PANORAMIC VIEW OF TRICKLING FILTERS, COLUMBUS, OHIO 



THE CONTACT BED 



Mr. W, J. 1 )il x I in , Chemist to the London County Council, was one 

 of the first to attempt to modify the sewage filter so that it would operate 

 at higher rates, and as a first step he naturally sought to build his beds of 

 coarser material. In a notable series of experiments at the Barking outfall 

 on the Thames, he found that the nitrifying bacteria could he grown on 

 fragments of coke or stone as well as on sand and that purification could 

 be effected in such beds if only the sewage were held in contact with the 

 material, instead of being allowed to stream directly through. In sand 

 filters, frictional resistance delays the passage of the sewage, SO that time 

 is given for the purifying process. With coarser materials, however, it is 

 necessary to regulate the How by making the beds water-tight and retain- 

 ing the sewage in them until purification is completed. This was in outline 

 the genesis of the contact bed. 



Beds of this type are simply concrete or masonry basins, filled with 

 crushed stone or coke or slag, in which sewage is allowed to stand for a period 

 of about two hours. After one dose is withdrawn the bed stands empty 

 for aeration for four hours or so and another dose is then introduced, three 

 fillings a day being perhaps an average. A single contact treatment does 

 not commonly yield an effluent sufficiently stable to discharge into a small 

 stream. It is the general practise therefore to use double contact, treating 

 the sewage first in a. bed of coarse stone, perhaps one and one-half to two 



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