Soils and Sewage 129 



The early experience with sewage-irrigation soon 

 taught that soils differ strikingly in their ability to effect 

 purification. It was also noted in those days that the 

 purifying power of peat and heavy clay soils was rather 

 limited. We readily see now why this should have been 

 so, for the acid character of the peat retards the growth 

 of decay bacteria, and, more particularly, of nitrifying 

 bacteria. The retarding action in the heavy clay soils is 

 due, on the other hand, to their compact nature and 

 improper aeration, which is a serious hindrance to the 

 aerobic nitrifying ferments. The best results in the 

 purification of sewage were obtained with light sandy 

 soils underlaid by a porous gravelly subsoil, and pro- 

 vided with a sufficient amount of lime. Additions of the 

 latter to the land were found to intensify its purifying 

 power. 



Broad irrigation. — The amount of sewage that may 

 be successfully treated by broad irrigation is limited by 

 the area available. Land to which too much sewage is 

 apphed, ceases to purify it. It becomes wet and foul; 

 the aerobic bacteria, whose function it is to accomphsh 

 much of the purification, are crowded out, and the sewage 

 runs off unpurified. Matters are made worse even in 

 suitable soils, by the deposition at the surface of various 

 organic materials. These form a felt-Hke layer and tend 

 to keep out the air. This circumstance necessitates the 

 occasional stirring of the surface soil, as well as the 

 limiting of the amounts applied per given area within a 

 limited time. It is estimated that one acre of land should 

 not receive the sewage from more than one hundred 

 persons. This is, of course, only an arbitrary measure, 



