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.
Which of the following combination of gases form Biogas?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → CH4, H2S and CO2
The passage says that the leftover activated sludge is anaerobically digested to produce a mixture of gases — methane, hydrogen sulphide and carbon dioxide — which together form biogas. Methane makes up the bulk of it and gives the fuel its calorific value.
Any option containing oxygen must be wrong, because the digestion is anaerobic; and neither ammonia nor nitrous oxide is a normal constituent of biogas.