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]

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

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^[4]
• $t \!$ 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]

Chlorine Concentration (mg/L)	1 log inactivation	2 log inactivation	3 log inactivation
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.

See also

• Chlorination
• Disinfectant

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" (PDF). The Journal of Hygiene 8 (1): 92–158. doi:10.1017/s0022172400006987. PMC 2167134. PMID 20474353. Retrieved 20 October 2013.
3. Watson, Herbert Edmeston (1908). "A Note on the Variation of the Rate of Disinfection with Change in the Concentration of the Disinfectant" (PDF). The Journal of Hygiene 8: 536. doi:10.1017/s0022172400015928. PMC 2167149. PMID 20474372. Retrieved 20 October 2013.
4. 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. Table E-2

External links

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