Identify the correct sequence of steps involved in sewage treatment.(…

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

(B) Physical removal of particles from the sewage through filtration and sedimentation — first. This is the primary treatment. It removes floating and settling solids before anything else is done.

(D) Effluent from the primary settling tank is taken for secondary treatment — second. The liquid left after the primary step goes to the aeration tank, where microbes grow and digest the organic matter.

(A) When the BOD of sewage is reduced significantly, the effluent is passed into a settling tank — third. The BOD (biochemical oxygen demand) is the oxygen the microbes need to break down the organic matter. Only after it has fallen are the flocs allowed to settle.

(C) Effluent from secondary treatment plant is generally released into natural water bodies — last. It is released only after the treatment is complete.

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. These are removed in stages; initially, floating debris is removed by sequential filtration. Then the grit (soil and small pebbles) are removed by sedimentation. All solids that settle form the primary sludge, and the supernatant forms the effluent. The effluent from the primary settling tank is taken for secondary treatment.”
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.”