222 Agricultural Bacteriology. 



logged. With the discharge at intervals, the entire line 

 of tile is filled and no part of the soil is kept in a satu- 

 rated condition. 



The tiles should be laid 18-24 inches below the sur- 

 face, the trench about the tile should be filled with cin- 

 ders or gravel so that the water may pass out of the tiles 

 freely. It is not necessary to place the tile below the 

 frost line, although they will be somewhat disintegrated 

 by freezing and the system will have to be dug up more 

 frequently than if freezing did not occur. If the tile are 

 laid too deeply, the amount of oxygen is small and nitrifi- 

 cation is retarded. In a loose, sandy soil, the tile can 

 be laid deeper than in a dense soil and a much shorter 

 line of tile will suffice. By increasing the length of the 

 tile drains, the system will work in a satisfactory manner 

 in a close clay soil. The tile can be laid beneath a cul- 

 tivated field or beneath the lawn since they need be re^ 

 moved and cleaned only at intervals of several years. 



The essential parts of the system are: (1) a supply of 

 water for the flushing of closets, etc. ; (2) the house so sit- 

 uated that a small amount of fall can be had away from 

 it in order that the septic tank can be emptied into the 

 drains; (3) a septic tank large enough to hold the quan- 

 tity of sewage produced in three or four days so that 

 ample time can be given for decomposition of the solid 

 matter; (4) a method of discharging a quantity of sew- 

 age at intervals. 



In the accompanying figure the general arrangement of 

 a sewage disposal plant is given. The main drain enters 

 the septic tank below the surface of the liquid so as to 

 avoid disturbing the scum on the surface or the sediment. 

 The* first compartment, the septic tank proper, should 

 hold at least three days sewage and is separated from the 



