CHAPTER X 



BACTERIAL PURIFICATION (continued) 



Capacity of filters — Nature and size of materials — Gases in filters — 

 Depth of beds — Aerating processes — Lowcock — Waring — 

 Ducat — Artificial warming — "Thermal aerobic" — Continuous 

 filtration — Salford — Stoddart's filter. 



The regulations of the Local Government Board as to filters 

 originally provided that each set of filters — i.e., both coarse and 

 fine — must be of sufficient capacity to contain the normal dry- 

 weather flow for twenty-four hours. Coarse-grain beds can 

 hold 25 per cent, sewage, and fine beds 33J per cent. This 

 means, taking an eight-hour cycle, that the beds will be large 

 enough to deal with three times the dry-weather flow — i.e., 

 I volume normal, 2 volumes storm water. 



As to material, its size and mode of arrangement have been 

 shown to be more important than its kind, and it has been 

 unfortunate that the Board have not during the past few years 

 allowed considerations of material and distribution to modify 

 their insistence on a fixed filter capacity. 



Coke breeze from its porosity exposing a larger surface was 

 recommended by the Barking experiments and has been 

 generally adopted, but it is somewhat expensive when required 

 in large quantities, therefore in many localities local material, 

 when properly screened and graded, can be employed. 



The Massachusetts Reports, 1898-99, comparing filter-beds 

 of ashes and cinders with those of sand and gravel, state that 

 the former have great advantage as regards rapidity and not 

 becoming clogged, that they are equal or even better in the 

 colour and chemical character of the effluent, though the 

 percentage of bacteria removed is less. 



In 1897 fine coal was used as a medium for the filtration of 

 chemically precipitated effluents, at Wolverhampton, Lichfield, 

 and other places. The sewage of the former town is heavily 



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