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Septic tank above
ground. |
A septic tank is a key component of the septic
system, a small-scale sewage treatment system
common in areas with no connection to main
sewage pipes provided by local governments or
private corporations.
Other
components, typically mandated and/or restricted
by local governments, optionally include pumps,
alarms, sand filters, and clarified liquid
effluent disposal means such as a septic drain
field, ponds, natural stone fiber filter plants
or peat moss beds. Septic systems are a type of
On-Site Sewage Facility (OSSF). In Canada,
approximately 25% of the population relies on
septic tanks; this can include suburbs and small
towns as well as rural areas. |
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Periodic preventive maintenance is required to
remove the irreducible solids that settle and
gradually fill the tank, reducing its
efficiency. In most jurisdictions this
maintenance is required by law, yet often not
enforced. Those who ignore the requirement will
eventually be faced with extremely costly
repairs when solids escape the tank and destroy
the clarified liquid effluent disposal means. A
properly maintained system, on the other hand,
can last for decades or possibly even a
lifetime. |
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A septic tank
generally consists of a tank (or sometimes more
than one tank) of between 4000 and 7500 liters
(1,000 and 2,000 gallons) in size connected to
an inlet wastewater pipe at one end and a septic
drain field at the other.
In general,
these pipe connections are made via a T pipe,
which allows liquid entry and exit without
disturbing any crust on the surface.
Today, the design
of the tank usually incorporates two chambers
(each of which is equipped with a manhole
cover), which are separated by means of a
dividing wall that has openings located about
midway between the floor and roof of the tank. |
Septic tank above
ground. |
Wastewater enters the
first chamber of the tank, allowing solids to settle and
scum to float. The settled solids are anaerobically
digested, reducing the volume of solids. The liquid
component flows through the dividing wall into the second
chamber, where further settlement takes place, with the
excess liquid then draining in a relatively clear condition
from the outlet into the leach field, also referred to as a
drain field or seepage field, depending upon locality.
The remaining impurities are trapped and eliminated in the
soil, with the excess water eliminated through percolation
into the soil (eventually returning to the groundwater),
through evaporation, and by uptake through the root system
of plants and eventual transpiration. A piping network,
often laid in a stone-filled trench (see weeping tile),
distributes the wastewater throughout the field with
multiple drainage holes in the network. The size of the
leach field is proportional to the volume of wastewater and
inversely proportional to the porosity of the drainage
field. The entire septic system can operate by gravity alone
or, where topographic considerations require, with inclusion
of a lift pump. Certain septic tank designs include siphons
or other methods of increasing the volume and velocity of
outflow to the drainage field. This helps to load all
portions of the drainage pipe more evenly and extends the
drainage field life by preventing premature clogging.
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