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

Conventional trench or bed designs should not be used for rapidly perme-
able soils with percolation rates faster than 1 min/in. (0.4 min/cm)
(11). The rapidly permeable soils may not provide the necessary treat-
ment to protect the groundwater quality. This problem may be overcome
by replacing the native soil with a suitably thick (greater than 2 feet)
layer of loamy sand or sand textured soil. With the liner in place, the
design of the system can follow the design of conventional trenches and
beds using an assumed percolation rate of 6 to 15 min/in. (2.4 to 5.9
min/cm).
Conventional trench or bed designs should also be avoided in soils with
percolation rates slower than 60 min/in. (24 min/cm). These soils can
be easily smeared and compacted during construction,reducing the soil's
infiltration rate to as little as half the expected rate (12). Trench
systems may be used in soils with percolation rates as slow as 120
min/in (47 min/cm), but only if great care is exercised during construc-
tion. Construction should proceed only when the soil is sufficiently
dry to resist compaction and smearing during excavation, This point is
reached when it crumbles when trying to roll a sample into a wire be-
tween the palms of the hands. Trenches should be installed so that con-
struction machinery need not drive over the infiltrative surface. A 4-
to 6-in. (lo- to 15-cm) sand liner in the bottom of the trench may be
used to protect the soil from compaction during placement of the aggre-
gate and to expose infiltrative surface that would otherwise be covered
by the aggregate (11)(13).
b. Geometry of the Infiltrative Surface
Sidewalls as Infiltrative Surfaces: Both the horizontal bottom area and
the vertical sidewalls of trenches and beds can act as infiltrative surfaces.
When a gravity-fed system is first put into service, the bottom
area is the only infiltrative surface. However, after a period of
wastewater application, the bottom can become sufficiently clogged to
pond liquid above it, at which time the sidewalls become infiltrative
surfaces as well. Because the hydraulic gradients and resistances of
the clogging mats on the bottom and sidewalls are not likely to be the
same, the infiltration rates may be different. The objective in design
is to maximize the area of the surface expected to have the highest
infiltration rate while assuring adequate treatment of wastewater and
protection of the groundwater.
Because the sidewall is a vertical surface, clogging may not be as se-
vere as that which occurs at the bottom surface, due to several fac-
tors: (1) suspended solids in the wastewater may not be a significant
factor in sidewall clogging; (2) the rising and falling liquid levels in
the system allow alternative wetting and drying of the sidewall while
the bottom may remain continuously inundated; and (3) the clogging mat
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