# 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. 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.

## Example CT Table

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

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.

## References

1. ^ Pine, Rob; Joe Savage. "Everything You Ever Wanted to Know About CT (and then some)" (PDF). New Mexico Environment Department. Retrieved 20 October 2013.
2. ^ 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.
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. 8 (4): 536–42. doi:10.1017/s0022172400015928. PMC 2167149. PMID 20474372.
4. ^ a b MWH (2005). Water Treatment: Principles And Design (2 ed.). Hoboken, NJ: John Wiley & Sons. ISBN 0471110183.
5. ^ Office of Drinking Water (1991). Guidance Manual for Compliance with the Filtration and Disinfection Requirements for Public Water Systems Using Surface Water Sources (PDF). United States Environmental Protection Agency. page 3-20
6. ^ Office of Drinking Water (1991). Guidance Manual for Compliance with the Filtration and Disinfection Requirements for Public Water Systems Using Surface Water Sources (PDF). United States Environmental Protection Agency. Table E-2