During which treatment stage are solids primarily removed?

The correct choice indicates that solids are primarily removed during the primary treatment stage of wastewater treatment. This initial phase is crucial for reducing the overall solid content in the wastewater before it undergoes further treatment processes. Primary treatment primarily involves physical processes such as sedimentation, where larger solids settle at the bottom of settling tanks, forming sludge. Additionally, some floating materials, like oils and grease, are skimmed off the surface.

This stage effectively removes a significant portion of suspended solids and organic matter, thereby reducing the load on subsequent treatment stages, which focus more on biological and chemical remediation. Understanding the role of primary treatment in solid removal is essential, as it lays the groundwork for the efficiency and effectiveness of the entire wastewater treatment process.