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Treatment of wastewater is… (Class 12 Biology 2023, Q26)

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BiologyCBSE Class 12 · Board exam 2023, set 57/1/1 · 3 marks · Short answer

CBSE Class 12 Biology 2023: question 26

Set in the CBSE Class 12 board exam of 2023, paper 57/1/1, section C, question 26, for 3 marks. Try it first; the hints and our step-by-step solution are below.

Q26 · 3 marks · Short answerMicrobes in Human Welfare

Treatment of wastewater is done in a sewage treatment plant to make it less polluting. Explain the following with reference to this treatment process :
(a) Primary sludge
(b) Activated sludge
(c) Anaerobic sludge digesters

Source: official question paper from cbse.gov.in (PDF). See it in the full 2023 board paper, set 57/1/1.

Hints

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

Hint 1

Primary treatment is physical; secondary treatment is biological.

Hint 2

Which microbes form flocs in aeration tanks, and what happens to BOD?

Hint 3

What gases do anaerobic bacteria produce when they digest the sludge?

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) Primary sludge: in primary treatment, floating debris is removed by filtration and grit (soil, pebbles) by sedimentation.

    ½ mark

  2. All the solids that settle down during this primary treatment form the primary sludge; the liquid left above is the primary effluent.

    ½ mark

  3. (b) Activated sludge: in secondary (biological) treatment, the primary effluent is aerated so that aerobic microbes grow into flocs (masses of bacteria with fungal filaments). These microbes consume organic matter and greatly reduce the BOD.

    ½ mark

  4. The effluent is then passed to a settling tank, where the bacterial flocs settle as activated sludge. Part of it is pumped back into the aeration tank as inoculum.

    ½ mark

  5. (c) Anaerobic sludge digesters: the remaining activated sludge is pumped into large tanks called anaerobic sludge digesters.

    ½ mark

  6. Inside, anaerobic bacteria digest the bacteria and fungi of the sludge and produce a mixture of gases such as , and , which form biogas.

    ½ mark

Answer: (a) Solids settling in primary treatment; (b) sediment of aerobic microbial flocs after secondary treatment, BOD reduced; (c) tanks where anaerobic bacteria digest activated sludge, producing biogas (CH₄, H₂S, CO₂)

Where the marks are

  • Primary sludge: solids that settle during primary treatment (½ + ½ mark)
  • Activated sludge: aerobic flocs reduce BOD and settle as sediment in secondary treatment (½ + ½ mark)
  • Anaerobic digesters: large tanks where anaerobic bacteria digest sludge (½ mark), producing CH₄, H₂S, CO₂/biogas (½ 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 paper2023 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.