I 



BACTERIAL PURIFICATION 



235 



failure of nitrification noticed in so many of these filters will 

 follow. 



In the Exeter experiments I sunk " compo " tubes to different 

 levels in filter No. 2, 5 feet deep, which had been in constant 

 work for several days, and aspirated the gas for analysis two or 

 three hours after the last discharge. The results were : — 



11 



Assuming the air in each empty filter to contain i per cent, 

 of COo, it follows that the volume of carbonic acid removed as 

 gas is also i per cent, of the volume of sewage dealt with in the 

 filters. The weight of organic carbon destroyed in this way is 

 therefore about 50 pounds per 1,000,000 gallons, or 0*5 part per 

 100,000, without reckoning the dissolved COo in the interstitial 

 iquid. 



The interference of this carbonic acid in deep filters seems to 

 account, even more than the insufficient time during which the 

 beds had been working, for the fact that the purification re- 

 ported by Dr. Clowes, even by his "secondary" (really triple) 

 treatment, is not equal to what has been attained elsewhere. 

 He states that "the purification effected by a single treatment 

 of the raw sewage in the coke beds amounts to a complete 

 removal of the suspended matters, and to a further removal of 

 at least 51*3 per cent, of the dissolved putrescible oxidizable 

 matter. The primary 6-foot coke bed actually removed on the 

 average 49*9 per cent, of dissolved impurity, and a second 

 process has effected thus far an additional purification of about 

 I9'3 per cent., giving a total average of purification of the 

 clarified raw sewage amounting to about 69*2 per cent." With 

 this deeper filter nearly 70 acres of filter 12 feet deep would be 

 required. 



Mr. Dibdin states^ that slate or tiles laid in layers and 

 separated by spaces of 2 to 4 inches give a coarse bed effluent 

 from which the solids have practically been removed, and at 

 the same time a working capacity equal to double that of the 



^ J. of preventive Medicine, July, 1905 ; J. Soc. CJum. Ind., May 15, 1906; also 

 Somerville, Public Health, September, 1904. 



