Question

A large quantity of urban sewage is drained to the nearby village river. Which among the given conditions would happen after mixing of sewage into the river?

(i) Biochemical oxygen demand (BOD) of receiving water body increases.
(ii) Dissolved oxygen of receiving water body decreases.
(iii) It will not cause mortality among fishes and other aquatic creatures.
(iv) It will lead to nutrient enrichment of receiving water body.

• A. (i), (ii) and (iii)
• B. (i), (ii) and (iv)
• C. (ii) and (iii)
• D. (iii) and (iv)

Answer 1

The correct option is B (i), (ii) and (iv)

Sewage is primarily wastewater that contains too much organic waste matter of humans and many living organisms such as faecal matter and nutrients. High levels of organic matter in water are decomposed by the action of microorganisms which uses oxygen in the process. The amount of oxygen consumed by these microorganisms in breaking down the waste is known as the biochemical oxygen demand or BOD. So, statement (i) is correct.

There is an inversely proportional relation between BOD level and dissolved oxygen of the water body. Higher BOD level means low dissolved oxygen and vice versa. Thus, statement (ii) is correct.

Since, the dissolved oxygen is very less in water bodies thus it may kill all living organisms due to hypoxia conditions. Hence statement (iii) is incorrect.

Sewage water contains lots of organic matter and nutrients, thus on mixing with the river it sums up all nutrients in the river. So, it leads to nutrient enrichment of the receiving water body. Thus statement (iv) is correct.