364 SEWAGE AND SEWAGE DISPOSAL 



tained between animal and plant life, offensive conditions do not 

 result, but where abnormal conditions are produced, as, for example, 

 by the discharge of excessive quantities of sewage or trade wastes 

 into a stream, a depletion of the dissolved oxygen may follow, or 

 there may be an overproduction of alga? so that the conditions 

 become offensive. It is coming to be realized that in order to 

 properly determine the dilution required in any particular case 

 the conditions required to bring about this condition of biologica) 

 equilibrium must be determined." 



Pathogenic Bacteria. Owing to the origin and nature of sewage 

 it may at any time contain pathogenic bacteria. The ones with 

 which the sanitarian is most concerned are the typhoid, cholera 

 and dysenteria, but it is always possible for many other species 

 to find their way into sewage and from it cause infection. This 

 is especially true of B. tuberculosis which is quite resistant to putre- 

 faction. With the exception of the cholera and dysentery organ- 

 isms, there is no evidence that they ever multiply in sewage, and 

 they produce no appreciable change in its composition. 



The majority of the pathogens soon die in sewage. The results 

 as reported by Whipple for typhoid bacilli are given in Fig. 44. 



The speed with which the typhoid bacilli disappear from water 

 varies with the vitality of the organism, the temperature of the 

 water, the organic matter, and the bacterial antagonism exerted 

 by other organisms. Typhoid bacilli seem to die more quickly in 

 sewage than in fairly pure water, probably because of the great 

 bacterial antagonism existing. Furthermore, the absence of 

 oxygen probably plays an important part, as Whipple found oxygen 

 necessary for longevity of typhoid bacilli. Jordan thus summarizes 

 our present knowledge of the longevity of typhoid bacilli: 



"Laboratory experiments have shown that the typhoid bacillus 

 can survive in sterile water in glass vessels for upward of three 

 months, and for possibly two or three weeks in unsterilized ground 

 or surface water. Other evidence indicates that the bacillus is 

 able to travel in water a distance of at least 140 kms., and to retain 

 its vitality in natural bodies of water for at least four or five days. 

 It is possible that water may continue to be the vehicle of infection 

 during a much longer period, but the available data point to a 

 comparatively short duration of life of the specific germ in the 

 water of flowing streams. Under ordinary conditions no multi- 

 plication of the typhoid bacillus takes place in water, even when 

 a considerable amount of organic matter is present, but, on the 

 contrary, a steady decline in numbers goes on. The history of 

 typhoid epidemics tends to show that sewage pollution is to be 

 feared chiefly when the sewage is fresh, and that the danger of 

 infection diminishes progressively with the lapse of time. 



"In soil in the fecal matter of privy vaults the duration of life 



