Effect of Chlorination on Inactivating Selected Pathogens
bacteria, viruses, and protozoa. The Ct factor can be used to compare the effectiveness of chlorine against different pathogens, and is calculated by multiplying the concentration of chlorine needed to inactivate a certain percentage of the pathogens by the time the pathogen was exposed to that concentration of chlorine. Higher Ct factors indicate relatively higher resistance to chlorine, while lower Ct factors indicate relatively low resistance to chlorine. The Ct factors shown in the table below were calculated from data in peer-reviewed research articles (references below). The efficacy of disinfection using chlorine is dependent not only on the pathogen itself, but also on the pH and temperature of the water. In general, disinfection is more effective at higher temperatures and lower pH. Attachment to particulate matter, aggregation, encapsulation of the pathogen, ingestion by protozoa, and water turbidity may also affect chlorine efficacy. The results below reflect conditions of low water turbidity (<1 NTU), demand-free water systems. The Safe Water System accounts for variations in water quality by doubling the chlorine used in turbid drinking water. The maximum Ct factor created by adding 1.875mg/L sodium hypochlorite to water for 30 minutes (the minimum chlorine dosage recommended by the Safe Water System for clear, non-turbid, demand-free water) is 56.25 mg·min/L (Lantagne, in press). For turbid water, the dose is doubled to 3.75mg/L, with a resulting maximum Ct factor of 112.5 mg·min/L.
Pathogen | From WHO Guidelines for Drinking-Water Quality | Concentration of chlorine (mg/L) | Time of chlorine exposure (min) | Ct factor | % Inactivation | Variables affecting Ct factor | Pathogen subclassification and/or experimental design | Source | |||||||
Health significance | Persistence in water supplies | Resistance to chlorine | Relative infectivity | Temp (°C) | pH | ||||||||||
Bacteria | |||||||||||||||
Low | May multiply | Low | Low | 1.0 | 60 | 60 | 99% | 22.0-25.0 | 6.25-7.0 | 45 pooled clinical and environmental samples | Howard, 1993 | ||||
High | Moderate | Low | Moderate | 0.1 | 5 | 0.5 | 99-99.9% | 25.0 | 8.0 | Serotypes PEN1, PEN2, PEN3 isolated from patients | |||||
High | Moderate | Low | Low | 0.5 | <0.5 | <0.25 | 99.999999% | 23.0 | 7.0 | Strain ATCC 11229 | |||||
(enterohemorrhagic) | High | Moderate | Low | High | 0.5 | <0.5 | <0.25 | 99.98-99.999999% | 23.0 | 7.0 | Strains isolated from six human patients | ||||
High | Moderate | Low | Low | 0.05 | 20 | 1 | 99.2% | 20-25 | 7.0 | Two isolates – one from patient blood sample | |||||
High | Short | Low | Moderate | 0.05 | <1 | <0.05 | 99.9% | 20-25 | 7.0 | Three isolates from patient stool samples | |||||
- | - | - | - | 0.5 | 1 | 0.5 | 99% | 25.0 | 7.0 | Water Engineering Research Laboratory isolate | |||||
(smooth strain) | High | Short | Low | Low | 0.5 | <1 | <0.5 | 100% | 20.0 | 7.0 | O1 El Tor Inaba strain N16961 | ||||
(rugose strain) | High | Short | Low | Low | 2.0 | 20 | 40 | 99.999% | 20.0 | 7.0 | O1 El Tor Inaba strain N16961/Ru | ||||
High | Long | Low | Low | 1.0 | >30 | >30 | 82-92% | 20.0 | 7.0 | 3 strains: ATCC 9610 O:8, 632 O:25,35 and IM 69/85 O:3 Lis VIII | |||||
Viruses | |||||||||||||||
Enteroviruses | |||||||||||||||
| High | Long | Moderate | High | 0.46-0.49 | 0.3 | 0.14-0.15 | 99% | 5.0 | 6.0 | Coxsackie A9 | ||||
| High | Long | Moderate | High | 0.48-0.50 | 4.5 | 2.16-2.25 | 99% | 5.0 | 7.81-7.82 | Coxsackie B5 | ||||
| High | Long | Moderate | High | 0.48-0.52 | 1.8 | 0.86-0.94 | 99% | 5.0 | 7.79-7.83 | Serotype 5 | ||||
| High | Long | Moderate | High | 0.41 | <1 | <0.41 | 99.99% | 25.0 | 8.0 | Strain from one patient sample | ||||
| High | Long | Moderate | High | 0.5 | 12.72 | 6.36 | 99.99% | 5.0 | 6.0 | Poliovirus type 1 | ||||
High | Long | Moderate | High | 0.17 | 4.41 | 0.75 | 99.99% | 5.0 | 7.0 | Adenovirus 40 | |||||
High | Long | Moderate | High | 1.0 | 0.07 | 0.07 | 99.99% | 5.0 | 7.0 | Feline calicivirus used as a model | |||||
High | Long | Moderate | High | 0.20 | 0.25 | 0.05 | 99.99% | 4.0 | 7.0 | Human rotavirus type 2 (Wa) | |||||
Protozoa | |||||||||||||||
High | Moderate | High | Low | 2.0 | 10 | 20 | 99% | 27-30 | 7 | Viability assessed by in vitro excystation assay | |||||
High | Moderate | High | Low | 1.5 | 10 | 15 | 99.9% | 25.0 | 7.0 | Viability assessed by excystation | |||||
High | Moderate | High | Unknown | 100 | 1440 | >144,0001 | - | 22.0 | 7.2 | Viability assessed by mouse bioassay | |||||
High | Long | High | Low | 80 | 90 | 7,200[1]Selected Pathogens | 99% | 25.0 | 7.0 | Viability assessed by excystation and mouse viability assays | |||||
[1]Toxoplasma oocysts and cryptosporidium oocysts are highly resistant to chlorine disinfection. Chlorine alone should not be expected to inactivate these pathogens in drinking water.
