Biological oxidation ponds in wastewater treatment How do oxidation ponds remove organic matter from polluted water under normal design and operating conditions?

Introduction / Context:
Oxidation ponds (stabilisation ponds) are low-cost, land-intensive systems that treat municipal wastewater using natural processes. Understanding their removal mechanisms helps in sizing, performance prediction, and seasonal operation planning.

Given Data / Assumptions:

• Ponds operate as facultative/aerobic systems depending on design and loading.
• Sunlight and algae provide oxygen; bacteria degrade organics.
• We focus on the essence of how organics are removed.

Concept / Approach:
In oxidation ponds, microorganisms (bacteria, algae, protozoa) drive removal. Organic matter is biologically oxidised primarily under aerobic conditions near the surface, with possible facultative zones below. Thus, both statements—microbial activity and aerobic oxidation—are jointly correct and complementary, making “Both (a) & (b)” the best choice.

Step-by-Step Solution:

Verification / Alternative check:
Design texts describe BOD reduction via aerobic heterotrophs supported by algal oxygen production; facultative layers allow some settling and further stabilisation.

Why Other Options Are Wrong:

• Only microorganisms or only aerobic oxidation: Incomplete alone; the processes are intertwined.
• Neither / purely physical sedimentation: Misses the biological core of oxidation ponds.

Common Pitfalls:
Overlooking the role of algae in oxygen supply; ignoring temperature dependence that affects microbial rates.

Final Answer:
Both (a) & (b)