Wastewater lagoons (oxidation/maturation ponds) for purifying polluted water Which statements about lagoon systems used for biological treatment are accurate regarding construction, operating principle, and achievable effluent quality when multiple ponds are placed in series?

Introduction / Context:
Lagoon systems, also called oxidation ponds or maturation ponds, are widely used for municipal and industrial wastewater polishing. They exploit natural biological processes in a controlled, shallow basin to reduce biochemical oxygen demand (BOD), suspended solids (SS), pathogens, and odour. This question checks your understanding of what lagoons are, how they work, and what level of effluent quality they can approach when multiple cells are arranged in series.

Given Data / Assumptions:

• Lagoons are engineered, shallow, earthen basins with long detention time.
• They host mixed communities of bacteria, algae, and protozoa.
• Oxygen is supplied by surface re-aeration and algal photosynthesis; optional mechanical aeration can be added.
• Series operation (facultative followed by maturation cells) improves clarity and stability.

Concept / Approach:
Lagoons achieve treatment by biological oxidation of organics. In facultative zones, bacteria oxidize carbonaceous matter while algae generate oxygen. Settling of biological flocs and algal-bacterial aggregates lowers suspended solids. Series ponds provide staged treatment: primary reduction, polishing, and pathogen die-off.

Step-by-Step Solution:

Verification / Alternative check:
Typical designs report progressively lower BOD and SS from cell to cell. Warm climates, adequate area, and proper hydraulic retention time yield excellent polishing, confirming the combined statements.

Why Other Options Are Wrong:

Common Pitfalls:
Assuming lagoons are purely sedimentation devices (they are biological); underestimating the benefit of series cells; forgetting the role of algal photosynthesis in oxygen supply.

Final Answer:
All (a), (b) & (c).