CHAPTER VII 



BACTERIOLOGICAL EXAMINATION OF WATER AND 



SEWAGE 



NATURAL waters are usually considered in three classes accord- 

 ing to their location ; namely, rain water, ground water, and surface 

 water. 



Practically all, from whatever source, contain bacteria, the num- 

 ber and kind varying under different conditions. Rain water 

 contains comparatively few excepting the first shower, which 

 washes the air and brings down most of the floating dust particles 

 and bacteria in its fall. 



Ground waters, which include springs, shallow wells, and deep 

 artesian wells, rank next from the standpoint of bacteriological 

 purity. The water as it percolates through the soil gradually 

 leaves behind its bacterial content in the upper layers of the ground 

 and finally emerges in a spring or well almost germ free. This is 

 especially true of artesian wells and springs. Shallow wells are 

 more liable to variation. Unclean surroundings, privy vaults, 

 or barns placed in such a position that drainage may take place 

 in the direction of the well may lead to contamination of the water 

 and a consequent increase in the number of bacteria. 



Surface waters include streams, rivers, ponds, and lakes, and 

 these of all natural supplies contain the most bacteria on account 

 of the exposure to contamination to which they are subjected. 

 During heavy rains soil washed down from the banks of rivers or 

 streams supply an additional number of bacteria ; wind currents 

 and waves stirring up the bottom mud may bring up bacteria 

 that have been sedimented ; sewage and trade wastes from near-by 

 towns may add enormously to the bacterial content. 



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