Question

Given below are two statements, one is labelled as Assertion A and the other is labelled as Reason R

Assertion A: Polluted rivers with very high values of Biological Oxygen Demand (BOD) may produce a foul smell

Reason R: Anaerobic bacteria produce sulfur dioxide and oxides of nitrogen

In light of the above statements, choose the most appropriate answer from the options given below

1. Both A and R are correct and R is the correct explanation of A
2. Both A and R are correct but R is NOT the correct explanation of A
3. A is correct but R is not correct
4. A is not correct but R is correct

Answer 1

Correct Answer - Option 2 : Both A and R are correct but R is NOT the correct explanation of A

Biological Oxygen Demand (BOD)

• Biochemical oxygen demand (BOD, also called biological oxygen demand) is the amount of dissolved oxygen needed by aerobic biological organisms to break down organic material present in a given water sample at certain temperature over a specific time period. 
• BOD is the amount of oxygen consumed by bacteria and other microorganisms while they decompose organic matter under aerobic (oxygen is present) conditions at a specified temperature. This implies that the more the organic waste, the more will be the oxygen demand or the BOD. Thus, a high BOD value indicates high pollution level. Many animals need high oxygen amounts to survive. If the B.O.D. is high, that means that microorganisms are using most of the Oxygen.
• Oxygen reaches water either through the atmosphere or from the process of photosynthesis carried out by many aquatic green plants during daylight. However, during the night, photosynthesis stops but the plants continue to respire, resulting in a reduction of dissolved oxygen. The dissolved oxygen is also used by microorganisms to oxidise organic matter.
• If too much of organic matter is added to water, all the available oxygen is used up. This causes oxygen-dependent aquatic life to die. Thus, anaerobic bacteria (which do not require oxygen) begin to break down organic waste and produce chemicals that have a foul smell and are harmful to human health. Aerobic (oxygen requiring) bacteria degrade these organic wastes and keep the water depleted in dissolved oxygen.
• Thus, polluted rivers with very high values of Biological Oxygen Demand (BOD) may produce a foul smell is correct but the statement of reason is incorrect.