Read the following passage carefully and answer the given questions.
During anaerobic sludge digestion, bacteria produce a mixture of gases such as methane, hydrogen sulphide and carbon dioxide. These gases form biogas and can be used as source of energy as it is inflammable. The bioactive molecule, cyclosporin A, that is used as an immunosuppressive agent in organ-transplant patients, is produced by the fungus Trichoderma polysporum. You must have noticed that bottled fruit juices bought from the market are clearer as compared to those made at home. This is because the bottled juices are clarified by the use of pectinases and proteases. Statins produced by the yeast Monascus purpureus have been commercialised as blood-cholesterol lowering agents. It acts by competitively inhibiting the enzyme responsible for synthesis of cholesterol. 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.
During anaerobic sludge digestion, bacteria produce the biogas. The composition of this biogas is -
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → Methane, hydrogen sulphide and carbon dioxide
Biogas is a mixture of gases made by bacteria that digest organic matter in the absence of oxygen. In a sewage plant this happens in large tanks called anaerobic sludge digesters.
Option (1) Methane, hydrogen and carbon dioxide — Incorrect. Hydrogen sulphide is missing, and hydrogen is not one of the three gases the book lists for biogas.
Option (2) Nitrogen, hydrogen sulphide and carbon dioxide — Incorrect. It leaves out methane, the main gas of biogas and the one that makes it burn. Nitrogen is not a part of the mixture.
Option (3) Methane, hydrogen sulphide and sulphur dioxide — Incorrect. Sulphur dioxide is formed when sulphur burns in oxygen. The digester has no oxygen, so the sulphur comes out as hydrogen sulphide. Carbon dioxide is also missing.
Option (4) Methane, hydrogen sulphide and carbon dioxide — Correct. These are the very gases of biogas. Methane makes it inflammable, so it can be used as a source of energy.
Note: 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.