Environmental engineering basics: A shallow basin where municipal sewage is retained for natural biological treatment (sunlight, algae–bacteria symbiosis, long detention time) is called a…

Introduction / Context:
Wastewater treatment can be achieved with low-energy, land-intensive systems that rely on sunlight and natural microbial communities. Recognizing unit names and their functions is fundamental for designing cost-effective treatment schemes for small towns and industrial estates.

Given Data / Assumptions:

• Shallow depth, large surface area, and long detention for stabilization.
• Natural aeration (wind), photosynthetic oxygenation by algae, and bacterial oxidation.
• Municipal sewage or similar biodegradable wastewater.

Concept / Approach:
Oxidation ponds (lagoons) are engineered basins where organic matter is degraded biologically under aerobic, facultative, or anaerobic zones depending on depth and loading. They are simple to operate, robust to shock loads, and economical where land is available. The terms “lagoon” and “oxidation pond” are often used interchangeably in practice.

Step-by-Step Solution:
Match the description “shallow, natural treatment, long detention” with known units.Oxidation pond/lagoon precisely fits these attributes.Therefore, select “Lagoon (oxidation pond)”.

Verification / Alternative check:
Textbook process flow diagrams list oxidation ponds as low-rate biological systems with hydraulic retention times of days to weeks and depths typically around 1–1.5 m for facultative ponds.

Why Other Options Are Wrong:
Oxidation ditch: a mechanical aeration system (activated sludge variant), not a shallow passive pond.Imhoff tank: two-story sedimentation with anaerobic digestion below; not an open, shallow, sunlight-driven process.Skimming tank: removes floatables like oil/grease; not for full biological stabilization.Trickling filter: fixed-film aerobic reactor with media and underdrains, not a shallow lagoon.

Common Pitfalls:
Confusing oxidation ditches (mechanized) with oxidation ponds (passive). The energy footprint and operational complexity differ greatly.

Final Answer:
Lagoon (oxidation pond)