Secondary Treatment & BOD Reduction — NEET Biology
📚 Practice ConceptSecondary treatment or Biological treatment: The primary effluent is passed into large aeration tanks 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.
Which of the following statements are correct regarding the secondary treatment stage of sewage in a Sewage Treatment Plant (STP)? S1: Aeration tanks facilitate the vigorous growth of useful aerobic microbes, which form large masses called flocs. S2: Flocs are composed of bacteria associated with fungal filaments, which digest a major part of the organic matter in the effluent. S3: The primary purpose of aeration and agitation in the secondary treatment is to maintain anaerobic conditions crucial for microbial growth. S4: A significant reduction in the BOD of the effluent indicates successful removal of most of the inorganic contaminants. S5: The activated sludge formed in the settling tank, after BOD reduction, is entirely pumped into anaerobic sludge digesters. S6: Anaerobic digestion of the activated sludge results in the production of inflammable gases like methane, hydrogen sulphide, and carbon dioxide.
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Correct answer: C — S1, S2, and S6 are correct
The secondary treatment stage of sewage involves several key microbial processes. S1: This statement is correct. Primary effluent is passed into aeration tanks where it is constantly agitated and air is pumped, allowing vigorous growth of useful aerobic microbes into flocs. S2: This statement is correct. Flocs are indeed masses of bacteria associated with fungal filaments that consume the major part of the organic matter in the effluent. S3: This statement is incorrect. The purpose of aeration and agitation is to promote *aerobic* conditions, not anaerobic, for the growth of useful aerobic microbes. S4: This statement is incorrect. BOD reduction indicates a decrease in *organic matter*, not inorganic contaminants. S5: This statement is incorrect. Only a *major part* of the activated sludge is pumped into anaerobic sludge digesters; a small part is pumped back into the aeration tank to serve as inoculum. S6: This statement is correct. In anaerobic sludge digesters, anaerobic bacteria digest the sludge, producing a mixture of gases such as methane, hydrogen sulphide, and carbon dioxide, which form biogas and are inflammable. Therefore, statements S1, S2, and S6 are correct. The tested concept is the detailed process and outcomes of secondary sewage treatment.
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Through deep analysis of NEET and NTA, 176 of the 180 ReNEET 2026 (June 21) questions were already in the MedicNEET question bank before the exam.
176/180MedicNEET's Biology question bank is built from the same NCERT lines NTA picks repeatedly. Not random MCQs — questions crafted exactly like NTA crafts them.