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.
Answers
Explanation:
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BOD ( biological oxeygen demand) measures amount of oxeygen consumed if all organic matter in one litre water were oxidised by bacteria.
Changes observed in river water when sewage is discharged :
• BOD refers to the amount of oxeygen consumed by the bacteria in oxidizing organic matter.
• on discharge of sewage in river water the level of dissolved oxeygen significantly gets reduced, as the oxeygen is required by the bacteria for oxidizing organic matter.
• a significant decrease of dissolved oxeygen causes a rapid increase in fish mortality rate. Thus, sewage water is treated till the BOD is reduced and it serves as a test to measure the rate of uptake of oxeygen by micro- organisms in a sample of water.