114 AGRICULTURAL BACTERIOLOGY 



The tank must be so arranged that the sewage will enter 

 without disturbing the sediment or the layer of scum. The 

 inlet must be below the surface, and cross-partitions should 

 be placed in the tank to prevent currents and to keep all 

 portions of the sewage in the tank for the same time. A 

 second chamber is provided, into which sewage enters from 

 the first tank. The septic tank proper is thus kept con- 

 tinually full. 



If the digested sewage is applied to the surface of the 

 soil or discharged into underdrains, it is necessary to dis- 

 charge it at intervals rather than constantly, so as to give 

 the water time to drain away and the air to enter in order 

 that the decomposition may be completed. The second tank 

 should hold from one third to one quarter of the daily 

 volume of sewage. It is called the dosing chamber, and 

 some means must be provided by which it can be emptied 

 when full. This can be accomplished by installing a gate- 

 valve at the bottom which can be opened and closed from 

 the surface. A more convenient arrangement is the auto- 

 matic syphon, by which the tank is emptied whenever the 

 sewage reaches a certain depth. 



The drains into which the sewage is discharged are placed 

 from eight to ten inches below the surface, arid are laid 

 to grade with open joints, as in the case of ordinary tile 

 drains. When the tank is emptied, the sewage flows into 

 the tile and fills the entire length of the drain. It passes 

 out of the open joints and percolates into the soil, where 

 the last steps in the decomposition of the organic matter 

 take place, just as in the filter beds used in the disposal 

 of municipal sewage. If the sewage were allowed to flow 

 constantly from the tank in a small stream, it would find its 

 way into the soil through the first few joints of the tile; 

 the soil in the immediate neighborhood would be kept water- 

 logged, and the oxidation processes could not go on. The 



