The tank should be large enough to retain the sewage at least 24 hours.
The size should be determined by the largest number of persons that may
live in the house, rather than by the number actually living there at
the time the tank is built. The additional cost of a large tank over a
small one is relatively little. If there is any question as to which
of two sizes should be built, it is wise to choose the larger. The
dimensions recommended in the table in figure 4 are based on an average
production of 50 gallons of sewage per person per day.
Unusually large quantities of sewage call for a tank of large capacity.
In village and suburban homes where there is less food preparation
than on farms and where the number of persons is more or less fixed,
slightly smaller sizes will serve. In no case should the capacity of
the tank below the flow line be less than 500 gallons. A tank length of
two to three times the width should be maintained, and it is advisable
to provide a depth of at least 4 feet below the flow line.
Allow about 1 foot of "freeboard," or air space, above the flow line
for the accumulation of gases. This space is generally vented through
the soil stack of the house.
A siphon (fig. 5) with a dosing chamber is not considered necessary
for a farm septic tank except for large installations (1,000 gallons
or more), for those in tight soils, and where the disposal field is
limited.
[Illustration]
+---------------------------------------------------------------------------+
| CAPACITIES, DIMENSIONS, AND CONCRETE MATERIALS |
| FOR SEPTIC TANKS SERVING INDIVIDUAL DWELLINGS |
+---------+--------+--------------------------------+-----------------------+
|_Maximum |_Liquid |_Recommended inside dimensions_ |_Materials for concrete|
|number of|capacity+-------+--------+-------+-------+ 1:2-1/2:4 mix_ |
|persons |of tank |_Width_|_Length_|_Liquid|_Total +-------+-------+-------+
|served_ | in | | | depth_| depth_|_Cement|_Sand |_Gravel|
| |gallons_| | | | | sacks_| cubic | cubic |
| | | | | | | | yards_| yards_|
+---------+--------+-------+--------+-------+-------+-------+-------+-------+
|4 or less| 500 | 3'-0" | 6'-0" | 4'-0" | 5'-0" | 16 | 1-1/2 | 2-1/2|
+---------+--------+-------+--------+-------+-------+-------+-------+-------+
|6 | 600 | 3'-0" | 7'-0" | 4'-0" | 5'-0" | 17 | 1-3/4 | 2-3/4|
+---------+--------+-------+--------+-------+-------+-------+-------+-------+
|8 | 750 | 3'-6" | 7'-6" | 4'-0" | 5'-0" | 19 | 2 | 3 |
+---------+--------+-------+--------+-------+-------+-------+-------+-------+
|10 | 900 | 3'-6" | 8'-6" | 4'-0" | 5'-0" | 21 | 2-1/4 | 3-1/4|
+---------+--------+-------+--------+-------+-------+-------+-------+-------+
|12 | 1100 | 4'-0" | 5'-6" | 4'-6" | 5'-6" | 24 | 2-1/4 | 3-1/2|
+---------+--------+-------+--------+-------+-------+-------+-------+-------+
|14 | 1200 | 4'-0" | 5'-0" | 4'-6" | 5'-6" | 25 | 2-1/2 | 3-3/4|
+---------+--------+-------+--------+-------+-------+-------+-------+-------+
|16 | 1500 | 4'-6" | 10'-0" | 4'-6" | 5'-6" | 28 | 2-3/4 | 4-1/4|
+---------+--------+-------+--------+-------+-------+-------+-------+-------+
Figure 4.--Single-chamber septic tank. Note alternate use of baffle
boards where sanitary tees are omitted at inlet and outlet.
The siphon provides intermittent discharge of effluent, which allows
time for the disposal area to rest and aerate between discharges. This
is more important where the discharge is nearly continuous than in
small installations.
The frequency and volume of the discharge into the tile field are
controlled by the sizes of the siphon and the dosage chamber. The
dealer should be informed of the size of the tank and the number of
persons in the household, in order that he may furnish the proper
unit. A 3- or 4-inch siphon will be adequate for almost any farmhouse
installation.