How many ppm alkalinity are required to convert 1 ppm nitrates to nitrogen?

• A. 7.2
• B. 0.7
• C. 72
• D. 1.72

Answer: (A)

Alkalinity acts as the buffering capacity that must be present when nitrate is removed from water. Converting nitrate to nitrogen (denitrification) consumes alkalinity, so you need a certain amount of alkalinity to keep the pH from dropping.

The standard rule of thumb used in practice is about 7.2 mg/L as CaCO3 of alkalinity for each 1 mg/L of NO3-N removed. That’s why 7.2 is the correct value for 1 mg/L of nitrate being transformed to nitrogen—the alkalinity requirement is roughly seven times the nitrate-N amount when expressed in these units.

In short, removing nitrate to nitrogen consumes alkalinity, and the commonly used conversion factor is around 7.2 mg/L as CaCO3 per mg/L NO3-N.