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When domestic sewage mixes with river water
A
Small animals like rats will die after drinking river water
B
The increased microbial activity releases micro-nutrients such as iron
C
The increased microbial activity uses up dissolved oxygen
D
The river water is still suitable for drinking as impurities are only about 0.1%
Detailed Solution
Domestic sewage contains a large amount of biodegradable organic matter.
Micro-organisms in the receiving water body decompose this organic matter, and in doing so they consume a lot of oxygen.
As a result there is a sharp decline in dissolved oxygen downstream from the point of sewage discharge (high BOD), which causes mortality of fish and other aquatic creatures.
Although the impurities are only about 0.1%, they include pathogens and make the water unfit for drinking.
Hence the increased microbial activity uses up dissolved oxygen.
Micro-organisms in the receiving water body decompose this organic matter, and in doing so they consume a lot of oxygen.
As a result there is a sharp decline in dissolved oxygen downstream from the point of sewage discharge (high BOD), which causes mortality of fish and other aquatic creatures.
Although the impurities are only about 0.1%, they include pathogens and make the water unfit for drinking.
Hence the increased microbial activity uses up dissolved oxygen.