Arrange the following steps involved in sewage treatment in correct sequence.
A. Primary effluent is passed into large aeration tanks
B. Physical removal of particles from the sewage through filtration and sedimentation
C. Remaining major part of sludge is pumped into anaerobic sludge digesters
D. Once the BOD of sewage is reduced, the effluent is then passed into a settling tank
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → B, A, D, C
(B) Physical removal of particles from the sewage through filtration and sedimentation — comes first. This is the primary treatment. Floating debris and then grit are removed, and the settled solids form the primary sludge.
(A) Primary effluent is passed into large aeration tanks — comes second. This is the secondary (biological) treatment. Air is pumped in, and aerobic microbes grow into flocs and use up the organic matter.
(D) Once the BOD of sewage is reduced, the effluent is then passed into a settling tank — comes third. The flocs settle there as activated sludge, which is the step that needs the BOD to be low first.
(C) Remaining major part of sludge is pumped into anaerobic sludge digesters — comes last. A small part of the activated sludge goes back to the aeration tank as inoculum. The rest is digested by anaerobic bacteria, which release biogas.
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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.”