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Large quantities of sewage… (Class 12 Biology 2024, Q33b)

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BiologyCBSE Class 12 · Board exam 2024, set 57/1/1 · 5 marks · Long answer

CBSE Class 12 Biology 2024: question 33(b)

Set in the CBSE Class 12 board exam of 2024, paper 57/1/1, section E, question 33(b), for 5 marks. Try it first; the hints and our step-by-step solution are below.

Q33(b) · 5 marks · Long answerCompetency-basedMicrobes in Human Welfare

Large quantities of sewage are generated every day in cities as well as in towns and are treated in Sewage Treatment Plants (STPs) to make them less polluting. Given below is the flow diagram of stages of STP.

Study the flow diagram and answer the questions that follow :

Primary effluent passed into large aeration tanks → Effluent passed into settling tanks to form 'sediment'

(i) (1) Why is primary effluent passed into large aeration tanks ? (1)
(2) What is the 'sediment' formed, referred to as ? Mention its significance. (1)
(3) Explain the final step in the settling tank before the treated effluent is released into water bodies. (1)
(ii) Name any two organisms commonly used as biofertilisers, belonging to different kingdoms. Write how each one acts as a biofertiliser. (2)

Figure in the paper: Flow diagram: box 'Primary effluent passed into large aeration tanks' with a downward arrow to box 'Effluent passed into settling tanks to form sediment'. See it in the official paper.

Source: official question paper from cbse.gov.in (PDF). See it in the full 2024 board paper, set 57/1/1.

Hints

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

Hint 1

Aeration supplies oxygen. Which microbes benefit, and what do they do to the organic matter?

Hint 2

The sediment contains microbes that are still active. Where could some of it be reused?

Hint 3

For biofertilisers, pick one bacterium and one fungus, and think about which nutrient each provides.

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. (i)(1) The primary effluent is constantly agitated and aerated in the aeration tanks. This allows vigorous growth of useful aerobic microbes into flocs (bacteria associated with fungal filaments). The flocs consume the major part of the organic matter in the effluent, which significantly reduces its BOD.

    1 mark

  2. (i)(2) The sediment is called activated sludge.

    ½ mark

  3. (i)(2) Significance: a small part of the activated sludge is pumped back into the aeration tank to serve as the inoculum of microbes.

    ½ mark

  4. (i)(3) Once the BOD has been reduced sufficiently, the effluent is held in the settling tank so that the bacterial flocs sediment as activated sludge. The clear supernatant is then released into water bodies. The major part of the sludge is pumped into anaerobic sludge digesters, where anaerobic bacteria digest it and produce biogas (CH₄, H₂S, CO₂).

    1 mark

  5. (ii) Rhizobium (bacterium, kingdom Monera): it lives symbiotically in the root nodules of leguminous plants. It fixes atmospheric nitrogen into organic forms that the plant uses as a nutrient.

    1 mark

  6. (ii) Glomus (fungus, kingdom Fungi): it forms a mycorrhizal symbiotic association with the roots of higher plants. It absorbs phosphorus from the soil and passes it to the plant.

    (Alternative: cyanobacteria such as Anabaena or Nostoc fix atmospheric nitrogen and add organic matter to soil, for example in paddy fields. Note that cyanobacteria are also Monera, so they cannot be paired with Rhizobium here.)

    1 mark

Answer: (i)(1) For growth of aerobic microbial flocs that consume organic matter and reduce BOD.
(i)(2) Activated sludge; a small part is used as inoculum in the aeration tank.
(i)(3) Bacterial flocs settle as sludge, and the clear effluent is released into water bodies; most of the sludge goes to anaerobic digesters.
(ii) Rhizobium (bacterium) fixes atmospheric nitrogen in legume root nodules; Glomus (fungus) absorbs phosphorus for the plant as mycorrhiza.

Where the marks are

  • (i)(1) Aerobic microbes and flocs grow, consume organic matter and reduce BOD (1 mark)
  • (i)(2) Names activated sludge (½ mark)
  • (i)(2) States its use as inoculum in the aeration tank (½ mark)
  • (i)(3) Explains that bacterial flocs are allowed to sediment before the effluent is released (1 mark)
  • (ii) Rhizobium (bacterium) fixing nitrogen symbiotically in legume nodules (1 mark)
  • (ii) Glomus (fungus) absorbing phosphorus as mycorrhiza, or cyanobacteria fixing nitrogen and adding 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 paper2024 board paper, set 57/1/1: 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.