i64 SEWAGE AND ITS PURIFICATION 



for regulating the supply of precipitant. The details, including 

 a *'flocculent flue," are very elaborate.^ 



The sludge or precipitate left after either subsidence or filtra- 

 tion putrifies very rapidly in warm weather, therefore requires 

 rapid removal from filters or tanks. It was formerly intended 

 to disinfect it at great cost. It is of very varying composition, 

 and is disposed of at the present time by many processes, 

 namely : 



1. Drying in open air on specially-prepared beds. 



2. Running on to land a few inches thick, and then ploughing 

 or digging in. 



3. Running into deep valleys. 



4. Transport to sea in specially-constructed barges. 



5. Mechanical pressing and disposal to farmers, or tipping. 



6. Drying mechanically, pressing, and using as a basis for 

 fertilizers. 



7. Centrifugal drying. 



The centrifugal drying-machine and the filter-press were 

 first introduced about 1855 at Leicester Sewage Works under 

 Wickstead. 



The Ashton-Booth sludge-remover now in course of installa- 

 tion at Bolton claims to take the place of the old methods of 

 manual labour. This remover is propelled by pressure from 

 the inflowing sewage, can be worked by two men, and takes 

 fifteen minutes to clean a tank 400 feet long. It is applicable 

 to any tank having a flat bottom, and claims to effect economy 

 in tank construction, owing to the absence of channels and 

 underground sludge-pipes. 



Pressed sludge from the London sewage at Crossness averages, 

 according to Dibdin's analysis — 



Water ... ... ... ... 58-06 



Organic matter ... ... ... 16-69 



Inorganic or ash ... ... ... 25-25 



1 00 -GO 



Saline ammonia ... ... .. 0*035 



Organic nitrogen ... ... ... 0-87 



The composition of the mineral matter was affected by the 

 treatment with lime and ferrous sulphate, being — 



1 For details see Moore's " Sanitary Engineering," p. 453. 



