126 AGRICULTURAL BACTERIOLOGY. 



paring favorably with ordinary river water. This process is 

 sometimes called filtering, although the name is quite erro- 

 neous. The sewage is not filtered in any sense but simply 

 decomposed by bacterial action. 



To allow this bacterial decomposition to take place as com- 

 pletely as possible, two different methods of treatment are at 

 the present time employed, which may be used separately but 

 are frequently combined to advantage. The first method is 

 by what is called the contact beds, sometimes called filters. 

 In this method the sewage is allowed to flow into large open 

 basins the bottoms of which are covered with a layer of coke 

 or furnace clinkers. The sewage remains in this bed for six 

 to twelve hours when it is conducted into a second of a 

 similar nature. After remaining here about the same length 

 of time it has become so much purified by the bacteria that it 

 may be allowed to flow into a neighboring stream without 

 excessive pollution. 



The second method is by the septic tank. This is a large 

 closed chamber, perhaps below the surface of the ground^ and 

 closed upon all sides and the top, with simply a vent pipe ex- 

 tending from the top to allow the escape of gases. The sew- 

 age is passed into one end of the tank in a somewhat slow but 

 constant stream, and the cavity of the tank is so divided by 

 partitions as to insure the sewage a slow uniform passage 

 through the tank, and a final exit at the other end by an efflu- 

 ent pipe. The flow is regulated so that each particle of sew- 

 age remains in the tank from 24 to 48 hours. The effluent 

 water is very much purer from organic matter than the inflow. 

 If this effluent is then conducted upon a contact bed the puri- 

 fication is still further increased. 



The change which takes place in the sewage is due to bac- 

 terial action and is dependent upon the principles already pointed 

 out. There are two quite different phases of the decomposi- 

 tion which follow each other without any sharp demarkation. 



