36 



BULLETIN 57, U. S. DEPARTMENT OF AGEICULTUKE. 



The discharge chamber should be of such capacity and depth as to 

 discharge about every 10 to 12 hours. It may be desirable to dis- 

 charge at more or less frequent mtervals, according to the nature of 

 the soil in the disposal area, and this may be controlled by the 

 arrangement of discharge chamber and siphon. Where little outlet 

 fall is available it is possible to so construct the discharge chamber 

 that its floor will be considerably above that of the settling chamber. 



The capacity and depth of discharge chamber and the size of siphon 

 will depend on the number of persons served and the means of disposal. 

 If a sand filter is used or a distribution system in heavy loam, the 

 discharge chamber must be larger and deeper, in order that the 

 discharge interval may be lengthened and the distribution system 

 be given ample time to aerate. If the distribution is in sandy or 

 very porous soil the discharge may be more frequent. 



The following table of dimensions of septic tanks suggests sizes of 

 settling and discharge chambers and the corresponding siphon sizes 

 to apply to various average conditions. The depths of siphon cham- 

 ber given are the minimum allowable. 



Dimensions of septic tanks. 











Siphon chamber. 





Settling chamber. 







Num- 

 ber per- 





Sand filter or heavy 



Sandy or porous soil 



Siphon 

 diam- 





loam distribution. 



distribution. 











ter 





Width 



Length 



Depth. 



Width 



Length 



Mini- 



AVidth 



Length 



Mini- 









inside. 



inside. 



inside. 



inside. 



depth. 



inside. 



inside. 



depth. 





Feet: 



Feet. 



Feet. 



Feei. 



Feet. 



Ft. in. 



Feet. 



Feet. 



Ft. in. 



Inches. 



6 



i 



6 



^ 



4 



3 



2 4 



3 



2 



2 4 



3 



8 



4 



6i 



4 



4 



4 



2 4 



3 



^ 



2 4 



3 



12 



4 



7 







4 







2 5 



3 



4 



2 5 



4 



15 



4 



8 







4 



6 



2 5 



3 



4 



2 5 



4 



25 



4 



10 







4 



6i 



3 2 



SI 



4 



3 2 







35 



i-J 



12 







4 



64 



3 2 



3| 



4i 



3 2 







The above table is computed on the basis that the inlet and outlet 

 of the settling chamber should be placed with theh inverts 12 inches 

 below the roof of the tank, thus making the depth of sewage m both 

 settling and discharge chamber 12 inches less than the mean inside 

 depth. 



The tank dimensions given in the above table, it should be remem- 

 bered, are for average cases only and are not standard for all such 

 cases. They are subject to such variations to suit local conditions 

 as the farmer's judgment indicates; yet care should be taken not to 

 vary any of the esssential dimensions and not to go below the given 

 minimum depth of siphon chamber. 



