266 MICROBIOLOGY OF WATER AND SEWAGE 



heat" count, is perhaps more important than the gelatin count, as 

 many water bacteria do not grow at blood heat, whereas sewage and 

 soil organisms grow readily at- this temperature. The agar count 

 eUminates the water flora, but obscures the sanitary results by reason 

 of the presence of soil bacteria. For deep waters, the agar count 

 should generally not exceed lo per c.c; and for surface waters, not 

 over loo per c.c. 



1 QuALiTATiVK STANDARDS. — The isolation and identification of 

 specifi(C disease organisms, such as tj^hoid and cholera microbes 

 from water, is suflELcient to condemn such a sample as unfit for use; 

 but, on account of many technical difficulties, it is practically impossible 

 to make such an examination. Apart from a few special cases, when 

 it may be necessary to attempt the isolation of these pathogenic 

 bacteria, the presence of the colon bacillus {B. coli) in small amounts 

 of water, is generally looked upon as significant and indicative of sew- 

 age pollution. The technical methods used in this isolation and 

 enumeration are many, and may be found in the works cited; but there 

 is considerable differenjce of opinion as to the number of B. coli which 

 should condemn a sample of water. Prescott and Winslow state 

 that if the colon bacillus is in "such abundance as to be isolated in a 

 large proportion of cases from i c.c. of water, it is reasonable proof 

 of the presence of serious pollution." Savage suggests that B. coli 

 should be absent from loo c.c. in the case of water from deep wells 

 and springs, and should be absent from lo c.c. in surface waters, such 

 as rivers used for drinking purposes, shallow wells, and upland surface 

 waters. 



The streptococcus examination is next in importance as an indi- 

 cator of sewage. Streptococci should be absent from the amounts 

 of water mentioned above for B. coli; and B. enteritidis sporogenes 

 should not be present in i,ooo c.c. of water from deep wells, nor in 

 100 c.c. from surface waters. 



Sedimentation, Filtration, and Purification of Water 



As areas become more and more thickly settled and towns and 

 cities increase in population, the problem of obtaining sanitary con- 

 trol over the water supply increases in importance. Very few towns 

 and cities are fortunate to obtain their water supply from an unpol- 



