Most septic tanks are built of concrete cast in place, since in this
way there is a minimum possibility of cracks developing. Concrete
blocks, however (not cinder blocks), stone, brick, or structural tile
are sometimes used. Prefabricated commercial tanks of concrete and
various other materials also are available.
[Illustration]
+--------------------------------------------------------------+
| SIPHON |
+--------------------------+-----------------------------------+
|_Diameter of siphon |_Clearance under bell |
| A - 3" or 4"_ | E - 2"_ |
+--------------------------+-----------------------------------+
|_Diameter of bell |_Distance across U-trap |
| B - 10"or 12"_ | F - 10" or 12"_ |
+--------------------------+-----------------------------------+
|_Bottom of outlet |_Bottom of outlet |
| to discharge line | to bottom of U-trap |
| C - 20-1/2" to 25-3/4"_| G - 12" or 13"_ |
+--------------------------+-----------------------------------+
|_Drawing depth |_Height above floor |
| D - 13" to 17"_ | H - 7-1/4"to 11-3/4"_ |
+--------------------------+-----------------------------------+
| DIMENSIONS OF DOSING CHAMBER |
+-----------------+-------------------+------------+-----------+
| _Number of |_Depth below | | |
| persons served_ | discharge line_[3]| _Width_[4] | _Length_ |
+-----------------+-------------------+------------+-----------+
| 4 or less | 16-1/4" to 20-1/4"| 3'-0" | 6'-0" |
+-----------------+-------------------+------------+-----------+
| 6 | " | 3'-0" | 7'-0" |
+-----------------+-------------------+------------+-----------+
| 8 | " | 3'-6" | 7'-6" |
+-----------------+-------------------+------------+-----------+
| 10 | " | 3'-6" | 8'-6" |
+-----------------+-------------------+------------+-----------+
| 12 | " | 4'-0" | 8'-6" |
+-----------------+-------------------+------------+-----------+
| 14 | " | 4'-0" | 9'-0" |
+-----------------+-------------------+------------+-----------+
| 16 | " | 4'-6" | 10'-0" |
+-----------------+-------------------+------------+-----------+
[3] Depending upon depth C of siphon.
[4] Same as single chamber tank fig. 4.
Figure 5.--Typical design for a concrete septic tank with a dosing
chamber and a siphon.
Masonry units should be laid in full beds of 1:3 cement mortar and
the walls and floor plastered with at least a 1/2-inch coat of 1:2
mortar. Cells of concrete blocks and tile must be filled with concrete.
Masonry walls are generally 8 inches thick, and care must be taken to
follow _inside_ dimensions given for concrete tanks. Directions for
laying structural tile, brick, and concrete blocks can be obtained from
dealers or trade associations.
Commercial tanks are suitable if they embody the essential features
given in this bulletin. Capacities should be as recommended in figure
4 for concrete tanks. Proper installation and periodic servicing also
are essential. Tanks badly damaged in handling should not be used.
Rapid corrosion of steel tanks will result if the asphalt coating is
impaired. Minor defects in precast masonry tanks may often be overcome
by plastering the interior with cement mortar.