C-T Contact Time
and Inactivation Calculations
for Chlorine Disinfection

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

The applet below uses this formula to solve for any desired parameter.


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 ConditionFactorDescription
Unbaffled0.1No baffling, low length to width ratio. Also applies to agitated basins (e.g. flocculation tanks)
Poor0.3unbaffled inlet/outlet. No baffles inside basin.
Average0.5Baffled inlet or outlet. Some inter-basin baffles.
Superior0.7Baffled inlet and outlet, serpentine inter-basin baffles.
Excellent0.9As above. Very high length-to-width ratio.
(plug Flow)
1.0Used for pipe flow

Shock Well Disinfection Protocols
Water Quality Help Guides
Glossary of Water Terminology
Cryptosporidium Website

EPA Interim Enhanced Surface Water Treatment Rule - EPA
Surface Water Filtration Rule PADEP

Term 1 Enhanced Surface Water Treatment Rule- PADEP
Drinking Water Technologies for GUDI -PADEP
New Drinking Water Regulations-PADEP


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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