CT Values are an important part of calculating disinfectant dosage for the chlorination of drinking water. A CT value is the product of the concentration of a disinfectant (e.g. free chlorine) and the contact time with the water being disinfected. It is typically expressed in units of mg-min/L.

The goal of disinfection is the inactivation of microorganisms. This depends on: the microorganism, the disinfectant being used, the concentration of the disinfectant, the contact time, and the temperature and pH of the water.^[1]

The disinfection kinetics are conventionally calculated via the Chick-Watson model, named for the work of Harriette Chick^[2] and H. E. Watson.^[3] This model is expressed by the following equation:^[4]

${\displaystyle \ln({\frac {N}{N_{0}}})=-\Lambda _{CW}C^{n}t\!}$

Where:

• ${\displaystyle ({\frac {N}{N_{0}}})\!}$ is the survival ratio for the microorganisms being killed
• ${\displaystyle \Lambda _{CW}\!}$ is the Chick-Watson coefficient of specific lethality
• ${\displaystyle C\!}$ is the concentration of the disinfectant (typically in mg/L)
• ${\displaystyle n\!}$ is the coefficient of dilution, frequently assumed to be 1^[4]
• ${\displaystyle t\!}$ is the contact time (typically in minutes or seconds)

The survival ratio is commonly expressed as an inactivation ratio (in %) or as the number of reductions in the order of magnitude of the microorganism concentration. For example, a situation where N₀=10⁷ CFU/L and N=10⁴ CFU/L would be reported as a 99.9% inactivation or "3-log₁₀" removal.

In water treatment practice, tables of the product C×t are used to calculate disinfection dosages. The calculated CT value is the product of the disinfectant residual (in mg/L) and the detention time (in minutes), through the section at peak hourly flow.^[5] These tables express the required CT values to achieve a desired removal of microorganisms of interest in drinking water (e.g. Giardia lamblia cysts) for a given disinfectant under constant temperature and pH conditions. A portion of such a table is reproduced below.

CT Values for the Inactivation of Giardia Cysts by Free Chlorine at 5 °C and pH ≈ 7.0:^[6]

Full tables are much larger than this example and should be obtained from the regulatory agency for a particular jurisdiction.

• Chlorination
• Disinfectant

• Earth Tech, Inc. (2005). Chlorine and Alternative Disinfectants Guidance Manual (PDF). Manitoba: Manitoba Water Stewardship.