10 



ELEMENTS OF WATER BACTERIOLOGY 



Effect of Storage upon Bacteria in Water. In stand- 

 ing waters all the tendencies which make for the reduc- 

 tion of bacteria are intensified, and when a river 

 passes into a natural or artificial reservoir a notable 

 reduction in numbers occurs. The table below shows 

 the striking effect produced upon the water of the 

 Potomac River by its successive passage through the 

 three reservoirs of the Washington water supply in 

 the first nine months of 1907. We owe these figures 

 to the courtesy of Mr. F. F. Longley, the engineer then 

 in charge of the Washington filter plant. 



REDUCTION OF BACTERIA IN WASHINGTON RESERVOIRS. 

 BACTERIA PER C.C., MONTHLY AVERAGE, 1907 



The still more striking results obtained at London 

 are indicated in the table on page n. 



When the water which enters a pond or a reservoir 

 has already undergone considerable storage and reached 

 a comparatively stable condition, the diminution due to 

 additional storage may be almost negligible. Thus 

 Philbrick (1905) found that the influent water of the 

 Chestnut Hill Reservoir of the Metropolitan Water 



