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.