ii6 



SEWAGE AND ITS PURIFICATION 



receiver a pipe conveyed the gas to the attendant's hut, where 

 it was used efficiently for light in an incandescent burner, or 

 for heat in a small gas-stove. The volume generated is tested 

 by this gasometer, and it is stated that about a cubic foot is 

 produced in twenty-four hours for every square foot of surface, 

 and that it is equal in heating power to good coal-gas^ (see 

 further, Chap. XL). 



The residual gaseous energy that is available in this way can 

 be approximately calculated from the consideration that the 

 organic matter removed from the sewage and converted into 

 gas in the tank is, for the most part, not oxidized or burnt 

 therein. The oxygen-consumed figure of the raw sewage, with 

 its suspended matter, less the oxygen-consumed figure of the 

 tank effluent, gives a measure of the combustibility of the gases 

 produced. For example : 



Parts per 100,000. 



It is easy to understand, bearing in mind ordinary burningj 

 how, in oxidation changes, energy is obtained for the continuance 

 of the reaction. In hydrolytic changes the source of energy ii 

 not so clear, but it is certain that there occurs a distinct evolu- 

 tion of heat, small in amount, and almost imperceptible in the 

 bulk of water, but sufficient to continue the reaction, which is" 

 therefore exothermic, or containing within itself the conditions 

 of its own propagation.^ Thus in the case of urea — 



CO(NH2)2+H20H-Aq = C02-F2NH3Aq 

 Heats of formation +77*5 +68-4 -i-97'6 2x( + 2o-4) 



\ 



145-9 



138-4 



but the 2NH3 and CO2 neutralize one another, resulting in a 

 further evolution of about 20 units. 



Water, June 16, 1902. 



^ Cf. Berthelot, Comptes rendus, lix., 901, 1864. 



