Oxidation pond design – detention time In sewage treatment, the typical detention period provided for facultative oxidation ponds ranges approximately from how many weeks?

Introduction / Context:
Oxidation ponds (also called waste stabilization ponds) are low-cost, land-intensive biological treatment units that rely on algae–bacteria symbiosis for BOD removal and pathogen die-off. Proper design requires choosing a detention time that balances treatment performance with land area and climatic conditions.

Given Data / Assumptions:

• Facultative pond (not aerated lagoon or maturation pond).
• Typical temperate to warm climate conditions and standard organic loading.
• Question seeks a representative design range in weeks.

Concept / Approach:
Detention time in facultative ponds is influenced by temperature, solar radiation, and loading. Textbook values often fall in the multi-week range to achieve substantial BOD reduction and pathogen control. Common design practice cites about 3–6 weeks; many exam references round this to 4–5 weeks as a representative figure, assuming moderate loading and typical climates. Therefore, among the options, 4 to 5 weeks captures the mainstream design window for such ponds.

Step-by-Step Solution:

Verification / Alternative check:
Design charts and case studies show BOD and coliform reductions improving significantly when detention exceeds roughly one month under favourable climates, aligning with 4–5 weeks.

Why Other Options Are Wrong:

• 1 to 2 weeks: Short for many climates; more typical of aerated lagoons or high-rate units.
• 9 to 10 or 15 to 20 weeks: Possible in cold climates or very conservative designs, but not typical.
• 2 to 3 weeks: Borderline; often insufficient for consistent pathogen reduction in standard facultative ponds.

Common Pitfalls:
Confusing aerated lagoon detention (days) with facultative pond detention (weeks); not accounting for seasonal temperature effects on sizing.

Final Answer:
4 to 5