A septic system is a highly efficient, self-contained, underground
wastewater treatment system. Because septic systems treat and dispose
of household wastewater onsite, they are often more economical than
centralized sewer systems in rural areas where lot sizes are larger and
houses are spaced widely apart.
Septic systems are also simple in design, which make them generally
less expensive to install and maintain. And by using natural processes
to treat the wastewater onsite, usually in a homeowner's backyard,
septic systems don't require the installation of miles of sewer lines,
making them less disruptive to the environment.
A septic system consists of two main parts: a septic tank and a drainfield.
The septic tank is a watertight box, usually made of concrete or
fiberglass, with an inlet and outlet pipe. Wastewater flows from the
home to the septic tank through the sewer pipe. The septic tank treats
the wastewater naturally by holding it in the tank long enough for
solids and liquids to separate. The wastewater forms three layers
inside the tank. Solids lighter than water (such as greases and oils)
float to the top forming a layer of scum. Solids heavier than water
settle at the bottom of the tank forming a layer of sludge. This leaves
a middle layer of partially clarified wastewater.
The layers of sludge and scum remain in the septic tank where bacteria
found naturally in the wastewater work to break the solids down. The
sludge and scum that cannot be broken down are retained in the tank
until the tank is pumped.
The layer of clarified liquid flows from the septic tank to the
drainfield or to a distribution device, which helps to uniformly
distribute the wastewater in the drainfield. A standard drainfield
(also known as a leachfield, disposal field, or a soil absorption
system) is a series of trenches or a bed lined with gravel or course
sand and buried one to three feet below the ground surface. Perforated
pipes or drain tiles run through the trenches to distribute the
wastewater. The drainfield treats the wastewater by allowing it to
slowly trickle from the pipes out into the gravel and down through the
soil. The gravel and soil act as biological filters.
Reprinted courtesy of NSFC.
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Septic Tanks 
