A treatment plant operator adding 200 lbs/day of alum at 48% purity to 2.5 MGD has an alum dose of?

A treatment plant operator adding 200 lbs/day of alum at 48% purity to 2.5 MGD has an alum dose of?

• A. 5 mg/L
• B. 10 mg/L
• C. 15 mg/L
• D. 20 mg/L

Answer: (B)

The key idea is to convert the daily amount of alum actually entering the plant into a concentration (mg/L) based on the daily water volume. First apply the purity: 200 lb/day × 0.48 = 96 lb/day of active alum. Then use the standard conversion for dosing: mg/L = lbs/day ÷ (MGD × 8.34). So the dose is 96 ÷ (2.5 × 8.34) ≈ 96 ÷ 20.85 ≈ 4.6 mg/L, which rounds to about 5 mg/L. If you didn’t account for purity, you’d get roughly 9.6 mg/L (about 10 mg/L).

The key idea is to convert the daily amount of alum actually entering the plant into a concentration (mg/L) based on the daily water volume. First apply the purity: 200 lb/day × 0.48 = 96 lb/day of active alum. Then use the standard conversion for dosing: mg/L = lbs/day ÷ (MGD × 8.34). So the dose is 96 ÷ (2.5 × 8.34) ≈ 96 ÷ 20.85 ≈ 4.6 mg/L, which rounds to about 5 mg/L. If you didn’t account for purity, you’d get roughly 9.6 mg/L (about 10 mg/L).