512 WATER SUPPLY AND SEWAGE DISPOSAL 



page 408) attack the sewage and by decomposing it soon cause 

 the solid material to dissolve, or, as we say, to LIQUEFY. When 

 working properly, this process requires but a short time to 

 liquefy most solid material in sewage. Even paper and cloth 

 are liquefied within a few weeks or months. The liquefying 

 action of these bacteria sets free considerable quantities of 

 carbon dioxide, ammonia, and other gases which escape around 

 the cover of the tank; it also breaks all fats into small particles, 

 which rise to the surface of the liquid and there form a tough, 

 leathery scum which completely excludes the air, thus pro- 

 ducing ideal conditions for the existence of anaerobic bacteria. 

 To prevent the still undissolved solids from being stirred up 



Underdrams 



f Plan / 

 Tank -= 



or Sand 



Onderdratn- 7 



FIG. 318. Plan and section showing septic tank and subsoil filter. 

 (Courtesy of Chas. Brossman.) 



by the in-rush of fresh sewage a partition, or BAFFLE PLATE, 

 is placed across the tank in front of the opening of the house 

 drain. 



From compartment 1 (Fig. 316) the liquid sewage passes 

 into compartment 2. In this compartment the bacterial ac- 

 tion still continues, but the chief purpose of the compartment 

 is to serve as a storage tank. It is called the DOSING TANK. 

 The sewage in this compartment should be fairly clear and 

 nearly free from sediment, but still contains large amounts of 

 undecomposed organic matter. The sewage accumulates in 

 this compartment till it is nearly full and is then drawn off 

 through the INTERMITTENT SIPHON in compartment 3 into the 



