Purification of Streams 81 



of bacteria capable of growing in the absence of air, and 

 is destroyed. The bubbles of gas which rise to the sur- 

 face testify to this. 



The decomposition of the organic matter in the water 

 and in the mud at the bottom of rivers, lakes and ponds, 

 as well as the destruction of the disease germs introduced 

 with the sewage cannot, however, be depended upon to 

 purify the water so as to make it absolutely safe. The 

 typhoid germs may be destroyed in a few hours or in a 

 few days in sewage-polluted waters, as numerous in- 

 vestigations have clearly demonstrated; yet, under 

 other conditions, they may survive for weeks, or even 

 months, and there is no assurance that twenty, thirty, 

 or even fifty miles of flow would be sufficient for their 

 removal under all circumstances. There is a general 

 agreement on this point among sanitarians and for this 

 reason the practice of pumping sewage-polluted water 

 directly in the city mains is strongly discouraged. 



The storing and filtration of river-water. — When brought 

 to rest in large reservoirs, river-water is subject to the 

 sedimentation process as it is in rivers, lakes or ponds. 

 The suspended particles in such stored waters settle 

 out gradually. Not only is there a subsidence of the 

 suspended particles, but, also, a partial decomposition 

 of the organic matter in solution, and a very marked 

 general reduction in the number of bacteria. 



The reservoirs for unfiltered water, or settling basins, 

 as they are often called, appear thus to be a very im- 

 portant means for the purification of drinking-water. 

 The settling basins of the city of Washington, which, 

 before the installation of the filtration plant, were the 



