Question

 A wastewater contains 8 mg/L O2, 10-3 mol/L NO3 - , and 10-2 mol/L organic matter [CH2O] (no significant exchanges with atmosphere occur) After bacterial metabolism is complete, which of the following statements will be true? a) No [CH2O] remains b) Some O2 remains c) Some NO3 - remains d) Denitrification consumed more [CH2O] than respiration e) The composition of water remains unchanged

Answer 1

NO₃^- is 10^-3 mol/L

organic matter [CH₂O] is 10^-2 mol/L

O₂ is 8 mg/L which is 8 x 10^-3g/32 g/mol = 0.25 x 10^-3 mol/L

Anaerobic respiration is another heterotrophic mode of metabolism in which a specific compound other than O2 serves as a terminal electron acceptor. Such acceptor compounds include NO₃^–, SO₄^2–,

Denitrification takes place under special conditions in both terrestrial and marine ecosystems. In general, it occurs where oxygen, a more energetically favourable electron acceptor, is depleted, and bacteria respire nitrate as a substitute terminal electron acceptor.

Denitrification generally proceeds through some combination of the following intermediate forms:

NO₃⁻ → NO₂⁻ → NO + N₂O → N₂ (g)

The complete denitrification process can be expressed as a redox reaction:

2 NO₃⁻ + 10 e⁻ + 12 H⁺ → N₂ + 6 H₂O

Since under the present condition concentration of NO₃^- is more than O₂ the correct answer will be

d) Denitrification consumed more [CH2O] than respiration