SEWAGE DISPOSAL 113 



of ammonia to nitrates, can go on only in the presence of 

 air. The sewage may he conducted on to filter beds of 

 porous material such as cinders, through which it is al- 

 lowed to trickle slowly. The final stages of decomposition 

 here take place, and the effluent from the filter beds is as 

 clear as water and is harmless when added to water in any 

 amount, as far as the water life is concerned. 



The decomposition may be carried on entirely under 

 aerobic conditions. This is accomplished by conducting 

 the sewage into tanks having false bottoms of a porous ma- 

 terial through which air can be forced. Under such con- 

 ditions there soon becomes established a bacterial flora that 

 is capable of immediately decomposing the organic matter. 

 After a period of aeration, the air is tunied off. The bac- 

 teria, settle rapidly, so that the upper two thirds of the con- 

 tents <.f the tank can be drained off, carrying the soluble 

 products, and leaving the bacteria, to act quickly and com- 

 pletely on the next quantity of organic matter presented 

 to them. This process is called the "activated sludge" pro- 

 cess. 



The farm home will find it most convenient to use the 

 septic tank for the disposal of the sewage, and to apply the 

 effluent of the tank either to the surface of the soil or be- 

 neath the surface by means of tile. It is necessary to 

 establish conditions that will be favorable to the growth of 

 the essential classes of bacteria. The installation of such a 

 plant is a separate problem for each home. All that can 

 here be done is to point out the necessary conditions that 

 must be established. The tank must be of such a size that 

 it will hold at least twice the amount of sewage produced 

 daily. Before the sewage passes into the septic tank, it is 

 desirable to have it flow through a grease trap to remove 

 the fat that is found in the kitchen waste ; for the grease is 

 quite resistant to decomposition and may clog the drain tile. 



