SANITATION 



389 



of light, heat, air, and current, the solid matter will be 

 decomposed and the waste from a septic tank will be much 

 less dangerous and objectionable than from a cesspool. As 

 the natural conditions for the proper working of a cesspool as 

 a septic tank seldom occur, it is desirable in the beginning to 



y 



1-^ lit _ \ ^ 



"*-"»//' = 



After University of Minnesota 



Fig. 303. General view of a septic tank 



construct a tank in such a manner that the bacterial growth 

 will be promoted. These bacteria thrive 'best where there 

 are a limited amount of light and air, a certain degree of heat, 

 and a surface unbroken by flowing water. These condi- 

 tions are secured by the discharge of the sewage into a tank 

 from which the light is excluded by a covering, air being 

 admitted through a small vent. Heat is supplied by the 



