GATE CE 2017 Set 1 — Question 14
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Environmental Engineering → Wastewater Treatment Processes → Oxygen Sag (Streeter-Phelps)
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Question
The wastewater from a city, containing a high concentration of biodegradable organics, is being steadily discharged into a flowing river at a location . If the rate of aeration of the river water is lower than the rate of degradation of the organics, then the dissolved oxygen of the river water
Correct answer
(D) is lowest at a point downstream of the location S.
Solution
This scenario is described by the Streeter-Phelps oxygen sag curve. When organic waste is discharged into a river at location , the dissolved oxygen (DO) level begins to drop because the rate of deoxygenation (due to microbial decomposition of organics) is initially higher than the rate of reaeration (replenishment of oxygen from the atmosphere). The DO level continues to decrease until it reaches a minimum point, known as the critical DO point. This critical point occurs at some distance downstream from the point of discharge . After this point, as the organic matter is depleted, the reaeration rate begins to exceed the deoxygenation rate, and the DO level starts to recover.
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