Chlorination of wastewater is the application of chlorine to a wastewater to accomplish some definite purpose.  The purpose of chlorination may not always be disinfection and may, in fact, involve odor control or some other objective which will be noted.  Chlorine may be applied in two general ways, gaseous and liquid.  In general, the effective chemical form of chlorine that either destroys the microbe or acts against odor, etc., is the same.  Gaseous forms of chlorine are generally first dissolved in water prior to addition to the wastewater stream, while liquid forms of chlorine (called hypochlorites) are sold in the form of water soluble salts.  Because chlorine gas generally costs less than hypochlorites, it is normally used in treatment plants except in rare instances where only a relatively small amount of chlorine is needed or where the possible danger from gaseous chlorine overrides economic considerations.  The application of chlorine is usually controlled by special devices which are known as chlorinators, chlorinizers or by similar names. 

Reactions of Chlorine in Wastewaters

In order to determine at what points in the treatment process, and how much chlorine should be applied to accomplish the purpose desired, it is necessary to know what reactions occur when chlorine is mixed with a wastewater.  When chlorine is mixed with pure water, it immediately dissolves, forming fist hypochlorous acid and then hypochlorites: 


The above two forms of chlorine (hypochlorous acid and hypochlorite ion) are called "free" residual chlorine, as opposed to the reaction products of chlorine with other compounds that can also be detected using analytical techniques that are called "combined" residual chlorine.  Free residual chlorine is a more effective disinfecting agent than combined residual chlorine, and generally hypochlorous acid is a much more effective disinfectant than hypochlorite ion.  In wastewaters, free residual chlorine is seldom detected and chlorine is usually found in the "combined" residual form. 

Chlorine is an extremely active oxidizing chemical that will react with many substances in wastewaters.  If small amounts of chlorine are added to wastewaters it will react rapidly and is thus consumed.  For example, chlorine first reacts readily with such substances as hydrogen sulfide, ferrous iron, manganese or thiosulfates, which may have their origin from industrial wastes.  However, almost any "reducing" compound capable of reacting with chlorine (an oxidizing compound), will react.  If all of the chlorine is consumed in these reactions, no disinfection will result. 

Chlorine generally reacts in a prescribed order, first with inorganic reducing compounds.  If enough chlorine is added to react with these substances, then the addition of more chlorine will result in reactions of chlorine with the organic matter that is present.  This forms chloroorganic compounds, which have little or no disinfecting action.  Again, if enough chlorine is added to react with all the reducing compounds and all the organic matter, then the addition of a little more chlorine will react with ammonia or other nitrogeneous compounds to produce chloramines or other combined forms of chlorine which also have disinfecting action but are not as effective as free chlorine. 

The continued addition of chlorine will result in the destruction of the chloramines and the formation of free chlorine.  While chlorine is seldom applied to this level in wastewater treatment, the addition of chlorine in sufficient dosages to where free chlorine is formed is called "breakpoint" chlorination. 

For effective chlorine disinfection both sufficient chlorine dosages as well as contact time are necessary.  Generally both of these factors must be worked out experimentally and other factors will affect the effectiveness of chlorination.  Among the principal factors are bacterial numbers, pH, temperature and contacting.  In "pure" systems bacterial kill at a particular chlorine dosage is directly related to the number of bacteria present when the chlorine is first added.  pH will affect the form of chlorine present and, generally, at neutral pH's hypochlorous acid, the more effective form of chlorine, is favored.  Temperature affects the speed with which chemical reactions take place and colder temperatures are less favorable for disinfection.  Proper contacting or mixing or agitation, is necessary to make sure that the chlorine applied contacts or reaches the vital parts of the microbial cell. 

 The precise mechanism of the disinfecting action of chlorine is not fully known.  However, chlorine is capable of undergoing a wide variety of reactions and probably reacts with the microbial cell at several levels.  At high concentrations, massive oxidation takes place and membranes and all organic components are affected.  At lower concentrations chlorine probably affects vital protein systems as well as membranes.  From the point of view of wastewater treatment, the mechanism of action of chlorine is much less important than its effects as a disinfecting agent. 

