Arrange the following steps involved in sewage treatment in correct s…

The correct answer is Option (1) → (C), (D), (A), (B)

(C) Physical removal of particles by filtration and sedimentation — first. This is the primary treatment. Floating debris is removed by filtration and the grit settles down. The liquid left above (primary effluent) goes on to the next stage.

(D) The primary effluent is passed into large aeration tanks for flocs formation — second. This starts the secondary (biological) treatment. Air is pumped in, and useful aerobic microbes grow into flocs (masses of bacteria and fungal threads). They eat up the organic matter and so reduce the BOD.

(A) Once the BOD is reduced, the effluent is passed into a settling tank — third. Here the flocs settle down. This sediment is the activated sludge. A small part of it goes back to the aeration tank as the inoculum.

(B) Remaining part of the sludge is pumped into anaerobic sludge digesters — last. In these tanks, bacteria that grow without oxygen digest the microbes in the sludge and give off gases such as methane, which form biogas.

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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). While growing, these microbes consume the major part of the organic matter in the effluent. This significantly reduces the BOD (biochemical oxygen demand) of the effluent. BOD refers to the amount of the oxygen that would be consumed if all the organic matter in one liter of water were oxidised by bacteria. The sewage water is treated till the BOD is reduced. The BOD test measures the rate of uptake of oxygen by micro-organisms in a sample of water and thus, indirectly, BOD is a measure of the organic matter present in the water. The greater the BOD of waste water, more is its polluting potential. 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.”