CHEMICAL PRECIPITATION 163 



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pressed out water must be taken back to the tanks for re- 

 treatment. 



The Dortmund Tank is circular and deep, with the lower 

 part conical, and with a vertical cylinder fixed in the middle. 

 The strained sewage, after treatment with lime and aluminium 

 sulphate, passes downwards through the central cylinder, and 

 is then distributed horizontally by specially constructed arms. 

 The sludge deposited in the cone is withdrawn by suction- 

 pumps through a 6-inch pipe opening near this bottom, at 

 a uniform rate of 15 feet per hour. This tank had its origin 

 in the Rockner-Rothe process, and was first used at Dortmund 

 in Germany, subsequently at the Chicago Exhibition, and at 

 Alfreton and Ilkeston in England. The deposition in conical 

 vessels has been long known in laboratories as a means of 

 concentrating precipitates. The idea aimed at in the Dort- 

 mund is timing the deposition with the withdrawal of clear 

 liquor. The fault of conical, as distinguished from cylindrical 

 vessels, is that the former allow deposition on their sides, 

 the greater in proportion to their low angle. Hence the 

 working is sometimes deficient, "the sides of the cone being 

 coated with filth, which decomposes, making the effluent very 

 unsatisfactory." 



At Essen (Rockner-Rothe principle) shallower tanks are 

 adopted with pneumatic raising. 



Cosham's Tank has the advantage of compactness, by means 

 of a radial arrangement with two concentric circles, the middle 

 one being divided into two, the outer space into eight compart- 

 ments, the whole arrangement being conical, so that the inner 

 two divisions are deepest, and the shallower outside ones 

 encircle them. The sewage passes into the centre and over- 

 flows gradually through the other compartments, with deposi- 

 tion in each. Arrangements are made by which the sediment 

 can be withdrawn from the bottom of each chamber, or passed 

 into the centre divisions and siphoned out collectively.^ A 

 rectangular form is also included. This system is said to have 

 worked satisfactorily in twenty-five places, amongst which is 

 Tibshalf, Derbyshire. 



The Ives' Tank (patent 16,724 of 1894) is also circular, and 

 includes arrangements for aeration and, as a preliminary, a 

 centrifugal reducer of coarser soHds, with a " chemical cage " 



^ For details see Moore's " Sanitary Engineering," p. 452. 



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