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

The liquid level in the trenches is established by the elevation of the
overflow invert leading to the succeeding drop box. If the elevation of
this invert is near the top of the rock in the trench, the entire trench
sidewall will be utilized, maximum hydrostatic head will be developed to
force the liquid into the surrounding soil, and evapotranspiration by
plants during the growing season will be maximized by providing a supply
of liquid to the overlying soil.
The drop box design has several advantages over single lines, closed
loop, and distribution box networks for continuously ponded systems. It
may be used on steeply sloping sites without surface seepage occurring
unless the entire system is overloaded. If the system becomes over-
loaded, additional trenches can be added easily without abandoning or
disturbing the existing system. Drop box networks also permit unneeded
absorption trenches to rest and rejuvenate. The lower trenches are
rested automatically when flows are low or infiltration capacity is
high. The upper trenches may be rested when necessary by plugging the
drop box lateral outlets.
c. Closed Loop
In absorption systems where the entire infiltrative surface is at one
elevation, such as in beds or multi-trench systems on level or nearly
level sites, closed loop networks may be used. The distribution pipe is
laid level over the gravel filled excavation and the ends connected to-
gether with additional pipe with ell or tee fittings. In beds, the
parallel lines are usually laid with 3 to 6 ft (0.9 to 1.8 m) spacings.
A tee, cross, or distribution box may be used at the inlet to the closed
system (See Figure 7-21).
d. Distribution Box
Distribution box networks may be used in multi-trench systems or beds
with independent distribution laterals. They are suitable for all gra-
vity-flow systems.
The distribution laterals in the network extend from a common watertight
box called the distribution box. The box may be round or rectangular,
with a single inlet, and an outlet for each distribution lateral. It
has an exposed, removable cover. Its purpose is to divide the incoming
wastewater equally between each lateral. To achieve this objective, the
outlet inverts must be at exactly the same elevation. The inlet invert
should be about 1 in. above the outlet inverts. Where dosing is em-
ployed or where the slope of the inlet pipe imparts a significant velo-
city to the wastewater flow, a baffle should be placed in front of the
inlet to prevent short-circuiting.
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