166 BACTERIA IN WATER 



allowed, it regains its adsorptive properties probably from the 

 oxidation of the material adsorbed. 



During this stage, as in the whole purification process, four, 

 and it may be five, processes are at work : First, the action of 

 ordinary bacteria splitting up the higher albuminous molecules ; 

 secondly, the action of nitrifying bacteria building up nitrites 

 and nitrates from ammoniacal products ; thirdly, the action of 

 denitrifying bacteria which reduce nitrates to lower gaseous 

 oxides and to free nitrogen (the presence of which in filter-beds 

 can be demonstrated) ; fourthly, the action of higher forms of 

 vegetable and animal life ; fifthly, it is possible that direct 

 chemical oxidation of the earlier products of bacterial action 

 may occur, and in any case the access of an abundant oxygen 

 supply to adsorbed material hastens its destruction. It is 

 possible, as is indicated by the work of Lorrain Smith and of 

 Mair, that perhaps too little weight has been attached to the 

 parts played by the two last processes specified, for in the later 

 stages of the purification process there is a very marked 

 diminution in the number of bacteria present in the filter. 

 Much further work, however, is necessary before the part to be 

 assigned to each factor in operation can be properly estimated. 



Further, the details of the essentially bacterial part of the 

 process are obscure, and the relative parts played, even in an 

 open purification process, by aerobes on the one hand, and 

 anaerobes on the other, are little understood. When sewage is 

 drained off to rest a filter-bed, great quantities of oxygen are 

 sucked in, but as to how long the bed thus remains aerated, 

 authorities differ some maintaining that oxidation processes per- 

 sist even after the bed has been recharged, while others state that 

 soon the oxygen in the resting bed is consumed, and its place 

 taken by carbon dioxide and nitrogen. Certainly, at certain 

 stages of the purification process, large amounts of free nitrogen 

 come off the bed, but whether at such periods anaerobic bacteria 

 are or are not in the ascendant, is not known. It is probable 

 that, from the practical standpoint, the later stages of purification 

 should take place with free oxidation, as when anaerobic bacteria 

 are active at this point a very offensive effluent is produced. 



Often the effluent from a sewage purification system contains 

 as many bacteria as the sewage entering, but there is often a 

 marked diminution. It is said by some that pathogenic 

 bacteria do not live in sewage. The typhoid bacillus has been 

 found to die out when placed in sewage, but it certainly can 

 live in this fluid for a much longer period than that embraced 

 by any purification method. Thus the constant presence of 



