Question

Assertion (A): Untreated sewage when discharged into natural water bodies may cause the death of fishes.
Reason (R): It increases the BOD of water.

• A. Both assertion and reason are true and reason is the correct explanation for the assertion
• B. Both assertion and reason are true but the reason is not the correct explanation for the assertion
• C. Assertion is true but reason is false
• D. Both assertion and reason are false

Answer 1

The correct option is A Both assertion and reason are true and reason is the correct explanation for the assertion

The large quantities of waste water that is generated every day in cities and towns is known as sewage. It contains human excreta, large amounts of organic matter (food waste) and pathogenic microbes. It needs to be treated in sewage treatment plants (STPs) before they are discharged into natural water bodies to reduce the organic matter in it and thereby minimise its polluting potential.

BOD stands for biochemical oxygen demand. It refers to the amount of the oxygen that would be consumed if all the organic matter in one liter of water were oxidised by bacteria. It measures the rate of uptake of oxygen by microorganisms that are present in a sample of water and thus, BOD is an indirect measure of the organic matter present in the water. The greater the organic matter present in a water sample, the greater would be its BOD.

When untreated sewage is discharged into water bodies, it increases the BOD of the waterbody by adding large amounts of organic matter into it. The heterotrophic microbes present in water consume dissolved oxygen from the water body to break down the organic matter, leading to reduction in the dissolved oxygen in the water body. It may eventually result in the death of fishes due to oxygen deficiency.


Figure: Effect of sewage on dissolved oxygen