TREATMENT OF CITY SEWAGE 85 
débris, animal or vegetable, with the confidence that his bacterial 
aids will in time place the nitrogenous material at his service as a 
fertilizer. ‘Thus, by the aid of his invisible allies the agriculturist 
will be able to make use of the wastes on his farm and in time re- 
turn to his soil a considerable portion of the nitrogen. 
SEWAGE AND ITS TREATMENT 
Composition of Sewage.—By sewage we ordinarily understand 
the material which collects in the sewerage system of our larger 
communities and which has no exact counterpart on the farm. It 
always contains the products of the life of men and animals, which 
are no longer useful; also large quantities of both animal and vege- 
table foods which have passed through the alimentary canals of 
men and animals unassimilated. It contains a large amount of 
urea which has come from the animal metabolism; and also woody 
matter, cellulose, fat, starch, and an indefinite series of other or- 
ganic bodies. Almost anything which enters the city may find its 
way eventually into the sewers where, mixed with large quantities 
of water, it contributes to the sewage. The sewage thus contains 
exactly the same sort of material as that found in the manure heap 
and the compost pile. Evidently the problem of the various steps 
of decomposition of this material will be nearly identical with that 
already considered. 
TREATMENT OF CITY SEWAGE 
As cities have grown, the matter of disposing of their sewage be- 
comes more and more difficult. In small communities the digging 
of cess-pools is satisfactory; but as larger numbers of people con- 
gregate together, this method becomes objectionable, and finally 
impossible. The treatment of city sewage has become a problem 
involving the ingenuity of the expert sanitary engineer. It cannot 
be said that any wholly satisfactory method has yet been devised 
for handling this difficult problem. 
