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

area (19). This design allows flexibility in operation. Two beds are
always in operation, providing 100% of the needed i-nfiltrative surface.
The third field is alternated in service on a semi annual or annual
schedule. Thus, each field is in service for one or two years and
"rested" for 6 months to one year to rejuvenate. The third field also
acts as a standby unit in case one field fails. The idle field can be
put into service immediately while a failed field is rehabilitated.
Larger systems should utilize some dosing or uniform application to
assure proper performance.
7.2.3 Seepage Pits
7.2.3.1 Description
Seepage pits or dry wells are deep excavations used for subsurface dis-
posal of pretreated wastewater. Covered porous-walled chambers are
placed in the excavation and surrounded by gravel or crushed rock (see
Figure 7-9). Wastewater enters the chamber where it is stored until it
seeps out through the chamber wall and infiltrates the sidewall of the
excavation.
Seepage pits are generally discouraged by many local regulatory agencies
in favor of trench or bed systems. However, seepage pits have been
shown to be an acceptable method of disposal for small wastewater flows
(21). Seepage pits are used where land area is too limited for trench
or bed systems; and either the groundwater level is deep at all times,
or the upper 3 to 4 ft (0.9 to 1.2 m) of the soil profile is underlain
by a more permeable unsaturated soil material of great depth.
7.2.3.2 Site Considerations
The suggested site criteria for seepage pits are similar to those for
trench and bed systems summarized in Table 7-l except that soils with
percolation rates slower than 30 min/in. (12 min/cm) are generally ex-
cluded. In addition, since the excavation sidewall is used as the
infiltrative surface, percolation tests are run ineach soil layer en-
counted. Maintaining sufficient separation between the bottom of the
seepage pit and the high water table is a particularly important con-
sideration for protection of groundwater quality.
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