Maker Label Studio

Chlorine CT Disinfection Calculator

Calculate Contact Time (T10) and Log Inactivation using EPA SWTR Guidance

System Parameters

Total operative volume in gallons (gal).
Must be a valid positive number.
Peak hourly flow through the basin in gallons per minute (gpm).
Must be greater than 0.
Represents short-circuiting potential.
Measured at the effluent in mg/L (or ppm).
Must be a valid positive number.
Target CT based on EPA tables for log inactivation at current pH/Temp in mg·min/L.
Must be greater than 0.

Calculated Results

Theoretical Detention Time (TDT)
-- min
Effective Contact Time (T10)
-- min
Calculated CT Achieved
-- mg·min/L
Inactivation Ratio (Calculated / Required)
--
Awaiting Input
Please enter all parameters

About the Chlorine CT Disinfection Contact-Time Calculator

A chlorine CT disinfection calculator allows water treatment operators and environmental engineers to verify regulatory compliance for drinking water safety. Accurately model the complex interaction between free chlorine residual, tank baffling factors, and effective contact time to guarantee sufficient log-inactivation of harmful Giardia and viruses.

How it works

  1. Input peak hourly flow rate, clearwell volume, and the tank's specific baffle factor.
  2. Enter the current water temperature, pH, and measured free chlorine residual.
  3. Compare the resulting CT actual value against the EPA-required CT to ensure your log-removal requirements are met.

Frequently asked questions

What does CT mean in water treatment?

CT is the product of the disinfectant residual Concentration in mg/L and the effective Contact Time in minutes. It is used to measure the effectiveness of pathogen inactivation.

How does a baffle factor affect contact time?

A baffle factor accounts for short-circuiting in a tank. A poorly baffled tank might have a factor of 0.1, meaning effective contact time is only 10% of theoretical detention time.

Why is pH important for chlorine disinfection?

Lower pH favors the formation of hypochlorous acid, which is a significantly stronger disinfectant than hypochlorite ion. This lowers the required CT for safe water.

What happens to required CT when water temperature drops?

Disinfection kinetics slow down in colder water. As water temperature decreases, the required CT value increases to achieve the same level of pathogen inactivation.

References