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Discussion

Dissolved oxygen in a river falls immediately downstream of a sewage outfall and then recovers. This is

  • A.the oxygen sag curve of self-purification
  • B.evidence that the effluent contained no organic matter
  • C.caused entirely by the temperature of the effluent
  • D.a sign that nitrification has been inhibited

Answer: A

Microbial degradation of the organic load draws oxygen down, and as the load is consumed reaeration and photosynthesis restore it, producing the characteristic sag and recovery. The length of river affected is a measure of the discharge's strength.

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