CT Value

Kinetics

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

: \ln(\frac{N}{N_0})=-\Lambda_{CW} C^n t !

Where:

• (\frac{N}{N_0}) ! is the survival ratio for the microorganisms being killed
• \Lambda_{CW} ! is the Chick-Watson coefficient of specific lethality
• C ! is the concentration of the disinfectant (typically in mg/L)
• n ! is the coefficient of dilution, frequently assumed to be 1
• 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. 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:

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

References

References

1. Pine, Rob. "Everything You Ever Wanted to Know About CT (and then some)". New Mexico Environment Department.
2. Chick, Harriette. (January 1908). "An Investigation of the Laws of Disinfection". The Journal of Hygiene.
3. Watson, Herbert Edmeston. (1908). "A Note on the Variation of the Rate of Disinfection with Change in the Concentration of the Disinfectant". The Journal of Hygiene.
4. MWH. (2005). "Water Treatment: Principles And Design". John Wiley & Sons.
5. Office of Drinking Water. (1991). "Guidance Manual for Compliance with the Filtration and Disinfection Requirements for Public Water Systems Using Surface Water Sources". United States Environmental Protection Agency.
6. Office of Drinking Water. (1991). "Guidance Manual for Compliance with the Filtration and Disinfection Requirements for Public Water Systems Using Surface Water Sources". United States Environmental Protection Agency.