chlorine is added to water, a variety of chemical
place. The chlorine reacts with compounds in the water and with
the water itself. Some of the results of these reactions (known
as the chlorine residual) are
kill microorganisms in the water. In the following sections, we
will show the chemical reactions which occur when chlorine is added to
When chlorine enters water, it immediately begins to react with compounds found in the water. The chlorine will react with organic compounds and form trihalomethanes. It will also react with reducing agents such as hydrogen sulfide, ferrous ions, manganous ions, and nitrite ions.
Let's consider one example, in which chlorine reacts with
sulfide in water. Two different reactions can occur:
Hydrogen Sulfide + Chlorine + Water → Sulfuric Acid + Hydrochloric Acid
H2S + 4Cl2 + 4 H2O → H2SO4 + 8 HCl
I have written each reaction using both the chemical formula
and the English name of each compound. In the first reaction,
hydrogen sulfide reacts with chlorine and oxygen to create elemental
sulfur, water, and chloride ions. The elemental sulfur
precipitates out of the water and can cause odor problems. In the
second reaction, hydrogen sulfide reactions with chlorine and water to
create sulfuric acid and hydrochloric acid.
Each of these reactions uses up the chlorine in the water, producing chloride ions or hydrochloric acid which have no disinfecting properties. The total amount of chlorine which is used up in reactions with compounds in the water is known as the chlorine demand. A sufficient quantity of chlorine must be added to the water so that, after the chlorine demand is met, there is still some chlorine left to kill microorganisms in the water.
Reactions of Chlorine Gas With Water
At the same time that chlorine is being used up by compounds in the water, some of the chlorine reacts with the water itself. The reaction depends on what type of chlorine is added to the water as well as on the the pH of the water itself.
Chlorine may be added as to water in the form of chlorine gas, hypochlorite, or chlorine dioxide. All types of chlorine will kill bacteria and some viruses, but only chlorine dioxide will effectively kill Cryptosporidium, Giardia, protozoans, and some viruses. We will first consider chlorine gas, which is the most pure form of chlorine, consisting of two chlorine atoms bound together.
Chlorine gas is compressed into a liquid and stored in metal cylinders. The gas is difficult to handle since it is toxic, heavy, corrosive, and an irritant. At high concentrations, chlorine gas can even be fatal.
When chlorine gas enters the water, the
following reaction occurs:
The chlorine reacts with water and breaks down
into hypochlorous acid and
hydrochloric acid. Hypochlorous acid may further break down,
↔ Hydrogen Ion +
HOCl ↔ H+ + OCl-
Note the double-sided arrows which mean that the reaction is reversible. Hypochlorous acid may break down into a hydrogen ion and a hypochlorite ion, or a hydrogen ion and a hypochlorite ion may join together to form hypochlorous acid.
The concentration of hypochlorous acid and
ions in chlorinated water will depend on the water's pH. A higher
facilitates the formation of more hypochlorite ions and results in less
hypochlorous acid in the water. This is an important reaction to
understand because hypochlorous acid is the most effective form of free chlorine residual, meaning that
it is chlorine available to kill microorganisms in the water.
Hypochlorite ions are much less efficient disinfectants.
So disinfection is more efficient at a low pH (with large quantities of
hypochlorous acid in the water) than at a high pH (with large
quantities of hypochlorite ions in the water.)
Instead of using chlorine gas, some plants apply chlorine to water as a hypochlorite, also known as a bleach. Hypochlorites are less pure than chlorine gas, which means that they are also less dangerous. However, they have the major disadvantage that they decompose in strength over time while in storage. Temperature, light, and physical energy can all break down hypochlorites before they are able to react with pathogens in water.
There are three types of hypochlorites -
sodium hypochlorite, calcium hypochlorite, and commercial bleach:
Acid + Calcium Hydroxide
Ca(OCl)2 + 2 H2O → 2 HOCl + Ca(OH)2
Sodium hypochlorite + Water
Hypochlorous Acid + Sodium Hydroxide
NaOCl + H2O → HOCl + NaOH
In general, disinfection using chlorine
gas and hypochlorites occurs in the same manner. The differences
lie in how the chlorine is fed into the water and on handling and
storage of the chlorine compounds. In addition, the amount of
each type of chlorine added to water will vary since each compound has
different concentration of chlorine.
