Arrange the steps of biological sewage treatment process in a sequence.
(A) Vigorous growth of flocs and significant reduction in BOD
(B) Activated sludge pumped into anaerobic sludge digester
(C) Effluent passed into an aeration tank and agitated mechanically
(D) Effluent transfer to settling tank followed by sedimentation of bacterial 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)
(C) Effluent passed into an aeration tank and agitated mechanically — first. After primary treatment the effluent is aerated so that aerobic microbes grow.
(A) Vigorous growth of flocs and significant reduction in BOD — second. Flocs (bacteria with fungal filaments) consume the organic matter, so the BOD falls.
(D) Effluent transfer to settling tank followed by sedimentation of bacterial flocs — third. The flocs settle as activated sludge.
(B) Activated sludge pumped into anaerobic sludge digester — last. Anaerobic bacteria digest it and produce biogas.
NCERT Extract:
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.”