Arrange the following steps of sewage treatment in correct sequence.
(A) Physical removal of particles from sewage through filtration and sedimentation
(B) Primary effluent is passed into large aeration tanks
(C) Remaining major part of sludge is pumped into anaerobic sludge digesters
(D) Part of the activated sludge is pumped back into the aeration tank
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → (A), (B), (D), (C)
(A) Physical removal of particles from sewage through filtration and sedimentation — this comes first. It is the primary treatment. Floating debris is filtered out and the grit settles.
(B) Primary effluent is passed into large aeration tanks — this comes second. In the secondary (biological) treatment, air is pumped in and useful aerobic microbes grow as flocs and use up the organic matter.
(D) Part of the activated sludge is pumped back into the aeration tank — this comes third. The flocs are allowed to settle as activated sludge, and a small part is returned to the tank as inoculum.
(C) Remaining major part of sludge is pumped into anaerobic sludge digesters — this comes last. Anaerobic bacteria digest the sludge there, and biogas is formed.
Note: A similar question was also asked in CUET (UG) 2026 on 07-06-2026 (Slot 2), on 31-05-2026 (Slot 2), on 20-05-2026 (Slot 2), on 18-05-2026 (Slot 1) and on 15-05-2026 (Slot 1).
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. This sediment is called activated sludge. A small part of the activated sludge is pumped back into the aeration tank to serve as the inoculum. The remaining major part of the sludge is pumped into large tanks called anaerobic sludge digesters.”