Identify the steps involved in the treatment of sewage in correct sequence.
A. Physical removal of particles through filtration and sedimentation
B. After BOD reduction, effluent is passed into settling tank
C. Primary effluent is passed into large aeration tanks
D. Remaining sludge is pumped into anaerobic sludge digesters
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → A, C, B, D
A. Physical removal of particles through filtration and sedimentation — first. This is the primary treatment. Floating debris is filtered out, and grit settles by sedimentation. The liquid left is the primary effluent.
C. Primary effluent is passed into large aeration tanks — second. This is the start of the secondary (biological) treatment. The tank is stirred and air is pumped in. Aerobic microbes grow as flocs (masses of bacteria with fungal filaments) and consume the organic matter, so the BOD falls.
B. After BOD reduction, effluent is passed into settling tank — third. This can happen only after the BOD has been reduced in the aeration tank. In the settling tank the flocs settle as sediment, called activated sludge.
D. Remaining sludge is pumped into anaerobic sludge digesters — last. A small part of the activated sludge goes back to the aeration tank as inoculum. The rest goes to digesters, where anaerobic bacteria digest it and produce 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.”