Skip to content

The Biological Oxygen… (Class 12 Biology SQP 2025-26, Q26)

questionbuddy.in/q/cbse-12-biology-2025-26-sqp-q26-biological-oxygen-demand-bod-primary

BiologyCBSE Class 12 · Sample paper, session 2025-26 · 3 marks · Short answer

CBSE Class 12 Biology sample paper 2025-26: question 26

From the official CBSE sample question paper for 2025-26, 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

The Biological Oxygen Demand (BOD) of a primary effluent during sewage treatment is reduced. Enlist the process of how this is achieved.

Source: official sample paper from cbseacademic.nic.in (PDF). See it in the full Sample paper 2025-26.

Hints

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

Hint 1

The BOD falls during the secondary (biological) treatment stage.

Hint 2

Follow the effluent from aeration tank to settling tank to sludge digester.

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. The primary effluent is passed into large aeration tanks, where it is constantly agitated mechanically and air is pumped in. This lets useful aerobic microbes grow into flocs: masses of bacteria held together by slime and fungal filaments in mesh-like structures.

    1 mark

  2. The microbes consume most of the organic matter, turning it into microbial biomass and releasing minerals. This sharply reduces the BOD. When the BOD has fallen to a small fraction of that of raw sewage, the effluent goes to a settling tank, where the flocs sediment. The sediment is called activated sludge.

    1 mark

  3. Part of the activated sludge is pumped back into the aeration tank as inoculum. The rest goes to anaerobic sludge digesters, where anaerobic bacteria digest the bacteria and fungi in the sludge, producing biogas (methane, HS, CO). The treated effluent is released into rivers or streams, and the leftover sludge can be used as manure.

    1 mark

Answer: Secondary (biological) treatment: aeration tanks with flocs consume organic matter (BOD falls) → settling tank (activated sludge) → part recycled as inoculum, rest digested anaerobically in sludge digesters producing biogas.

Where the marks are

  • Aeration tank and formation of flocs (1 mark)
  • Microbial consumption of organic matter lowers BOD; settling gives activated sludge (1 mark)
  • Activated sludge used as inoculum and in anaerobic sludge digesters (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 paperSample paper 2025-26: 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.