Chlorine Disinfection




What is Disinfection?

Before water treatment became common, waterborne diseases could spread quickly through a population, killing or harming hundreds of people.  The table below shows some common, water-transmitted diseases as well as the organisms (pathogens) which cause each disease.  


Disease Caused




Escherichia coli

E. coli infection

Myobacterium tuberculosis



salmonellosis, paratyphoid

Vibrio cholerae



Hepatitis Virus

Hepatitis A

Polio Virus





Giardia lamblia



The primary goal of water treatment is to ensure that the water is safe to drink and does not contain any disease-causing microorganisms.  The best way to ensure pathogen-free drinking water is to make sure that the pathogens never enter the water in the first place.  However, this may be a difficult matter in a surface water supply which is fed by a large watershed.  Most treatments plants choose to remove or kill pathogens in water rather than to ensure that the entire watershed is free of pathogens. 

Pathogens can be removed from water through physical or chemical processes.  Sedimentation and filtration can remove a large percentage of bacteria and other microorganisms from the water by physical means.  Storage can also kill a portion of the disease-causing bacteria in water. 

Disinfection is the process of selectively destroying or inactivating pathogenic organisms in water usually by chemical means.  Disinfection is different from sterilization, which is the complete destruction of all organisms found in water and which is usually expensive and unnecessary.  Disinfection is a required part of the water treatment process while sterilization is not. 




Chlorination is the application of chlorine to water to accomplish some definite purpose.  We will be concerned with the application of chlorine for the purpose of disinfection, but you should be aware that chlorination can also be used for taste and odor control, iron and manganese removal, and to remove some gases such as ammonia and hydrogen sulfide.
Chlorination is currently the most frequently used form of disinfection in the water treatment field.  However, other disinfection processes have been developed.



Prechlorination and Postchlorination

Like several other water treatment processes, chlorination can be used as a pretreatment process (prechlorination) or as part of the primary treatment of water (postchlorination).  Treatment usually involves either postchlorination only or a combination of prechlorination and postchlorination. 

Prechlorination is the act of adding chlorine to the raw water.  The residual chlorine is useful in several stages of the treatment process - aiding in coagulation, controlling algae problems in basins, reducing odor problems, and controlling mudball formation.  In addition, the chlorine has a much longer contact time when added at the beginning of the treatment process, so prechlorination increases safety in disinfecting heavily contaminated water. 

Postchlorination is the application of chlorine after water has been treated but before the water reaches the distribution system.  At this stage, chlorination is meant to kill pathogens and to provide a chlorine residual in the distribution system.  Postchlorination is nearly always part of the treatment process, either used in combination with prechlorination or used as the sole disinfection process. 

Until the middle of the 1970s, water treatment plants typically used both prechlorination and postchlorination.  However, the longer contact time provided by prechlorination allows the chlorine to react with the organics in the water and produce carcinogenic substances known as trihalomethanes.  As a result of concerns over trihalomethanes, prechlorination has become much less common in the United States.  Currently, prechlorination is only used in plants where trihalomethane formation is not a problem.



Location in the Treatment Process

During prechlorination, chlorine is usually added to raw water after screening and before flash mixing.  Postchlorination, in contrast, is often the last stage in the treatment process.  After flowing through the filter, water is chlorinated and then pumped to the clearwell to allow a sufficient contact time for the chlorine to act.  From the clearwell, the water may be pumped into a large, outdoor storage tank such as the one shown below.  Finally, the water is released to the customer. 



Forms of Chlorine

Elemental Chlorine

Elemental chlorine is either liquid or gaseous in form.
In its liquid form, it must be under extreme pressure.
In its gaseous form, it is 2.5 times as heavy as air.



Forms of Chlorine in Solution

There are two forms of chlorine in solution:

Hypochlorous Acid


Hypochlorite Ion



Liquid Chlorine

Liquid chlorine is a clear, amber colored liquid.

Common properties of chlorine are listed in the following table:


Vapor Pressure

Vapor pressure is a function of temperature and is independent of volume. The gage pressure of a container with 1 pound of chlorine will be essentially the same as if it contained 100 pounds, at the same temperature conditions.

Vapor pressure increases as the temperature increases, as demonstrated in the following figure:





Gaseous Chlorine

Gaseous chlorine is a greenish, yellow gas.

