Question

A large quantity of urban sewage is drained into nearby village river. Which among the given conditions would happen after mixing of sewage into the river? (i) $\quad$Biochemical oxygen demand (BOD) of receiving water body increases. (ii)$\quad$ Dissolved oxygen of receiving water body decreases. (iii) $\quad$It will not cause mortality among fishes and other aquatic creatures. (iv) $\quad$It will lead to nutrient enrichment of receiving water body.

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

Answer 1

Answer: (B)

(i), (ii) and (iv)

Answer 2

When sewage, having biodegradable organic matter is released into the water body, microorganisms involved in biodegradation of organic matter (in the receiving water body) consume a lot of oxygen to decompose the sewage and as a result there would be a sharp decline in dissolved oxygen downstream from the point of sewage discharge and biological oxygen demand (BOD) would increase. Presence of sewage causes nutrient enrichment (large amount of nutrients) in water which causes excessive growth of planktonic (freefloating) algae, called algal bloom. Algal bloom causes deterioration of water quality and fish mortality.