Question 151 markSection A

$Assertion (A) : Flocs are masses of bacteria associated with fungal filaments in secondary treatment of sewage. Reason (R) : Flocs help in digestion of solid waste by anaerobic respiration.$

Q: Assertion (A) : Flocs are masses of bacteria associated with fungal filaments in secondary treatment of sewage. Reason (R) : Flocs help in digestion of solid waste by anaerobic respiration.

**Correct option: (C)** — Assertion is true, but Reason is false. **Why A is true:** - In secondary (biological) treatment, useful **aerobic microbes grow into flocs** — masses of **bacteria associated with fungal filaments** forming mesh-like structures. This matches NCERT exactly. **Why R is false:** - Flocs work under **vigorous aeration** and consume organic matter by **aerobic respiration**, which reduces the **BOD** of the effluent — not by **anaerobic respiration**. - **Anaerobic** digestion of the settled sludge happens **later, in anaerobic sludge digesters** (by different bacteria), not by the flocs. So the Reason is incorrect.

Microbes in Human Welfare

Sewage Treatment — Flocs

Options

(A)

Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation for Assertion (A).

(B)

Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation for Assertion (A).

(C)

Assertion (A) is true, but Reason (R) is false.

(D)

Assertion (A) is false, but Reason (R) is true.

Official answer

Correct option: (C) — Assertion is true, but Reason is false. Why A is true: In secondary (biological) treatment, useful aerobic microbes grow into flocs — masses of bacteria associated with fungal filaments forming mesh-like structures. This matches NCERT exactly. Why R is false: Flocs work under vigorous aeration and consume organic matter by aerobic respiration, which reduces the BOD of the effluent — not by anaerobic respiration . Anaerobic digestion of the settled sludge happens later, in anaerobic sludge digesters (by different bacteria), not by the flocs. So the Reason is incorrect.

flocs

secondary treatment

aerobic microbes

aeration tank

BOD reduction

activated sludge

anaerobic sludge digester

sewage treatment

Marking scheme

1. $1 mark: correct code (C) — Assertion true, Reason false.$
2. $Accept justification that flocs act aerobically to reduce BOD, while anaerobic digestion occurs separately in sludge digesters.$

Hint

$Flocs form in aeration tanks — do they work with oxygen or without it? Check whether 'anaerobic' fits.$

Quick oral answer

$The Assertion is correct — flocs are masses of bacteria linked with fungal filaments formed during secondary treatment — but the Reason is wrong, because flocs act aerobically in aeration tanks to reduce BOD; anaerobic respiration happens separately in the sludge digesters, so the answer is code C.$

Common mistakes

1. $Marking (A) by assuming any microbial digestion is anaerobic; flocs actually work aerobically in aeration tanks to reduce BOD.$
2. $Confusing the aerobic floc step with the anaerobic sludge-digester step that produces biogas.$
3. $Thinking BOD increases after treatment; correct flocs action lowers BOD, indicating cleaner water.$

Analysis and explanation

Concept — Secondary (biological) sewage treatment: After primary treatment removes solids by settling, the effluent goes to aeration tanks . Air is pumped in vigorously, so aerobic heterotrophic microbes multiply rapidly and form flocs (bacteria + fungal filaments in a mesh). These flocs oxidise organic matter aerobically, greatly reducing the Biochemical Oxygen Demand (BOD) of the sewage. Falling BOD is the sign treatment is working. Where anaerobic respiration actually fits: The activated sludge (mostly flocs) that settles in the settling tank is passed to anaerobic sludge digesters . There, different anaerobic bacteria digest the sludge and produce biogas. So anaerobic respiration is a separate, later step — not what flocs do. Why the code is (C): Assertion (definition of flocs) is textbook-correct, but the Reason wrongly attributes anaerobic respiration to flocs. Since flocs act aerobically, R is false — giving code C. Exam trap: Many students conflate the aeration-tank (aerobic, flocs) step with the sludge-digester (anaerobic, biogas) step. Keep them separate: flocs = aerobic BOD reduction; digesters = anaerobic biogas production.

Interesting facts

$A high BOD means water is heavily polluted; the greater the BOD, the more polluting the waste — flocs bring BOD down before discharge.$

$The activated sludge left after settling is used partly as inoculum for fresh sewage and the rest is sent to anaerobic digesters that yield biogas.$

$The biogas from anaerobic sludge digesters (a mix including methane) can be used as a source of energy, making sewage plants partly self-powered.$

Question details

Q15 of 37 questions

Board: CBSE
Class: Class 12
Subject: Biology
Year: 2026
Section: Section A
Marks: 1
Chapter: Microbes in Human Welfare
Topic: $Sewage Treatment — Flocs$
Bloom's level: understand
Syllabus topic: $Unit VIII: Microbes in Human Welfare — Microbes in Sewage Treatment (Secondary/Biological treatment, Flocs)$
Author: CBSE
Reviewed by: Padhantu Academic Team

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Frequently asked questions

$Do flocs work aerobically or anaerobically?$

$Aerobically. Flocs form in vigorously aerated tanks where aerobic microbes oxidise organic matter and reduce the BOD. Anaerobic respiration occurs later and separately in the sludge digesters, not in the flocs.$

$Why is the Assertion true but the Reason false?$

$The definition of flocs (bacteria with fungal filaments in secondary treatment) is correct, but attributing anaerobic respiration to them is wrong. Flocs act aerobically, so only the Reason is false, giving code C.$