Question

The process of self purification process of polluted waters can be noted by the

• A. Physical changes
• B. Chemical changes
• C. Biological changes
• D. Physical, chemical and biological changes.

Hint:

The "oxygen sag curve" is a classic way to monitor the self-purification of a river downstream from a pollution source.
It shows how dissolved oxygen levels dip and then recover as these physical, chemical, and biological processes clean the water.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
Self-purification is a natural process where a water body (such as a river or lake) recovers its original clean state over time after being polluted.
This recovery is driven by a combination of physical, chemical, and biological processes.

Step 3: Detailed Explanation:
Let us analyze how these different processes contribute to self-purification:
1. Physical changes:
These include dilution of pollutants by incoming water, sedimentation of suspended solids, solar UV radiation disinfecting pathogens, and aeration (the absorption of atmospheric oxygen at the water's surface).
2. Chemical changes:
These include the oxidation of organic pollutants by dissolved oxygen, reduction of chemical compounds, and precipitation of heavy metals or other chemicals out of the water column.
3. Biological changes:
Microorganisms (primarily aerobic and anaerobic bacteria and fungi) decompose and mineralize organic pollutants, turning them into harmless inorganic compounds.
Algae consume dissolved nutrients, and aquatic plants absorb pollutants.
These physical, chemical, and biological changes work together as an integrated system to purify the water.

Step 4: Final Answer:
The self-purification process of polluted waters involves physical, chemical, and biological changes.
Therefore, the correct option is (D).