A water sample from a river near a sewage outfall shows a very high BOD value. What does this indicate about the water quality, and why?

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A very high BOD value in a river sample near a sewage outfall indicates that the water is heavily polluted with biodegradable organic matter -- exactly what would be expected if inadequately treated or untreated sewage is entering the river at that point. This matters because BOD directly measures how much dissolved oxygen bacterial decomposition of that organic matter will consume: a high BOD means a correspondingly large amount of dissolved oxygen will be drawn down as naturally occurring bacteria in the river decompose the organic load. Since fish and other aquatic organisms depend on adequate dissolved oxygen to survive, this oxygen depletion can severely stress or kill aquatic life in the affected stretch of river, and generally si …

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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. …
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Q.A water sample from a river near a sewage outfall shows a very high BOD value. What does this indicate about the water quality, and why?

Concept understanding — Sewage Treatment

Q.A water sample from a river near a sewage outfall shows a very high BOD value. What does this indicate about the water quality, and why?

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