Wastewater treatment basics — lagooning (stabilization ponds): In municipal and industrial sanitation, the lagooning process (also called oxidation ponds or stabilization ponds) is used primarily for which main purpose?

Introduction / Context:
Lagooning, also known as oxidation ponds or stabilization ponds, is a widely used, low-energy wastewater treatment method. It relies on natural processes involving sunlight, algae, and bacteria to degrade organic matter. The question asks for the principal function of lagooning within the overall treatment train.

Given Data / Assumptions:

• Lagooning involves large, shallow basins with long detention times.
• Processes rely on microbial activity supported by oxygen from photosynthesis and atmospheric reaeration.
• Applications include domestic sewage and some industrial effluents.

Concept / Approach:
In facultative or aerated lagoons, heterotrophic bacteria oxidize dissolved and colloidal organics. Algae supply oxygen via photosynthesis; bacteria consume oxygen while converting carbonaceous substrates to biomass, carbon dioxide, and water. The dominant role is the biological treatment (stabilization) of wastewater organics measured by BOD and COD reduction. While solids settle in lagoons, and some sludge accumulates over time, sludge management is not the primary design intent. Flow equalization is incidental at best and handled better by dedicated tanks.

Step-by-Step Solution:
Identify the main engineered function: reduce organic load via microbial oxidation.Map this function to choices: “biological treatment of wastes” matches exactly.Confirm that other listed functions are secondary or unrelated.

Verification / Alternative check:
Design texts describe lagoon series (anaerobic → facultative → maturation) achieving staged removal of BOD and pathogens. Detention times of days to weeks confirm the biological stabilization focus, not short-term flow control or sole sludge disposal.

Why Other Options Are Wrong:
Sludge disposal: Lagoons accumulate sludge but are not primarily designed for sludge final disposal.Reduction of excessive flow: Not a primary objective; equalization basins perform that role.None of these: Incorrect because lagoons clearly perform biological treatment.

Common Pitfalls:

• Confusing lagoon sludge accumulation with intentional sludge disposal.
• Assuming any large basin provides flow equalization; hydraulic design matters.
• Overlooking the algae–bacteria symbiosis that supplies oxygen and removes organics.

Final Answer:
biological treatment of wastes