Approximately three and one-half million North Carolinians use on-site systems to treat and dispose of their household wastewater, according to the 1990 U.S. Census. These systems generally rely on a wastewater receiving tank, or septic tank, and a treatment and disposal field for proper treatment and disposal of the sewage. Most of the purification of the wastewater occurs in the soil beneath the drainfield.
Using a soil absorption system for disposal and treatment of wasteater taJc:es advantage of the physical, chemical, and biological processes in the soil. Soil can absorb and treat the wastewater and its constituents.
Not all soils and sites will adequately treat and dispose of wastewater. Soil and site evaluations are necessary in order to locate on-site systems on appropriate sites. This section presents soil concepts that are required to conduct a soil and site evaluation.
The Use of Soil for On-Site Systems
There are many methods to treat and dispose of wastewater. On-site systems usually rely on the soil because it provides an inexpensive and reliable medium for wastewater treatment and disposal. The porous nature of soil and the biological activity in the soil are key characteristics in absorbing and treating wastewater.
Because of large variations in soil characteristics, not all soils can suitably treat and dispose of the waste. The challenge for those involved with on-site systems is to design and install wastewater systems that optimize a soil's treatment potential. The following concepts should be considered when selecting sites for on-site systems.
Processes that purify wastewater include physical filtration by the soil particles, chemical treatment through ion exchange and transformation in chemical reactions, biological oxidation and decomposition by micro-organisms, and uptaJc:e of nutrients by plants.
Soils can vary greatly over short distances, even from one side of a lot to the other. Because of their spatial variability, understanding soil becomes critical in the selection and evaluation of the sites where on-site systems will be located.
All soils are composed of mineral matter, organic matter, and voids or spaces that can be filled with either water or air. Soil water and soil air are inversely related to each other because water and air compete for the same void space in the soils ( see Figure 4.4.1).
A winding flow path through soil voids that is neither too rapid nor too slow provides for maximum treatment of wastes by natural soil processes.
Figure 4.4.1 The relationship between soil solids, soil wetness, and void space.
From the North Carolina Onsite Guidance Manual