Which one of the following stage of sewage treatment consists of temporarily holding the sewage in a quiescent basin where heavy solids can settle to the bottom while oil, grease and lighter solids float to the surface?

Explanation

The stage of sewage treatment described is called "primary sedimentation." It involves holding the sewage in a quiescent basin, allowing heavier solids to settle to the bottom (forming sludge) and lighter solids like oil and grease to float to the surface (forming scum). This stage is not called "primary treatment" because primary treatment typically refers to the initial phase of sewage treatment, which involves physical processes such as screening to remove large objects and grit, followed by primary sedimentation. So, primary sedimentation is part of the primary treatment stage, but it's specifically referred to as "primary sedimentation" to distinguish it from other processes within primary treatment.