Biochemical Oxygen Demand (BOD) standard test: Over how many days at 20°C is the standard BOD measured for wastewater characterization?

Introduction / Context:
BOD quantifies the oxygen consumed by microorganisms while oxidizing biodegradable organics in wastewater. The standard BOD test is foundational for sewage treatment design, regulatory compliance, and stream impact assessment.

Given Data / Assumptions:

• Incubation temperature 20°C.
• Dark conditions, absence of nitrification (or inhibition used).
• Standard dilution water and seeding where required.

Concept / Approach:

The “standard” BOD, commonly referred to as BOD5, measures oxygen demand over a 5-day incubation at 20°C. Historically, 5 days approximated the time a river in temperate climates required to carry wastes to the sea, making it a practical compromise between accuracy and test duration.

Step-by-Step Solution:

Verification / Alternative check:

Standards (e.g., APHA, IS) recognize BOD5 at 20°C as the reference measure; ultimate BOD (BODu) can be inferred via kinetics but is not the standard regulatory test.

Why Other Options Are Wrong:

• 1–4 days: Underestimate stabilized oxygen demand in most wastewaters.

Common Pitfalls:

• Nitrification during the test artificially increases BOD unless inhibited.
• Poor dilution selection can drive DO to zero, invalidating results.

Final Answer:

5 days.