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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.{{cite web|last=Pine|first=Rob|title=Everything You Ever Wanted to Know About CT (and then some)|url=http://www.nmenv.state.nm.us/dwb/whats_new/documents/CT.pdf|publisher=New Mexico Environment Department|accessdate=20 October 2013|author2=Joe Savage}}

Kinetics

The disinfection kinetics are conventionally calculated via the Chick-Watson model, named for the work of Harriette Chick{{cite journal|last=Chick|first=Harriette|title=An Investigation of the Laws of Disinfection|journal=The Journal of Hygiene|date=January 1908|volume=8|issue=1|pages=92–158|doi=10.1017/s0022172400006987|pmid=20474353|pmc=2167134}} and H. E. Watson.{{cite journal|last=Watson|first=Herbert Edmeston|title=A Note on the Variation of the Rate of Disinfection with Change in the Concentration of the Disinfectant|journal=The Journal of Hygiene|year=1908|volume=8|issue=4|pages=536–42|doi=10.1017/s0022172400015928|pmid=20474372|pmc=2167149}} This model is expressed by the following equation:{{cite book|last=MWH|title=Water Treatment: Principles And Design|year=2005|publisher=John Wiley & Sons|location=Hoboken, NJ|isbn=0471110183|edition=2}}

: \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
  • 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.{{cite book|last=Office of Drinking Water|title=Guidance Manual for Compliance with the Filtration and Disinfection Requirements for Public Water Systems Using Surface Water Sources|year=1991|publisher=United States Environmental Protection Agency|url=https://www.epa.gov/sites/production/files/2015-10/documents/guidance_manual_for_compliance_with_the_filtration_and_disinfection_requirements.pdf|quote=page 3-20}} 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:{{cite book|last=Office of Drinking Water|title=Guidance Manual for Compliance with the Filtration and Disinfection Requirements for Public Water Systems Using Surface Water Sources|year=1991|publisher=United States Environmental Protection Agency|url=https://www.epa.gov/sites/production/files/2015-10/documents/guidance_manual_for_compliance_with_the_filtration_and_disinfection_requirements.pdf|quote=Table E-2}}

class="wikitable"
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

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External links

  • {{cite book|last=Earth Tech, Inc.|title=Chlorine and Alternative Disinfectants Guidance Manual|year=2005|publisher=Manitoba Water Stewardship|location=Manitoba|url=https://www.gov.mb.ca/waterstewardship/odw/reg-info/approvals/odw_chlorine_and_alternative_disinfectants.pdf}}

Category:Water treatment

Category:Chlorine