Filtering water supplies is recommended to physically remove the oocysts before chlorination if these pathogens are of concern.
References
Blaser, M. J., P. F. Smith, et al. (1986). "Inactivation of Campylobacter jejuni by chlorine and monochloramine." Appl Environ Microbiol 51(2): 307-11.
Butterfield, C. T., W. Wattie, et al. (1943). "Influence of pH and temperature on the survival of coliforms and enteric pathogens when exposed to free chlorine." Public Health Rep. 58(51): 1837-1880.
Engelbrecht, R. S., M. J. Weber, et al. (1980). "Comparative inactivation of viruses by chlorine." Appl Environ Microbiol 40(2): 249-56.
Grabow, W. O., V. Gauss-Muller, et al. (1983). "Inactivation of hepatitis A virus and indicator organisms in water by free chlorine residuals." Appl Environ Microbiol 46(3): 619-24.
Howard, K. and T. J. Inglis (2003). "The effect of free chlorine on Burkholderia pseudomallei in potable water." Water Res 37(18): 4425-32.
Jarroll, E. L., A. K. Bingham, et al. (1981). "Effect of chlorine on Giardia lamblia cyst viability." Appl Environ Microbiol 41(2): 483-7.
King, C. H., E. B. Shotts, Jr., et al. (1988). "Survival of coliforms and bacterial pathogens within protozoa during chlorination." Appl Environ Microbiol 54(12): 3023-33.
Korich, D. G., J. R. Mead, et al. (1990). "Effects of ozone, chlorine dioxide, chlorine, and monochloramine on Cryptosporidium parvum oocyst viability." Appl Environ Microbiol 56(5): 1423-8.
Lantagne, D.S. (2008). “Sodium hypochlorite dosage for household and emergency water treatment.” JAWWA, in press.
LeChevallier, M. W. and K.-K. Au (2004). Water treatment and pathogen control : process efficiency in achieving safe drinking-water. London, Published on behalf of the World Health Organization by IWA.
Morris, J. G., Jr., M. B. Sztein, et al. (1996). "Vibrio cholerae O1 can assume a chlorine-resistant rugose survival form that is virulent for humans." J Infect Dis 174(6): 1364-8.
Paz, M. L., M. V. Duaigues, et al. (1993). "Antimicrobial effect of chlorine on Yersinia enterocolitica." J Appl Bacteriol 75(3): 220-5.
Stringer, R. P., W. N. Cramer, et al. (1975). Comparison of bromine, chlorine, and iodine as disinfectants for amoebic cysts, p. 193-209. In J. D. Johnson (ed.), Disinfection: water and wastewater. Ann Arbor Science Publishers, Inc. Ann
Arbor, Mich.
Thurston-Enriquez, J. A., C. N. Haas, et al. (2003). "Chlorine inactivation of adenovirus type 40 and feline calicivirus." Appl Environ Microbiol 69(7): 3979-85.
Vaughn, J. M., Y. S. Chen, et al. (1986). "Inactivation of human and simian rotaviruses by chlorine." Appl Environ Microbiol 51(2): 391-4.
Wainwright, K. E., M. A. Miller, et al. (2007). "Chemical inactivation of Toxoplasma gondii oocysts in water." J Parasitol 93(4): 925-31.
World Health Organization (2006). Guidelines for drinking-water quality: incorporating first addendum, 3rd edition. Geneva, WHO Press.
Zhao, T., M. P. Doyle, et al. (2001). "Chlorine inactivation of Escherichia coli O157:H7 in water." J Food Prot 64(10): 1607-9.
Complied December 20, 2007 by Amelia Kasper. Contact: safewater@cdc.gov
Resources
The issues in this article are addressed on the USAID Environmental Health web pages. For more topics relating to environmental health, visit the web site at: http://www.ehproject.org
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