With the same temperature and time, at pH 7.0 the combined residual must be increased to 0.6 ppm, to accomplish the same degree of bacterial kill. Of all the waterborne diseases, those caused by bacteria are the most easily prevented by chlorine disinfection. At the other extreme, certain pathogenic organisms such as the cysts of the protozoa E. histolytica and Giardia lamblia are the most resistant. Therefore, two parallel recommendations for chlorine residuals are often made. The lower one for bactericidal purposes and the higher one for cysticidal purposes.
Available information suggests that cysticidal residuals are also viricidal.
Bactericidal levels are routinely used for all water supplies at Navy and Marine Corps installations in the United States since waterborne bacteria are likely to be the most prevalent organisms. Cysticidal levels will be used whenever epidemiological evidence shows the presence of nonbacterial waterborne diseases such as amebiasis, viral hepatitis A, or giardiasis. The contact time of the organisms with the chlorine. The temperature of the water. At lower temperatures, bacterial kill tends to be slower and higher residuals are needed.