On-Site Wastewater Treatment and Disposal Systems - Forced ...

The acceptable depth to an impermeable layer or rock strata is site spe-
cific. Sufficient depth must be available to channel the percolating
wastewater away from the mound (see Figure 7-10). If not, the soil
beneath the mound and the fill material may become saturated, resulting
in seepage of effluent on the ground surface. The suggested depths to
an impermeable layer given .in Table 7-7 may be adjusted in accordance
with the site characteristics. Soil permeability, climate, slope, and
mound layout determine the necessary depth. Slowly permeable soils
require a greater depth to remove the liquid than do permeable soils.
Frost penetration reduces the effective depth and therefore a greater
depth is required in areas with severe winters. Level sites require a
greater depth because the hydraulic gradients in the lateral direction
are less than on sloping sites. Finally, mound systems extended along
the contour of a sloping site require less depth than a square mound.
Not enough research information is available to give specific depths for
these various conditions. Until further information is available,
mounds on slowly permeable soils should be made as long as possible,
with the restricting layer at least 3 ft (0.9 ml below the natural soil.
b. Influent Wastewater Characteristics
The wastewater entering the mound system should be nearly free from set-
tleable solids, greases, and fats. Septic tanks are commonly used for
pretreatment and have proved to be satisfactory. Water softener wastes
are not harmful to the system nor is the use of common household chemi-
cals and detergents (9)(10).
7.2.4.3 Design
a. Fill Selection
The mound design must begin with the selection of a suitable fill mater-
ial because its infiltrative capacity determines the required absorption
bed area. Medium texture sands, sandy loams, soil mixtures, bottom ash,
strip mine spoil and slags are used or are being tested (24). To keep
costs of construction to a minimum, the fill should be selected from
locally available materials. Very permeable materials should be
avoided, however, because their treatment capacity is less and there is
a greater risk of surface seepage from the base of the mound when used
over the more slowly permeable soils. Commonly used fill materials and
their respective design infiltration rates are presented in Table 7-8.
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