Discussion
In which zone of pollution in river stream, the presence of dissolved oxygen will be minimum?
Answer: B
Dissolved oxygen is lowest in the Zone of active decomposition.
Below a sewage outfall a river passes through four zones. In the zone of degradation, DO starts to fall as decomposition begins. In the zone of active decomposition, bacterial action is most intense and DO falls to its minimum, often near zero, with anaerobic conditions and foul gases.
In the zone of recovery reaeration exceeds oxygen demand and DO rises again, and in the zone of clear water conditions return to normal.
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