During sewage treatment, in which stage are microbes used?

The correct answer is Option (2) → In secondary treatment.

Sewage is treated in two stages. Primary treatment is physical: floating debris is removed by filtration, and grit and solids by sedimentation. Secondary treatment is biological: the effluent is sent to aeration tanks, where useful aerobic microbes grow into flocs and eat up most of the organic matter.

Option (1) In primary treatment. — Incorrect. Primary treatment only removes particles physically, by filtration and sedimentation. Microbes are not used for it.

Option (2) In secondary treatment. — Correct. Secondary treatment is also called biological treatment. Air is pumped into the tanks so that aerobic microbes grow and consume the organic matter, which lowers the BOD (biochemical oxygen demand) of the effluent.

Option (3) Both primary and secondary treatment — Incorrect. Microbes act only in the secondary stage. The primary stage is physical, so “both” is wrong.

NCERT Extract:
Microbes in Sewage Treatment – “… This treatment is carried out in two stages: Primary treatment : These treatment steps basically involve physical removal of particles – large and small – from the sewage through filtration and sedimentation. … Secondary treatment or Biological treatment : The primary effluent is passed into large aeration tanks (Figure 8.6) where it is constantly agitated mechanically and air is pumped into it. This allows vigorous growth of useful aerobic microbes into flocs (masses of bacteria associated with fungal filaments to form mesh like structures).”