132 Bacteria in Relation to Country Life 



Preliminary treatment of sewage for sewage- farms. 

 The clogging of the surface soil by the suspended matter 

 in the sewage has led, in some places, to its preliminary 

 treatment with chemicals. The suspended solids are 

 precipitated by means of lime, or salts of alumina and 

 iron, and the resulting deposit (sludge) is removed. 

 The clear liquid still contains a considerable amount of 

 organic matter in solution. When allowed to stand, it 

 undergoes decomposition and gives rise to the same 

 offensive conditions created by the untreated sewage. 

 When applied to the land, however, it decomposes more 

 readily, does not clog the soil to such an extent, and is 

 evidently more suited than untreated sewage to promote 

 the activities of the nitrifying bacteria. This is par- 

 ticularly true of the sewage that has been clarified by 

 lime, for the latter promotes the desirable changes. 



The advantages of preliminary treatment, aside 

 from those already mentioned, include the greater 

 capacity of any given area for sewage-purification. 

 For instance, the official regulations in England required 

 one acre of gravelly loam soil for every one hundred 

 persons when the sewage was untreated, but allowed 

 one acre for every four hundred persons when prelimi- 

 nary treatment was carried out. The favorable influence 

 of the preliminary treatment of the decomposition of 

 the sewage in the soil is offset by the accumulation of 

 large quantities of sludge that results. When left to 

 itself, the sludge does not dry rapidly, but undergoes 

 putrefaction and creates a nuisance. It must be disposed 

 of in one way or another, thus involving considerable 

 expense to the community. 



