purpose of the tank is two fold, first as a filter and second
as a treatment device. Most tanks are 1,000-2,000 gallons.
on average produce about 75 gallons per day of waste water.
If you have a house with 4 people and a 1500 gallon tank,
the water would stay in the tank for about 5 days. During
these 5 days things happen.
is settling. Every thing heavier than water sinks to the
bottom and forms the sludge layer (cellular debris, cellulose,
silica, borax). Everything lighter floats to the top (fats,
oils, grease) and forms the scum layer. What remains in
between is called effluent and is the only acceptable discharge
into the drainfield. The tank is a series of baffles and
sumps that ensure only effluent can pass. If too much solid
waste is allowed to accumulate in the tank it begins to
pass through to the drainfield, eventually clogging it to
the point of failure.
bacteria in the septic tank digest most of the solid waste
that flows into the tank, and converts it into liquid and
gas. Some solids can't be digested fast enough by the bacteria,
and they begin to collect in the tank. As the tank begins
to fill up with undigested solids it effectively becomes
smaller. Because of reduced tank capacity the retention
time is reduced, as a result the waste strength is increased.
Increased waste strength over extended periods of time ALWAYS
results in reduced system capacity and life.
Treated waste water exits the tank with vastly reduced waste
strength (TSS, BOD, nitrate, sulfate, phosphates) and passes
through an outlet pipe to a distribution box. The distribution
box is small, (compared to the septic tank) often about
12"-18" across. It's purpose is to maintain equal
distribution of waste water to the leach lines or pits (dry
drainfield is the area of ground that absorbs the treated
waste water and may contain leach lines, pressure lines,
dry wells, a soil mound, or a bed style absorption area.
lines are trenches in the ground. They are usually 1-3 feet
wide and 2-3 feet deep (deep trench leach lines can be 8
feet and deeper). Most leach line trenches are filled with
gravel and are lined with perforated pipe that can conduct
wastewater the length of the trench. The gravel is covered
with filter-fabric to prevent it from being loaded up with
soil back fill, usually the top 6-12 inches of the trench.
Waste water soaks into the trench floor and sidewalls, and
percolates down through the soil and eventually, joins with
other ground water to complete the cycle. During this process
it is filtered and purified.
wells are very similar, but they are vertical bore holes,
usually 3-4 feet in diameter and 25-45 feet deep.
is merely a brief overview of the basic concepts of conventional
gravity fed systems. More challenging site and elevation
conditions give rise to more complex systems that use pumps,
electronic timer controls, electric valves, and exotic filters.
However the basics still apply to all...filtration, waste
strength reduction, and leaching into the ground.