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

7.2 Subsurface Soil Absorption
7.2.1 Introduction
Where site conditions are suitable, subsurface soil absorption is
usually the best method of wastewater disposal for single dwellings
because of its simplicity, stability, and low cost. Under the proper
conditions, the soil is an excellent treatment medium and requires
little wastewater pretreatment. Partially treated wastewater is
discharged below ground surface where it is absorbed and treated by the
soil as it percolates to the groundwater. Continuous application of
wastewater causes a clogging mat to form at the infiltrative surface,
which slows the movement of water into the soil. This can be beneficial
because it helps to maintain unsaturated soil conditions below the
clogging mat. Travel through two to four feet of unsaturated soil is
necessary to provide adequate removals of pathogenic organisms and other
pollutants from the wastewater before it reaches the groundwater.
However, it can reduce the infiltration rate of soil substantially.
Fortunately, the clogging mat seldom seals the soil completely.
Therefore, if a subsurface soil absorption system is to have a long
life, the design must be based on the infiltration rate through the
clogging Mt. that ultimately forms. Formation of the clogging mat
depends primarily on loading patterns, although other factors may impact
its development.
7.2.1.1 Types of Subsurface Soil Absorption Systems
Several different designs of subsurface soil absorption systems may be
used. They include trenches and beds, seepage pits, mounds, fills, and
artificially drained systems. All are covered excavations filled with
porous media with a means for introducing and distributing the waste-
water throughout the system. The distribution system discharges the
wastewater into the voids of the porous media. The voids maintain expo-
sure of the soil's infiltrative surface and provide storage for the
wastewater until it can seep away into the surrounding soil.
These systems are usually used to treat and dispose of septic tank ef-
fluent. While septic tank effluent rapidly forms a clogging mat in most
soils, the clogging mat seems to reach an equilibrium condition through
which the wastewater can flow at a reasonably constant rate, though it
varies from soil to soil (l)(2)(3)(4). Improved pretreatment of the
wastewater does not appear to reduce the intensity of clogging, except
in coarse granular soils such as sands (4)(5)(6).
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