CHLORINE DISINFECTION AND HEALTH EFFECTS
Chlorine as a disinfectant
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 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.
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 lengthening the life-expectancy of
humans.
For more
information about pathogens in aquatic systems, please take a look at pathogens in freshwater ecosystems
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.
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.
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
behavior. 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.
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.
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%, whereas 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.
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.
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 account 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.
What doses of chlorine does one apply?
When dosing chlorine one has to take into account 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 by products, such as trihalomethanes (THM) and halogenated acetic
acids (HAA).
Chlorine can be added for disinfection in several different ways. When ordinary
chlorination is applied, the chlorine is simply added to the water and no prior
treatment is necessary.
Pre- and
post-chlorination means adding chlorine to water prior to and after other
treatment steps. Re-chlorination 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 up to 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.
What is breakpoint chlorination?
"Breakpoint chlorination" consists of a continual addition of chlorine to the water up to 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.
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 adds 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.
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)
|
The reaction of the human body to chlorine depends on the concentration of chlorine present in the 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 respiratory 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.
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.
source: lenntech.org
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