264 BACTERIOLOGICAL EXAMINATIONS 



varies in many ways, according to the amount of pollution, or albu- 

 minous matter in the water, while depth, and the swiftness with 

 which it flows are conditions that modify bacterial contents. The 

 water in a reservoir becomes almost free from bacteria during the 

 first ten days. The number of bacteria diminishes 10 percent per 

 day for the first five or eight days, due no doubt to gravitation 

 of the bacteria to the bottom, also in part to the action of light, 

 which plays an important role in the destruction of the bacteria 

 of water supplies. 



In general, water containing less than 100 bacteria per i c.c. 

 is considered to be from a deep source, and uncontaminated by 

 drainage. Deep artesian wells often contain but from 5 to 15 

 bacteria per cubic centimeter, water from rivers often contain 

 12,000 or 20,000, depending somewhat upon the season of the 

 year. Rains cause an augmentation of the bacterial content. 

 Summer causes a diminution. 



In identifying a certain water supply as the cause of an epidemic 

 of typhoid, the number of bacteria is of great value in locating the 

 place of infection. 



The efficiency of filters in large municipal water supplies is only 

 known by the bacterial content of the effluent. In good sand and 

 mechanical (alum) filters, the reduction in the number of bacteria 

 is often over 95 percent (sometimes 99 percent). Plate cultures 

 should be made daily from every filter in order to determine how 

 each filter is performing. Sand filters should not filter more than 

 1,000,000 gallons per acre a day. They should be at least one 

 metre thick; the upper half inch of the sand performs over 90 percent 

 of the filtration, due to a certain zooglea, or growth of bacteria. 

 Cracks, or imperfections in the filtei beds are quickly detected by 

 the rapid increase of the number of the bacteria in the effluent. 

 It is supposed that not only are bacteria filtered by the sand but 

 that destructive changes occur in the filter which greatly diminish 

 the number of bacteria. A filter must be used for a few days 

 before it becomes efficient or "ripe." After a time it becomes 



