Chlorine is one of the most widely used disinfectants. It is very applicable and very effective for the deactivation of pathogenic microorganisms. Chlorine can be easily applied, measures and controlled. Is is fairly persistent and relatively cheap.
Chlorine has been used for applications, such as the deactivation of pathogens in drinking water, swimming pool water and wastewater, for the disinfection of household areas and for textile bleaching, for more than two hundred years. When chlorine was discovered we did not now that disease was caused by microorganisms. In the nineteenth century doctors and scientists discovered that many diseases are contagious and that the spread of disease can be prevented by the disinfection of hospital areas. Very soon afterward, we started experimenting with chlorine as a disinfectant. In 1835 doctor and writer Oliver Wendel Holmes advised midwifes to wash their hands in calcium hypochlorite (Ca(ClO)2-4H2O) to prevent a spread of midwifes fever.
However, we only started using disinfectants on a wider scale in the nineteenth century, after Louis Pasteur discovered that microorganisms spread certain diseases.
Chlorine has played an important role in lenghthening the life-expectancy of humans.

For more information about pathogens in aquatic systems, please take a look at pathogens in freshwater ecosystems

Chlorine as a bleach

Surfaces can be disinfected by bleaching. Bleach consists of chlorine gas dissolved in an alkali-solution, such as sodium hydroxide (NaOH). When chlorine is dissolved in an alkalic solution, hypochlorite ions (OCl-) are formed during an autoredox reaction. Chlorine reacts with sodium hydroxide to sodium hypochlorite (NaOCl). This is a very good disinfectant with a stable effect.
Bleach cannot be combined with acids. When bleach comes in contact with acids the hypochlorite becomes instable, causing poisonous chlorine gas to escape. The accompanying underchloric acid is not very stable.

Figure 4: chlorine is often used as a bleach

Bleaching powder (CaOCl2) can also be used. This is produced by directing chlorine through calcium hydroxide (CaOH). The benefit of bleaching powder is that it is a solid. This makes it easier to apply as a disinfectant in medical areas, next to its use as a bleach. When bleaching powder dissolves, it reacts with water to underchloric acid (HOCl) and hypochlorite ions (OCl-).

How does chlorine disinfection work?

Chlorine kills pathogens such as bacteria and viruses by breaking the chemical bonds in their molecules. Disinfectants that are used for this purpose consist of chlorine compounds which can exchange atoms with other compounds, such as enzymes in bacteria and other cells. When enzymes come in contact with chlorine, one or more of the hydrogen atoms in the molecule are replaced by chlorine. This causes the entire molecule to change shape or fall apart. When enzymes do not function properly, a cell or bacterium will die.

When chlorine is added to water, underchloric acids form:
Cl2 + H2O -> HOCl + H+ + Cl-

Depending on the pH value, underchloric acid partly expires to hypochlorite ions:
Cl2 + 2H2O -> HOCl + H3O + Cl-
HOCl + H2O -> H3O+ + OCl-

This falls apart to chlorine and oxygen atoms:
OCl- -> Cl- + O

Underchloric acid (HOCl, which is electrically neutral) and hypochlorite ions (OCl-, electrically negative) will form free chlorine when bound together. This results in disinfection. Both substances have very distinctive behaviour. Underchloric acid is more reactive and is a stronger disinfectant than hypochlorite. Underchloric acid is split into hydrochloric acid (HCl) and atomair oxygen (O). The oxygen atom is a powerful disinfectant.
The disinfecting properties of chlorine in water are based on the oxidising power of the free oxygen atoms and on chlorine substitution reactions.

Figure 5: the neutral underchloric acid can better penetrate cell walls of pathogenic microorganisms that the negatively charged hypochlorite ion

The cell wall of pathogenic microorganisms is negatively charged by nature. As such, it can be penetrated by the neutral underchloric acid, rather than by the negatively charged hypochlorite ion. Underchloric acid can penetrate slime layers, cell walls and protective layers of microorganisms and effectively kills pathogens as a result. The microorganisms will either die or suffer from reproductive failure.

The effectivity of disinfection is determined by the pH of the water. disinfection with chlorine will take place optimally when the pH is between 5,5 and 7,5. underchloric acid (HOCl) reacts faster than hypochlorite ions (OCl-); it is 80-100% more effective. The level of underchloric acid will decrease when the pH value is higher. With a pH value of 6 the level of underchloric acid is 80%, whereass the concentration of hypochlorite ions is 20%. When the pH value is 8, this is the other way around.
When the pH value is 7,5, concentrations of underchloric acid and hypochlorite ions are equally high.

