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Chlorine Disinfection Contact Time and Inactivation Calculations (Advanced)

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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 (CT) required to inactivate Giardia using chlorine is approximated by the following formula.

Equation 1: Peter Martin, December 1993
CT = 0.2828 * ( pH2.69 ) * ( X0.15) * (0.933(T-5) ) * L

CT = Product of Free Chlorine Residual and Time required [mg-minute/L or (mg/L)*minute]

pH = pH of water

X = Free Chlorine residual, mg/L

T = Temperature, degrees C

L = Log Removal Desired  (The amount of the log reduction is a function of the anticipated concentration of the target organism. The higher the concentration of the organism, the greater the log reduction requirement to get to 0 (zero). Log Reduction: 1 log removal (90% reduction); 2 log removal (99% reduction); 3 log removal (99.9% reduction); 4 log removal (99.99% reduction).

Note: Public water supply systems that require chlorination because of the presence of E. coli or the potential presence of other pathogens would need a disinfection system that can provide 4 log reduction.

Log  Reduction Percent Reduction (%)
1 - Log 90
2 - Log 99
3 - Log 99.9
4 - Log 99.99
5 - Log 99.999

This formula was developed by Peter Martin, an associate Engineer with the Contra Costa Water District, and was published in an AWWA Journal (AWWA 85:12:12 Dec 1993). The CT concept was developed specifically for surface water, with the assumption that water suppliers would be trying to inactivate Giardia.

Equation 2: Using the EPA Regression Equation

This method is calculated using regression equations developed by Smith et al. (1995). The equations can be found in Appendix E of the US EPA Disinfection Profiling and Benchmarking Guidance Manual (EPA 815-R-99-013), August 1999, and the calculation is for the inactivation of Giardia.

If Temperature < 12.5 °C:

CT = (0.353L)* (12.006+e^(2.46-(0.073T))+(0.125*X) + (0.389(pH))); where e = 2.7183, the base for the natural logarithm.

Log Inactivation Desired (L)
Free Chlorine (X) - mg/L
Water Temperature (T) - Celsius
pH- Units

If Temperature > or equal to 12.5 °C:

CT = (0.361L)*(-2.261+e^(2.69-(0.065T)+(0.111X)+(0.361(pH))); where e = 2.7183, the base for the natural logarithm.

The PDFs below use this formula to solve for any desired parameter.

CT Made Simple

CT Lookup Table

EPA Guidance Manual LT1ESWTR Disinfection Profiling and Benchmarking

Standards and Guidelines for Municipal Waterworks, Wastewater and Storm Drainage Systems (2012)

PADEP Giardia Inactivation Calculator Document


The CT concept was developed specifically for surface water, with the assumption that water suppliers would be trying to inactivate both Giardia and Viruses. 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, but this should also address any issue with Viruses.

If a well tests positive for E. coli Bacteria, it is more likely to test positive for viruses and other waterborne pathogens as well. It seems reasonable, therefore, that the disinfection of a well that tests positive for Coliform Bacteria should be effective in inactivating Viruses, but it may be wise to provide a multiple barrier approach that includes disinfection followed by ultrafiltration.

The next question is how do you 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, ideally with baffles to lengthen the flow path which would also increase the contact time. 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 the 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.
Perfect  (plug flow) 1 Used for pipe flow

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