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.
- is the survival ratio for the microorganisms being killed
- is the Chick-Watson coefficient of specific lethality
- is the concentration of the disinfectant (typically in mg/L)
- is the coefficient of dilution, frequently assumed to be 1
- 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:
|Chlorine Concentration (mg/L)||1 log inactivation||2 log inactivation||3 log inactivation|
Full tables are much larger than this example and should be obtained from the regulatory agency for a particular jurisdiction.
- Pine, Rob; Joe Savage. "Everything You Ever Wanted to Know About CT (and then some)". New Mexico Environment Department. Retrieved 20 October 2013.
- Chick, Harriette (January 1908). "An Investigation of the Laws of Disinfection". The Journal of Hygiene 8 (1): 92–158. doi:10.1017/s0022172400006987. PMC 2167134. PMID 20474353. Retrieved 20 October 2013.
- 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 8: 536. doi:10.1017/s0022172400015928. PMC 2167149. PMID 20474372. Retrieved 20 October 2013.
- MWH (2005). Water Treatment: Principles And Design (2 ed.). Hoboken, NJ: John Wiley & Sons. ISBN 0471110183.
- 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.
- Earth Tech, Inc. (2005). Chlorine and Alternative Disinfectants Guidance Manual. Manitoba: Manitoba Water Stewardship.