Standard biochemical oxygen demand (BOD) test: Over how many days is the BOD of a water sample measured under standard conditions?

Introduction / Context:
Biochemical oxygen demand (BOD) is the oxygen required by microorganisms to oxidize biodegradable organic matter. A standard test period ensures comparability of results across laboratories and systems.

Given Data / Assumptions:

• Standard conditions are implied (most commonly 20°C in the laboratory).
• We are asked for the standard number of days used internationally for BOD determination.

Concept / Approach:
The conventional BOD test is BOD5, meaning 5-day BOD, performed at 20°C in the dark to prevent photosynthesis. This historical choice approximated river travel time in temperate climates and became a standard benchmark for wastewater strength.

Step-by-Step Solution:
1) Recall the standard test designation: BOD5.2) BOD5 indicates measurement over 5 days at 20°C.3) Select the corresponding option.

Verification / Alternative check:
Many regulations and design references specify BOD5 as the basis for effluent limits and treatment performance, confirming the answer.

Why Other Options Are Wrong:
1, 2, 3 days: Too short to represent standard BOD5.10 days: Sometimes used for ultimate BOD estimation trends but not the standard test period.

Common Pitfalls:

• Confusing BOD5 with ultimate BOD (BODu), which extrapolates beyond 5 days.
• Assuming temperature other than 20°C; the standard prescribes 20°C.

Final Answer:
5 days