Standard BOD₅ test condition: The 5-day Biochemical Oxygen Demand (BOD₅) of wastewater is conventionally measured at which incubation temperature?

Introduction / Context:
BOD₅ is a fundamental indicator of biodegradable organic strength in wastewater. Standardizing the incubation temperature ensures comparability of results across laboratories and seasons by controlling microbial activity and oxygen solubility.

Given Data / Assumptions:

• Conventional 5-day BOD test.
• Dark incubation to prevent photosynthesis.
• Standard dilution water and seed where required.

Concept / Approach:
International practice has settled on 20°C for five days as a practical compromise reflecting historical European conditions where the test originated. This temperature provides reproducible microbial kinetics and oxygen saturation properties suitable for routine analysis.

Step-by-Step Solution:
Recall the standard condition: BOD₅ at 20°C for 5 days.Compare to options and select 20°C.

Verification / Alternative check:
Standard methods and textbooks explicitly state 20°C as the incubation temperature for BOD₅, confirming the choice.

Why Other Options Are Wrong:
0°C and 15°C: too cold; metabolic rates suppressed and results not comparable.25°C or higher: accelerates microbial activity, deviating from standard.

Common Pitfalls:

• Confusing BOD₅ (5 days at 20°C) with other biochemical tests run at different temperatures.
• Not ensuring darkness, leading to oxygen production by algae.

Final Answer:
20°C