Chemistry: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]
- [math]\displaystyle{ \ln(\frac{N}{N_0})=-\Lambda_{CW} C^n t \! }[/math]
Where:
- [math]\displaystyle{ (\frac{N}{N_0}) \! }[/math] is the survival ratio for the microorganisms being killed
- [math]\displaystyle{ \Lambda_{CW} \! }[/math] is the Chick-Watson coefficient of specific lethality
- [math]\displaystyle{ C \! }[/math] is the concentration of the disinfectant (typically in mg/L)
- [math]\displaystyle{ n \! }[/math] is the coefficient of dilution, frequently assumed to be 1[4]
- [math]\displaystyle{ t \! }[/math] 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]
| Chlorine Concentration (mg/L) | 1 log inactivation (mg·L−1·min) | 2 log inactivation (mg·L−1·min) | 3 log inactivation (mg·L−1·min) |
|---|---|---|---|
| 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
References
- ↑ Pine, Rob; Joe Savage. "Everything You Ever Wanted to Know About CT (and then some)". New Mexico Environment Department. http://www.nmenv.state.nm.us/dwb/whats_new/documents/CT.pdf. 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. PMID 20474353.
- ↑ 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. PMID 20474372.
- ↑ 4.0 4.1 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. https://www.epa.gov/sites/production/files/2015-10/documents/guidance_manual_for_compliance_with_the_filtration_and_disinfection_requirements.pdf. "page 3-20"
- ↑ 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. https://www.epa.gov/sites/production/files/2015-10/documents/guidance_manual_for_compliance_with_the_filtration_and_disinfection_requirements.pdf. "Table E-2"
External links
- Earth Tech, Inc. (2005). Chlorine and Alternative Disinfectants Guidance Manual. Manitoba: Manitoba Water Stewardship. https://www.gov.mb.ca/waterstewardship/odw/reg-info/approvals/odw_chlorine_and_alternative_disinfectants.pdf.
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