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Question
Assertion (A): There is a sharp decline in the dissolved oxygen (DO) in the area of the water body where the sewage is discharged
Reason (R): Microorganisms involved in the degradation of organic matter in the receiving waterbody consume a lot of oxygen
Assertion (A): There is a sharp decline in the dissolved oxygen (DO) in the area of the water body where the sewage is discharged
Reason (R): Microorganisms involved in the degradation of organic matter in the receiving waterbody consume a lot of oxygen
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Solution
The correct option is A Both assertion and reason are true and reason is the correct explanation of assertion
Domestic sewage is rich in biodegradable organic matter and the decomposition of the organic matter by microorganisms requires oxygen. So, there is always a sharp decline in dissolved oxygen when the sewage (rich in organic matter) is discharged as shown in the image.
Figure: Effect of sewage discharge on some important characteristics of a water body
Simultaneously, there is a sharp rise in BOD. BOD (Biochemical oxygen demand) is the amount of oxygen that would be consumed if all the organic matter in one litre of water were oxidised by bacteria. Therefore, the amount of biodegradable organic matter in sewage can be indirectly measured by BOD.
Greater the BOD of waste water, the higher is the amount of organic matter present in it. Higher BOD indicates that more oxygen is utilised for the breakdown of organic matter. It also indicates that this water is not suitable for the survival of aquatic organisms like fishes, as the dissolved oxygen is less. This causes mortality of fish and other aquatic organisms in this area of sewage discharge.
Further downstream of the sewage flow, the water has been cured of organic matter, so its dissolved oxygen levels return to normal, and BOD also comes back to baseline. This part of the water supports life. Low BOD means less oxygen is being utilised from water, so water is generally cleaner and suitable for aquatic organisms with higher dissolved oxygen levels as shown in the image.
Domestic sewage is rich in biodegradable organic matter and the decomposition of the organic matter by microorganisms requires oxygen. So, there is always a sharp decline in dissolved oxygen when the sewage (rich in organic matter) is discharged as shown in the image.
Figure: Effect of sewage discharge on some important characteristics of a water body
Simultaneously, there is a sharp rise in BOD. BOD (Biochemical oxygen demand) is the amount of oxygen that would be consumed if all the organic matter in one litre of water were oxidised by bacteria. Therefore, the amount of biodegradable organic matter in sewage can be indirectly measured by BOD.
Greater the BOD of waste water, the higher is the amount of organic matter present in it. Higher BOD indicates that more oxygen is utilised for the breakdown of organic matter. It also indicates that this water is not suitable for the survival of aquatic organisms like fishes, as the dissolved oxygen is less. This causes mortality of fish and other aquatic organisms in this area of sewage discharge.
Further downstream of the sewage flow, the water has been cured of organic matter, so its dissolved oxygen levels return to normal, and BOD also comes back to baseline. This part of the water supports life. Low BOD means less oxygen is being utilised from water, so water is generally cleaner and suitable for aquatic organisms with higher dissolved oxygen levels as shown in the image.
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