CHAPTER V OBJECTIONABLE LIMITS OF VARIOUS SUBSTANCES AND THEIR RELATION TO PRACTICAL PROBLEMS Prior to about 1890 attempt was made to judge the sanitary quality of water chiefly on its chemical analysis with more or less consideration to conditions at the source. With the development of bacteriological methods, however, the chemical analyses, except for comparative purposes, have received less and less attention. The chemical composition of water is subject to the effects of so many varying factors which have no sanitary significance whatever, that it is difficult, without making an extensive series of tests at fairly frequent intervals, to say what is the normal chemical content for water from a given region. The substances determined and reported in the sanitary chemical water analyses are of themselves harmless even in high concentrations. Since it was recognized long before the development of modern bacteriology that the pollution of water with human excreta was one of the most menacing sources of disease, chemists attempted to devise tests by which it would be possible to detect such pollution. A brief description of the substances determined in the sanitary chemical analysis and their significance are given below. The "oxygen consumed" test is a measure of the oxidizable matter contained in water and indicates the presence of certain forms of organic material or other oxidizable substances. The test serves to compare the relative amounts of carbonaceous organic matter. or other oxidizable constituents in samples of water from various sources. Since fecal matter is known to be composed of that class of biological substance known as protein and characteristically to contain nitrogen, the sanitary chemical water analysis includes the determination of four different forms of nitrogen-as albuminoid ammonia (or Kjeldahl nitrogen), as free ammonia, as nitrites and as nitrates. The relative amount of these substances found in water indicates in a comparative way the quality of the samples analyzed. The former indicate crude organic matter, nitrates, completely mineralized matter; and free ammonia and nitrites, intermediate steps in the cycle of nitrogen changes. Albuminoid nitrogen includes the nitrogen which is still a part of the protein-like organic substance. It is the nitrogenous organic matter of either animal or vegetable origin in an undecomposed condition. Free ammonia nitrogen is nitrogen which has undergone the first stages of mineralization; that is, it is no longer combined with organic substances. The next step in mineralization produces nitrite. Since nitrites are rather unstable, they are not ordinarily found except where an active chemical change is going on. This form of nitrogen is in fact the transitional stage of the conversion of the complex organic nitrogen into the fully mineralized form. Nitrates represent the final stage of oxidation of nitrogen in the completely mineralized form. The nitrate content of a water indicates somewhat its past history, and serves as an index of the so-called "remote pollution." Nitrates in ground water or from deep sources are frequently excessive due to mineral deposits and under these conditions have no sanitary significance. Chlorides are a measure of the chloride salts of which sodium chloride, ordinary table salt, is by far the major constituent. This substance is perfectly harmless in itself, but since sewage contains relatively large amounts of salt, pollution may be detected by the abnormally high chloride content of a water. Interpretation of the sanitary significance of chlorides in water depends, therefore, upon local conditions and the quantity of chloride present in the sample. In sections where the normal chloride content of the water is high, due to saline deposits in the ground or to industrial wastes, the chloride content as a sanitary factor is to be excluded or discounted. The turbidity is a measure of the physical condition of the water indicating the presence of suspended solids of an organic or inorganic nature. Waters containing a large amount of turbidity are unsuitable for domestic use and for many industrial processes. The total solids represent the total amount of matter carried by the water, as determined by evaporation. There is no definite limit as to the amount of solids that may be present in the water which determines its potability. The alkalinity of water is a measure of the salts that neutralize acids. These may be present in a number of different forms, but the ones generally encountered in waters are carbonates, bi-carbonates and hydrates. Except under unusual conditions, the alkalinity of a water has no sanitary significance. The hardness of water is a measure of its soap consuming power. This phenomenon is caused by the presence of certain soluble constituents in the water, of which calcium and magnesium are the most important. |