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]
Where:
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[4]
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
References
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External links
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