The Well-Studied Water Treatment Benefits of Chlorine

The chlorination of public drinking water has been hailed as one of the most significant advancements in public health of the last century. Since its adoption in the U.S. in 1908, the initiative has helped to virtually eradicate most waterborne diseases in the industrialized world—an achievement that’s been central to the economic and social development of these nations.

Today, the majority of municipal water supplies in the developed world are disinfected using chlorine-based solutions, either alone or in combination with others. Because of this, chlorine is thoroughly researched and scientifically well understood. Long has it proven to be a safe, effective, economical and widely applicable water treatment method.

It’s because of chlorine’s advantages and numerous benefits that MSR Global Health’s water treatment technologies have focused on its applications.

How Chlorine Works

Chlorine occurs in several forms, but for the purposes of water treatment, it’s most commonly used as either: compressed gas, as liquid sodium hypochlorite (bleach), as solid calcium hypochlorite, or as a tablet of sodium dichloroisocyanurate (NaDCC).

When each of these forms is added to water, it generally reacts by forming what is called free available chlorine (FAC). FAC is a balance of differing levels of hypochlorous acid and hypochlorite ion depending the pH level of the water. 

As these FAC agents come into contact with waterborne microorganisms, they attack the pathogens at various levels, either killing them or rendering them incapable of reproducing, thus providing you with microbiologically safe drinking water.

Chlorine’s Advantages

By 1890, scientists had discovered that chlorine served as a suitable defense against waterborne microorganisms. Today that benefit combines with chlorine’s many other advantages to make it a viable solution to combating waterborne disease around the world.

Potent Disinfectant: Chlorine reduces the threat of nearly all types of waterborne microorganisms (the most notable exceptions being Cryptosporidium and Mycobacteria), making it a highly effective and efficient way to treat drinking water.

In addition, it can neutralize the foul tastes and odors from organic substances, such as decaying plant matter and sulfides. It also inhibits mold and algae growth, and oxidizes inorganic chemicals like iron and manganese, making it a versatile disinfectant.

Safe & Simple to Use: “
Chlorine is very well studied, which means science tells us how to use it effectively and safely,” says Zac Gleason, a microbiologist in MSR’s water research lab. “We know it takes a very small amount of chlorine to kill the pathogens—a level that is still very safe for human consumption.”

Chlorine’s simple to use nature also increases its safety. For water treatment, it requires only three steps: add the dose, agitate the water, wait 30 minutes. “Science tells us the correct dosage needed, so we simply increase that dose based on the number of liters we’re treating,” says Zac. “It’s easily scalable.”


Residual Effects: Chlorine continues to disinfect after its initial treatment. “It has residual effects,” says Zac. “This means it continues to work until all the chlorine molecules have been exhausted. How long that takes depends on storage conditions (light, heat, etc.). But it will continue to kill any microbes introduced into the treated water until all the chlorine molecules are used up. This chlorine residual provides extra protection against re-contamination.”

Efficient for Treating Large Volumes: These residual qualities make chlorine great for treating large volumes of water as well. The larger the volume, or the farther that water must travel to reach its end-user, the more likely it is to be re-contaminated. Chlorine’s residual qualities help protect against this threat.

In addition, a relatively small amount of chlorine goes a long way when compared to the size of the filter that would be required to treat vast amounts of water without quickly clogging. Because of this, chlorine is a very efficient way to treat water for thousands of people.


Low Cost: Though its cost varies with circumstances, chlorine treatment is still overall one of the least expensive treatment methods available—this is one reason it’s by far the most commonly used water disinfection method in the world. The resources it requires are widely available and low-cost, and the tools needed to use it are elementary. Because of this, chlorine lacks many of the expensive maintenance, repair, and operating costs associated with other types of water treatment methods.

Practical for Developing Nations

These multifarious benefits of chlorine make it the smart and preferred method for water treatment in low-resource countries, where treatment facilities are often distant or do not exist at all. The World Health Organization agrees: “Of the drinking water disinfectants, free chlorine is the most widely used, the most easily used and the most affordable,” it says.

Because all classes of waterborne pathogens (bacteria, protozoa and viruses) can exist nearly anywhere, chlorine’s ability to combat each provides users protection against the threat of cholera, typhoid fever, dysentery, hepatitis A, and other potential diseases using a single treatment technique. For those in the developing world living on less than $5 a day, this, along with chlorine’s reasonably low-cost, makes it a very practical, economic clean-water solution.

Chlorine’s effective potency is an advantage as well. In a mere 30 minutes, entire community tanks of contaminated water can be rendered drinkable using considerably little of the resource.


More, in rural and remote villages, a family’s daily water supply is often collected from a distant natural source. The risk of re-contamination increases during transport back to the home. Chlorine’s residual effects help neutralize this threat. As it’s carried for hours and then stored, water treated with chlorine remains disinfected and safe.

And in these settings, perhaps most significant is chlorine’s ability to be generated on site (as liquid sodium hypochlorite). Even in rural communities, this can be done by the process of electrolysis of salt, which turns a simple brine of water and salt into chlorine by electrifying the molecules and causing a chemical reaction.

MSR’s SE200™ Community Chlorine Maker conducts this process. With the device, community members can quickly and easily make their own supplies of chlorine to be used not just for water treatment but also for critical WASH applications in the home, in schools, medical clinics and in other communal places.

MSR Applications

This ability to produce chlorine on the spot is a significant boon to low-resource communities; it provides them a way make clean water on demand. With the SE200™ Chlorine Maker, users can treat 200 liters with just one batch of chlorine made in a mere five minutes with the device. Thus, the small chlorine maker is perfect for the medium-scale daily clean-water needs of rural communities.


In places where even electricity is absent, such as areas struck by disaster, another chlorine technology that MSR distributes offers particular advantages. Aquatabs purification tablets utilize the chlorine form NaDCC. These tablets come in simple-to-use pre-dosed sizes that are easy to drop into small quantities of water, say 5 or 10 liters at a time. The tablets’ ultimate convenience and five-year shelf life can be beneficial when chlorine can’t be made on the spot.

As you can see, at MSR, we’re working with chlorine’s various forms and their distinct advantages to identify the right solutions for diverse users, scenarios and environments. Today, science tells us that the chlorination of drinking water remains one of the best ways to improve access to one of life’s most vital resources. As water treatment and sanitation continue to be a focal point of global health efforts, we’ll strive to research the forefront of innovation that can help.

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