It was the great bacteriologist Koch who first demonstrated that filtered water would prevent water-borne epidemics. In 1893 he was asked to determine why, when both Hamburg and Altona drank the waters of the river Elbe, the former had a cholera outbreak while the latter remained free from it. Koch was not content with pointing out the fact that Altona filtered its water while Hamburg did not. He first isolated the cholera microbe from the Elbe using a compound microscope. Then he showed it to be present in the water mains and taps of Hamburg, but absent in those of Altona. To clinch the argument, he also showed that the microbe that was examined under a compound microscope, though absent from the filtered Altona water, was present in its unfiltered water.
There are two ways of determining the purity of a water supply. One is chemical, the other bacteriological. But even the chemical tests are in the last analysis of significance only because it is known that some of the microbes found in human and animal excreta produce ammonia and urea as end products of their protein putrefaction. The nitrifying bacteria of water into nitrites in turn change such urea and ammonia. Later other microbes change the nitrites to nitrates, producing chlorides as by-products.
In the chemical test the water sample first is tested for albuminoid ammonia by boiling it with alkaline permanganate pf potash. When the latter comes into contact with organic nitrogenous matter such as is if Quad-in sewage, it forms-the substance called albuminoid ammonia and indicates that human or animal sewage, recently has polluted the water. The absence of albuminoid ammonia from a water sample does not rule out, however, the possibility of an older pollution; for, by the time the water is tested, the ammonias and ureas may already have been changed to nitrites and nitrates. So a test must be made for these also. But it may happen that the pollution is so old or so distant that no abnormal amount even of nitrates can be detected. In that case, another test is made this time for chlorides, which are the by-products of the change into nitrites and nitrates and do not disappear as quickly. If chlorides are present in unusual amounts it is considered presumptive scientific evidence that animal nitrogen in the form of sewage have reached the water.
It is true that both chemical and bacteriological routine water examinations are mostly quantitative, and when positive are only presumptive evidence. It is therefore necessary to know what may be considered a safe proportion, both of organic nitrogenous material and saprophytes, per volume of water taken from different points in the watershed. Such examinations are part of the regular routine of any water department laboratory. A proportion of more than one part of nitrites to one hundred million parts of water is a matter of suspicion. Or if more than an occasional colony of the bacteria Bacillus coli aerogenes develops in one out of five cultures each containing 10 c.c. of water when examined under compound microscopes, pollution is in evidence.
In the detection of water pollution, bacteriological methods are more accurate, in some respects, than chemical, and this despite certain difficulties. While many kinds of bacterial pathogens, for instance, may be expelled with human solid and fluid excreta, it is frequently almost impossible to isolate them in the laboratory using compound microscopes. Moreover, the typhoid bacillus, for example, dies quickly in water, though often not quickly enough to prevent the transmission of disease. Polluted water, indeed, may have caused disease, and yet by the time a sample of it reaches the laboratory it may have become free of the microbe. Despite such disadvantages, however, the bacteriological method is usually superior to the chemical.
By way of illustrating the point, let us consider the Bacillus typhosus, the active agent in typhoid fever. It has close relatives to be found in the same sewage, which do not cause disease. These are the group of colon bacilli; and they are much hardier and much more easily isolated than Bacillus typhosus when both are examined under a compound microscope. Thus, instead of searching for typhoid fever bacilli (since their probable absence does not disprove their former presence, we search for colon bacilli. If we find them in unusual numbers, it indicates that the water supply has been polluted by sewage.
What constitutes unusual numbers is a quantity varying with the season, the recent rainfall and snowfall, the nature of the watershed, and many more. Nevertheless, many bacteriologists look askance at water, which, when observed under a compound microscope, has as much as one thousand miscellaneous saprophytes per milliliter Others go further and distrust a water having half that number, and are willing to approve only such water as yields not more than one hundred insignificant microbes in fifteen drops.
When in 1854 Dr. Ward and the Rev. Whitehead in England, and later Pettenkofer on the mainland, came to the conclusion that sewage might possibly be a source of disease, they did not specify the bacterial pathogens as the enemy, for as yet these had not been discovered. Instead, they blamed the complex substances, which form in sewage and which subsequently, may be washed into the drinking water. That they were only partly correct in their surmise is due to the fact that they antedated Pasteur and his discoveries. We know now that cholera, typhoid fever, dysentery, is caused by specific pathogenic microbes, and not by the complex chemical by-products of sewage, although these may prove harmful, of course, in other ways.
Before great populations crowded into small areas, it perhaps was less dangerous to let the sewage drain into adjoining streams, lakes, and rivers; for the enormous dilution reduced much of the hazard. Nevertheless, in view of the epidemics that constantly harassed our ancestors, we may doubt whether such a practice was not always highly dangerous.
