112 



AGRICULTURAL BACTERIOLOGY 



solid matter settles on the bottom and forms a sludge. 

 There soon accumulates on the surface a scum in which a 

 portion of the gases formed in the 'decomposition is held. 

 This excludes the air in a very perfect manner. If the 



Fig. 22. A Septic Tank 



The larger compartment is the septic tank proper in which the organic matter 



of the sewage is exposed to bacterial action. The second compartment or 



dosing chamber is gradually filled from the septic tank and then emptied by tho 



automatic syphon into tile drains 



sewage is allowed to remain in the tank from twenty-four 

 to forty-eight hours, the solid matter will be liquefied, the 

 protein changed to ammonia, the carbohydrates to acids 

 and gases. The tanks are frequently called digestion tanks 

 because of the nature of the changes that go on in them. 

 The effluent from the tank is turbid and has a disagreeable 

 odor. This partially decomposed material is often turned 

 into a body of water, or it may be so treated that the de- 

 composition will be more complete. The more complete 

 changes, which include such processes as the oxidation 



