134 



SEWAGE AND ITS PURIFICATION 



per acre per day), notes that the latter contained 8'i per cent, 

 of carbonate of lime, the former under 2 per cent. ; and we 

 now know that the lime is favourable to the nitrifying organism. 

 Effluents which have been chemically treated with lime may 

 acquire sufficient alkaline base to favour the growth of the 

 nitrifying organisms, even when the soil or the filter-bed is 

 originally devoid of such base. In the case of a sewage farm 

 in Surrey, where the soil, a ferruginous sandstone, is very 

 deficient in lime, I found that the calcium carbonate increased 

 through the treatment of the land with sewage, and the quality 

 of the effluent also improved : 



I have obtained the following results in an inquiry in which 



alternative sites were available for a sewage farm : 



A. B. c. 



Percentage of water 6-75 1-90 3*05 



Parts of nitric nitrogen produced per 



100,000 parts of soil in five days on 



dilute urine ... ... ... ... o'i68 0*504 0*36 



Showing that the dryest of the soils was the most active 

 bacteriologically. 



Both a mechanical and chemical analysis of samples of soil 

 is necessary in the selection of proposed sites. The most un- 

 suitable conditions are stig _tenacious clay s, pe aty or bogg y 

 ground, and coarse gravel with hard cong^lomer ated lay ers. In 

 India, where the temperature is higher, all bacterial change, 

 including nitrification, occurs more rapidly, the growth con- 

 tinues for a longer time in the year, and the sewage is generally 

 less voluminous than in England^ ; hence a less area per person 



^ A. E. Silk instances a sewage at Calcutta which consisted of faecal matter 

 and urine mixed with only 3 gallons of water per head. 



