516 WATER SUPPLY AND SEWAGE DISPOSAL 



a piece of a larger tile is laid over each joint. The liquid sew- 

 age readily passes out into the soil through these open joints. 

 Here it is attacked by AEKOBIC BACTERIA (Art. 465) and is 

 completely decomposed, i.e., it is completely mineralized. 



Since aerobic bacteria live and multiply only in the presence 

 of an abundance of air, they are to be found in large numbers 

 only near the surface of well-drained soil. It is because the 

 aerobic bacteria can not survive without an abundant supply 

 of air that the intermittent siphon is used to empty the septic 

 tank. If the discharge from the tank were constant and steady, 

 the ground surrounding the upper end of the drain would con- 

 stantly be water-soaked, thus preventing air from entering the 

 soil, and therefore preventing the sewage from being acted 

 upon by aerobic bacteria. By using the siphon, the contents of 

 the tank are completely discharged into the drain once in from 

 6 to 24 hours, and the volume discharged, at one time, is 

 sufficient to fill the drain its entire length. The area covered 

 by the drain is intended to be great enough to insure the com- 

 plete oxidation and mineralization of the sewage of one dis- 

 charge before the next discharge occurs. 



607. The Contact Filter Bed. When the amount of sewage 

 to be handled is too great, or the character of the soil is such as 

 not to permit of the successful use of the subsoil drain, contact 

 beds are provided (Figs. 319 and 321). These beds are merely 

 beds of gravel, broken stone, or coarse sand. The sewage is 

 run out from the dosing tank upon the surface of these beds. 

 As it soaks down through the sand or gravel, the organic matter 

 adheres to, or is deposited upon, the surface of the rock particles, 

 where it is attacked and destroyed by aerobic bacteria. The 

 sewage is retained in the contact bed for a fixed period of time 

 and then is drawn off, thus permitting air to enter all spaces 

 between the rock particles, a necessary condition for the growth 

 and multiplication of aerobic bacteria. The water drawn off 

 from such a bed should be practically free from organic or other 

 injurious matter. 



