C-T Contact Time and Inactivation Calculations for Chlorine Disinfection

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Boil Water Advisories - Surface Water Treatment Rule - GWUDI


Chlorine is the primary disinfectant used in the United States. In order to be effective, the chlorine must be given time to react with the microorganisms.

The time required depends on the temperature and the pH of the water. Chlorine works best in water with a low pH and a high temperature. The concentration and contact time required to inactivate Giardia using chlorine is approximated by the following formula.

CT=.2828 * ( pH^2.69 ) * ( Cl^.15 ) * (.933^(T-5)) * L

  • CT = Product of Free Chlorine Residual and Time required
  • pH = pH of water
  • Cl = Free Chlorine residual, mg/l
  • T = Temperature, degrees C
  • L = Log Removal

The PDFs below use this formula to solve for any desired parameter.
- CT Made Simple
- CT Lookup Table


The CT concept was developed specifically for surfacewater, with the assumption that water suppliers would be trying to inactivate both Giardia and viruses. Since the CT required to provide 3 log inactivation of Giardia is at least enough to provide the required 4 log inactivation of viruses, the EPA just set the standard for Giardia and ignored viruses.

If a well tests positive for e. coli bacteria, it is very likely that it will test positive for viruses as well.  It seems reasonable, therefore, that the disinfection of a well that tests positive for coliform bacteria should be effective in inactivating viruses.

Just in case the Applet is not working - Here is a summary table with some inactivation and CT data as a function of pH, chlorine concentration, and log inactivations.


Water pH 6.0 at 0.5 C

Chlorine Conc  1.0 log 2.0 log 3.0 log 
0.4 mg/L 46 (CT value) 91 137
1.0 mg/L 49 99 148
2.0 mg/L 55 110 165

Water pH 7.0 at 0.5 C

Chlorine Conc 

1.0 log

2.0 log

3.0 log 

0.4 mg/L

65 (CT value)



1.0 mg/L




2.0 mg/L





Water pH 8.0 at 0.5 C

Chlorine Conc 

1.0 log

2.0 log

3.0 log 

0.4 mg/L

92 (CT value)



1.0 mg/L




2.0 mg/L





The next question is how do know how much contact time you are providing? 

Unfortunately, it is not as easy as dividing the storage volume by the flow rate. In order to count at all, there must be a separate inlet and outlet to the tank, widely separated. Even with this provided, the volume would then be discounted by one of the following baffling factors.


Baffling Condition Factor Description
Unbaffled 0.1 No baffling, low length to width ratio. Also applies to agitated basins (e.g. flocculation tanks)
Poor 0.3 unbaffled inlet/outlet. No baffles inside basin.
Average 0.5 Baffled inlet or outlet. Some inter-basin baffles.
Superior 0.7 Baffled inlet and outlet, serpentine inter-basin baffles.
Excellent 0.9 As above. Very high length-to-width ratio.
(plug Flow)
1.0 Used for pipe flow

Shock Well Disinfection Protocols

Water Quality Help Guides
Glossary of Water Terminology
Cryptosporidium Website

EPA Enhanced Surface Water Treatment Rule - EPA


Note: (This formula was provided by Peter Martin, an associate Engineer with the Contra Costa Water District, and was published in the AWWA Journal AWWA 85:12:12 Dec 1993).The applet above uses this formula to solve for any desired parameter. The applet may not be conservative for high pH, high chlorine residuals, and high temperatues.



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