What does a conventional subsurface sewage disposal system primarily use to pre-treat sewage?

A conventional subsurface sewage disposal system primarily uses a septic tank to pre-treat sewage. The septic tank serves as an underground storage and treatment container where the wastewater from the home enters first. Within the septic tank, solids settle to the bottom, forming sludge, while lighter materials, such as grease and oils, float to the top, creating a scum layer. This separation allows the liquid effluent in the middle to be distributed to the drain field or leach field for further treatment and dispersal into the soil.

The design and function of the septic tank are critical to the effectiveness of the subsurface sewage disposal system. The tank not only provides initial treatment but also helps to reduce the number of pathogens and organic matter before the effluent reaches the soil. This pre-treatment is essential for protecting groundwater and ensuring the overall functioning of the sewage disposal system, thus making the septic tank the correct answer.