For every 100 ppm of BOD removed, how much nitrogen and phosphorus are required?

• A. 10 Nitrogen and 2 Phosphorus
• B. 5 Nitrogen and 1 Phosphorus
• C. 5 Nitrogen and 5 Phosphorus
• D. 1 Nitrogen and 5 Phosphorus

Answer: (B)

Microorganisms need nutrients beyond the carbon they consume. In biological wastewater treatment, nitrogen and phosphorus are the key nutrients that support microbial growth as they break down BOD. A common design guideline uses a rough ratio of 100 parts BOD to 5 parts nitrogen to 1 part phosphorus. So for every 100 ppm of BOD removed, about 5 ppm of nitrogen and 1 ppm of phosphorus are required. This ratio helps ensure microbes have enough N and P to grow and metabolize the BOD efficiently; too little N or P can slow removal, while excess amounts aren’t typically necessary for standard operations. Other ratios would either oversupply nitrogen or phosphorus or under-supply the other nutrient, making them less appropriate for typical biological BOD removal.