What Is a Septic System?

In comparison to a city sewage system---where the waste from many buildings and homes flows to a large central location for treatment---a septic system typically handles only one house or building. Wastewater enters the system when toilets are flushed or water is run down drains. The septic system's job is to cleanse the wastewater and render it suitable to reenter the surrounding environment.

House Plumbing

In a building served by a septic system, all the sink drains, bathtub and shower drains and toilets have plumbing that leads into one septic pipe. Along the course from the drain to the central pipe (and including the central pipe), these drain pipes are pitched at an angle. This ensures flow away from the drain and toward the outside septic system. Recurved pipes---called traps---just below the drains form water seals that prevent septic gasses from traveling backward and escaping into the house from the drain pipes.

External Pipe

Once in the main septic pipe, waste drains out of the building where the main septic pipe exits below the ground surface. This pipe leads to a septic tank.

Septic Tank

The main septic pipe delivers waste to an underground septic tank. The septic tank is a concrete, plastic or fiberglass tank with an inlet connected to the drain pipe from the building. The tank receives waste and allows it to separate into a floating layer of scum and grease and a sediment layer of sludge at the bottom. Between these two layers is mostly water, though the water contains contaminants that include pathogenic microbes. At the opposite end of the tank from the inlet is an outlet pipe. It is positioned so that the middle layer of wastewater drains out while the floating layer and the sludge remain in the tank. There are access ports at the tank's top to allow inspection and periodic pumping to remove the grease and sludge.

Distribution Box

The tank's outlet pipe leads to a box called a distribution box (sometimes called a D-box for short). The distribution box has an inlet where the pipe from the septic tank delivers wastewater. It also has a set of outlets that are connected to perhaps three or more pipes that deliver the wastewater to a leach field.

Leach Field

Sometimes called a drain field or leaching area, the leach field is an array of several subsurface trenches. Perforated pipes leading from the D-box carry the wastewater into the trenches and allow it to seep into the soil. Once in the soil, microbial action essentially "treats" the wastewater and purifies it as it percolates down through soil layers. By the time the water from the leach field reaches an area where it may flow into existing groundwater or surface water, it has been renovated by the septic system and is--- at least theoretically---clean.