What is the proper nutrient ratio for carbon, nitrogen, and phosphorus?

• A. 50 carbon, 5 nitrogen, 1 phosphorus
• B. 100 carbon, 5 nitrogen, 1 phosphorus
• C. 100 carbon, 10 nitrogen, 2 phosphorus
• D. 100 carbon, 5 nitrogen, 2 phosphorus

Answer: (B)

Microorganisms in biological wastewater treatment need carbon as an energy and building-block source, nitrogen for proteins and nucleic acids, and phosphorus for energy transfer and nucleic acids. For balanced microbial growth in typical wastewater, a standard ratio is about 100 units of carbon to 5 of nitrogen to 1 of phosphorus. This 100:5:1 balance ensures carbon is available in enough quantity to fuel growth while nitrogen and phosphorus are present in just enough amounts to support cellular needs without creating excess nutrients in the effluent.

If carbon is too low relative to nitrogen and phosphorus (like a 50:5:1 ratio), growth becomes carbon-limited and slows down the treatment process. If nitrogen or phosphorus are too high relative to carbon (such as 100:10:2 or 100:5:2), you’re providing more nutrients than the microbes can use efficiently, which can lead to wasted nutrients or potential issues with nutrient removal processes.

Therefore, the proper nutrient ratio is 100 carbon, 5 nitrogen, and 1 phosphorus.