Water operator math tutorial
Chemical Feed Pump Setting
Find the mL/min a metering pump must deliver for a target dose.
Metering pumps are set in mL/min or gal/day. This formula works from the plant flow, the dose you want, and the strength of the stock solution.
Solution strength in mg/mL is a common way to write concentration. A 1% solution is about 10 mg/mL.
The formula
feed (mL/min) = [flow (MGD) × 1,000,000 gal × 3.785 L/gal × dose (mg/L)] ÷ [strength (mg/mL) × 1,440 min/day]
| Variable | Unit |
|---|---|
| Plant flow | MGD |
| Dose | mg/L |
| Solution strength | mg/mL |
| Result: Pump setting | mL/min |
Conversion factors used: 3.785 L per gallon; 1,440 minutes per day; 1 MGD = 1,000,000 gpd.
Applies to: Water Treatment, Water Distribution.
Common exam tip: Do the unit chain on paper: MGD to gal, gal to L, mg per day, then per minute.
Calculator
Pump setting
262.8 mL/min
- 1. Chemical needed = 0.5 × 1,000,000 × 3.785 × 2 = 3,785,000 mg/day
- 2. Feed = 3,785,000 ÷ (10 × 1,440) = 262.8 mL/min
For instructional use. Results assume the standard conversion factors shown on this page. Actual field calculations may need chemical strength, density, temperature, and other adjustments, so check your plant’s procedures.
Worked examples
Example 1
0.5 MGD, 2 mg/L dose, 10 mg/mL solution.
- Chemical needed = 0.5 × 1,000,000 × 3.785 × 2 = 3,785,000 mg/day
- Feed = 3,785,000 ÷ (10 × 1,440) = 262.8 mL/min
Answer: 262.8 mL/min
Example 2
1.2 MGD, 1.5 mg/L dose, 15 mg/mL solution.
- Chemical needed = 1.2 × 1,000,000 × 3.785 × 1.5 = 6,813,000 mg/day
- Feed = 6,813,000 ÷ (15 × 1,440) = 315.4 mL/min
Answer: 315.4 mL/min
Common mistakes
Wrong result: 15,770.8 mL/min
dividing by 24 instead of 1,440 and reporting per hour as per minute
Wrong result: 2,628.5 mL/min
not dividing by the solution strength
Practice questions
1. 0.8 MGD, 2.5 mg/L, 10 mg/mL. Pump setting in mL/min?
- A31,541.7 mL/min
- B525.7 mL/min
- C5,256.9 mL/min
- D5,257 mL/min
Show answer and explanation
Correct answer: B. Chemical needed = 0.8 × 1,000,000 × 3.785 × 2.5 = 7,570,000 mg/day Feed = 7,570,000 ÷ (10 × 1,440) = 525.7 mL/min Answer: 525.7 mL/min. A common slip is dividing by 24 instead of 1,440 and reporting per hour as per minute, which gives 31,541.7 mL/min.
2. 3.0 MGD, 1.0 mg/L, 20 mg/mL. Pump setting in mL/min?
- A23,656.3 mL/min
- B7,885.4 mL/min
- C394.3 mL/min
- D3,943 mL/min
Show answer and explanation
Correct answer: C. Chemical needed = 3 × 1,000,000 × 3.785 × 1 = 11,355,000 mg/day Feed = 11,355,000 ÷ (20 × 1,440) = 394.3 mL/min Answer: 394.3 mL/min. A common slip is dividing by 24 instead of 1,440 and reporting per hour as per minute, which gives 23,656.3 mL/min.
3. 0.25 MGD, 3.0 mg/L, 10 mg/mL. Pump setting in mL/min?
- A11,828.1 mL/min
- B1,971.4 mL/min
- C1,971 mL/min
- D197.1 mL/min
Show answer and explanation
Correct answer: D. Chemical needed = 0.25 × 1,000,000 × 3.785 × 3 = 2,838,750 mg/day Feed = 2,838,750 ÷ (10 × 1,440) = 197.1 mL/min Answer: 197.1 mL/min. A common slip is dividing by 24 instead of 1,440 and reporting per hour as per minute, which gives 11,828.1 mL/min.
Where this formula applies
This calculation can appear in the disciplines below. Exact exam coverage depends on your state, certification level, and official exam outline.
Water Treatment
Water Distribution
Study guides
Keep going with the full guide
Water Operator Math Made Simple walks through operator math step by step and includes printable references plus a full practice exam.
Water Operator Math Made Simple
A Step-by-Step Guide to the Math Every Water Operator Needs to Know