BACTERIOLOGY OF SEWAGE 163 



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 

 examine the bacterial content of sewage, especially in connection 

 with the efficiency of purification works. The main lines of 

 inquiry are here the same as for water, and the general methods 

 are identical, the only modification necessary being that, in 

 consequence of the high bacterial content, much smaller 

 quantities of the raw material must be worked with. With 

 regard to the numbers of bacteria in sewage, these may run 

 from a million to ten millions or even more per c.c., and here 

 of course the question of the presence of intestinal organisms 

 of the coli group is of great importance. The numbers of these 

 are large, and members of the group may be detected in a 

 000001 c.c. or less. The numbers present are frequently 

 considerably reduced by purification methods, but it is to be 

 noted that, even when such methods are most successful, b. coli 

 may yet be present in considerable quantities. This is especially 

 true in Britain, where sewage is much more concentrated than 

 it apparently is in America. In the latter country, purification 

 may yield effluents in which b. coli can be detected in only 

 '001 c.c. By no purification method has the production of a 

 potable water been attained, and the high content of effluents 

 in b. coli makes the passage of typhoid bacilli through a purifica- 

 tion system possible although the organism has perhaps never 

 been certainly demonstrated. 



The part which bacteria play in the purification of sewage 

 constitutes a question of great interest, to which much attention 

 has been directed. The methods adopted for sewage purification 

 may be divided into two groups. In the first of these, the 

 sewage coming from the mains is run on to a bed of gravel, 

 clinker, or coke, on which it is allowed to stand for some hours. 

 The effluent is then run out through the bottom of the bed, 

 which is then allowed to rest for some hours before being 

 recharged. In a modification of this method the sewage is 



