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How does a septic system work? |
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A septic tank is simply a big concrete or steel tank that is buried in
the yard. The tank might hold 1,000 gallons (4,000 liters) of water.
Wastewater flows into the tank at one end and leaves the tank at the
other. The tank looks something like this in cross-section:
In this picture, you can see three layers. Anything that floats rises
to the top and forms a layer known as the scum layer. Anything heavier
than water sinks to form the sludge layer. In the middle is a fairly
clear water layer. This body of water contains bacteria and chemicals
like nitrogen and phosphorous that act as fertilizers, but it is
largely free of solids.
Wastewater comes into the septic tank from the sewer pipes in the house, as shown here:
A septic tank naturally produces gases (caused by bacteria breaking
down the organic material in the wastewater), and these gases don't
smell good. Sinks therefore have loops of pipe called P-traps that hold
water in the lower loop and block the gases from flowing back into the
house. The gases flow up a vent pipe instead -- if you look at the roof
of any house, you will see one or more vent pipes poking through.
As new water enters the tank, it displaces the water that's already
there. This water flows out of the septic tank and into a drain field.
A drain field is made of perforated pipes buried in trenches filled
with gravel. The following diagram shows an overhead view of a house,
septic tank, distribution box and drain field:
A typical drain field pipe is 4 inches (10 centimeters) in diameter and
is buried in a trench that is 4 to 6 feet (about 1.5 m) deep and 2 feet
(0.6 m) wide. The gravel fills the bottom 2 to 3 feet of the trench and
dirt covers the gravel.
The water is slowly absorbed and filtered by the ground in the drain
field. The size of the drain field is determined by how well the ground
absorbs water. In places where the ground is hard clay that absorbs
water very slowly, the drain field has to be much bigger.
A septic system is normally powered by nothing but gravity. Water flows
down from the house to the tank, and down from the tank to the drain
field. It is a completely passive system.
You may have heard the expression, "The grass is always greener over
the septic tank." Actually, it's the drain field, and the grass really
is greener -- it takes advantage of the moisture and nutrients in the
drain field.
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Septic Tanks 