Some plants use chloramines rather than hypochlorous acid to disinfect the water. To produce chloramines, first chlorine gas or hypochlorite is added to the water to produce hypochlorous acid. Then ammonia is added to the water to react with the hypochlorous acid and produce a chloramine.
Three types of chloramines can be formed in water - monochloramine, dichloramine, and trichloramine. Monochloramine is formed from the reaction of hypochlorous acid with ammonia:
→ Monochloramine + Water
NH3 + HOCl → NH2Cl + H2O
Monochloramine may then react with more hypochlorous acid to form a dichloramine:
Monochloramine + Hypochlorous Acid → Dichloramine + Water
→ NHCl2 + H2O
Finally, the dichloramine may react with
acid to form a trichloramine:
Dichloramine + Hypochlorous
→ Trichloramine + Water
NHCl2 + HOCl → NCl3 + H2O
Chloramines are weaker than chlorine, but are more
stable, so they are often used as the disinfectant in the distribution
lines of water treatment systems. Despite their stability,
chloramines can be broken down by bacteria, heat, and light.
Chloramines are effective at killing bacteria and will also kill some
protozoans, but they are very ineffective at killing viruses.
The number of these reactions which will take place in any given situation depends on the pH of the water. In most cases, both monochloramines and dichloramines are formed. Monochloramines and dichloramines can both be used as a disinfecting agent, called a combined chlorine residual because the chlorine is combined with nitrogen. This is in contrast to the free chlorine residual of hypochlorous acid which is used in other types of chlorination.
The graph below shows what happens when chlorine
(either chlorine gas
a hypochlorite) is added to water. First (between points 1 and
water reacts with reducing compounds in the water, such as hydrogen
sulfide. These compounds use up the chlorine, producing no
Next, between points 2 and 3, the chlorine reacts with organics and ammonia naturally found in the water. Some combined chlorine residual is formed - chloramines. Note that if chloramines were to be used as the disinfecting agent, more ammonia would be added to the water to react with the chlorine. The process would be stopped at point 3. Using chloramine as the disinfecting agent results in little trihalomethane production but causes taste and odor problems since chloramines typically give a "swimming pool" odor to water.
In contrast, if hypochlorous acid is to be used as the chlorine residual, then chlorine will be added past point 3. Between points 3 and 4, the chlorine will break down most of the chloramines in the water, actually lowering the chlorine residual.
Finally, the water reaches the breakpoint, shown
4. The breakpoint
is the point at which the chlorine demand has been totally satisfied -
the chlorine has reacted with all reducing agents, organics, and
ammonia in the water. When more chlorine is added past the
the chlorine reacts with water and forms hypochlorous acid in direct
proportion to the amount of chlorine added. This process, known
as breakpoint chlorination,
is the most common form of chlorination, in which enough chlorine is
added to the water to bring it past the breakpoint and to create some
free chlorine residual.
There is one other form of chlorine which can be used for disinfection - chlorine dioxide. We have not discussed chlorine dioxide previously because it disinfects using neither hypochlorous acid nor chloramines and is not part of the breakpoint chlorination process.
Chlorine dioxide, ClO2, is a very effective form of chlorination since it will kill protozoans, Cryptosporidium, Giardia, and viruses that other systems may not kill. In addition, chlorine dioxide oxidizes all metals and organic matter, converting the organic matter to carbon dioxide and water. Chlorine dioxide can be used to remove sulfide compounds and phenolic tastes and odors. When chlorine dioxide is used, trihalomethanes are not formed and the chlorination process is unaffected by ammonia. Finally, chlorine dioxide is effective at a higher pH than other forms of chlorination.So why isn't chlorine dioxide used in all systems? Chlorine dioxide must be generated on site, which is a very costly process requiring a great deal of technical expertise. Unlike chlorine gas, chlorine dioxide is highly combustible and care must be taken when handling the chlorine dioxide.