Comprehension:
Municipal waste water, also called as sewage, contains large amounts of organic matter, pathogens and other pollutants. Before being discharged into the natural water bodies, the sewage is treated in the sewage treatment plants (STPs) to undergo primary (physical removal of large and small particles) and secondary (removal of organic load and pathogens) treatment. During secondary treatment, vigrous growth of useful aerobic microbes consume the major portion of organic matter in the sewage water, thereby significantly reducing the BOD of treated water. The bacterial flocs formed during this process are allowed to settle in the bottom of the sedimentation tank as “activated sludge”. A small portion of activated sludge is used as inoculum and pumped back into the aeration tank, while the rest is anaerobically digested to produce a mixture of gases such as methane, hydrogen sulphide and carbon dioxide, which form ‘Biogas’. A similar process of anaerobic digestions using methanogens is used for production of ‘Gober gas’ from cattle dung.
Choose the incorrect statement
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → Activated sludge always remain afloat in water tank in STPs.
This statement is incorrect. The passage says the bacterial flocs are “allowed to settle in the bottom of the sedimentation tank as activated sludge” — the sludge sinks, it does not float.
The other three follow the passage: Gobar gas is made by anaerobic digestion of cattle dung using methanogens, bacterial flocs do form during secondary treatment, and a portion of the activated sludge is pumped back as inoculum into the aeration tank for the treatment process.