What Is a Basic Septic Tank System Layout?

KevinDerrick/iStock / Getty Images Plus/Getty Images

A basic septic tank system layout includes a pipe leading from the house to the septic tank, an underground tank and a drain field. Most residential septic tank systems use gravity to move the water through the system.

A pipe carries waste water from the plumbing system of the house to a tank buried underground. Inside the tank, heavier solids sink to the bottom, while grease and other materials float to the top. In the middle is a layer of relatively clear water that is free of solids. The tank is designed so that bacteria breaks down the solids while the clear water is sent to the drain field. The breakdown of the solids produces a foul-smelling gas that is vented to the outside.

As water from the home enters the tank, an equivalent amount of water is forced out and into the drain field. The drain field consists of a series of pipes in gravel-lined trenches that extend out from the tank. As the water is forced from the tank, it flows into a distribution box that splits the flow into two or more pipes. As the water travels through the pipes, it is absorbed into the soil where further breakdown of organic material occurs. The ability of the soil to absorb water determines how big the drain field needs to be.