Arrange the following steps in correct sequence which are involved in sewage treatment.
A. Primary effluent is passed into aeration tanks for growth of aerobic microbes.
B. Remaining part of the sludge is pumped into anaerobic sludge digesters.
C. Physical removal of particles from the sewage through filtration and sedimentation.
D. Once the BOD of sewage is reduced, the effluent is passed into settling tank.
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → C, A, D, B
(C) Physical removal of particles from the sewage through filtration and sedimentation. — First. This is primary treatment. Floating debris is removed by filtration and grit by sedimentation. The solids that settle form the primary sludge. The liquid above it is the primary effluent.
(A) Primary effluent is passed into aeration tanks for growth of aerobic microbes. — Second. This begins secondary (biological) treatment. Air is pumped in, and aerobic microbes grow as flocs. They eat most of the organic matter, and this lowers the BOD.
(D) Once the BOD of sewage is reduced, the effluent is passed into settling tank. — Third. In the settling tank the flocs settle down. This sediment is called activated sludge. A small part of it is sent back to the aeration tank as a starter.
(B) Remaining part of the sludge is pumped into anaerobic sludge digesters. — Fourth. Here anaerobic bacteria digest the bacteria and fungi in the sludge. This produces biogas, a mix of methane, hydrogen sulphide and carbon dioxide.
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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.”