EXAMINATION OF WATER AND SEWAGE 75 



Relative Purity. An arbitrary standard of relative purity is 

 almost impossible to fix. Several hundred bacteria per cubic centi- 

 meter might be normal in a river water, whereas the same number 

 found in well water would immediately arouse suspicion. Accord- 

 ing to certain authorities water containing less than 100 bacteria 

 per c.c. is presumably uncontaminated by surface drainage, one 

 with 500 bacteria per c.c. is open to suspicion, one with 1000 

 per c.c. is presumably contaminated by sewage or surface drainage. 

 A practical classification from a sanitary point of view is as follows : 

 (1) good, as determined by bacteriological and chemical analyses, 

 physical inspection, and a sanitary survey of the watershed ; (2) 

 contaminated, if organic waste of either animal or vegetable origin 

 be present (a contaminated water is suspicious but not necessarily 

 dangerous) ; (3) infected, if the water contains specific organisms 

 causing disease. 



Significance of the Presence of Colon Bacilli. Enumeration 

 of bacteria gives an approximate idea of the degree of pollution 

 with organic material, but it gives no idea of the kind of bacteria 

 present. Unfortunately there is no reliable method whereby 

 pathogenic organisms such as the typhoid and dysentery bacillus 

 can be isolated from water with any degree of certainty, even 

 though it is actually known that the organisms are or have been 

 present because of cases of disease which have developed from 

 drinking it. The bacteria may be present in such small numbers 

 that though an ordinary tumbler of water might contain sufficient 

 to cause infection it would be a rare chance if one or two of them 

 should be in the small quantity taken for examination. Another 

 likely reason for failure is the fact that they do not live many 

 days in water. It is known, however, that certain organisms, 

 such as the colon bacilli, which are normally present in the intes- 

 tines, have a longer life in water than those which are present only 

 in diseased conditions. Moreover, it is practically sure that all 

 the pathogenic organisms which give rise to water-borne infections 

 find their way into the water supply by means, of intestinal dis- 

 charges from human beings. Under these conditions it is practi- 

 cally safe to assume that the absence of the colon bacilli in water 



