Water · Worked step by step
How many pounds of alum for 3 MGD at 12 mg/L?
If math has never been your thing, you are in the right place. Try it on paper first, then we write it out with you, one small step at a time, and explain every step in plain words so you never lose the thread.
The problem
You treat 3 MGD of water with alum at a dose of 12 mg/L. How many pounds of alum per day?
Grab a pen and paper first
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Press play and I’ll write it out with you, one small step at a time.
Every step gets a plain-words explanation here, so you always know exactly what just happened and why.
The written version, step by step
- 1
Write the numbers
flow = 3 MGD • dose = 12 mg/L
- 3 million gallons a day of water.
- 12 mg/L of alum.
- Then the usual 8.34.
- 2
Multiply across
pounds = 3 × 12 × 8.34 • pounds ≈ 300 lb/day
- 3 × 12 = 36.
- 36 × 8.34 = 300.24.
- So about 300 pounds of alum a day.
Quick trick: 36 × 8 is 288, plus a bit more is about 300. Quick sanity check.
Remember this for good
The pounds formula works for any chemical: flow (MGD) times dose (mg/L) times 8.34.
Same chant, different chemical: flow times dose times 8.34. Alum, chlorine, lime, it does not matter; the pounds formula is always the same.
Solve it fast next time
- Pounds per day = flow times dose times 8.34, for any chemical.
- Keep flow in MGD and dose in mg/L.
- Just three numbers multiplied.
The answer: About 300 pounds per day.