Which of the following does NOT belong to primary wastewater treatment?

Correct option is C

Explanation:
A trickling filter is a biological treatment process used in secondary wastewater treatment, not in primary treatment. In primary treatment, physical processes are used to remove large solids and settleable organic and inorganic matter.
Information Booster:
Primary wastewater treatment includes bar screens, grit chambers, and primary clarifiers, all of which use physical separation methods. In contrast, trickling filters rely on microbial activity to degrade organic matter in the wastewater, making them part of secondary (biological) treatment.