164 BACTERIA IN WATER 



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 

 '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. 



. 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 allowed to rest for some hours before being 

 recharged. In a modification of this method the sewage is 

 allowed to percolate slowly through a bed consisting of large 

 porous objects, such as broken bricks or large pieces of coke, and 

 here the percolation may be constant, no interval of rest being 

 given. The bacterial processes which take place in these two 

 methods are, however, probably closely similar. In the second, 

 the essential feature is a preliminary treatment of the 

 sewage in more or less closed tanks ("septic tanks"), where 

 the conditions are supposed to be largely anaerobic. This 

 method has been adopted at Exeter, Sutton, and Yeovil in this 

 country, and very fully worked at in America by the State 

 Board of Health of Massachusetts. In the explanation given 

 of the rationale of this process, sewage is looked on as exist- 

 ing in three stages. (1) First of all, fresh sewage the newly 



