Differentiate between primary and secondary (biological) treatment of sewage, stating clearly which step involves the action of microorganisms.

Question

Accepted Answer

Primary treatment of sewage is a physical process: sewage is passed through filters to remove large floating debris, then allowed to stand in settling tanks where sand, grit and other dense solids settle out as primary sludge, while lighter material is skimmed from the surface. No microbial action is involved at this stage -- it simply removes solid material mechanically. Secondary, or biological, treatment is where microorganisms take over: the primary-treated effluent is passed into aeration tanks and continuously agitated with air, allowing naturally occurring aerobic bacteria and filamentous fungi to grow into flocs that consume the remaining dissolved and suspended organic matter as food, dramatically lowering the sewage's Biochemical Oxygen Demand (BOD).

[!ANSWER] Primary treatment physically removes solids with no microbial involvement; secondary treatment uses aerated bacterial and fungal flocs to biologically break down dissolved organic matter and reduce BOD.

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] Primary treatment physically removes solids with no microbial involvement; secondary treatment uses aerated bacterial and fungal flocs to biologically break down dissolved organic matter and reduce BOD.

Q.Differentiate between primary and secondary (biological) treatment of sewage, stating clearly which step involves the action of microorganisms.

Concept understanding — Sewage Treatment

Q.Differentiate between primary and secondary (biological) treatment of sewage, stating clearly which step involves the action of microorganisms.

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