When chlorine is added to water, a variety of chemical processes take 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 able to kill microorganisms in the water.  In the following sections, we will show the chemical reactions which occur when chlorine is added to water. 

Chlorine Demand

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 hydrogen 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, depending on pH:

Hypochlorous Acid ↔ Hydrogen Ion + Hypochlorite 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 hypochlorite ions in chlorinated water will depend on the water's pH.  A higher pH 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.) 

Hypochlorites

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:

• Sodium hypochlorite (NaOCl) comes in a liquid form which contains up to 12% chlorine.  
• Calcium hypochlorite (Ca(OCl)_²), also known as HTH, is a solid which is mixed with water to form a hypochlorite solution.  Calcium hypochlorite is 65-70% concentrated.  
  
• Commercial bleach is the bleach which you buy in a grocery store.  The concentration of commercial bleach varies depending on the brand - Chlorox bleach is 5% chlorine while some other brands are 3.5% concentrated.

Calcium hypochlorite  + Water → Hypochlorous 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 a different concentration of chlorine. 

Chloramines

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:

Ammonia + Hypochlorous Acid → Monochloramine + Water
NH₃ + HOCl → NH₂Cl + H₂O

Monochloramine may then react with more hypochlorous acid to form a dichloramine:

Monochloramine + Hypochlorous Acid → Dichloramine + Water

NH₂Cl + HOCl → NHCl₂ + H₂O

Finally, the dichloramine may react with hypochlorous acid to form a trichloramine:

Dichloramine + Hypochlorous Acid → Trichloramine + Water
NHCl₂ + HOCl → NCl₃ + H₂O

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. 

Breakpoint Chlorination

The graph below shows what happens when chlorine (either chlorine gas or a hypochlorite) is added to water.  First (between points 1 and 2), the water reacts with reducing compounds in the water, such as hydrogen sulfide.  These compounds use up the chlorine, producing no chlorine residual. 

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 at point 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 breakpoint, 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. 

Chlorine Dioxide

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.