Common properties of gaseous chlorine are listed in the following table:


Reactions in Aqueous Solution

Chlorine added to chemically pure water forms a mixture of hypochlorous (HOCl) and hydrochloric (HCl) acids, as indicated in the following chemical equation:

Cl2 + H2O ↔ HOCl + H+ + Cl-


At ordinary temperatures, the reaction is essentially complete within a few seconds.

Hypochlorous acid dissociates into hydrogen and hypochlorite ions almost instantaneously:

HOCl ↔ H+ + OCl-


The degree of dissociation is dependent on both pH and temperature.


The normal pH of water supplies is within range where chlorine may exist as both hypochlorous acid and hypochlorite ion. This is indicated in the following figure. HOCl is a stronger oxidant and disinfectant than OCL-, which is why disinfection is more effective at lower pHs.



Chlorine Handling and Safety

Personnel Safety Protection

Basic Equipment

Forced air ventilation is required for all chlorine storage and feed rooms.


There are two types of gas masks – a canister type with a full face piece and a self contained breathing apparatus.


Protective clothing.

Emergency showers and eye-wash stations.

Automatic leak detection.



First Aid

The following guidelines should be adhered to in the event of exposure to chlorine.



Remove the injured party to an uncontaminated outdoor area. Use appropriate respiratory equipment during rescue—do not become another victim.

Check for breathing and pulse. If not breathing, give artificial respiration. If breathing is difficult, have trained personnel administer oxygen as soon as possible. If no pulse, perform CPR.

Call for medical assistance as soon as possible.

Check for other injuries.

Keep the injured party warm and at rest.



Skin Contact

Immediately shower with large quantities of water.

Remove protective clothing and equipment while in shower.

Flush skin with water for at least 5 minutes.

Call for medical assistance.

Keep affected area cool.



Eye Contact

Immediately shower with large quantities of water while holding eyes open.

Call a physician immediately.

Transfer promptly to medical facility.




Do not induce vomiting.

Give large quantities of water.

Call physician immediately.

Transfer promptly to a medical facility.



Chlorine Leaks and Response

Potential Points of Chlorine Leaks

Leaks can occur anywhere in the pressurized supply, including connections and piping joints, cylinders or containers and feed equipment.


Leak Detection

The sense of smell can detect chlorine concentrations as low as 4 parts per million (ppm).

Portable and permanent automatic chlorine detection devices can detect at concentrations of 1 ppm or less.

A rag saturated with strong ammonia solution will indicate leaks by the presence of white fumes.



Leak Repair

In the event of a chlorine leak, the following guidelines should be followed.

Activate the chlorine leak absorption system, if available.


Repair leaks immediately or they will become worse.


If the leak is in the chlorine supply piping:


If the leak is in the equipment:


If the leak is in a cylinder or container:



Other Chlorine Emergency Measures


Chlorine will not burn in air. It is a strong oxidizer and contact with combustible materials may cause fire. When heated, chlorine is dangerous and emits highly toxic fumes.
In the event of a fire caused by chlorine, the following fire fighting measures should be adhered to:



Risk Management Plan

An emergency plan for chlorine is essential and should include the following:

Training of personnel.

Periodic training drills.

A list of assistance available in the event of an emergency. The supplier’s name, address and emergency telephone number should be posted.




Storage Requirements

Separate rooms for storage and feed facilities should be provided.

Storage and feed rooms need to be separate from other operating areas.

Rooms should have an inspection window to permit equipment to be viewed without entering the room.

All openings between rooms and the remainder of the plant need to be sealed.
Storage for a 30 day supply should be available.



Types of Storage Containers

100 and 150 lb. Cylinders

Position and store vertically.

Restraint chains are necessary to prevent accidents



Ton Containers

Provide storage area with 2 ton capacity monorail or crane for cylinder movement and placement.

Roller trunions are necessary to properly position cylinders.

Cylinder valves must be positioned vertically. Gas flows from the top valve and liquid flows from the bottom valve.


Tank Cars

Tank cars are generally only provided for the largest plants.

Rail siding is required.




