BACTERIAL PURIFICATION 245 



Health Act, 1875. It is hoped that the scheme will be com- 

 pleted in 1907. 



It is important to notice that these " bacteria filters " are 

 only dealing with an effluent already artificially purified by 

 subsidence, straining, and precipitation. They were protected 

 by a roughing filter of fine gravel, to arrest any floating fats or 

 any precipitated sludge which might find its way through the 

 subsidence tanks. " This roughing filter required to be cleaned 

 every one or two days. . . . The Salford process will then be 

 threefold : first, precipitation by the lime or other chemical process ; 

 second, clarification and interception of all suspended matters by 

 roughing filters of coarse gravel ; third, purification by means of 

 bacterial filters on the lines above described.'' The expenses and 

 sludge of the older processes are thus retained. 



The works are designed for pumping and fully treating per 

 day 30,000,000 gallons for two or three days together, or about 

 20,000,000 gallons for some weeks, the ordinary flow being 

 11,000,000 to 12,000,000, from a population of 210,000, with a 

 water-supply of 4,500,000 to 5,000,000 gallons, or 25 gallons 

 per head. '' The balance, therefore, of half the ordinary sewage 

 is subsoil water, and the sewage requires about 4 grains of 

 oxygen per gallon to oxidize the putrescible matter on arrival 

 at the works." 



Of the original twelve tanks, ten, in two sets of five each, 

 were formed into two large tanks, of a total capacity of 4,750,000 

 gallons ; the other two tanks were altered into six roughing 

 filters, of 2,040 square yards area, where the precipitation tank 

 effluent passes through 3 feet depth of fine gravel. " The final 

 filters or * bacteria beds ' are 26,000 square yards in extent ; 

 during dry weather the flow will be about 300 gallons per 

 square yard per day, but in wet weather three or fourfold this 

 flow can be satisfactorily purified, and the filtrate averages well 

 within the standard of the Joint Board of the Irwell and Mersey 

 Watershed. The cost of the works, when fully completed, will 

 amount to about ^80,000, so the whole cost from first to last 

 will amount to about ;f 198,000, to serve a population of, say, 

 250,000, or i6s. per head."^ 



The filter plant has been several times modified. In 1906 

 Mr. Corbett writes, " the distributing pipes are placed over the 

 beds. . . . We support the gridiron of pipes on brick piers, 

 and make a central channel for the filtrate below the central 

 piers," which each contain a 9" x 9" flue with openings at the 



1 Report of Mr. J. Corbett, Borough Engineer to Salford Corporation, 1902. 



