Understanding Chlorine: How It Works to Disinfect Water

Introductory Note:  The article below is adapted from information from Blue-White Industries.  It provides a good explanation for the frequently misunderstood terms free chlorine, total chlorine and combined chlorine.

For over a century, chlorine has been used to provide clean drinking water to communities in the U.S. and across the world. In the correct doses, chlorine can kill a broad range of pathogens while remaining safe for people and animals to consume. The key is dosage, as too little chlorine will not have the disinfecting power required to eliminate the most critical pathogens. Too much chlorine can cause water to taste and/or smell unappealing, or worse, have long-term negative impact.

Measuring chlorine isn’t a simple matter of saying, “I have X parts per million in my water.” After all, once chlorine enters water, it begins to change, and when it interacts with pathogens and other matter, it changes again.  There are technically three measurements that must be considered: free chlorine, combined chlorine, and total chlorine.

How Chlorine Works

In water, chlorine breaks down into smaller chemicals such as hypochlorite ion and hypochlorous acid. It is these substances that kill bacteria, viruses, and other microorganisms. They do this by either collapsing proteins in bacterial cells or damaging the outer membrane of viruses and similar pathogens. Not every pathogen is equally vulnerable to chlorine, however. Protozoa such as Giardia lamblia and Cryptosporidium are chlorine-resistant. Fortunately such pathogens are large and can be easily removed via filtration.

Free Chlorine

“Free chlorine” is the amount of chlorine that is available to combine or oxidize contaminants in water. The greater the amount of free chlorine, the greater the disinfection potential. In a drinking water system, the amount of free chlorine should generally be kept between 2 ppm and 4 ppm. When free chlorine levels rise above 4 ppm, the water may take on a strong “swimming pool” smell or taste.  However, too little chlorine means there may not be enough chlorine available to disinfect pathogens.

Combined Chlorine

When hypochlorite ions and hypochlorous acid interact with contaminants, they form new compounds. Generally speaking, these new compounds are no longer available for disinfection. The amount of combined chlorine measures how many pathogens or other contaminants have been using chlorine, which helps to understand how dirty the water is (or was).

Of course, not all combined chlorine chemicals are inert. When chlorine combines with nitrogen, it can form chloramines. These compounds do have some disinfection power, but they are not likely present in high enough quantities to be considered in disinfection potential (unless operators deliberately added ammonia to the system with the intent of forming chloramines).

Other types of combined chlorine include disinfection byproducts (DBPs), such as trihalomethane and haloacetic acid. These substances occur when chlorine reacts with natural organic matter in the water. DBPs can be harmful to human health and are regulated by the U.S. EPA.