What is a septic system?

There are a number of different septic systems, each with its own design. The conventional system is the one most commonly used in North Carolina (Figure 1). It consists of three main parts: the septic tank, the drainfield, and the soil beneath the drainfield.

The septic tank is a watertight concrete box about 9 feet long and 5 feet tall. It is buried in the ground just outside the home. The tank is usually precast from reinforced concrete and can be purchased from concrete manufacturers. While typically designed with a 1,000-gallon liquid capacity, the size of the tank is legally determined by the number of bedrooms in the home. The tank temporarily holds household wastes and allows a small amount of pretreatment to take place (Figure 2). 

The tank is connected to the drainfield by a buried pipe. A typical drainfield consists of two to five trenches excavated into the subsoil. In many systems, a distribution box or a flow divider helps move wastewater to each trench. In most conventional septic systems, the trenches are 3 feet wide, 2 to 3 feet deep, and 9 feet apart. In each trench, a 1-foot thick layer of washed gravel or stone is placed around a 4-inch-diameter perforated distribution pipe. After the trenches are covered with soil, the area must be landscaped to keep surface waters from ponding over the drainfield. 

The drainfield has also been called the nitrification field or the soil absorption field. The sole purpose of the drainfield is to deliver wastewater to the soil. The soil purifies the wastewater by removing the germs and chemicals before they reach the groundwater or any adjacent surface waters such as rivers, lakes, and estuaries.


This article was originally featured in Soil Facts by NC State University, please see the source below for continued reading.


  Figure 1  . A conventional septic system.

Figure 1. A conventional septic system.

  Figure 2  . A two-compartment septic system.

Figure 2. A two-compartment septic system.