204 MICROBIOLOGY OF WATER AND SEWAGE. 



The number of organisms which develop on beef peptone aga 

 incubated at blood heat, commonly termed the "agar" or "blood-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 eliminates 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 

 10 per c.c. ; and for surface waters, not over 100 per c.c. 



QUALITATIVE STANDARDS. The isolation and identification of 

 specific disease organisms, such as typhoid and cholera microbes from 

 water, is sufficient 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 pres- 

 ence of the colon bacillus (B. coli) in small amounts of water, is generally 

 looked upon as significant and indicative of sewage pollution. The 

 technical methods used in this isolation and numeration are many, and 

 may be found in the works cited; but there is considerable difference 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 100 c.c. in the case of water 

 from deep wells and springs, and should be absent from 10 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 indicator 

 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 1,000 c.c. of water from deep wells, nor in 100 c.c. from sur- 

 face 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 control over 

 the water supply increases in importance. Very few towns and cities 

 are fortunate enough to obtain their water supply from an unpolluted 



