i66 SEWAGE AND ITS PURIFICATION 



number slowly diminishes, reaching 90 millions after twenty- 

 four hours, and falling to 7 millions in three months. With 

 regard to sludge cake, the further addition of lime, together 

 with abstraction of water by the presses, renders the mass 

 almost sterile at first, but when exposed to the weather and to 

 dust a ripening process takes place on the development of 

 bacterial life. 



Bradford, Yorkshire, has met with great difficulty on account 

 of the large quantity of grease, mainly wool-fat, amounting 

 sometimes to 20 per cent, of the dry solids, preventing the 

 squeezing out of more than 25 per cent, of water. The usual 

 percentage of water in the wet sludge of other towns is 

 go per cent., the increase from 90 to 98 per cent, at Brad- 

 ford making a vast difference in the total bulk. Thus, wet 

 sludge 



with go per cent, water = g vols. H2O to i of solids; 



)f g5 >5 >) ^^ ^9 " >' ^ " 



>» 98 „ „ =• 4g ,, „ I ,, 



Therefore, 100 tons with go per cent, water become 200 tons 

 with g5, and 500 tons with gS per cent., so that the watery 

 character of the Bradford sludge caused its volume to be 

 increased five times. Being so thin and greasy, it was difficult 

 to press, and after being pressed in the most improved 

 machines it left fully 75 per cent, of water in the cake. 



Sludge cake can be brought to 50 per cent, water by pressing, 

 and to 12 per cent, by air-drying. As the value, either as 

 manure or fuel, is inversely proportional to the amount of 

 water present, it follows that in all cases air-drying should be 

 used before disposal. In the table (p. 167) the monetary value 

 of air-dried sewage sludges is given, while their calorific value 

 is roughly proportional to the organic matter. Since the latter 

 is largely nitrogenous in character, its value as fuel is much 

 lower than that of coal, and, of course, the remaining water 

 associated with it must be evaporated before any energy is 

 available. Santo Crimp estimated that in an efficient chemical 

 process 10 cwt. per head per year of wet sludge with go per 

 cent, water was obtained, equal to 2 cwt. of dried sludge with 

 50 per cent, moisture. 



The following are analyses of air-dried sludges as given by 

 Professor Robinson : 