Instead of using chlorine gas, some plants apply chlorine to water as a hypochlorite, also known as 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:


Hypochlorites and bleaches work in the same general manner as chlorine gas.  They react with water and form the disinfectant hypochlorous acid.  The reactions of sodium hypochlorite and calcium hypochlorite with water are shown below:


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. 



Storage Facilities

Basic Facilities and Housing



Entry and Exit Requirements












Chlorine Scrubbers

Description of Equipment



Description of Process

Two chlorine scrubbing processes are available: one uses a caustic solution and the other uses solid media.



Caustic Solution Type

A caustic soda is used to neutralize the chlorine:

Cl2 + 2 NaOH → NaOCl + NaCl + H2O




Solid Media Type



Chlorination Mechanics and Terminology

An enteric virus is one that lives in the intestines.

Chlorine demand is the amount of chlorine required to react with all the organic and inorganic material. In practice, the chlorine demand is the difference between the amount of chlorine added and the amount remaining after a given contact time. Some reactive compounds have disinfecting properties and others do not.

Chlorine residual is the total of all compounds with disinfecting properties and any remaining free chlorine.

Chlorine Residual (mg/l) = Combined Chlorine Forms (mg/l) + Free Chlorine (mg/l)
The residual should contain free chlorine since it has the highest disinfecting ability.
The presence of measurable chlorine residual indicates that all chemical reactions have been satisfied and that sufficient chlorine is present to kill microorganisms.

Chlorine dose is the amount of chlorine needed to satisfy the chlorine demand plus the amount of chlorine residual needed for disinfection.

Chlorine Dose (mg/l) = Chlorine Demand (mg/l) + Chlorine Residual (mg/l)

Breakpoint chlorination is the addition of chlorine until all chlorine demand has been satisfied. It is used to determine how much chlorine is required for disinfection.
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. 

Breakpoint curve 


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. 




Residual and Dosage

A variety of factors can influence disinfection efficiency when  using breakpoint chlorination or chloramines.  One of the most important of these is the concentration of chlorine residual in the water.

The chlorine residual in the clearwell should be at least 0.5 mg/L.  This residual, consisting of hypochlorous acid and/or chloramines, must kill microorganisms already present in the water and must also kill any pathogens which may enter the distribution system through cross-connections or leakage.   In order to ensure that the water is free of microorganisms when it reaches the customer, the chlorine residual should be about 0.2 mg/L at the extreme ends of the distribution system.  This residual in the distribution system will also act to control microorganisms in the distribution system which produces slimes, tastes, or odors. 

Determining the correct dosage of chlorine to add to water will depend on the quantity and type of substances in the water creating a chlorine demand.  The chlorine dose is calculated as follows:

Chlorine Dose = Chlorine Demand + Chlorine Residual


So, if the required chlorine residual is 0.5 mg/L and the chlorine demand is known to be 2 mg/L, then 2.5 mg/L of chlorine will have to be added to treat the water. 

The chlorine demand will typically vary over time as the characteristics of the water change.  By testing the chlorine residual, the operator can determine whether a sufficient dose of chlorine is being added to treat the water.  In a large system, chlorine must be sampled every two hours at the plant and at various points in the distribution system. 

It is also important to understand the breakpoint curve when changing chlorine dosages.  If the water smells strongly of chlorine, it may not mean that too much chlorine is being added.  More likely, chloramines are being produced, and more chlorine needs to be added to pass the breakpoint. 



Contact Time

Contact time is just as important as the chlorine residual in determining the efficiency of chlorination.  Contact time is the amount of time which the chlorine has to react with the microorganisms in the water, which will equal the time between the moment when chlorine is added to the water and the moment when that water is used by the customer.  The longer the contact time, the more efficient the disinfection process is.  When using chlorine for disinfection a minimum contact time of 30 minutes is required for adequate disinfection. 

The CT value is used as a measurement of the degree of pathogen inactivation due to chlorination.  The CT value is calculated as follows:

CT = (Chlorine residual, mg/L) (Contact time, minutes)


The CT is the Concentration multiplied by the Time.  As the formula suggests, a reduced chlorine residual can still provide adequate kill of microorganisms if a long contact time is provided.  Conversely, a smaller chlorine residual can be used as long as the chlorine has a longer contact time to kill the pathogens. 



