Environmental engineering – wastewater treatment overview: Which of the following are recognized biological treatment methods used in municipal and industrial wastewater treatment plants?

Introduction / Context:
Biological wastewater treatment relies on microorganisms to remove dissolved and particulate organic matter, nutrients, and some toxic compounds. Common engineered systems include stabilization lagoons, the activated sludge process, and oxidation ditches. This question checks recognition of the major categories used in practice.

Given Data / Assumptions:

• Municipal or industrial wastewater containing biodegradable organics.
• Operating under aerobic conditions for carbon removal.
• Typical secondary treatment step following primary clarification.

Concept / Approach:
All three listed options are biological processes. Lagoons (oxidation ponds) use large basins with long detention times, sunlight, algae–bacteria symbiosis, and natural aeration. Activated sludge uses mechanical aeration to maintain a suspended floc of biomass in a reactor with a secondary clarifier for solids separation and recycle. Oxidation ditches are extended-aeration variants with a looped channel providing long sludge age and stable nitrification/denitrification when configured properly.

Step-by-Step Solution:
Identify whether each listed technology depends on microbial metabolism.Lagoon: biological removal with natural/aerated oxygen supply → yes.Activated sludge: aerobic suspended growth with recycle → yes.Oxidation ditch: extended-aeration form of activated sludge → yes.Therefore, the correct selection that includes all true methods is 'All of these'.

Verification / Alternative check:
Typical design textbooks classify secondary biological treatment into suspended growth (activated sludge, oxidation ditch) and attached growth (trickling filters, MBBR). Lagoons are also categorized as biological treatment with long detention times.

Why Other Options Are Wrong:
Each individual option (a, b, or c) is correct by itself, but choosing only one would omit the other valid biological methods.

Hence, only 'All of these' captures the full set.

Common Pitfalls:

• Confusing biological processes with purely physical–chemical methods (e.g., coagulation or air stripping).
• Assuming lagoons are not 'engineered' because they are passive; they are standardized biological systems.

Final Answer:
All of these