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

can slough off the sidewall. These factors tend to make the sidewall
clogging less severe than the bottom surface. However, the hydraulic
gradient across the sidewall mat is also less. At the bottom surface,
gravity, the hydrostatic pressure of the ponded water above, and the
matric potential of the soil below the mat contribute to the total
hydraulic gradient. At the .sidewall, the gravity potential is zero, and
the hydrostatic potential diminshes to zero at the liquid surface. Be-
cause the matric potential varies with changing soil moisture condi-
tions, it is difficult to predict which infiltrative surface will be
more effective.
In humid regions where percolating rainwater reduces the matric poten-
tial along the sidewall, shallow trench systems are suggested (41. The
bottom area is the principal infiltrative surface in these systems.
Shallow trenches often are best because the upper soil horizons are usu-
ally more permeable and greater evapotranspiration can occur. In dry
climates, the sidewall area may be used to a greater extent. The bottom
area may be reduced as the sidewall area is increased. Common practice
is not to give credit to the first 6 in. (15 cm) of sidewall area mea-
sured from the trench bottom, but any exposed sidewall above 6 in. (15
cm1 may be used to reduce the bottom area (3)(11). The infiltration
rates given in Table 7-2 may be used for sidewall areas.
Trench versus Bed Design: Because beds usually require less total land
area and are less costly to construct, they are often installed instead
of trenches. However, trenches are generally more desirable than beds
(4)(11)(12)(13)(14). Trenches can provide up to five times more side-
wall area than do beds for identical bottom areas. Less damage is
likely to occur to the soil during construction because the excavation
equipment can straddle the trenches so it is not necessary to drive on
the infiltrative surface. On sloping sites, trenches can follow the
contours to maintain the infiltrative surfaces in the same soil horizon
and keep excavation to a minimum. Beds may be acceptable where the site
is relatively level and the soils are sands and loamy sands.
Shallow versus Deep Absorption Systems: Shallow soil absorption systems
offer several advantages over deep systems. Because of greater plant
and animal activity and less clay due to eluviation, the upper soil ho-
rizons are usually more permeable than the deeper subsoil. Also, the
plant activity helps reduce the loading on the system during the growing
season by transpiring significant amounts of liquid and removing some
nitrogen and phosphorus from the waterwater. Construction delays due to
wet soils are also reduced because the upper horizons dry more quickly.
On the other hand, deep systems have advantages. Increased depths per-
mit increased sidewall area exposure for the same amount of bottom area.
They also permit a greater depth of liquid ponding which increases the
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