36 
BULLETIN 57, U.- S. DEPARTMENT OF AGRICULTURE. 
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 intervals, 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. 
Num- 
ber per- 
sons. 
Siphon chamber. 
Settling chamber. 
Sand filter or heavy 
loam distribution. 
Sandy or porous soil 
distribution. 
Siphon 
diam- 
ter. 
Width 
inside. 
Length 
inside. 
Depth. 
Width 
inside. 
Length 
inside. 
Mini- 
mum 
depth. 
Width 
inside. 
Length 
inside. 
Mini- 
mum 
depth. 
6 
8 
12 
15 
25 
35 
Feet. 
4 
4 
4 
4 
4 
41 
Feet. 
6 
61 
7 
8 
10 
12 
Feet. 
3| 
4 
5 
5 
5 
5 
Feet. 
4 
4 
4 
4 
4 
4 
Feet. 
3 
4 
5 
6 
6£ 
6| 
Ft. in. 
2 4 
2 4 
2 5 
2 5 
3 2 
3 2 
Feet. 
3 
3 
3 
3 
3| 
31 
Feet. 
2 
2| 
4 
4 
4 
4* 
Ft. in. 
2 4 
2 4 
2 5 
2 5 
3 2 
3 2 
Inches. 
3 
3 
4 
4 
5 
5 
The above table is computed on the basis that the inlet and outlet 
of the settling chamber should be placed with their inverts 12 inches 
below the roof of the tank, thus making the depth of sewage in 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. 
