What Is Chlorination: A Simple Explanation

We are so used to the chlorine taste in our drinking water and smell in our swimming pools that most of us hardly notice it is there. Chlorination is a popular water disinfection method, largely due to its long history and the fact that it works reliably at scale. We all know chlorination exists, however, an overall idea of how the process works are something that is not common knowledge, this led me to come up with this succinct article on the topic.

Chlorination is a method of water disinfection using chlorine to oxidize and eliminate microorganisms from a water source. It is widely used globally for drinking water and general use largely due to its scalable nature and longer-term disinfection properties.

What is Chlorine?

Chlorine is a chemical element with an atomic number of 17 and an atomic mass of 35.5. It is one of the most common substances on Earth however pure chlorine rarely exists in nature. At room temperature and sea level pressure, chlorine takes the form of a yellow-green gas. As a pure gas, chlorine is highly poisonous and even lethal. In the past chlorine gas has been used as a chemical weapon in war.

Although in its pure form chlorine may be dangerous, it can be combined with other elements to become perfectly safe. For instance, combining chlorine with sodium creates sodium chloride which is the scientific name of common table salt. Chlorine is an ingredient in many products mostly due to its disinfectant and bleaching properties, this is why it is used to treat water and as an ingredient in cleaning products. The reason water is chlorinated is to kill microorganisms like bacteria and parasites which have the potential to cause water-borne infections in humans.

Sodium Chloride, also known as table salt is put on some hot chips

Chlorination’s history with water treatment.

Chlorine was discovered by Swedish chemist Carl Wilhelm Scheele in 1744. In 1835, chlorine was originally used to help remove odours from water, however, chlorine disinfectant properties weren’t discovered in 1890. After the discovery of chlorination’s disinfectant properties, the process was adopted in Great Britain and then expanded to the United States in 1908 and over to Canada in 1917. In current times, chlorination is still the most popular method of large-scale water disinfection in the world.

How Does Chlorination Work?

Chlorine kills harmful microorganisms such as viruses and bacteria through the destruction of chemical bonds within their molecules. Chlorine-based disinfectants used for this purpose utilize chlorine oxidizing properties to exchange atoms with compounds contained in enzymes of bacteria and other cells. As a general rule, when chlorine comes into contact with enzymes, one or more hydrogen atoms from the enzyme molecules are replaced with a chlorine atom. When this replacement occurs, it completely disrupts the molecular structure of the enzyme, causing it to warp into a different shape or simply fall apart. Once this replacement process takes place with a sufficient number of enzymes within a cell or bacteria, it will simply cease to function and die.

When chlorine is added to water it can behave in a variety of ways, depending on the temperature of the water, the pH and the length of time it has been present in the water for. Initially, when chlorine is added to water it forms underchloric acid (HOCl) which is electrically neutral, depending on pH levels this will then break down into hypochlorite ions (OCl-) a negatively charged ion.

Cl2 + H2O -> HOCl + H+ + Cl-

Initial underchloric acid formation.

Cl2 + 2H2O -> HOCl + H3O + Cl-
HOCl + H2O -> H3O+ + OCl-

Underchloric acid breaks down into hypochlorite ions.

OCl- -> Cl- + O

Hypochlorite eventually breaks down into chlorine and oxygen atoms.

Each of the forms of chlorine in water behaves very differently and has varied disinfection qualities. Chlorine’s ability to disinfect water centres on the oxidation ability of chlorine and chlorine-based atoms as well as the free oxygen atoms produced. One point to note is underchloric acid’s effectiveness at penetrating the negatively charged cell walls of pathogens due to its neutrally charged nature. Being neutrally charged, underchloric acid can pass through protective layers within the cell walls of microorganisms, effectively killing the pathogens as a result. This penetration of the cell walls and protective layers would not be possible by hypochlorite (OCl-), Chloride (Cl-) or oxide (O-) as they too are all negatively charged and would be electrically repelled by the negative charge in the cell wall.

The pH of the water being chlorinated is a primary driver of disinfection effectiveness. The optimal pH levels for chlorination are between 5.5 to 7.5, at this level, there are equally high concentrations of both hypochlorite and underchloric acid. Underchloric acid values drop off significantly as pH increases and Hypochlrote does the same when pH decreases. Due to this phenomenon, the pH of the water is carefully managed and kept within this relatively neutral zone of 5.5 to 7.5 during the process of chlorination.

Does Chlorination Kill ALL microorganisms?

Chlorination does not kill all microorganisms and pathogens, it is relatively ineffective against a class of bacteria called Protozoan Cysts. Although very effective against most bacteria and viruses, chlorine cannot disinfect all waterborne pathogens, particularly protozoan cysts with Giardia and cryptosporidium being two examples. Both Giardia and Cryptosporidium pose huge health risks due to the level of sickness they can cause, so in water high in either of these pathogens an alternative to chlorination should be deployed. Thankfully both pathogens are much more susceptible to other treatment techniques like UV water treatment and Ozonation.

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