For municipal wastewater with low biochemical oxygen demand (low-BOD), which treatment unit is generally the most economical primary option for stabilization and polishing?

Introduction / Context:
When BOD levels are relatively low, capital- and energy-intensive processes may be unnecessary. Oxidation ponds provide an economical, natural-treatment alternative that uses solar energy and microbial ecology to polish effluents before discharge or reuse.

Given Data / Assumptions:

• Influent has low BOD (e.g., partially treated municipal wastewater).
• Adequate land availability and favorable climate for pond operation.
• Objective: cost-effective polishing and pathogen reduction.

Concept / Approach:
Oxidation ponds provide long detention times, allowing sedimentation of fine particulates, aerobic oxidation of organics, and photolytic/UV inactivation of pathogens. For low-BOD influent, the required area decreases, making ponds particularly economical versus mechanical aeration or advanced processes.

Step-by-Step Solution:
Compare units: sedimentation tanks/clarifiers primarily remove settleable solids, not dissolved organics.Sludge digesters are anaerobic units for concentrated sludges, not low-BOD wastewater.Oxidation ponds address residual soluble BOD and pathogens at low cost, matching the need.Conclusion: oxidation ponds are the most economical option under the given conditions.

Verification / Alternative check:
Design manuals consistently recommend stabilization ponds for small communities and low-strength wastewater due to low OPEX and simplicity, provided land is available.

Why Other Options Are Wrong:
Sedimentation/clarifier: insufficient for dissolved BOD removal.
Sludge digester: intended for sludge stabilization, not dilute wastewater.

Common Pitfalls:
Underestimating land requirement or seasonal performance; not providing disinfection where reuse requires pathogen limits.

Final Answer:
oxidation ponds