Consider the following statements regarding nitrification and denitrification:

Question

Consider the following statements regarding nitrification and denitrification:

I. Nitrification converts ammonium (NH4^+) to nitrate (NO3^-) under aerobic conditions.

II. Denitrification converts nitrate (NO3^-) to nitrogen gas (N2) under anaerobic/anoxic conditions.

III. Both processes are performed by the same group of autotrophic bacteria.

Which of the statements given above is/are correct?

1. A. I and II only (Correct answer)
2. B. I and III only
3. C. II and III only
4. D. I, II and III

Correct Answer

Option A — I and II only

Detailed Solution & Explanation

The correct answer is I and II only.

Key Points

• Statement I is correct. Nitrification is an aerobic, two-stage bacterial oxidation: *Nitrosomonas* converts ammonium $\text{NH}_4^+$ to nitrite $\text{NO}_2^-$, and *Nitrobacter* converts nitrite to nitrate $\text{NO}_3^-$.
• Statement II is correct. Denitrification reduces nitrate to nitrogen gas, which escapes to the atmosphere and so genuinely removes nitrogen from the wastewater.
• Statement III is incorrect, and it is the statement that decides the answer.

Additional Information

• The two processes need opposite conditions, which is the whole design problem in biological nitrogen removal. Nitrification requires dissolved oxygen (about 2 mg/L) and is performed by autotrophs; denitrification requires an anoxic zone — nitrate present but free oxygen absent — and is performed by heterotrophs that need a carbon source.
• Plants therefore alternate zones or phases: the Modified Ludzack-Ettinger layout places an anoxic tank first and recycles nitrate back to it, using the incoming sewage's own BOD as the carbon source rather than dosing methanol.
• Nitrification consumes alkalinity — roughly 7.14 mg as $\text{CaCO}_3$ per mg of ammonia-N oxidised — so poorly buffered wastewater can lose pH and stall the process. Denitrification returns about half of it.
• Nitrifiers grow slowly and are sensitive to low temperature and to toxins, so nitrification is usually the first function lost when a plant is upset.