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Calculators / CT Value

CT Value calculator

CT measures disinfection: the disinfectant residual multiplied by the contact time. Exams usually give you the residual at the end of the contact basin and the T10 contact time.

CT (mg·min/L) = Residual (mg/L) × Contact time (min)

    Worked example

    The free chlorine residual leaving a clearwell is 1.2 mg/L and the T10 contact time is 30 minutes. What is the CT?

    1. CT = 1.2 × 30 = 36 mg·min/L

    Answer: 36 mg·min/L

    Common mistakes

    • Using the theoretical detention time when the question gives T10.
    • Using the dose instead of the residual.

    Check your exam's reference sheet for its exact constants. Some use 3.14 instead of 3.1416, which changes answers in the last digit.