a factor in these electrical phenomena. Light Light is a powerful agent in the self-purification of streams. Mention has already been made of the effect of the solar energy in causing photo-synthesis. But it does more than this. It kills bacteria and it causes physico-chemical changes in organic matter. Colored waters exposed to the light are bleached that is, their color is reduced. It hastens other natural coagulations. The solar rays are effective only as they enter the water. Even in clear waters they are rapidly absorbed and their energy decreases logarithmically with depth. In colored waters and especially in turbid waters decrease is very rapid. Algae growths in reservoirs stimulated by sunlight make the water turbid and tend to shut out the light and thus hasten their own end. Aeration Aeration occurs naturally when water flows over riffles or breaks into droplets as it falls over dams or ledges; it also occurs when water is exposed in thin films to the air or is blown by the wind into waves or churned by tidal action. Any exposure of the surface of water to the air induces aeration and the greater the surface exposed the more rapid the action. Aeration gives opportunity for an adjustment of the gas pressures in air and the amounts soluble in water. If water containing less dissolved oxygen than it can hold under its condition of temperature and pressure is exposed to the air, it will tend to absorb more oxygen. If it contains more than its natural quota, it will tend to lose oxygen. The rates of absorption and liberation are subject to known laws, but are complicated and diffi |