Sewage Contamination 73 



place, the sedimentation process carries down large 

 numbers of bacteria which have become entangled in 

 the organic matter. 



The advantages of sedimentation in reducing the 

 number of bacteria in water have long been recognized 

 in municipal sanitation. Many towns and cities employ- 

 either settling basins alone, or settling basins in connec- 

 tion with filter beds for the purification of their drinking- 

 water. The efficacy of sedimentation in removing the 

 bacteria from water has been demonstrated by careful 

 investigation. 



Dilution. — The influence of sedimentation and of 

 the other factors in reducing the number of bacteria in 

 water, is reinforced by that of the dilution. The water 

 of any river or lake containing large numbers of bacteria, 

 shows a relative decrease of the latter when pure water 

 is added to it from any source. Under actual conditions, 

 the purer water of tributaries frequently serves to reduce, 

 to a very considerable extent, the relative number of 

 bacteria in large rivers. 



The contamination of drinking-water by sewage. — 

 The vast number of bacteria in sewage, averaging, at 

 times, more than 10,000,000 per cubic centimeter, is 

 frequently a direct source of increase in the number of 

 bacteria in drinking-water. Sewage-polluted streams 

 and other bodies of water show a high bacterial content 

 for the double reason that they are supplied with both 

 the bacteria and the organic matter serving as food for 

 the latter. However, the mere increase of bacteria in 

 water used for drinking purposes cannot be accepted as 

 an infallible proof of sewage contamination, for it has 



