tially slower and less effective than its bactericidal action. The killing mechanism is believed to involve attacking many protein sites rather than one critical site of the virus. The chlorine treatment, designed to kill bacteria, does not necessarily kill viruses. Chlorine is not effective in normal concentrations to kill the cyst Endamoeba histolytica, the cause of amoebic dysentery, a protozoan disease that invades the body by a parasitic organism through the intestinal tract. Fortunately, it is a relatively rare disease.
Chlorine is also ineffective against nematodes, a free-living microorganism present in surface water supplies. Nematodes, although nonpathogenic, are capable of ingesting and harboring potentially dangerous organisms.
Minimum bactericidal chlorine residual was determined by the Public Health Service in terms of free available chlorine, using a 10-min contact time, and in terms of combined available chlorine (free chlorine and chloramines), using a 60-min contact time. The free available chlorine necessary for disinfection is 0.2 ppm at pH 6-8 and 0.4 ppm at pH 8-9. The corresponding concentrations with combined available chlorine are 1.5 and 1.8 ppm.
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