A typical septic system has four main components: a pipe that exits the building, a septic tank, a drainfield, and the soil.
Your wastewater exits your home through the pipe to the septic tank, a buried watertight container typically made of concrete, fiberglass or polyethylene. The tank holds the wastewater long enough to allow solids to settle out (sludge) and oil and grease to float to the surface (scum).
Compartments and a T-shaped outlet in the tank prevent the sludge and scum from leaving the tank and traveling into the drainfield area. Some newer tanks have risers with lids at the ground surface to allow easy location, inspection and pumping of the tank.
The wastewater exits the tank and is discharged into the drainfield for further treatment by the soil. The partially treated wastewater is pushed along into the drainfield for further treatment every time new wastewater enters the tank.
Wastewater then percolates into the soil, which provides final treatment. Microbes in the soil digest or remove most contaminants from wastewater before it eventually reaches groundwater.