What process occurs in a septic tank as wastewater moves through?

The correct answer is the settling of solids. In a septic tank, when wastewater enters, it undergoes a physical process where heavier solids settle at the bottom. This sedimentation forms a layer of sludge, while lighter materials, such as fats and oils, float to the top, creating a scum layer. The liquid effluent, which is a mixture of water with dissolved and suspended solids, remains in the middle and flows out to the drain field for further treatment.

This settling process is crucial as it helps separate the solid waste from the liquid, allowing for a more efficient treatment of wastewater. By removing the bulk of the solids, the septic tank ensures that fewer particulates are discharged into the drain field, reducing the risk of clogging and allowing for better soil filtration and eventual breakdown of any remaining contaminants by bacteria and other microorganisms in the soil.

While evaporation, filtration, and oxidation of waste are processes that can occur in various wastewater treatment systems, they are not the primary functions of a septic tank. Evaporation may happen in a broader context of wastewater treatment but is not a controlled process within a septic tank. Filtration generally refers to the passage of water through soil or other media and occurs later in the treatment process, while oxidation is a