236 



FARM MECHANICS 



nearly air-tight as possible. The scientific working of a 

 septic tank depends upon the destructive work of two 

 kinds of microscopic life known as aerobic and anarobic 

 forms of bacteria. Sewerage in the first tank is worked 

 over by aerobic bacteria, the kind that require a small 

 amount of oxygen in order to live and carry on their 



Figure 238. Brass Valves. Two kinds of globe valves are used in 

 farm waterworks. The straight valve shown to the left and the 

 right angle valve to the right. Either one may be fitted with a long 

 shank to reach above ground when pipes are laid deep to prevent 

 freezing. 



work. The second compartment is inhabited by anaero- 

 bic bacteria, or forms of microscopic life that work 

 practically without air. The principles of construc- 

 tion require that a septic tank shall be large enough to 

 contain two days ' supply of sewerage in each compart- 

 ment; thus, requiring four days for the sewerage to 

 enter and leave the tank. 



Estimating 75 gallons daily of sewerage for each in- 

 habitant of the house and four persons to a family, the 

 septic tank should be large enough to hold 600 gallons, 



