34 



BULLETIN 57, U. S. DEPARTMENT OP AGEICULTURE. 



l/&r>fifo*i'oi>t 





hours givos the best results and the least annoyance. The soHd 

 matter settles out m such a tank and, accordmg to the theory at 

 present accepted, it is partially liquefied, deodorized, and destroyed 



by countless numbers 

 of bacteria, which 

 thrive in filth and live 

 without air. Some 

 authorities assert that 

 these bacteria also 

 slightly affect the dis- 

 solved organic matter 

 in raw Hquid sewage. 

 In such a tank a 

 thick scum forms on 

 the surface of the sew- 

 age, which protects 

 the bacteria from the 

 incoming air and is 

 evidence of good bac- 

 terial action. The 

 breaking up or dis- 

 turbance of this scum 

 will destroy the bac- 

 terial action for the 

 time being and is 

 likely to cause con- 

 siderable annoyance 

 by bad odors. 



FINAL TREATMENT. 



It is found that the 

 septic tank effects 

 only about 40 per cent 

 purification. The 

 liquefying action in 

 the tank, however, 

 makes it possible to 

 subject th0 sewage to 

 a final treatment by 

 filtration or distribu- 

 tion in a natural soil. 

 This final purification 

 is effected by means of bacteria which work in the air. Therefore it is 

 necessary that the sewage be applied to the final disposal system in 

 ixitermittent doses so that the system may have a chance to air out. 



7^ *S G wvc?/- -^ 



Fig. 27.— Plumbing system for .sewage disposal. 



