Which pond type is used in series after secondary treatment to provide tertiary treatment?

• A. Stabilization ponds- raw wastewater
• B. Oxidation ponds- ponds in series, following primary treatment, provide additional clarification, BOD removal, and disinfection
• C. Polishing ponds- used in series, follow secondary treatment, provide form of tertiary treatment
• D. Anaerobic ponds- no DO, treatment through fermentation

Answer: (C)

Polishing ponds are designed to provide the final refinement after secondary treatment, making the effluent safer and clearer before discharge. When used in series, they extend detention time and use natural processes like sedimentation, sunlight, and microbial activity to remove any remaining suspended solids and some BOD, and to help reduce pathogens. This is exactly the tertiary treatment step—a final polishing of the wastewater.

Stabilization ponds are for initial stabilization of waste, often focusing on digestion and odor control, not the final polishing after secondary. Oxidation ponds are a form of secondary treatment that use both aerobic and anaerobic zones to remove more BOD and pathogens, but they aren’t the designated polishing step after secondary. Anaerobic ponds are for sludge digestion without oxygen, not for polishing treated effluent. So the polishing pond option best fits the role of tertiary treatment after secondary.