BACTERIOLOGY OF SEWAGE 163 



of chemical analysis and with the inspection of the locality. 

 The difficulty is greatest when dealing with water derived from 

 sewage-contaminated rivers, from agricultural land, and from 

 surface wells. With regard to the first two sources, the water 

 should never be used in an unfiltered condition, and with regard 

 to the last, every case must be considered on its own merits. It 

 may be said that under ordinary circumstances an inspection 

 of the surroundings and an unfavourable chemical analysis are 

 sufficient to condemn a water, even if a bacteriological examina- 

 tion showed the absence of b. coli in large samples ; and further, 

 if in a suspicious locality the bacteriological analysis yielded a 

 bad result, the water ought to be condemned even if from the 

 chemical analysis it could be passed. No principle of general 

 application can be laid down as to what in such circum- 

 stances is to be looked on as a bad result. It is probably, 

 however, safe to say that when excremental organisms are 

 found in 10 c.c. or less of the water it is unsafe for human 

 consumption. 



The examination for the presence of b. coli finds its best 

 application in determining the efficiency of a filtration process, 

 and here it is extraordinarily delicate. While again it is difficult 

 to lay down a standard of purity, the filtration methods in use 

 are, if properly worked, capable of delivering an effluent which 

 does not yield b. coli in amounts less than 100 c.c., and such 

 a degree of efficiency should in all cases be aimed at. 



In connection with the derivation of b. coli from animal 

 sources, it may be stated that birds, especially gulls, may by 

 defiling themselves with garbage act as carriers of human 

 excremental bacteria to water reservoirs. 



As the b. coli is fairly widespread in nature, Klein and 

 Houston hold that valuable supporting evidence is found in 

 the presence of the b. enteritidis sporogenes and of strepto- 

 cocci, both of which are probably constant inhabitants of the 

 human intestine. The spores of the former usually number 

 100 per c.c. in sewage, and the presence of the latter can always 

 be recognised in '001 grm. of human faeces. The deductions 

 to be drawn from the presence of these in water are the same 

 as those to be drawn from their presence in soil. 



It may be said that in water artificially polluted with sewage 

 containing intestinal bacteria, these can be detected by bacterio- 

 logical methods in mixtures from ten to a hundred times more 

 dilute than those in which the pollution can be detected by 

 purely chemical methods. 



Bacteriology of Sewage. It is sometimes necessary to 



