Section III. SEPTIC TANK PRACTICE
a. Reasons for Septic Tanks. Wastewater treatment presents special
problems for very small installations or small, isolated units. It is not always
economically feasible to install separate wastewater treatment facilities for such
installations and units. Whenever possible, a small installation should make
arrangements to discharge its wastewater into an adequate public or community
system. When no such system is available, where soil and site conditions are
favorable, and where it is not prohibited by local and/or state ordinances, a properly
constructed and installed septic tank system can be expected to give satisfactory
b. Septic Tank Systems. The basic principle of a septic tank system (see
Figure 1-9) follows. The liquid contents of the house sewer (A) are discharged into the
septic tank (B) where they undergo primary treatment by sedimentation and anaerobic
decomposition (secondary treatment) of the sludge that settles out. The treated effluent
is discharged into an absorption field (C) where it receives secondary treatment by
aerobic bacterial action and is disposed of by percolation through soil. Therefore,
before a septic tank system is installed, three important criteria must be met.
(1) It must be determined that the installation of such a system is permitted
under all local, state, and Federal ordinances governing the particular locality. This
determination may be made by liaison with the local health authorities.
(2) It must be determined that the absorptive capacity of the soil in the
locality is sufficient to permit disposal of the effluent without creating a nuisance, such
as ponding because of the inability of the soil to absorb the effluent.
(3) It must be determined that the area available for installing the system is
sufficient to provide for adequate disposal while maintaining the required distances from
underground water sources, buildings, and property lines.
1-13. SUITABILITY OF SOIL
a. General. The first step in the design of a septic tank system is to determine
whether the soil is suitable for the absorption of septic tank effluent and, if so, how much
area is required. The soil must have an acceptable percolation rate without interference
from ground water or impervious strata below the level of the absorption system. In
general, the percolation rate should not exceed 60 minutes per inch of fall in water level
(not over 30 minutes for seepage pits (see para 1-17a(2)) and the depth of the
maximum seasonal elevation of the ground water table or impervious strata (rock
formation, etc.) should be at least 4 feet below the bottom of the absorption trench.