Water · Worked step by step
How many pounds of chlorine to disinfect a 12,000 gallon main?
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
A new water main holds 12,000 gallons. To disinfect it, you want a chlorine dose of 50 mg/L. How many pounds of chlorine do you need?
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
Change gallons to million gallons
12,000 gal = 0.012 MG
- The pounds formula uses million gallons.
- 12,000 divided by 1,000,000 slides the decimal 6 places left.
- So 12,000 gallons is 0.012 MG.
Quick trick: Divide gallons by a million: 12,000 becomes 0.012.
- 2
Use the pounds formula
pounds = 0.012 × 50 × 8.34
- Pounds = volume in MG times dose times 8.34.
- Drop in 0.012 MG, the 50 mg/L dose, and 8.34.
- This is the same pounds formula you use for a daily flow.
- 3
Multiply it out
pounds = 5.0 • pounds ≈ 5 lb
- 0.012 times 50 is 0.6.
- 0.6 times 8.34 is 5.004.
- So you need about 5 pounds of chlorine.
Quick trick: 0.6 × 8.34 is about 5. Quick sanity check.
Remember this for good
Pounds of chlorine = volume in million gallons (MG) times dose (mg/L) times 8.34. Change the gallons to MG first.
It is the same pounds formula as daily flow. A new main is a one-time fill, so the volume in MG plays the part of the flow.
Solve it fast next time
- Change gallons to MG by dividing by 1,000,000 (slide the decimal 6 places left).
- Then pounds = MG times dose times 8.34.
- New mains are disinfected at a high dose, often around 50 mg/L, then flushed and cleared before use.
The answer: About 5 pounds.