What does BOD measure?

• A. The acidity of water
• B. The amount of oxygen bacteria need to break down organic matter
• C. The level of turbidity
• D. The amount of dissolved carbon dioxide

Answer: (B)

Biochemical Oxygen Demand measures the amount of oxygen microorganisms require to break down organic matter in water. It reflects how much organic pollution is present and the potential for the water to lose dissolved oxygen as microbes decompose that material, which can harm aquatic life. The test typically shows how much oxygen is consumed over a set period (often five days at around 20°C). This concept is distinct from acidity (pH), turbidity (cloudiness from suspended solids), or dissolved carbon dioxide, which are different water quality aspects. So the option describing the oxygen bacteria need to break down organic matter best captures what BOD quantifies.