54 



Elements of Water Bacteriology. 



of its type, as the Washington plant is the newest and 

 most perfectly equipped of slow sand filters. 



REMOVAL OF BACTERIA BY NATURAL SAND FILTERS 

 AND MECHANICAL FILTERS. BACTERIA PER C.C. IN 

 APPLIED WATER AND EFFLUENT. 



MONTHLY AVERAGES. 



In well-managed purification plants the bacteria in the 

 effluent are determined daily, and any deviation from the 

 normal value at once reveals disturbing factors which may 

 impair the efficiency of the process. In Prussia official 

 regulations demand such systematic examinations, and 

 prescribe 50 as the maximum number of bacteria allowable 

 in the filtered water. In the same way the condition of 

 an unpurified surface supply may be determined by daily 

 bacteriological analyses and warnings of danger issued 

 to the public, as has been done at Chicago and other 



