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Study the figure given below… (Class 12 Biology 2025, Q29)

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BiologyCBSE Class 12 · Board exam 2025, set 57/7/1 · 4 marks · Case-based

CBSE Class 12 Biology 2025: question 29

Set in the CBSE Class 12 board exam of 2025, paper 57/7/1, section D, question 29, for 4 marks. Try it first; the hints and our step-by-step solution are below.

Q29 · 4 marks · Case-basedCompetency-basedMicrobes in Human Welfare

Study the figure given below and answer the questions that follow.

  1. (a)

    Identify the figure and state its importance.

    1 mark
  2. (b)

    Why is air diffused into the aerator tank ? Explain.

    2 marks
  3. (c)(i)

    What changes take place in the settling tank ?

    1 mark

  4. OR

    (c)(ii)

    How is BOD related to organic matter present in the water ?

    1 mark

Figure in the paper: Flow diagram of sewage treatment: Influent (from Primary Clarifier) enters an Aerator Tank with air bubbles and Air Diffusers below; output goes to Settling Tank/Secondary Clarifier giving Effluent at the top and Activated sludge at the bottom; activated sludge splits into Return Activated Sludge (RAS) returning to the aerator tank and Waste Activated Sludge (WAS). See it in the official paper.

Source: official question paper from cbse.gov.in (PDF). See it in the full 2025 board paper, set 57/7/1.

Hints

Open one hint at a time and try again before the next.

Hint 1

Look at the labels influent, effluent and activated sludge; what is being treated?

Hint 2

Aerobic microbes need oxygen. What do they form, and what do they break down?

Step-by-step solution

Each step shows the marks it earns, as an examiner would award them. Checked by an independent re-solve from the question paper.

  1. (a) The figure shows secondary (biological) treatment in a sewage treatment plant (STP).

    ½ mark

  2. Importance: it treats sewage water and removes its organic matter before it is released into natural water bodies, so the water bodies are not polluted.

    ½ mark

  3. (b) Air is pumped into the aerator tank to allow vigorous growth of useful aerobic microbes. They form flocs, which are masses of bacteria joined with fungal filaments into mesh-like structures.

    1 mark

  4. These aerobic microbes use oxygen to consume and decompose most of the organic matter in the effluent, which greatly reduces its BOD.

    1 mark

  5. (c)(i) In the settling tank, the bacterial flocs settle down and form a sediment called activated sludge. A small part of it is sent back to the aerator tank as inoculum, and the rest goes to anaerobic sludge digesters.

    1 mark

  6. (c)(ii) BOD is the amount of oxygen consumed if all the organic matter in one litre of water were oxidised by bacteria. So BOD is directly related to the amount of organic matter: the greater the BOD, the more organic matter the water contains and the greater its polluting potential.

    1 mark

Answer: (a) Sewage treatment plant (secondary treatment), which treats sewage before release into water bodies; (b) air supports aerobic flocs that decompose organic matter; (c)(i) flocs settle as activated sludge; (c)(ii) BOD is directly proportional to organic matter.

Where the marks are

  • Identifies the sewage treatment plant (½ mark)
  • Importance: treats sewage before release into water bodies (½ mark)
  • Air promotes growth of aerobic flocs (1 mark)
  • Flocs decompose organic matter using oxygen (1 mark)
  • (c)(i) Flocs settle to form activated sludge (1 mark), OR (c)(ii) BOD directly related to organic matter (1 mark)

Revise the chapter

Practise this chapterBoard-style questions from Microbes in Human Welfare, one at a time, marked against the answer key and marking points.Back to the full paper2025 board paper, set 57/7/1: every question, by section.

The question is transcribed from the official paper; figures are described in words. The solution, hints and chapter tag are ours. Spotted an error? Write tosupport@questionbuddy.in.