CH. xi] Activated Sewage Sludge 205 



have not been very satisfactory. The usual course of 

 events is that farmers are first induced to purchase it 

 but finally have to be paid to take it away. Methods 

 have therefore been devised for improving the sludge, 

 perhaps the commonest being to add a certain pro- 

 portion of lime and then to force the mass into presses 

 when it forms a cake containing roughly 50 per cent, 

 of water, 15 to 25 per cent, of organic matter and 

 25 to 35 per cent, of mineral matter much of which 

 is lime, and about 1 per cent, each of nitrogen and of 

 P2O5. Several of these pressed sludges were tested on 

 field crops during the years 1905-8, but the results were 

 not good: only in the wetter districts of the North of 

 England did they seem to have much value^. In some 

 places, e.g., Glasgow, Kingston, etc., other materials are 

 added to enrich the sludge. These and other sludges 

 are sent out in good mechanical condition ready for 

 distribution: but some of them suffer from the draw- 

 back that they are sold at prices considerably in excess 

 of their real value. At Bradford and Huddersfield a 

 process is at work to extract the fat, grease, etc., which 

 in modern times have become too precious to lose even 

 in sewage ; the resulting products contain respectively 

 2 and 3| per cent, of nitrogen and are of distinct 

 fertiliser value. 



A recent process consists in blowing air through the 

 sewage; this gives a sludge very different in character 

 from the older types and containing as much as 6 per 

 cent, or more of nitrogen and over 4 per cent, of P2O5. 

 The material is called activated sludge and is said to be 

 of high manurial value 2. 



^ 5th Report of the Sewage Commission, 8th Appendix, Cd. 4286, 1908. 

 A good summary is given in Journ. Board of Agric, 1908, xv. p. 690. 

 2 E. Ardern, Journ. Soc. Chem. Ind., Jan. 31, 1917 and July 31, 1917. 



