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Chlorine and its requirement

Microbiological contamination of drinking water generally comes from three types of organisms:
  • virus
  • bacteria
  • and protozoa.

Viruses and bacteria are smaller organisms that are killed or inactivated by chlorine if the water is not turbid.

Cryptosporidium is recognized as the toughest to kill and is one of a group of organisms called protozoa. Protozoa are larger (3 – 8 micron) than bacteria and viruses and can effectively be removed by our filtration in the LWTS. However many protozoa, most notably cryptosporidium, are fairly resistant to chlorine. The CDC often uses a parameter called the Ct factor to describe the tolerance of microorganisms to chlorine. The Ct factor is simply the chlorine concentration (in ppm) times the exposure time (in minutes). Thus exposure to 2 ppm chlorine for 30 minutes would give a Ct factor of 2 x 30 = 60. CDC has data showing many viruses and bacteria will have inactivation levels of 99% or greater at Ct factors less than 60, with many being less than 1.0. On the other hand the data shows that a Ct factor of 7200 is required to get 99% in activation of cryptosporidium. Thus at 2 ppm it would require about 3600 minutes (60 hours) to achieve a 99% kill of cryptosporidium at 25 C (77F). This should be used as a rough guideline, as other factors impact the inactivation process. At higher temperature, the inactivation process will be quicker and at lower temperatures will slow down the inactivation. The pH of the water also has an impact. Lower pH speeds up the inactivation and higher pH slows down the process. The above data from CDC was collected a pH = 7.0. Also, other components in the water stream can have an impact.

Please note that there are many items that chlorine is preventing from causing issues. They include but are not limited to the following:
  • Typhoid Fever
  • Cholera
  • Gastroenteritis
  • Schistosomiasis
  • Worms
  • Tularemia
  • Viral Diseases
  • Adenoviruses
  • Enteroviruses

For more information, please see the need for chlorination powerpoint Pat presented in the 2008 Country Directors meeting. Also available is the United Nations Environment Programme on Disinfectants and Disinfectant By-Products. To see the chlorine concnetrations and the contact time required to inactive pathogens, please download this document.

Shock Chlorination

Shock chlorination occurs when a chlorine residual of 2 mg/L is maintained for 7-14 days (then maintain 0.25-0.5 mg/L residual afterwards). This will wipe out even the most resistant pathogens (like cryptosporidium).

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