In a septic-tank system the sewage flows by gravity from the farmhouse
through the sewer into the tank, where it should remain at least 24
hours. While passing through the tank the solids are acted upon by
anaerobic bacteria, which work only in the dark and where there is
little air. The heavy particles settle to the bottom as sludge, the
lighter particles float as scum, and the remainder passes out of the
tank through the effluent sewer to the disposal field. The gas released
in the process escapes through a vent provided either in the =T= to the
house sewer or the effluent sewer.
A tank that is too small may fill up with solids in a short while,
because sufficient time is not allowed for breaking them down by
fermentation, or the sewage may be pushed right through into the
disposal field and clog it.
The effluent may contain even more disease germs than the original
sewage, and though it may be as clear as spring water it is far from
pure and may cause foul odors if discharged or allowed to pool on the
surface of the ground.
The final disposition of the effluent into the upper layer of the soil
exposes it to the action of aerobic bacteria. These bacteria, unlike
those in the tank, need air and cannot work in saturated soil or live
much more than 3 feet below the surface of the ground. The "living
earth," or upper stratum, teems with these bacteria, which convert the
dangerous sewage and disease germs into harmless matter and thus tend
to purify the effluent if it remains long enough in the top layers of
soil before seeping into the subsoil and thence to the ground water.
Effluent discharged deep in the soil does not receive the benefit of
this purifying action.
Several types of septic tanks are in common use. The one described in
this bulletin is the single-chamber type, which can be built with or
without siphon. This should meet all average farm needs where there
are not more than 16 members in the household. It would be advisable
to consult the authorities of the State agricultural college or local
health department as to their recommendations because frequently local
conditions and larger establishments require special installations.