■ two hours' exposure pu- 

 trefaction took place on 

 afterwards incubating.^ 

 In 1892 investigations 



twere begun at Lawrence, 

 Mass., with the artificial 

 aeration of coarse gravel 

 filters to which sewage at 

 relatively high rates was 

 IP apphed, with a view of 

 purifying it partly so that 

 final purification could 

 take place rapidly in a 

 second filter of sand, 

 cinders, or coke. Air was 

 drawn through either up- 

 wards or downwards, but 

 in all cases these filters 

 clogged in a manner which 

 indicated that this artificial 

 aeration, while beneficial 

 to a certain degree,was not 

 helpful in proportion to its 

 cost. 



Mr. Lowcock, at Mal- 

 vern, in 1892, forced in air 

 at a mean pressure of 

 4J inches of water. He 

 used '^ a pressure varying 

 from 3*4 to 6 inches, but 

 bearing no relation to the 

 volume of liquid which 

 flowed continuously 

 through the bed. At Mal- 

 vern the filter was made 

 of sand and gravel (Fig. 22), 

 and later, at Wolverhamp- 

 ton, of sand and coke 

 breeze. The sewage had 

 been screened and chemically 



BACTERIAL PURIFICATION 



237 



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' See also Chapter V., p. 122. 

 - Proc. Inst. C.E., 1893. 



