Arrange the following steps of sewage treatment in a sequential order.
(A) Primary effluent passed in aeration tank
(B) Activated sludge is formed
(C) Filtration and sedimentation of sewage
(D) Aerobic microbes grow into flocs
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → (C), (A), (D), (B)
Primary treatment removes particles by filtration and sedimentation (C); the primary effluent goes into aeration tanks (A), where aerobic microbes grow into flocs (D); the flocs then settle as activated sludge (B).
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.”