CHAPTER XIV 



AGRICULTURAL VALUE OF BACTERIAL EFFLUENTS- 

 TRADE EFFLUENTS 



Conservation of the valuable constituents of sewage — Economic 

 value of nitrated effluents — Trade effluents — Classification — 

 Chemical and mechanical treatment — Recovery of products — 

 Wool grease — Relation to the bacterial process — Recommenda- 

 tions of the Royal Commission. 



The earlier attempts to utilize sewage and its sludge for agri- 

 cultural purposes have been discussed in Chapter VI. Sludge 

 in its various forms has never been of great value, and the 

 analyses show that it is not rich in plant food, especially when 

 large quantities of lime have had to be added in order to make 

 it into a marketable cake. Sewage effluents, on the other hand, 

 contain all the soluble constituents of the original sewage, and 

 after oxidation the nitrogen in the form of nitrates is easily and 

 quickly assimilated by plants. Maerker,^ from agricultural 

 experiments in Germany, considered loo parts of nitrogen, as 

 albumen, equal to 90, as ammonia, and 60, as nitrate, so that 

 for economic utilization the organic nitrogen should be nitrated 

 as far as possible ; and denitrification changes, as already seen, 

 must also be avoided in order to prevent loss. Sir William 

 Crookes estimated that in the sewage and drainage of towns 

 we " hurry down to the sea fixed nitrogen to the value of 

 ^16,000,000 per annum. "'^ 



Estimates of the agricultural value of human excreta. Annual 

 value per adult 6s. 6d. to 20s. (various authorities), i lb. human 

 excreta =15 lb. horse dung or 6 lb. cow dung (Macaire and 

 Marcet). Mechi ranks the droppings of a sheep at the same 

 value as the total excreta of one human adult. Voelcker 

 reckons the yearly excreta of an adult as equal to 75 lb. of 



^ Kew Bulletin, 1899, No. 144. 



2 " The Wheat ProlDlem," Murray, London, 1899. 



330 



