What is meant by the term "Biochemical Oxygen Demand (BOD)" and why is it used as a measure of the extent of organic pollution in water?

Question

Accepted Answer

Biochemical Oxygen Demand (BOD) is defined as the amount of dissolved oxygen that would be consumed if all the biodegradable organic matter present in one litre of a water sample were fully oxidised (broken down) by bacteria. It is used as the standard measure of organic-pollution load in a water body precisely because it captures, in a single number, how much decomposition activity -- and hence how much oxygen depletion -- that water's organic content will demand as it breaks down naturally. A high BOD value signals that a large amount of oxygen-demanding organic matter is present; if such water reaches a natural water body, the resulting bacterial decomposition will consume dissolved oxygen faster than it can be replenished, suffocating fish and other aquatic organisms that depend on that oxygen.

[!ANSWER] BOD measures the oxygen bacteria would need to decompose the organic matter in a water sample; a high BOD indicates heavy organic pollution because it predicts severe dissolved-oxygen depletion in the receiving water.

Same / Similar Concept — real previous-year questions on the same or a closely similar concept, not this exact question.

CBSE 2026Set 57/3/11 markMCQ
$Q. For Questions number 13 to 16, two statements are given - one labelled as Assertion (A) and the other labelled as Reason (R). Select the correct answer to these questions from the codes (A), (B), (C) and (D) as given below. (A) Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A). (B) Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of the Assertion (A). (C) Assertion (A) is true, but Reason (R) is false. (D) Assertion (A) is false, but Reason (R) is true. Assertion (A) : Activated sludge contains large population of bacteria. Reason (R) : These bacteria help in anaerobic digestion of human waste in biogas plant.$
›Reveal solutionSolution

The Assertion is true, but the Reason is false — activated sludge bacteria work aerobically in a sewage treatment plant, not anaerobically in a biogas plant.

Let’s place this in context. In a modern sewage treatment plant, the water that enters after primary treatment — where solid debris and grit are removed — still contains a heavy load of dissolved organic matter. To break this down, the plant relies on a biological step called the activated sludge process. Here, air is bubbled vigorously through the effluent in a large aeration tank. This oxygen-rich environment encourages the rapid growth of aerobic bacteria and other microbes. These microorganisms form flocs — sticky, clumpy masses — and they consume the organic pollutants as food. The result is a thick, dark liquid teeming with bacteria: that is the activated sludge.

So the Assertion (A) — that activated sludge contains a large population of bacteria — is perfectly correct. The NCERT textbook explicitly states that “the effluent is passed into an aeration tank where it is constantly agitated mechanically and air is pumped into it. This allows vigorous growth of useful aerobic microbes into flocs.” Those flocs are the activated sludge, and they are indeed packed with bacteria.

Now look at the Reason (R). It claims that these bacteria help in the anaerobic digestion of human waste in a biogas plant. That is a confusion of two entirely different processes. The bacteria in activated sludge are aerobic — they need oxygen to function. A biogas plant, on the other hand, operates under anaerobic conditions (no oxygen), where a different set of microbes, such as methanogens, break down organic matter to produce methane. The NCERT makes this distinction very clear: the activated sludge process is an aerobic treatment step in sewage management, while biogas production from human waste (or cattle dung) is a separate, anaerobic process carried out in a biogas plant.

Note

The activated sludge is not sent to a biogas plant. Instead, a portion of it is pumped back into the aeration tank to maintain the bacterial population, and the rest is sent to an anaerobic sludge digester — but even there, the digestion is done by anaerobic bacteria, not by the same aerobic bacteria from the activated sludge.

Therefore, the Reason (R) is false. It incorrectly attributes an anaerobic function to bacteria that are fundamentally aerobic, and it mixes up two distinct stages of waste treatment.

✓Final answer
$In short, Assertion (A) is true, but Reason (R) is false — so the correct choice is (C).$

[!ANSWER] BOD measures the oxygen bacteria would need to decompose the organic matter in a water sample; a high BOD indicates heavy organic pollution because it predicts severe dissolved-oxygen depletion in the receiving water.

Q.What is meant by the term "Biochemical Oxygen Demand (BOD)" and why is it used as a measure of the extent of organic pollution in water?

Concept understanding — Sewage Treatment

Q.What is meant by the term "Biochemical Oxygen Demand (BOD)" and why is it used as a measure of the extent of organic pollution in water?

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