Other Influencing Factors

Within the disinfection process, efficiency is influenced by the chlorine residual, the type of chemical used for chlorination, the contact time, the initial mixing of chlorine into the water, and the location of chlorination within the treatment process.  The most efficient process will have a high chlorine residual, a long contact time, and thorough mixing. 

Characteristics of the water will also affect efficiency of chlorination.  As you will recall, at a high pH, the hypochlorous acid becomes dissociated into the ineffective hypochlorite ion.  So lower pH values result in more efficient disinfection. 

Temperature influences chlorination just as it does any other chemical reaction.  Warmer water can be treated more efficiently since the reactions occur more quickly.  At a lower water temperature, longer contact times or higher concentrations of chemicals must be used to ensure adequate disinfection. 

Turbidity of the water influences disinfection primarily through influencing the chlorine demand.  Turbid water tends to contain particles which react with chlorine, reducing the concentration of chlorine residual which is formed.  Since the turbidity of the water depends to a large extent on upstream processes (coagulation, flocculation, sedimentation, and filtration), changes in these upstream processes will influence the efficiency of chlorination.  Turbidity is also influenced by the source water - groundwater turbidity tends to change slowly or not at all while the chlorine demand of surface water can change continuously, especially during storms and the snow melt season. 

Finally, and most intuitively, the number and type of microorganisms in the water will influence chlorination efficiency.  Since cyst-forming microorganisms and viruses are very difficult to kill using chlorination, the disinfection process will be less efficient if these pathogens are found in the water. 



Regulatory Requirements

Continuous disinfection is required of all public water systems.

For surface water supplies:


Log inactivation is defined as follows:


Chlorination equipment must be capable of maintaining a chlorine residual which achieves a minimum of 1 log Giardia cyst inactivation following filtration.

Contact time can be thought of as a residual disinfectant concentration C in mg/L which is multiplied by a time T in minutes. The time T is measured between the point of application of the disinfectant and the measurement of the residual.

For groundwater supplies not under the influence of surface water intrusion:


For chlorine residual requirements:


The exact mechanism of chlorine disinfection is not fully known.


Chlorine added to water containing organic and inorganic chemicals reacts with these materials to form chlorine compounds.



Process Calculations

There are two basic chlorination process calculations: chlorine dosage and chlorine demand.



Chlorine Dosage Calculation

To perform the calculation, you will need to know the amount of chlorine being added and the amount of water being treated.

Chlorine Dosage (mg/l) = Chlorine Feed (lb/day)
        [Flow (mgd) x 8.34 (lb/gal)]



Chlorine Demand Calculation

A sufficient amount of chlorine must be added so that the chlorine demand is met and the desired chlorine residual is provided.

Chlorine Demand (mg/l) = Chlorine Dose (mg/l) – Chlorine Residual (mg/l)



Sample Calculations

Example 1.

The chlorinator at a water treatment plant operating at a flowrate of 1.0 million gallons per day is set to feed 20 pounds in a 24 hour period. The chlorine residual in the finished water leaving the plant after a 20 minute contact period is 0.5 mg/l. Calculate the chlorine demand of the water.

Known: Flow, (mgd) = 1.0 MGD
  Chlorinator setting = 20 pounds/day
  Finished water chlorine residual = 0.5 mg/l

 Find:   Chlorine Dosage (mg/l) and Chlorine Demand (mg/l)


Step 1: Calculate chlorine dosage in mg/l

Chlorine Dose (mg/l) = Chlorine Feed (lb/day)
                                     [Flow (mgd) x 8.34 (lb/gal)]

Chlorine Dose (mg/l) = [20 lb Cl/day]
                                     [1.0 (mgd) x 8.34 (lb/gal)]

                                     = 20 lb Cl/day
                                        (8.34 (million lb water /day)

                                     = 2.4 lb Cl/million lb water

                                     = 2.4 Parts Per Million (ppm)

                                     = 2.4 mg/l


Step 2: Calculate Chlorine Demand in mg/l

Chlorine Demand (mg/l) = Chlorine Dose (mg/l) - Chlorine Residual (mg/l)

Chlorine Demand (mg/l) = 2.4 (mg/l) – 0.5 (mg/l)

                                       = 1.9 mg/l



Read about the Free Chlorine Amperometric Method


Read about the Free Chlorine Spectrophotometer Method