# CT Value

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]

## Kinetics

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. 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.

## Example CT Table

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

0.6 48 95 143
1.2 51 101 152
1.8 54 108 162
2.4 57 115 172

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