Question

Explain the changes that can be observed in the characteristics of river water when sewage is discharged into it and a few weeks after the discharge with respect to : 

(i) level of dissolved oxygen 

(ii) population of fresh water organisms.

Answer 1

Criteria /Time of Sewage discharged	When sewage is discharged	Few weeks after discharge
(i)   Level of dissolved oxygen	Microorganisms involved in bio-degradation of organic matter consume a lot of oxygen, resulting in sharp decline in dissolved oxygen.	Level of dissolved oxygen increases as sewage decreases.
(ii) Population of fresh water organisms	Decline in dissolved oxygen causes mortality of fresh water organisms.	Reappearance of population fresh water organisms.