Which type of treatment is primarily concerned with removing oil and settleable solids?

The correct choice is primary treatment, as it specifically focuses on the first stage of wastewater treatment processes. During primary treatment, the primary goal is to remove large particles, including oil and settleable solids, from sewage through physical processes. This typically involves processes such as sedimentation, where heavier solid particles settle to the bottom of a tank, forming sludge, while lighter materials like oil can rise to the top as scum.

In contrast, secondary treatment generally involves biological processes designed to remove dissolved organic matter and improve the quality of the effluent. Tertiary treatment, on the other hand, is an advanced stage intended to further polish effluent to achieve high-quality standards, often involving chemical or filtration methods, and is not mainly focused on oils or settleable solids. Equalization treatment is a process aimed at balancing the flow and concentrations of pollutants in wastewater but does not specifically target the removal of oil and settleable solids. Thus, primary treatment is characterized by its foundational role in effectively capturing these contaminants right at the outset of the wastewater treatment process.