Arrange the given steps involved in sewage treatment in correct seque…

The correct answer is Option (1) → (D), (A), (C), (B)

(D) Removal of large and small particles through filtration and sedimentation — first. This is the primary treatment. It is a physical step. Floating debris is filtered out first, and then grit and fine particles settle down.

(A) The primary effluent is passed into large aeration tanks — second. This is the start of the secondary (biological) treatment. Air is pumped in and aerobic microbes grow into flocs, which consume the organic matter.

(C) The effluent is then passed into a settling tank — third. The flocs settle here as activated sludge.

(B) The effluent is released into natural water bodies like rivers and streams — last. Only after the treatment can the water be let out.

NCERT Extract:
Microbes in Sewage Treatment – “… Primary treatment : These treatment steps basically involve physical removal of particles – large and small – from the sewage through filtration and sedimentation.”
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).”
Microbes in Sewage Treatment – “… Once the BOD of sewage or waste water is reduced significantly, the effluent is then passed into a settling tank where the bacterial ‘flocs’ are allowed to sediment.”
Microbes in Sewage Treatment – “… The effluent from the secondary treatment plant is generally released into natural water bodies like rivers and streams.”