BACTERIAL PURIFICATION 273 



best advantage, the purification of the effluent being completed 

 by denitrification of the mixed Hquids. 



The Manchester Experiments. 



The disposal of the sewage of 564,000 inhabitants, with much 

 manufacturing refuse, has been a difficult problem, which has been 

 carefully investigated. Formerly it was treated at Davyhulme 

 on the London plan of screening, adding milk of lime, then 

 ferrous sulphate, sedimentation, and discharge of the clarified or 

 filtered liquid into the Ship Canal, while the sludge was stored 

 in two tanks, holding 1,000 tons each, thence flowing at intervals 

 into a sludge steamer which carried it to sea. Pollution of the 

 Canal led to interference from the Mersey and Irwell Joint 

 Committee, and to the necessity of the adoption of some other 

 system. The Manchester Corporation in 1898 instructed 

 Baldwin Latham, Percy Frankland, and W. H. Perkin, to 

 examine into the merits of various processes, following a report 

 of the Rivers Committee : — 



1. "That filtration by land is altogether impracticable, as no 

 land obtainable in the district is suitable for such process," as 

 proved by the experiments which, at great cost, had been made 

 upon 25 acres of land at the works. Visits to the works of 

 other authorities had shown that *'in all cases the land filtra- 

 tion is ineffective, and is, in many cases, to be superseded by 

 artificial methods of filtration. The imposition of conditions 

 by the Local Government Board, making the purchase of large 

 areas of land compulsory, should be removed." 



2. " That no practicable system of precipitation by chemicals 

 alone has been laid before them which will meet the require- 

 ments of the Mersey and Irwell Joint Committee." [The 

 experiments had been very elaborate, and are detailed in the 

 City Reports of 1897 and of previous years.] 



3. They agreed that the method nearest to natural action and 

 " most reasonably practicable and reasonable for adoption " 

 was the biological filter or bacteria bed, such as had been seen 

 in operation at various places. 



The land available at Davyhulme for all disposal purposes is 

 165^ acres, and the three experts soon concluded that this area 

 was ample for the necessary works to purify the sewage includ- 

 ing storm-water — the works existing at the time occupied 

 27!^ acres. They pronounced adversely on the alternative pro- 

 posals of treatment of a tank effluent on land, and the Culvert 



18 



