In many places around the United States, chlorine is regularly put into the water in municipal water plants before it is sent out to customers. But why is chlorine added to water? This chemical is a powerful disinfectant, killing many bacteria, protozoa, and other microorganisms that could make people sick. Adding chlorine is an effective way to help ensure that the water you get out of the tap is safe to drink.
Chlorine has been used to clean water for over a century, with 1908 marking the first continuous use of chlorine in water treatment in the US. Modern municipal water treatment plants may use chlorine gas, a sodium hypochlorite solution, or solid calcium hypochlorite to disinfect water. In the United States, the Safe Drinking Water Act requires that water be disinfected to ensure that the water sent out to customers does not contain dangerous levels of viruses, bacteria, or other pathogens. Chlorine and related chemicals (like chloramines) are the most common disinfection method in the country, although they are not used in all water treatment plants.
The use of chlorine in water treatment is widespread for a number of reasons. Chlorine is a relatively inexpensive chemical, and it can be made and shipped to wherever it's needed. The actual disinfection process does take time, but it's simple to do. Chlorine is added to the water, and given adequate "contact time" to mix and react with microorganisms and other contaminants. The disinfection step is typically the final stage of the water treatment process, so the water is piped out to customers once this stage is complete.
Chlorine is an oxidant, which means that it can take electrons from other substances. So, how does chlorine kill bacteria? When the chlorine encounters organic material in the water – including microorgansims, like bacteria – it takes electrons from the cell membranes. When the membranes lose their electrons, they also lose their structure and the cell breaks down and dies.
Because chlorine is a very strong oxidant, it can kill a wide variety of potentially dangerous pathogens in water. Different types of microorganisms require different exposures, however, so it's important that the proper amount of chlorine is added and that it's allowed sufficient contact time to be effective. One of the benefits of using chlorine over other disinfection methods, such as ultraviolet light, is that it continues working even after the water has left the treatment plant.
How much chlorine is in your drinking water depends somewhat on where you live and any additional water treatment systems you use. The Environmental Protection Agency (EPA) allows up to 4 parts per million (ppm) of chlorine in tap water. At this level, the EPA says that there is no known health risk. If you use additional water treatment methods at home, such as carbon filters or reverse osmosis, you may have little or no chlorine at all in your drinking water.
Chloramines, which are a disinfectant related to chlorine, are also used to clean drinking water. As with chlorine, the EPA standard for chloramines is no more than 4 ppm.
Although the EPA says that chlorine at 4 ppm poses no health risk, many people still question the safety of chlorine in drinking water. After all, chlorine is a toxic gas and can be extremely dangerous. In addition, when chlorine and chloramines are added to water, they do more than just kill germs – they also react to organic and inorganic materials and metals in the water. The reaction with organic matter, in particular, creates what are called disinfection byproducts (DBPs), like trihalomethanes (THMs), that could be dangerous. A number of DBPs are considered carcinogenic, meaning that they could cause cancer, and have been linked to other serious health problems.
There's no question that chlorine is an extremely effective disinfectant, but many water quality professionals recommend using a carbon filter to remove any residual chlorine and DBPs once the water has reached your home. In addition to removing potentially hazardous chemicals, these filters can also make your water taste and smell better. US Water Systems offers a complete line of whole house water filters to remove chlorine, as well as many undersink drinking water filters that remove or reduce chlorine as well as having their own unique disinfecting media.