Which range represents the organic nitrogen content in wastewater?

• A. 40-50 ppm
• B. 25-30 ppm
• C. 15-20 ppm
• D. 5-15 ppm

Answer: (C)

Organic nitrogen is the portion of nitrogen that’s bound up in organic compounds, like proteins and amino acids, rather than in inorganic forms such as ammonia or nitrates. In typical municipal wastewater, the organic-N level as nitrogen generally sits in the tens of milligrams per liter, commonly around 15 to 20 mg/L. That makes the 15–20 mg/L range the best fit.

Ranges like 40–50 mg/L would usually indicate a higher concentration of inorganic nitrogen or total nitrogen, not primarily organic-N. A range of 25–30 mg/L is higher than the common organic-N portion you’d expect in ordinary wastewater, and 5–15 mg/L tends to be on the low side for organic-N.