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A
septic tank is a receptacle designed to treat wastewater
by settling out solid particles, then breaking down and storing
solids. Typical retention time for the wastewater ranges from
24 to 48 hours. Wastewater enters the tank through an inlet;
solids (sludge) settle to the bottom where some digestion
occurs, and grease and fats (scum) float to the top of the
water level. A clearer layer (effluent) is developed between
the sludge and scum layers and eventually passes on to the
next treatment step through an outlet device. Inlet and outlet
devices can be either baffles or tees (or a combination),
with the outlet device approximately three to five inches
lower than the inlet. A septic tank can be single or multi-compartment,
as shown. Several states now require effluent filter devices
in the outlet of new septic tanks or retrofitted to existing
septic tanks for further solids removal. This helps ensure
that solids do not reach the drainfield where they can cause
clogging.
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