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.
BOD is a measure of-
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → Organic matter present in the water
BOD (biochemical oxygen demand) is the amount of oxygen that would be used up if all the organic matter in one litre of water were oxidised by bacteria. Microbes use oxygen to break down the organic matter they feed on.
Option (1) Solid waste present in the water — Incorrect. Floating and settling solids are removed in primary treatment by filtration and sedimentation. BOD does not weigh the solids; it measures the oxygen that microbes use.
Option (2) Heavy metals present in the water — Incorrect. Heavy metals such as mercury and lead are not food for microbes. Microbes do not use oxygen to break them down, so the BOD test does not detect them.
Option (3) Inorganic matter present in the water — Incorrect. Inorganic salts are not oxidised by microbes for energy. Only organic matter takes up oxygen in the BOD test.
Option (4) Organic matter present in the water — Correct. The more organic matter the water holds, the more oxygen the microbes use, and the higher the BOD. So a high BOD means the water is badly polluted.
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.