VIII.] 



SEWAGE SLUDGES 



247 



tain the smaller and least valuable portion of the 

 nitrogenous material, also the phosphates but not the 

 potash of the sewage. Table LXXVl. gives a series of 

 analyses of such sludges, made for the Royal Commis- 

 sion on Sewage Disposal in 1906, which may be taken 

 as typical of this class of material : — 



Table LXXVL— Composition of Sewage Sludges. 



Of these sewage sludges No. i represents the 

 material sold as "native guano," 2 and 3 are lime 

 sludges, while for 4 the precipitant had chiefly been 

 sulphates of iron and alumina. It will be seen that in 

 no case is the material possessed of much fertilising 

 value, for not only are the percentages of nitrogen and 

 phosphoric acid low, but they must be combined in 

 extremely inactive forms. Field trials show that the 

 action of these sludges as manures is very small, below 

 that of equivalent amounts of nitrogen and phosphoric 

 acid in commercial fertilisers, so small in fact to be 

 negligible unless the material is applied in very large 

 quantity. Indeed, we can only conclude that these 

 sludges possess little or no value as fertilisers, though 

 they may be valuable for the lime they contain, especi- 

 ally on light sandy land where they will also add some 

 water-retaining humus and improve the texture of the 

 soil. 



