How Halogens Purify Water

Halogens in water


Every home in the developed world has chemistry running from its sinks. To enjoy the simple luxury of constant access to clean water, chemists have had to find ways to make our water supply safe for mass consumption. 


Water is not considered clean straight from the source. Rather, water is treated using halogens.


Known as group VIIA (17), halogens are a group of reactive nonmetallic elements on the periodic table. The group consists of fluorine, chlorine, bromine, iodine, and astatine. Tennessine, a synthetic chemical, may also be considered to be part of group VIIA. Each element in the halogen group is an oxidant with seven electrons in its outer shell. Halogens form salt when they react with metals. In fact, the word “halogen” means “salt-producing.”  


This elemental group is unique in that it is the only group of related elements that contain representatives of each state of matter. At normal pressure and room temperature, fluorine and chlorine are gases, bromine is a liquid, and iodine and astatine are solid. 


Halogens are used in a multitude of applications. But there are a few ways in which chemists use group VIIA to make our water safe for consumption. 



Chlorine is the most common form of water disinfectant worldwide. It is used mainly in the purification of municipal water supplies. But it is also used to preserve water during distribution. The World Health Organization recommends, assuming water has turbidity (cloudiness) of less than 1 NTU, water should be treated with a minimum free chlorine concentration of 0.5 mg/l for 30 minutes at a pH of less than 8. However, this is a general guideline. Most water treatment plants take a site-specific approach to meet the needs of their local community.  


There are a variety of methods for chlorinating water. In some cases, chlorine is added as a gas. It is delivered to water treatment plants in a liquefied form, contained in pressurized drums or cylinders. Using a system of pressurized vacuums, the chlorine is injected directly into the water supply. 


In other instances, water treatment plants use a compound made from chlorine called sodium hypochlorite. It is easier and safer to use than gaseous chlorine and can even be manufactured on-site. The compound is injected into the water supply using diaphragm metering pumps. 


Smaller facilities and service reservoirs are known to use the compound calcium hypochlorite. This compound is often purchased as a tablet or white powder. 



Bromine is another halogen used to disinfect water. Though not approved for use in municipal drinking water, bromine has been recognized by the USEPA as an effective and permissible disinfectant for drinking water since 1976. In fact, it is considered more effective in poor quality water than chlorine. 


Despite its effectiveness, bromine is not permitted for use in water treatment plants for two primary reasons. First, it is costly. Chiefly, the use of bromine in the water supply could allow the formation of brominated disinfection byproducts (DBPs). These byproducts are highly carcinogenic. Even water treated with chlorine, when exposed to bromine, is subject to the formation of DBPs. 


Rather than finding its way into municipal drinking water, bromine is typically used as an alternative disinfectant for pools and spas. 


Whether we’re taking a swim at the local pool or enjoying a refreshing sip from our local tap, chemistry helps to keep us safe. Through the diligence of those working in local municipalities and the use of high-quality chemicals, like those supplied by Noah Technologies, we can keep our communities happy and healthy. 


Noah Chemicals provides clients the purest chemicals. Discover a wealth of halogen compounds in our massive online catalog. To speak with a qualified chemist about custom chemicals and bulk ordering contact us today!