Arrange the following steps of sewage treatment in the correct sequence:
A. Grit (soil and small pebbles) is removed by sedimentation.
B. Floating debris is removed by sequential filtration.
C. The primary effluent is agitated in aeration tanks, where useful aerobic microbes grow into flocs.
D. The bacterial flocs are allowed to sediment in a settling tank, forming activated sludge.
E. The major part of the sludge is pumped into anaerobic sludge digesters.
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → B, A, C, D, E
Sewage is treated in two stages. Primary treatment removes particles physically, first by filtration and then by sedimentation. Secondary (biological) treatment uses aerobic microbes in aeration tanks, after which the microbial flocs are settled and the sludge is sent for anaerobic digestion.
B – Floating debris is removed first, by sequential filtration.
A – Then the grit (soil and small pebbles) is removed by sedimentation. The settled solids form the primary sludge and the liquid above forms the primary effluent.
C – The primary effluent is passed into aeration tanks, agitated and supplied with air, so that useful aerobic microbes grow into flocs and consume the organic matter.
D – When the BOD has fallen enough, the effluent goes to a settling tank, where the flocs sediment as activated sludge.
E – A small part of the activated sludge returns to the aeration tank as inoculum; the major part is pumped into anaerobic sludge digesters.
The correct order is B, A, C, D, E.
NCERT Extract:
Microbes in Sewage Treatment – “… These are removed in stages; initially, floating debris is removed by sequential filtration. Then the grit (soil and small pebbles) are removed by sedimentation. … 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). … 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.”