For which soil profile is soil classification used to determine the size of the system based?

The correct choice is related to the least permeable soil because soil classification is fundamentally concerned with understanding the characteristics that impact wastewater treatment and dispersal. In systems for private sewage, knowing the permeability of the soil is crucial as it determines how well effluent can percolate through the ground and be treated by the soil microbes, which are essential for breaking down contaminants.

When designing a sewage system, engineers prioritize the least permeable soils when sizing the system because these soils can inhibit the movement of water and sewage, leading to potential system overload or failure. The ability of soil to absorb water affects the sizing of the treatment and dispersal systems. Systems in areas with low permeability may need to be designed with larger components or additional treatment methods to ensure effective treatment and prevent surface water contamination.

In contrast, while the most permeable soils allow water to drain quickly, they may also require different engineering solutions to prevent rapid effluent dispersal without adequate treatment. Soils with high or low organic content can influence the biological activity within the system, but permeability remains the central focus for determining the size of sewage systems. Thus, understanding the characteristics of the least permeable soils is critical in designing an effective and efficient sewage system.