For each 1 mg/L of nitrate-nitrogen denitrified to nitrogen gas, how much alkalinity is produced as calcium carbonate?

• A. 1.0
• B. 3.57
• C. 0.5
• D. 5.0

Answer: (B)

Denitrification increases alkalinity. When nitrate-nitrogen is reduced to nitrogen gas, the process consumes hydrogen ions and generates hydroxide, which shifts the carbonate buffering system toward more carbonate species. In terms of alkalinity measured as calcium carbonate, the established stoichiometry gives about 3.57 mg/L as CaCO3 of alkalinity produced for each 1 mg/L of nitrate-nitrogen removed as N2. This is a standard rule-of-thumb in wastewater treatment for estimating how much alkalinity is generated by denitrification. If you remove 10 mg/L NO3-N, you’d expect roughly 35.7 mg/L as CaCO3 alkalinity. The other values don’t match this fixed ratio.