Given below are two statements: one is labelled as Assertion (A) and the other is labelled as Reason (R).
Assertion (A): Municipal waste-water (sewage) cannot be discharged directly into natural water bodies like rivers and streams.
Reason (R): Treatment of waste water is done by the heterotrophic microbes naturally present in the sewage.
In the light of the above statements, choose the most appropriate answer from the options given below:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Both (A) and (R) are correct but (R) is not the correct explanation of (A).
Sewage is the municipal waste-water of towns and cities. Its main part is human excreta. It is treated in sewage treatment plants (STPs) before it is let out.
Assertion (A): Municipal waste-water (sewage) cannot be discharged directly into natural water bodies like rivers and streams. — True. Sewage cannot be discharged directly into rivers and streams. It holds large amounts of organic matter and microbes, many of which are pathogenic, so it is first treated in STPs to make it less polluting.
Reason (R): Treatment of waste water is done by the heterotrophic microbes naturally present in the sewage. — True. The treatment of waste water is done by the heterotrophic microbes naturally present in the sewage.
Why R does not explain A? A is about why raw sewage must not go straight into rivers. The cause is what sewage contains: organic matter and pathogenic microbes that would pollute the water and spread disease. R is about something else: who carries out the treatment once the sewage reaches the STP. Knowing the agents of treatment does not tell us why untreated sewage is harmful. So both statements are correct, but R is not the explanation of A.
NCERT Extract:
Microbes in Sewage Treatment – “… This municipal waste-water is also called sewage. It contains large amounts of organic matter and microbes. Many of which are pathogenic. … This cannot be discharged into natural water bodies like rivers and streams directly – you can understand why. Before disposal, hence, sewage is treated in sewage treatment plants (STPs) to make it less polluting. Treatment of waste water is done by the heterotrophic microbes naturally present in the sewage.”