504 



AGRICULTURAL ENGINEERING 



as a means of subirrigation, furnishing the growing plants on 

 the surface with moisture and fertility. It is to be noted that 

 the discharge into the filter system should be intermittent, in 

 order that the bacteria at work shall not be drowned. 



Another plan of filtering which is used to some extent is 

 to allow the discharge to trickle down through a bed of sand, 

 which is placed over a perforated cover, to a second tank in 

 which the water level is maintained several inches below. 

 The dripping of the sewage through the air corresponds quite 



Fig. 309. 



Section of a septic tank made entirely of concrete. 

 a siphon and a filter bed of sand and gravel. 



It has 



closely to the sprinkling system of sewage disposal which is 

 used to some extent in city plants. 



Size of Septic Tank. The septic tank should be suffi- 

 ciently large to hold the entire discharge for about one day, 

 in which case the best bacterial action will be obtained. 

 Another rule tried out more or less by practice is to provide 20 

 gallons' capacity for each person in the household. There will 

 be a settlement amounting to several pailfuls in the septic 

 tank each year, and provision must be made for its removal. 



Construction of the Septic Tank. Concrete is the best 

 material for the septic tank. The tile line to the tank from 

 the house should be of vitrified bell-mouthed tile with 

 cemented joints. 



