Water operator math tutorial
Percent Removal
Express how much of a pollutant a process removes.
Percent removal is the standard way to report treatment efficiency for BOD, TSS, turbidity, and more.
It is the amount removed divided by the amount that came in, times 100. Never divide by the effluent value.
The formula
percent removal = [(in − out) ÷ in] × 100
| Variable | Unit |
|---|---|
| Influent concentration | mg/L |
| Effluent concentration | mg/L |
| Result: Removal | % |
Applies to: Water Treatment, Wastewater Treatment.
Common exam tip: A good check: removal can never exceed 100% or be negative for a working process.
Calculator
Removal
85 %
- 1. (200 − 30) ÷ 200 × 100 = 85%
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
Influent BOD is 200 mg/L and effluent BOD is 30 mg/L.
- (200 − 30) ÷ 200 × 100 = 85%
Answer: 85 %
Example 2
Influent TSS is 240 mg/L and effluent TSS is 18 mg/L.
- (240 − 18) ÷ 240 × 100 = 92.5%
Answer: 92.5 %
Common mistakes
Wrong result: 566.7 %
dividing by the effluent value instead of the influent
Wrong result: 15 %
reporting what remains instead of what was removed
Practice questions
1. In 180 mg/L, out 27 mg/L. Percent removal?
- A85 %
- B566.7 %
- C15 %
- D850 %
Show answer and explanation
Correct answer: A. (180 − 27) ÷ 180 × 100 = 85% Answer: 85 %. A common slip is dividing by the effluent value instead of the influent, which gives 566.7 %.
2. In 310 mg/L, out 46 mg/L. Percent removal?
- A573.9 %
- B85.2 %
- C14.8 %
- D852 %
Show answer and explanation
Correct answer: B. (310 − 46) ÷ 310 × 100 = 85.2% Answer: 85.2 %. A common slip is dividing by the effluent value instead of the influent, which gives 573.9 %.
3. In 95 mg/L, out 9.5 mg/L. Percent removal?
- A900 %
- B10 %
- C90 %
- D9 %
Show answer and explanation
Correct answer: C. (95 − 9.5) ÷ 95 × 100 = 90% Answer: 90 %. A common slip is dividing by the effluent value instead of the influent, which gives 900 %.
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
Wastewater Treatment
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