Which type of pond is not classified with the other three in secondary treatment?

The correct answer, which identifies the type of pond not classified with the others in secondary treatment, is the settling pond. Settling ponds, also known as sedimentation or clarification ponds, are primarily utilized for the removal of solids from wastewater through gravitational settling. This process primarily focuses on physical treatment and does not engage in biological processes, which are essential in secondary treatment.

In contrast, polishing ponds, oxidation ponds, and aerobic ponds are all integral to the secondary treatment category as they involve biological processes aimed at further breaking down organic matter in the wastewater. Polishing ponds provide a final stage treatment, reducing residual contaminants after primary and secondary processes. Oxidation ponds use natural processes involving aerobic bacteria and sunlight to decompose organic material. Aerobic ponds also facilitate aerobic digestion, encouraging microorganisms to convert organic substances into simpler substances.

By understanding the differences in function, it becomes clear why the settling pond stands apart from the other types, as it does not utilize biological mechanisms characteristic of secondary treatment methods.