284 BACTERIOLOGICAL AND ENZYME CHEMISTRY 



sewage. The passage of sewage througli sewers results in the 

 mechanical breaking up and emulsification of faecal matter, 

 and the setting up of ammoniacal fermentation of the urea 

 present in the urine. The extent to which these changes take 

 place depends on the length of sewer to be traversed, and also 

 on the state and construction of the sewer. New sewers 

 with smooth surfaces will not readily allow the formation of 

 deposits of sewage matter, and consequent further fermenta- 

 tive change. With well-laid sewers, only the initial stages 

 of fermentation of nitrogenous matter wiU have set in by the 

 time the sewage reaches the works, and such sewage should 

 therefore be comparatively inoffensive. Moreover, in designing 

 tanks for simple sedimentation, they should be of such a size 

 that the sewage will not remain in them sufficiently long for 

 any but the preliminary stages of decomposition to take place. 

 The sludge or deposit from such a sedimentation tank will, 

 in consequence of what has been said, also need to be very 

 frequently removed, if offensive decomposition is not to take 

 place, and not only must it be quickly removed, but it must 

 for the same reason be quickly disposed of, e.g., by trenching 

 into the ground. 



(b) Anaerobic Decomposition.— A tank designed to facili- 

 tate anaerobic decomposition differs from a sedimentation 

 tank chiefly in being relatively larger, and so allowing time 

 for decomposition to take place under anaerobic conditions. 

 Such a tank has been variously termed a cesspool, a septic 

 tank, a liquefying tank, or a hydrolytic tank. The differences 

 in design are mainly structural, to facilitate deposition and 

 removal of the solid matters, and to control more or less the 

 character and extent of the chemical changes taking place. 

 These chemical changes may at this point be usefully con- 

 sidered in detail. 



Decomposition of cellulose. — The anaerobic decomposition 

 of cellulose has been considered in Chapter X. It was there 



