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

7.2.1.2 System Selection
The type of subsurface soil absorption system selected depends on the
site characteristics encountered. Critical site factors include soil
profile characteristics and. permeability, soil depth over water tables
or bedrock, slope, and the size of the acceptable area. Where the soil
is at least moderately permeable and remains unsaturated several feet
below the system throughout the year, trenches or beds may be used.
Trenches and beds are excavations of relatively large area1 extent that
usually rely on the upper soil horizons to absorb the wastewater through
the bottom and sidewalls of the excavation. Seepage pits are deep exca-
vations designed primarily for lateral absorption of the wastewater
through the sidewalls of the excavation; they are used only where the
groundwater level is well below the bottom of the pit, and where beds
and trenches are not feasible.
Where the soils are relatively impermeable or remain saturated near the
surface, other designs can be used to overcome some limitations. Mounds
may be suitable where shallow bedrock, high water table, or slowly per-
meable soil conditions exist. Mounds are beds constructed above the
natural soil surface in a suitable fill material. Fill systems are
trench or bed systems constructed in fill material brought in to replace
unsuitable soils. Fills can be used to overcome some of the same limi-
tations as mounds. Curtain or underdrain designs sometimes can be used
to artifically lower groundwater tables beneath the absorption area so
trenches or beds may be constructed. Table 2-l presents the general
site conditions under which the various designs discussed in this manual
are best suited. For specific site criteria appropriate for each, refer
to the individual design sections in this chapter.
7.2.2 Trench and Bed Systems
7.2.2.1 Description
Trench and bed systems are the most commonly used method for onsite
wastewater treatment and disposal. Trenches are shallow, level excava-
tions, usually 1 to 5 ft (0.3 to 1.5 m) deep and 1 to 3 ft (0.3 to 0.9
m) wide. The bottom is filled with 6 in. (15 cm) or more of washed
crushed rock or' 'gravel over which is laid a single line of perforated
distribution piping. Additional rock is placed over the pipe and the
rock covered with a suitable semipermeable barrier to prevent the back-
fill from penetrating the rock. Both the bottoms and sidewalls of the
trenches are infiltrative surfaces. Beds differ from trenches in that
they are wider than 3 ft (0.9 m) and may contain more than one line of
distribution piping (see Figures 7-1 and 7-2). Thus, the bottoms of the
beds are the principal infiltrative surfaces.
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