on-site sewage basics

A conventional septic system consists of two main parts: the septic tank and the soil drainfield (also referred to as a leachfield, absorption bed or absorption field). At the head of the drainfield a distribution box or a manifold distributes wastewater to several absorption trenches. Some locations require that newly installed drainfields include a designated replacement area should the existing septic system need an addition, repair or replacement, the replacement area can then be used.

How the system works

Figure 1: Septic System
Courtesy National Small Flows Clearinghouse

The septic tank. A septic tank is a large, underground, watertight container, typically about 9 feet long, 4-5 feet wide and 5 feet tall that is connected to the home's sewer line. 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. Septic tanks may be rectangular or cylindrical and may be made of concrete, fiberglass or polyethylene.

Raw waste water from the bathroom, kitchen and laundry room flows into the tank where the solids separate from the liquid. Light solids, such as soap suds and fat, float to the top and form a scum layer. This layer remains on top and gradually thickens until you have the tank cleaned. The liquid waste goes into the drainfield, while the heavier solids settle to the bottom of the tank where they are gradually decomposed by bacteria. But some non-decomposed solids remain, forming a sludge layer that eventually must be pumped out.

Septic tanks may have one or two compartments. Two-compartment tanks do a better job of settling solids and are required for new installations. Tees or baffles at the tank's inlet pipe slow the incoming wastes and reduce disturbance of the settled sludge. A tee or baffle at the outlet keeps the solids or scum in the tank.

Figure 2: A Two-Compartment Septic Tank
Courtesy National Small Flows Clearinghouse

Figure 3 Wastewater Treatment and Disposal In the Soil
Courtesy North Carolina Extension Service

The Drainfield. Further treatment of wastewater occurs in the soil beneath the drainfield. The drainfield consists of long underground perforated pipes or chambers connected to the septic tank. Liquid waste or effluent flows out of the tank and is evenly distributed into the soil through the underground system. The soil below the drain-field provides the final treatment and disposal of the septic tank effluent. After the effluent has passed into the soil, most of it percolates downward and outward, eventually entering the groundwater. A small percentage is taken up by plants through their roots, or evaporates from the soil.

The soil filters the effluent as it passes through the pore spaces. Chemical and biological processes treat the effluent before it reaches groundwater, or a restrictive layer, such as hardpan, bedrock, or clay soils. These processes work best where the soil is somewhat dry and permeable, and contains plenty of oxygen for several feet below the drain field. The size and type of drainfield depends on the estimated daily wastewater flow and soil conditions.