salts makes the water unpleasant to use for washing, and also for drinking, to persons who have been accustomed to softer water, although, on the other hand, persons who are accustomed to the harder waters may find the softer waters less agreeable to their taste. While it is open to question whether it would be justifiable to require the dilution of hard water by distilled water in order to keep within the limits specified herein, it would be proper to require carriers to select the local supplies which most nearly fulfill the requirements of the standards with respect to mineral content. In so far as the chemical composition of the water may cause inconvenience by its irritating effect upon the intestinal canal, or by any more serious effect upon well-being, the certifying authority will be justified in demanding that due regard be paid to the matter by common carriers. Unfortunately, it is difficult to secure reliable information concerning the physiological activity of salts as found in waters. Idiosyncracy is important. It is universally admitted, of course, that poisonous metals such as lead and copper should not be allowed in water for drinking or culinary purposes, but the difficult points are concerned with the less poisonous substances, or salts which are normally present. The effect of sulphates, especially of magnesium sulphate, is, however, well recognized, and it would be desirable to avoid the use of waters in which the concentration of these salts is sufficiently high to be annoying. Where waters are treated with chemicals in order to soften them, or to purify them in any way, it is desirable that any excess of the chemicals used shall be avoided. Caustic alkalinity from excess of lime, more than a trace of the aluminium compounds added, or free chlorine, are objectionable in the effluent from a purification plant. In general, it is considered proper to insist that the effort should be made to find waters which are as satisfactory as possible from the standpoint of chemical characteristics but with due regard to the region within which the supply must be obtained. CHAPTER VI SELF-PURIFICATION OF STREAMS, LAKES, AND RESERVOIRS "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 |