Water and wastewater · Worked step by step
How many pounds of chlorine for 2 MGD at 3 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 2 MGD (million gallons a day) of water at a dose of 3 mg/L. How many pounds of chlorine do you need per day?
Grab a pen and paper first
Give it a real try on paper before you watch. If you get stuck, take a hint. If you go blank, that is fine, press play and we will walk it through together.
Watch it solved, with you
Every step is written out slowly and explained in plain words, so you always know what just happened. Nobody skips ahead and leaves you behind.
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 two numbers you were given
flow = 2 MGD • dose = 3 mg/L
- We push 2 million gallons a day through the plant. That is the flow.
- We want 3 mg/L of chlorine in the water. That is the dose.
- Two numbers so far. We need one more: the 8.34.
Quick trick: MGD means million gallons per day. If flow were in plain GPD, divide by 1,000,000 to get MGD.
- 2
Write the pounds formula
pounds = 2 × 3 × 8.34
- The pounds formula is flow × dose × 8.34.
- Drop your numbers in: 2 (flow) × 3 (dose) × 8.34.
- That 8.34 is the weight of a gallon of water, and it is what turns this into pounds.
Quick trick: This one formula covers most chemical-feed questions. Learn it once, use it forever.
- 3
Multiply across
pounds = 50.04 • pounds ≈ 50 lb/day
- Multiply left to right. First 2 × 3 = 6.
- Then 6 × 8.34 = 50.04.
- So you need about 50 pounds of chlorine a day.
Quick trick: Fast check: 6 × 8 = 48, plus a little more is about 50. Any answer near 50 is right.
Remember this for good
Pounds per day = flow (MGD) × dose (mg/L) × 8.34. The 8.34 is what turns it into pounds.
8.34 is the weight of one gallon of water, in pounds. This is the famous "pounds formula." Say it like a chant: flow, times dose, times eight-point-three-four.
Solve it fast next time
- Whenever a problem asks for POUNDS of a chemical, reach for flow × dose × 8.34.
- Flow has to be in MGD and dose in mg/L for this to work.
- It is just three numbers multiplied together. Don't overthink it.
The answer: About 50 pounds per day.