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Biology · 2025 · 4 marks
CBSE 2025 · Region 7 · Set 1 · Q29
Study the figure given below and answer the questions that follow. (a) Identify the figure and state its importance. (b) Why is air diffused into the…
Study the figure given below and answer the questions that follow.
(a)
Identify the figure and state its importance.
(b)
Why is air diffused into the aerator tank ? Explain.
(c)(i)
What changes take place in the settling tank ?
OR
(c)
(ii)
How is BOD related to organic matter present in the water ?
Marking-scheme solution
(a)
• Sewage treatment plant
• used to treat sewage water before it can be released into water
body.
(b)
Air helps in growth of flocs (aerobic bacteria along with fungal
filaments), which decompose organic matter in the sewage using oxygen.
(c)(i)
Bacterial ‘flocs’ settle down to form activated sludge.
OR
(c)
(ii)
BOD is directly related to the measure of the organic matter
present in water/ Greater the BOD more is the polluting potential
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CBSE Class 12 Biology past-paper question from the 2025 board exam, with the answer as CBSE’s own marking scheme gives it. Where our answers come from.