Chlorination
Chlorination is the application of chlorine to water to accomplish some definite purpose. On this page, we will be concerned with the application of chlorine for the purpose of disinfection, but you should be aware that chlorination can also be used for taste and odor control, iron and manganese removal, and to remove some gases such as ammonia and hydrogen sulfide.
Like several other water treatment processes, chlorination can be used as a pretreatment process (prechlorination) or as part of the primary treatment of water (postchlorination). Treatment usually involves either postchlorination only or a combination of prechlorination and postchlorination.
Prechlorination is the act of adding chlorine to the raw water after screening and before flash mixing. The residual chlorine is useful in several stages of the treatment process - aiding in coagulation, controlling algae problems in basins, reducing odor problems, and controlling mudball formation. In addition, the chlorine has a much longer contact time when added at the beginning of the treatment process, so prechlorination increases safety in disinfecting heavily contaminated water.
Postchlorination is the application of chlorine after water has been treated but before the water reaches the distribution system. After flowing through the filter, water is chlorinated and then pumped to the clearwell to allow a sufficient contact time for the chlorine to act. At this stage, chlorination is meant to kill pathogens and to provide a chlorine residual in the distribution system. Postchlorination is nearly always part of the treatment process, either used in combination with prechlorination or used as the sole disinfection process.