514 WATER SUPPLY AND SEWAGE DISPOSAL 



solid matter settles through this opening into the sludge 

 chamber below. It is chiefly in the sludge chamber that 

 LIQUEFACTION or DIGESTION takes place as a result of the action 

 of the anaerobic bacteria. This type of septic tank is con- 

 sidered superior because the contents of the sludge chamber are 

 but slightly disturbed by, or mixed with, the constant in-flow of 

 fresh sewage. The bacterial action is, therefore, more certain 

 and perfect. 



FIG. 320. Imhoff tank showing sludge formation at sides. Settling 

 chamber in center. Dosing chamber in foreground. Sewer inlet shown at 

 far end. (Julietta; Ind.) (Courtesy of Chas. Brossman.) 



604. Sludge and Its Disposal. Even when operating at its 

 best, considerable insoluble material accumulates in a septic 

 tank. This accumulation is known as SLUDGE. Occasionally 

 the sludge must be removed from a septic tank. It is claimed 

 that the Imhoff type of septic tank produces a sludge more 

 solid and more readily handled than that produced by tanks of 

 the type shown in Figs. 316 and 317. Sludge from septic 

 tanks is valuable as fertilizer. 



605. The Complete Oxidation of the Sewage. In the 

 septic tank only partial oxidation of the organic matter in the 

 sewage ever takes place. While the outflowing sewage from 

 the septic tank should be fairly clear and free from sediment, 

 it still contains large amounts of undecomposed organic matter 



