140 BACTERIA IN WATER 



are required, though on this point there may be some difference 

 of opinion. Certainly very fair results are obtained when 

 apparently the conditions chiefly favour aerobic organisms alone. 

 This is usually effected by running the sewage on to beds of sand, 

 or preferably of coke, allowing it to stand for some hours, slowly 

 running the effluent out through the bottom of the bed, and 

 leaving the bed to rest for some hours before recharging. The 

 final result is better if the effluent be afterwards run over another 

 similar coke-bed. According to some authorities the sewage, as 

 it runs into the first bed, takes up from the air considerable free 

 oxygen, which, however, soon disappears during the stationary 

 period, so that on leaving the first bed the sewage contains little 

 oxygen. In the latter part of its stay it has thus been submitted 

 to anaerobic conditions. Further, while by the passage of the 

 effluent out of the first bed oxygen is sucked in, this rapidly dis- 

 appears, and during the greater part of the resting stage the 

 interstices of the bed are filled with carbonic acid gas, with 

 nitrogen partly derived from the air, partly from putrefactive 

 processes, and thus in the filter anaerobic conditions prevail, 

 under which the bacteria can act on the deposit left on the coke. 

 On this latter point there is difference of opinion, for, in examin- 

 ing London sewage, Clowes has found oxygen present in 

 abundance from four to forty hours after the sewage has been 

 run off. Sometimes the treatment of the sewage consists in 

 allowing it continuously to trickle through sand or gravel or coke 

 beds. Probably the best results in sewage treatment are obtained 

 when it is practicable to introduce a step where there can be no 

 doubt that the conditions are anaerobic. This involves as a pre- 

 liminary stage the treatment of the sewage in what is called a 

 septic tank, and the 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 explana- 

 tion given of the rationale of this process, sewage is looked on as 

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

 mixed and very varied material as it enters the main sewers. 

 (2) Secondly, stale sewage the ordinary contents of the main 

 sewers. Here there is abundant oxygen, and as the sewage flows 

 along there occurs by bacterial action a certain formation of 

 carbon dioxide and ammonia which combine to form ammonium 

 carbonate. This is the sewage as it reaches the purification works. 

 Here a preliminary mechanical screening may be adopted, after 

 which it is run into an air-tight tank the septic tank. (3) 

 It remains there for from twenty-four to thirty-six hours, and 

 becomes a foul-smelling fluid the septic sewage. The chemical 



