Leachfields, Absorption Fields and Mounds
terms for the soil treatment system are: drainfield,
leachfield, seepage bed, soil absorption field and mound.
The soil treatment system is where the final treatment
and disposal of the septic tank effluent takes place.
A properly designed and installed soil treatment system
will destroy all disease-causing pathogens and filter
out the fine solids contained in the septic tank effluent.
two most commonly used types of soil treatment systems
are trenches and mounds.
Drain field trenches effectively treat liquid flowing
from the septic tank. A drainfield trench is generally
18 to 36 inches wide and up to 100 feet long. The trench
contains a perforated pipe in a bed of 3/4-inch to 2-inch
diameter rock covered by natural or synthetic permeable
fibers. Some soil treatment systems use large plastic
tubing or some other chamber wrapped with fabric in
the trench in place of rock. A 6- to 12-inch deep layer
of topsoil covers the trench. Sewage flows through the
holes in the distribution pipe, to the rock (or tube),
through the biomat, and into the soil. Bacteria and
fine sewage solids are removed or destroyed in this
trench system may be laid out in one of many configurations
to allow for the necessary square feet of surface. There
are often inspection pipes on one or both ends of the
pipes. These can be cut off at ground level and capped
for easier lawn maintenance. The ground surface level
of the soil treatment area should always be level or
slightly raised above the surrounding ground to avoid
excess rainfall flooding the system.
(Elevated Seepage Bed): A sewage treatment mound is
a seepage bed raised with clean sand to provide adequate
separation between the wastewater in the mound and the
saturated soil or an impermeable hardpan of soil or
bedrock beneath. The mound is carefully constructed
to provide adequate treatment of sewage. It is equally
as effective in treating sewage as a trench system as
long as it is properly constructed and operated, and
the septic tank is maintained correctly. The mound system,
as illustrated in Figure 3, has a pressurized distribution
system of 1-1/2 or 2-inch perforated pipe in a layer
of small rock. A layer of sand covers the rock. The
mound is covered with topsoil and planted to grass.
The grass should be mowed regularly.
treatment of wastewater occurs in the soil. Uncompacted,
unsaturated soil should surround the trenches of the
drainfield. There are millions of naturally-occurring
beneficial microscopic organisms in every square inch
of soil. These organisms complete the wastewater treatment
process by killing disease-causing organisms in the
sewage and removing nutrients.
beneficial bacteria in the soil need air to live. Therefore,
a zone of unsaturated soil must be present below the
drainfield for complete treatment.
biomat is a black tar-like substance made up of organic
material, microorganisms, and bacteria. As a drain field
matures the biomat grows along the walls and bottom
of the drain field trenches. The billions of living
organisms naturally occurring in the biomat act as a
filter for wastewater traveling through the drain field.
While effluent passes slowly through the biomat the
bacteria living there act as a filter digesting much
of the organic matter before it passes into the surrounding
soil. When drain field soils flood, or the organic balance
is upset the biomat can grow into an impermeable barrier
causing water to back up into the septic tank and eventually
into the household plumbing.