If sewage water is added to river water, it will:

The correct answer is Option (2) → Increase the BOD of river water

Sewage carries a large amount of organic matter. When it is added to a river, the river water gets this extra organic matter. Bacteria start breaking it down and use up the dissolved oxygen.

Option (1) Increase the BOD of sewage water — Incorrect. Mixing does not add organic matter to the sewage. The sewage is diluted by the river water, so its BOD cannot go up.

Option (2) Increase the BOD of river water — Correct. The organic matter of the sewage raises the oxygen demand of the river water. The fall in dissolved oxygen can kill fish and other aquatic animals.

Option (3) Decrease the BOD of river water — Incorrect. BOD falls only when organic matter is removed, as in sewage treatment plants. Adding sewage adds organic matter, so the BOD rises.

Option (4) Not affect the BOD of river water and sewage water — Incorrect. Any change in the organic matter changes the BOD. That is why sewage is treated until its BOD is low before it is let into rivers.

NCERT Extract:
Microbes in Sewage Treatment – …