However, low levels of chlorine, normally used to disinfect water, are not an effective treatment for giardia cysts. Chlorine has been used since 1908 to disinfect water supplies in the United States to protect public health. The effectiveness of chlorination depends on the chlorine demand of the water, the concentration of the chlorine solution added, the time that chlorine is in contact with the organism, and water quality. As the concentration of the chlorine increases, the required contact time to disinfect decreases.
Chlorination is more effective as water temperature increases.
The amount of chlorine that reacts with the other chemicals plus the amount required to achieve disinfection is the chlorine demand of the water. The safest way to be sure that the amount of chlorine added is sufficient is to add a little more than is required. This will result in a free chlorine residual that can be measured easily. This chlorine residual must be maintained for several minutes depending on chlorine level and water quality. Table 4 lists the free chlorine residual level needed for different contact times, water temperatures and pH levels.