154 



SEWAGE AND ITS PURIFICATION 



"sludge," which is the greater in amount as the straining 

 medium is finer, and is also increased by the precipitant. A 

 great difficulty in dealing with sludge is that its bulk is swelled 

 by its containing 92 to 98 per cent, of water. If it be tried to 

 obtain it in a denser condition by longer deposition, obnoxious 

 gases are certain to be produced. Spreading it over the soil to 

 dry, and finally digging or ploughing in, is in some places 

 possible, but for filling up low-lying land ^ it is unsuited through 

 its density, wetness, and unsanitary character. 



In 1886 the adoption of destructor furnaces at Southampton 

 and Ealing led to experiments at Leyton and Cardiff, founded 

 on the hope that sludge could be burned remuneratively and 

 without offence with the aid of a certain amount of coal to 

 dry the cake, the ammonia evolved being collected, and the 

 volatile matters passed through the fire. The net cost of 

 incineration was not to exceed sixpence per ton, the coke pro- 

 duced paying for the coal and working expenses. But in all 

 the numerous attempts at utilization of sludge either as manure 

 or by chemically extracting some of its constituents, it was 

 found that the agricultural value was disappointing, while in 

 combustion processes the cost of machinery and fuel absorbed 

 all the profits. 



The object now being merely to reduce the bulk and avoid 

 nuisance, sludge was compressed in filter-presses of various 

 constructions to a cake containing 25 to 50 per cent, of water, 

 with usually an addition of lime or other substance to facilitate 

 pressing. The cost was still great, and the product nearly 

 worthless. The following are analyses of two examples : 



^ L. Flower and others, Royal Commission on Metropolitan Sewage, 187; 



