THE SEWAGE DISPOSAL PROBLEM 185 



piece of land on the sewage works. Then it is dried by 

 simple exposure to the air in pits and furrows. Another 

 method is to dry the sludge in hydraulic presses by which 

 the moisture is reduced to about fifty per cent., and a solid 

 cake is formed which has some slight manurial value and may 

 be sold or given away to farmers according to the demand. 

 Recently, Dr. Grossmann, of Manchester, has suggested a 

 method of dealing with sewage sludge by non-destructive dis- 

 tillation. He points out that especially in some towns — e.g. 

 Bradford — the sludge contains a large amount of fatty matter, 

 grease and soap, arising from household waste, etc., and 

 estimates that the sludge cake should contain at least five 

 per cent, of the fatty acids, which ought easily to repay 

 recovery if a practicable process could be found. Further- 

 more, the removal of these fatty matters would render the 

 remainder of the sludge more suitable for manurial purposes. 

 He has been experimenting during the last twelve months 

 at Oldham, where the sewage is a fair example of an ordinary 

 town sewage. The sludge contains about ninety per cent, of 

 water, which is reduced to fifty per cent, by passing it through 

 presses. The sludge cake is dried and mixed with a suitable 

 amount of sulphuric acid. Then the mixture passes through 

 a retort consisting of a cylinder, in which revolves a hollow 

 shaft bearing perforated paddles. The sludge is thoroughly 

 agitated whilst steam is made to permeate the mass. The 

 steam carries with it the fatty acids which are condensed in 

 a tower into which water is injected. The condensed fat 

 collects on the surface of a tank. The residue from the retort 

 varies in the amount of nitrogen and organic matter which it 

 contains owing to the constant variations in the composition 

 of the sewage sludge. But it is important to notice that the 

 composition of this residue shows that it contains some forty 

 per cent, of partly decomposed organic matter, and thus 

 resembles humus. Dr. Grossmann puts the intrinsic value of 

 this manure at 25s. per ton, but for the purpose of his own 

 calculations assumes that it is not more than 105. per ton, and 

 it is found that about j\ cwt. of this manure are obtained 

 from every ton of pressed sludge containing fifty per cent, 

 water; therefore this at 105. per ton represents a value of 

 35. gd. The intrinsic value of the crude grease is put down 

 at ;£i2 per ton, but it is only calculated at £y per ton. The 



