If 150 ppm BOD is removed, how much nitrogen and phosphorus are required based on the ideal ratio?

• A. 5 Nitrogen and 1 Phosphorus
• B. 3 Nitrogen and 0.5 Phosphorus
• C. 7.5 Nitrogen and 1.5 Phosphorus
• D. 10 Nitrogen and 2 Phosphorus

Answer: (C)

Nutrients for biological BOD removal are added in a fixed nitrogen-to-phosphorus ratio, about 5 parts nitrogen for every 1 part phosphorus. This 5:1 ratio scales with how much BOD you’re treating. For 150 ppm BOD to be removed, you apply the ratio by making phosphorus 1.5 ppm and nitrogen 5 times that, which is 7.5 ppm. That gives N:P = 7.5:1.5 = 5:1, matching the ideal ratio. So the required amounts are 7.5 ppm nitrogen and 1.5 ppm phosphorus.