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How It Works

The CT concept links disinfectant concentration and contact time to pathogen inactivation. A higher CT value means more aggressive disinfection -- either by raising the chemical dose or extending exposure time. The constant n accounts for how sensitive a given pathogen is to concentration versus time.

Regulatory agencies publish required CT values for specific pathogens. For example, achieving 3-log (99.9%) inactivation of Giardia with free chlorine at pH 7 and 25°C requires a CT of about 36 mg·min/L.

Example Problem

A water plant maintains 2 mg/L free chlorine residual with n = 1. How many minutes of contact time are needed to achieve a CT of 36?

  1. k = Cn × t, so t = k / Cn
  2. t = 36 / 21 = 18 minutes

Frequently Asked Questions

What is CT value in water disinfection?

CT value is the product of disinfectant residual concentration (mg/L) and contact time (minutes). It quantifies the disinfection intensity needed to achieve a target level of pathogen inactivation in drinking water treatment.

What CT value is needed to kill Giardia?

For 3-log (99.9%) inactivation of Giardia cysts with free chlorine at pH 7 and 25°C, the required CT is about 36 mg·min/L. At lower temperatures or higher pH, the required CT increases significantly.

Is higher disinfectant concentration always better?

Not necessarily. Higher concentrations reduce the required contact time but can produce harmful disinfection byproducts like trihalomethanes. Water utilities balance dose and time to meet both disinfection and byproduct standards.

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