146 SEWAGE AND ITS PURIFICATION 



can see from them that — (i) The reduction of the total N by 

 about 75 per cent, (making allowance for dilution) is not 

 accounted for by the somewhat meagre production of nitrate 

 and nitrite. (2) Since the sewage " gradually sank into the soil 

 as it flowed," this improvement can only be partially due 

 to volatilization of free ammonia, of which soils, as is known, 

 are very retentive. (3) The organic nitrogen may at first have 

 been largely absorbed by the soil ; but as the analyses extended 

 over nearly a year and a half, and the later ones showed the 

 same changes, this mechanical absorptive active is of minor 

 importance. The explanation is rather to be found in the 

 life of the soil bacteria, acting by the process of denitrification, 

 in which free nitrogen and lower oxides of nitrogen are 

 generated from both ammonia and organic matter, and evolved 

 as gas. In fact, the whole process, instead of being, as it was 

 then considered, partly mechanical and partly chemical, was 

 in its essence bacterial. But as this sewage was admittedly 

 weak, Frankland over-estimated the efficiency of the method 

 when he stated that "the application of the sewage of more 

 than 1,000 persons to an acre of land is consistent with the 

 growth of crops and a superabundant purification of the effluent 

 water," and that " the sewage of a much larger number could 

 be effectually purified on an acre if the growth of crops 

 were given up." The Local Government Board, on the 

 other hand, prescribed " for intermittent filtration without pre- 

 cipitation, through sandy gravel, i acre for every 100 to 300 

 persons." 



c. Irrigation with Filtration or Precipitation, — From the 

 faults and difficulties we have mentioned, it is rare for any 

 sewage system to depend on the land solely. Even in the 

 Merthyr Tydvil trials the raw sewage was previously treated 

 with lime, and "a roughing filter" of gravel, coke, broken 

 ballast, or some other suitable material, is almost universally 

 used, and often by itself effects considerable bacterial improve- 

 ment in proportion to the time the liquid remains in contact, 

 although its functions are primarily to strain off the solids. 

 At Leicester, fair success was attained ^ by broad irrigation 

 on clay land after clarifying by coarse banks of clinker, f to 

 2 inch size, from the refuse destructor. 



^ Society of Engineers, December, 1898. 



