224 SEWAGE AND ITS PURIFICATION 



tion, causing clogging and the close consolidation of rather 

 small material. Present settling-tank capacity 58,000 gallons ; 

 average dry-weather flow 350,000 gallons per day. The new 

 scheme provides open sedimentation tanks holding slightly 

 more than a day's flow, with additional beds for storm water. 



Leeds, with the Sutton method in 1898, experienced much 

 difliculty owing to *' sludging-up " of the beds, but by increasing 

 the periods of rest, and by the introduction of finer screens, 

 which remove a greater portion of the suspended solids (sludge) 

 to be otherwise dealt with, better results were for a time 

 obtained. The increase of capacity gained by a long rest was 

 rapidly lost on renewal of working ; thus the capacity regained 

 by a rest of thirty-eight days fell again in a fortnight from 

 56,500 to 45,800 gallons. Probably the long aeration had 

 destroyed or enfeebled the anaerobes, and the liquefaction was 

 therefore suspended until an anaerobic state was restored. The 

 drying up of the spongy matter during lengthened rests accounts 

 to a great extent for the increase of capacity. Another cause 

 of diminution of capacity in rough beds of clinker was found 

 to be that the material had sunk and become consolidated, in 

 consequence of the alternate filling and emptying, and the 

 slight rise and settlement at each turn of the work. This is 

 liable to occur with all materials in intermittent filtration. 



After three years, in 1900, the decrease in capacity in contact- 

 beds was still so serious that " they could not be regarded as 

 suitable for Leeds sewage without preliminary treatment. "^ 

 Besides the disintegration and settlement of the coke and 

 clinker, there was an accumulation of sediment in the interstices, 

 much of it irreducible, and therefore unaffected by the periods 

 of rest. If the beds must still be used, " the problem w^ould be 

 (i) to find material of sufficiently even size not liable to degra- 

 dation ; (2) to reduce, as far as possible, the solid matter put on 

 to the rough beds ; and (3) to exclude and treat separately all 

 iron liquors." 



" Sutton," or " Dibdin," beds were adopted at a large number 

 of places. At Manchester experimental filter-beds on the same 

 principle were named " double contact-beds." 



As it became gradually evident that the two beds, coarse and 

 fine, even with preliminary screening or sedimentation, were not 

 exactly adapted to the three processes of bacterial change that 

 we have mentioned, a third bed, or " triple treatment," was in 

 many places adopted. In the Manchester inquiry of 1899 it 



