78 THE MANURE HEAP AND SEWAGE. 



material may be obtained from seaweeds, if they are at hand. In- 

 deed, any abundant vegetable substances may thus be heaped into a 

 mass, and, if moistened sufficiently by rains, the bacteria will be 

 sure to work within it, gradually transforming the nitrogens, and 

 converting them finally into nitrates for plant food. Into his 

 garbage heap, then, the farmer may throw all sorts of organic 

 debris, 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 return 

 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 com- 

 munities and which has no exact counterpart on the farm. It al- 

 ways contains the products of the life of men and animals, which are 

 no longer useful; also large quantities of both animal and vegetable 

 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 organic 

 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- 



