234 



SEWAGE AND ITS PURIFICATION 



secondary beds were matured, or inoculated, by frequently 

 charging with crude sewage for about three months to seed 

 them with bacteria. 



The aeration of the 6 and 15 foot beds was tested by sinking 

 vertical pipes into the bed and aspirating the gas. The amounts 

 are given as follows : — 



Later experiments (report of October 26, 1899) agree with 

 the above, and in a third report (July, 1900) the average of 

 almost daily analyses of the air in a " primary " coke bed about 

 10 feet deep showed after ** resting empty" periods of twenty- 

 one hours, oxygen 10*3 ahd CO2 5*7 per cent. ; after five hours' 

 rest, oxygen 8'0, CO2 57 : air containing normally 21 per cent, 

 of oxygen and 0*04 per cent, of COg. 



At Lawrence, Mass., in 1899, a cinder filter which had become 

 clogged and was resting, had air drawn through it constantly 

 for two months except at certain intervals, at a rate suffi- 

 cient to change the air-contents every three hours. The 

 gas in the filter gave in averages per cent. : {a) aspirator work- 

 ing continuously : CO2 0*25, O 20 ; {b) aspirator shut off for 

 some hours : CO2 1*3 to 2*6, O less than i*o. The quantities 

 are irregular, but show that a reduction of the free oxygen 

 occurs from the 21 per cent, normally present in air. The 

 carbonic acid produced usually corresponds to an equivalent 

 diminution of the organic carbon. A point to notice is that 

 besides the CO2 in the gases an additional quantity, propor- 

 tional to the vapour tension, is retained dissolved by the liquid 

 remaining in the coke. Several observers have proved the 

 inhibiting action of carbonic acid on bacteria, especially those 

 which are oxidizing, therefore it is important when the third or 

 oxidizing stage is reached that the carbonic acid should be 

 removed by free circulation of air as soon as formed, or the 



