What is the primary function of the aeration tanks in the secondary t…

The correct answer is Option (2) → To allow the growth of aerobic microbes that consume organic matter

Option (1) To remove solid waste through filtration — Incorrect. Large solid particles are removed in primary treatment, by filtration and sedimentation, before the aeration tanks.

Option (2) To allow the growth of aerobic microbes that consume organic matter — Correct. The effluent is stirred and air is pumped in, so aerobic microbes grow fast into flocs and use up most of the organic matter. This cuts down the BOD.

Option (3) To reduce the BOD of the effluent using chemicals — Incorrect. The BOD (biochemical oxygen demand) does fall here, but by the work of microbes, not by chemicals. That is why this step is called biological treatment.

Option (4) To settle the activated sludge — Incorrect. The flocs are allowed to settle later, in a settling tank; this sediment is the activated sludge.

NCERT Extract:
Microbes in Sewage Treatment – “… 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). While growing, these microbes consume the major part of the organic matter in the effluent. This significantly reduces the BOD (biochemical oxygen demand) of the effluent.”