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

7.2.8.1 Design
a. Single Line .
Single-line distribution networks are trenches loaded by gravity or
dosing. The distribution line is a 3- to 4-in. (8- to lo-cm) diameter
perforated pipe laid level in the center of the gravel-filled excavation
(see Figure 7-19). The pipe is usually laid such that the holes are at
or near the invert of the pipe. Where the length of single lines
exceeds 100 ft (30 m), it is preferable to locate the wastewater inlet
toward the center of the line.
b. Drop Box
Drop box networks are typically used for continuously ponded multi-
trench systems on level to maximum sloping lots. It is a network that
serially loads each trench to its full hydraulic capacity.
A drop box is a small, circular or square box with a removable cover.
It has an inlet, one or two distribution lateral outlets, and an over-
flow. The lateral outlet inverts are located at or near the bottom of
the box, all of the same diameter pipe. The overflow invert can be the
same elevation as the crown of the lateral outlet, or up to 2 in. above
it, to cause the full depth of the trench to flood. The inlet invert of
the drop box may be at the same elevation as the overflow invert or a
few inches above. An elevation difference of 1 to 2 in. (3 to 5 cm)
between trenches is all that is needed to install a drop box network.
The boxes may be buried, but it is suggested that the covers be left
exposed for periodic inspection and maintenance (see Figure 7-20).
Drop boxes are installed at the wastewater inlet of each trench. The
inlets may be located anywhere along the trench length. A solid wall
pipe connects the overflow from the higher box to the inlet of the lower
box. The first box in the network receives all the effluent from the
pretreatment tank and distributes it into the first trench. When the
first trench fills, the box overflows into the next trench. In this
manner, each trench in the system is used successively to its full capa-
city. Thus, only the portion of the system required to absorb the
wastewater is used. During periods of high flow or low absorptive capa-
city of the soil, more trenches will be used. When flows are low or
during the hot dry summer months, the lower trenches may not be needed,
so they may drain and dry out, automatically resting more trenches,
which rejuvenates their infiltrative surfaces (11).
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