The method
Before using the result
- Effective contact time must be supplied independently. No required CT or treatment-compliance result is inferred from nominal tank volume.
Worked examples
These original training examples illustrate the method for California SWRCB Water Treatment Operator T3. Use the matching “Load example” button above to explore a variation; example values are not equipment settings or prescribed criteria.
1. Worked example
Multiply measured disinfectant residual by an independently supplied effective contact time. Record where the time came from before discussing any treatment requirement.
- Measured residual (mg/L)
- 1.2
- Effective contact time
- 30 min
- Source of effective contact time
- Worked example: supplied effective contact time
- Time = 30 minutes.
- CT = 1.2 × 30 mg·min/L.
- Calculated CT: 36 mg·min/L
- Time source: Worked example: supplied effective contact time
2. Comparison example
Compare a different supplied condition using the same method. For Water Certification Exam, the important check is that the input meaning, units and method assumptions still match the problem.
- Measured residual (mg/L)
- 1.5
- Effective contact time
- 30 min
- Source of effective contact time
- Worked example: supplied effective contact time
- Time = 30 minutes.
- CT = 1.5 × 30 mg·min/L.
- Calculated CT: 45 mg·min/L
- Time source: Worked example: supplied effective contact time
Check your understanding
1. Which relationship is used?
CT = residual concentration (mg/L) × effective contact time (minutes).
2. Which assumption should you check first?
Effective contact time must be supplied independently. No required CT or treatment-compliance result is inferred from nominal tank volume.
3. How should you verify a different unit system?
Convert every input to the displayed calculation units first. Equivalent physical inputs must give equivalent results; a unit label alone does not convert a number.
Continue with the matching exam topic
This tool connects to California SWRCB Water Treatment Operator T3 free practice. Work through a question independently, then use the calculation steps to identify where your method differs. The tool stays free whether or not you choose paid preparation.