During denitrification, which gas is produced as a byproduct?

• A. Oxygen
• B. Carbon dioxide
• C. Methane
• D. Nitrogen gas

Answer: (D)

Denitrification is the process where bacteria use nitrate as an electron acceptor in the absence of oxygen, converting nitrate (NO3-) into nitrogen gas (N2) and releasing it to the atmosphere. This makes nitrogen gas the byproduct of the process and is why denitrification helps remove nitrogen from wastewater. Oxygen isn’t produced in this anaerobic reaction. Methane comes from methanogenesis, not denitrification, and carbon dioxide is released in many metabolic pathways but isn’t the defining byproduct of denitrification. (In some incomplete cases, nitrous oxide can be formed, but the primary gaseous product associated with denitrification is nitrogen gas.)