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

7.2.3.5 Maintenance
A well-designed and constructed seepage pit requires no routine mainte-
nance. However, failure ocassionally occurs. Pumping and resting is
the only reasonable rehabilitation technique available.
7.2.4 Mound Systems
7.2.4.1 Description
The mound system was originally developed in North Dakota in the late
1940's where it became known as the NODAK disposal system (23). The
mound was designed to overcome problems with slowly permeable soils and
high water tables in rural areas. The absorption bed was constructed in
coarse gravel placed over the original soil after the topsoil was re-
moved. Monitoring of these systems revealed that inadequate treatment
occured before the groundwater was reached, and seepage often occurred
during wet periods of the year. Successful modifications of the design
were made to overcome these limitations (4). Mound systems are now used
under a variety of conditions.
A mound system is a soil absorption system that is elevated above the
natural soil surface in a suitable fill material. The purpose of the
design is to overcome site restrictions that prohibit the use of conven-
tional soil absorption systems (4)(24). Such restrictions are: (1)
slowly permeable soils, (2) shallow permeable soils over creviced or
porous bedrock, and (3) permeable soils with high water tables. In
slowly permeable soils, the mound serves to improve absorption of the
effluent by utilizing the more permeable topsoil and eliminating con-
struction in the wetter and more slowly permeable subsoil, where smear-
ing and compaction are often unavoidable. In permeable soils with
insufficient depth to groundwater or creviced or porous bedrock, the
fill material in the mound provides the necessary treatment of the
wastewater (see Figure 7-10).
The mound system consists of: (1) a suitable fill material, (2) an ab-
sorption area, (3) a distribution network, (4) a cap, and (5) top soil
(see Figure 7-11). The effluent is pumped or siphoned into the absorp-
tion area through a distribution network located in the upper part of
the coarse aggregate. It passes through the aggregate and infiltrates
the fill material. Treatment of the wastewater occurs as it passes
through the fill material and the unsaturated zone of the natural soil.
The cap, usually a finer textured material than the fill, provides frost
protection, sheds precipitation, and retains moisture for a good vegeta-
tive cover. The topsoil provides a growth medium for the vegetation.
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