BACTERIAL TREATMENT OF SEWAGE. 139 



out of the first bed oxygen is sucked in, it rapidly disappears, 

 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 examining London 

 sewage, Clowes has found oxygen present in abundance from 

 four to forty hours after the sewage has been run off. 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 prelimi- 

 nary 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 England, and very fully worked out 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, (i) First of all, fresh sewage the 

 newly mixed and very varied material as it enters the main sew- 

 ers. (2) Secondly, stale sewage the ordinary contents of the 

 main sewers. Here there is abundant oxygen, and as the sew- 

 age flows along there occurs by bacterial action a certain forma- 

 tion 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 changes which take place in the 

 septic tank are of a most complex nature. The sewage entering 

 it contains little free oxygen, and therefore the bacteria in the 

 tank are probably largely anaerobic, and the changes which 

 they originate consist of the formation of comparatively simple 

 compounds of hydrogen with carbon, sulphur, and phosphorus. 

 As a result there is a great reduction in the amount of organic 

 nitrogen, of albuminoid ammonia, and of carbonaceous matter. 

 The latter fact is important, as the clogging of ordinary filter 

 beds is largely due to the accumulation of such material, and of 

 matters generally consisting of cellulose. One further impor- 

 tant effect is that the size of the deposited matter is decreased, 



