Secondary treatment or biological treatment: The primary effluent is passed into large tanks where air is pumped into it. This allows vigorous growth of aerobic microbes. While growing, these microbes consume the major part of the organic matter in the effluent. This significantly change the BOD (biochemical oxygen demand) of the effluent.
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 sludge digesters. Here, other kinds of bacteria grow anaerobically in the sludge. The biogas produced can be used source of energy as it is inflammable. The effluent from the secondary treatment plant is generally released into natural water bodies like rivers and streams.
The greater the BOD of water
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → More is its pollution level
BOD (biochemical oxygen demand) is the amount of oxygen consumed if all the organic matter in one litre of water were oxidised by bacteria. More organic matter means more oxygen used and a higher BOD.
Option (1) Less is its pollution level — Incorrect. NCERT says the greater the BOD of waste water, the more is its polluting potential. So a high BOD cannot go with less pollution.
Option (2) Less is its oxygen consumption — Incorrect. BOD measures oxygen consumption; a high BOD means more is used.
Option (3) More is its pollution level — Correct. BOD is the amount of oxygen consumed if all the organic matter in one litre of water were oxidised by bacteria. The greater the BOD, the more organic matter the water carries, so the more is its pollution level.
Option (4) No relation between oxygen consumption and BOD — Incorrect. BOD is defined by oxygen use: it is the oxygen that would be consumed if all the organic matter in one litre of water were oxidised by bacteria. So the two are directly related.
Note 1: The passage is taken from NCERT. NTA has put lines of the NCERT text together to make it. Most CUET (UG) questions come straight from NCERT, so study the book line by line, including figures and their titles.
Note 2: A similar question was also asked in CUET (UG) 2025 on 27-05-2025 (Slot 2).
NCERT Extract:
Microbes in Sewage Treatment – “… 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.”