How a Septic System Functions
The most common way to treat household waste in rural
Wisconsin is with an on-site disposal system or septic system. These systems
treat most waste through a natural process, the balance of the waste must be
periodically pumped out.
There are three main stages in the treatment of household
waste. The first is the septic tank. Wastewater exits the house via a sewer pipe
and enters the septic tank. The septic tank acts as a holding tank allowing the
solids to settle-out. The larger solids sink to the bottom where naturally
occurring anaerobic bacteria begin breaking them down and destroying pathogens.
The lighter solids, like grease, float to the top and form a scum layer. The
relatively clear layer in the middle is the effluent.
Next, liquid effluent flows from the tank through the
distribution box to the absorption area. The inlet and outlet to the septic tank
are covered by baffles which prevent the scum layer from entering the absorption
area where it would clog the field. A hydraulic pump is sometimes needed if the
absorption site is higher than the septic tank, or if an elevated sand mound is
Finally the effluent arrives at the absorption field or
drainfield where it is distributed to the soil for treatment. The drainfield
field consists of several lateral pipes either in a rectangular bed or in
individual trenches partially filled with gravel and covered with topsoil. Holes
positioned along the length of the pipe allow the effluent to slowly flow out.
As the effluent enters the drainfield it percolates through the gravel bed where
a large portion of the pathogens are destroyed. As the effluent exits the
drainfield microorganisms in the natural soil consume the organic pollutants and
complete the treatment process. This percolation process is a natural biological
one that can safely treat the liquid before it reaches groundwater, if the
system has been properly designed and maintained.
If a soil has shallow bedrock or a high water table, the
effluent can reach the groundwater before it is fully treated. To avoid this a
mound system design is used to maintain 3 feet of separation from your septic
system to these limiting factors. The system will require a hydraulic pump to
lift the effluent up into the mound.