Underchloric acid (left) : hypochlorite ions (right)

What is free and bound active chlorine?

When chlorine is added to water for disinfection purposes, it usually starts reacting with dissolved organic and inorganic compounds in the water. Chlorine can no longer be used for disinfection after that, because is has formed other products. The amount of chlorine that is used during this process is referred to as the ‘chlorine enquiry’ of the water.
Chlorine can react with ammonia (NH3) to chloramines, chemical compounds which contain chlorine, nitrogen (N) and hydrogen (H). These compounds are referred to as ‘active chlorine compounds’ (contrary to underchloric acid and hypochlorite, which are referred to as ‘free active chlorine’) and are responsible for water disinfection. However, these compounds react much more slowly than free active chlorine.

What doses of chlorine does one apply?

When dosing chlorine one has to take into acount that chlorine reacts with compounds in the water. The dose has to be high enough for a significant amount of chlorine to remain in the water for disinfection. Chlorine enquiry is determined by the amount of organic matter in the water, the pH of the water, contact time and temperature. Chlorine reacts with organic matter to disinfection byporducts, such as trihalomethanes (THM) and halogenated acetic acids (HAA).
Chlorine can be added for disinfection in several different ways. When ordinary chlorination is apllied, the chlorine is simply added to the water and no prior treatment is necessary. Pre- and postchlorination means adding chlorine to water prior to and after other treatment steps. Rechlorination means the addition of chlorine to treated water in one or more points of the distribution system in order to preserve disinfection.

What is breakpoint chlorination?

Breakpoint chlorination consists of a continual addition of chlorine to the water upto the point where the chlorine enquiry is met and all present ammonia is oxidized, so that only free chlorine remains. This is usually applied for disinfection, but it also has other benefits, such as smell and taste control. In order to reach the breakpoint, a superchlorination is applied. To achieve this, one uses chlorine concentrations which largely exceed the 1 mg/L concentration required for disinfection.

Which chlorine concentration is applied?

Chlorine gas can be obtained as fluid gas in 10 bar pressure vessels. It is highly water soluble (3 L chlorine/ 1 L water). To kill bacteria little chlorine is required; about 0,2-0,4 mg/L. the concentrations of chlorine added to the water are usually higher, because of the chlorine enquiry of the water.
Nowadays chlorine gas is only used for large municipal and industrial water purification installations. For smaller applications one usually ads calcium or sodium hypochlorite.

Which factors determine the effectivity of chlorine disinfection?

Factors which determine chlorine disinfection effectivity:
Chlorine concentrations, contact time, temperature, pH, number and types of microorganisms, concentrations of organic matter in the water.

Table 1: disinfection time for several different types of pathogenic microorganisms with chlorinated water, containing a chlorine concentration of 1 mg/L (1 ppm) when pH = 7,5 and T = 25 °C Disinfection time of fecal pollutants with chlorinated water
E. coli 0157 H7 bacterium < 1 minute
Hepatitis A virus about 16 minutes
Giardia parasite about 45 minutes
Cryptosporidium about 9600 minutes (6,7 days)

What are the health effects of chlorine?

The reaction of the human body to chlorine depends on the concentration of chlorine present in air, and on the duration and frequency of exposure. Effects also depend on the health of an individual and the environmental conditions during exposure.
When small amounts of chlorine are breathed in during short time periods, this can affect the respirational system. Effects vary from coughing and chest pains, to fluid accumulation in the lungs. Chlorine can also cause skin and eye irritations. These effects do not take place under natural conditions. When chlorine enters the body it is not very persistent, because of its reactivity.
Pure chlorine is very toxic, even small amounts can be deadly. During World War I chlorine gas was used on a large scale to hurt or kill enemy soldiers. The Germans were the first to use chlorine gas against their enemies.
Chlorine is much denser than air, causing it to form a toxic fume above the soil. Chlorine gas affects the mucous membrane (nose, throat, eyes). Chlorine is toxic to mucous membranes because it dissolves them, causing the chlorine gas to end up in the blood vessels. When chlorine gas is breathed in the lungs fill up with fluid, causing a person to sort of drown.