BACTERIAL PURIFICATION 223 



I 



■■bhemical and mechanical actions in contact-beds, 1 and give 

 " curves of the changes agreeing with the explanations I have 

 given. They emphasize the importance of adsorption^ by the 

 zoogloeal jelly, on which Dunbar and Thumm lay so much stress, 

 but the phenomenon is common to all kinds of bacterial filters. 

 They found the loss of nitrogen in contact-beds to be 29 to 

 50 per cent. (Clark found 38 to 50 per cent.) ; in continuous or 

 trickling filters it was only i to 6 per cent, (see Chapter X., p. 270.) 

 In 1903, the Sutton U.D.C. Surveyor, Mr. Chambers Smith, 

 reported that " the net cost for the year of treating the sewage, 

 including pumping, was £775, equivalent to a cost of ^4 is. y^d. 

 per 1,000,000 gallons, or ii"07d. per head of population, whilst 

 the charge on the rates amounted to ifd. only, a figure which 

 compares very favourably with the cost of similar work in other 

 districts. The land on which the sewage disposal works are 

 situated is stiff clay, but a portion has been rendered suitable 

 for sewage farming by ploughing domestic ashes into it. About 

 8 acres are given up to the production of peppermint, one of 

 the most profitable crops which can be grown on a sewage 

 farm, and from this source alone £iy^ 15s. was realized in the 

 year 1902-1903." 



At Oswestry, in 1898,^ material for beds on the Sutton system 

 was obtained by screening from an old refuse tip, in which 

 "everything excepting hard carbonaceous matter had dis- 

 appeared," the coarser portions being used for the primary 

 beds, 4|- feet deep, and the intermediate portions for the 

 secondary filters, 4 feet deep. The total cost of the screened 

 refuse was about is. 3d. per cubic yard. The crude sewage 

 was previously clarified by subsidence in a large settling tank 

 in two sections, used alternately. About half the sludge 

 settled in these tanks, and was removed weekly, mixed with the 

 dust screened out of the town refuse, and sold as manure. 



The Surveyor writes in 1906 that three primary beds have 

 since been filled in the same way, but that the refuse of latter 

 years contains a much less proportion of large cinders and is 

 not the best material. The beds have lost a great deal in 

 capacity, chiefly through insufficient settling-tank accommoda- 



^ Research Laboratory, Mass. Inst, of Technology ; see Journ. of Infect, Diseases, 

 Chicago, 1905. 



'^ The property by which certain substances, notably colloids, remove dissolved 

 material from solution. 



^ Population, 10,000; dry-weather sewage, 300,000 gallons per day; water- 

 supply, 20 gallons per head; "total cost of works, ;^i,8oo ; annual working 

 expenses, about /80." 



