Question

What does the term "BOD" (Biochemical Oxygen Demand) refer to in sewage analysis?

• A. The amount of oxygen needed by aquatic organisms
• B. The rate of bacterial growth in sewage
• C. The oxygen content in fresh water bodies
• D. The amount of oxygen required by bacteria to decompose organic matter in sewage

Hint:

Think of BOD as an indirect measure of biodegradable organic pollution: high organic load $\rightarrow$ high bacterial activity $\rightarrow$ high oxygen consumption (BOD).

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
Biochemical Oxygen Demand (BOD) is an empirical bioassay parameter used to measure the quantity of biodegradable organic matter present in water or sewage.

Step 2: Detailed Explanation:
Sewage typically contains high amounts of organic waste, which serves as a food source for heterotrophic, aerobic bacteria.
When these bacteria decompose the organic matter in sewage, they consume dissolved oxygen from the water.
The BOD value is determined by measuring the dissolved oxygen concentration in a water sample before and after a standard incubation period (typically 5 days at $20^{\circ}\text{C}$).
A higher BOD value indicates a greater quantity of biodegradable organic pollution in the sample, which leads to a faster depletion of oxygen in the receiving aquatic habitat.

Step 3: Final Answer:
Thus, BOD specifically measures the oxygen required by aerobic microbes to biologically decompose organic wastes in sewage.