The quantity of reducing substances, both organic and inorganic, in wastewaters, varies, so that the amount of chlorine that has to be added to wastewater for different purposes will also vary.  The chlorine used by these organic and inorganic reducing substances is defined as the chlorine demand.  Chlorine demand is equal to the amount of chlorine added minus that remaining as combined chlorine after a period of time, which is generally 15 minutes.  This relationship can be written as: 

Chlorine Demand  =  (Applied Chlorine Dose)  -  (Chlorine Residual) 

It is important to note that disinfection is carried out by that amount of chlorine remaining after the chlorine demand has been satisfied.  This quantity of chlorine in excess of the chlorine demand is defined as residual chlorine and expressed as milligrams per liter.  For example, if a chlorinator is set to feed 50 lbs. of chlorine per 24 hours and the wastewater flow is at a rate of 0.85 mgd and the chlorine as measured after 15 minutes contact is 0.5 mg/L, the chlorine feed or dose is: 


Chlorine dose in mg/L
Chlorine residual in mg/L
-   0.5
Chlorine demand in mg/L

Theoretically, while microorganisms are killed as the chlorine demand is being satisfied, disinfection is generally the result of chlorine residual or the amount of chlorine remaining after the chlorine demand has been satisfied.  Thus, measurement of chlorine residual is an important part of the operator's duties. 

Chlorine is seldom applied to wastewaters to reach "breakpoint" levels.  This is because the amount of chlorine required prior to observing free available chlorine would be very high (approximately 150 mg/L).  Generally chlorine is applied only to give a combined residual.  It should be noted that in some of the more recent advanced wastewater treatment processes with high quality effluents where reduced inorganics and organic compounds are produced, it may be possible to chlorinate to sufficient dosages to have free available chlorine while at lower chlorine dosages, and at the same time affect the removal of ammonia from the wastewater. 

Purposes of Chlorination

Chlorine is added to wastewater for a number of different purposes and chlorine dosages and management will vary with the specific purpose.  In general, chlorine applied before any treatment is given (pre-chlorination), during treatment (plant chlorination), or after normal treatment measures have been carried out (postchlorination).  A few of the more important purposes of chlorination are listed below. 

  1. Disinfection 

    Chlorine is a very effective disinfecting agent and has been the agent of choice in reducing bacterial numbers in wastewater effluents.  As noted, neither primary nor secondary methods of wastewater treatment can completely eliminate pathogenic bacteria which are always potentially present.  When wastewaters or treated effluents are discharged to bodies of water which are, or may be used as a source of public water supply, or for recreational purposes, treatment or disinfection for the destruction of pathogenic organisms is required to minimize the health hazards of pollution to these receiving waters. 

    Chlorination for disinfection requires that essentially all of the pathogens in the wastewater plant effluent be destroyed.  At the same time it should be noted that many but not all of the nonpathogenic microorganisms are also destroyed.  As noted, no attempt is made to sterilize wastewater and this is not only unnecessary but impractical.  In some instances sterilization might be detrimental where other treatment dependent upon microbial activity may follow chlorination.  Fortunately pathogenic microorganisms are less resistant to chlorine than most nonpathogens so that disinfection can be effected without sterilization.  Chlorination as commonly practiced in wastewater treatment is insufficient to inactivate all of the enteric (intestinal) viruses which may be present in wastewater. 

    To accomplish disinfection, sufficient chlorine must be added to satisfy the chlorine demand and leave a residual chlorine that will destroy bacteria.  Special laboratory equipment is necessary to measure the destruction of bacteria and the tests require several days to complete.  Thus, bacteriological examinations are not practical for the day-to-day control of the application of chlorine.  Laboratory experiments and actual plant experience have shown that if sufficient chlorine is added to wastewater so that 15 minutes after the chlorine has been added, a residual chlorine concentration of 0.5 mg/L is present in the wastewater, disinfection will usually be accomplished.  This follows the general pattern of toxicity of most disinfectants and both concentration and contact time are important.  Generally a small concentration acting over a long period of time would have the same effect as a large concentration acting over a short period of time.  For the elimination of all entering viruses, for example, both longer contact times and higher chlorine dosages than now used must be employed.  In actual operation the practical control of chlorination for disinfection is by measurement of the residual chlorine.  By this means, test results can be obtained in a few minutes and the chlorinators adjusted to the proper feed rate. 

    Disinfection of wastewater is defined in terms of fecal coliforms.  The requirements are that fecal coliform levels shall not exceed 400 organisms per 100 ml at any time or exceed a monthly geometric average of 200 organisms per 100 ml when disinfection is required to protect the best intended uses of the water in question. 

    It may be that a 0.5 mg/L residual after 15 minutes will not meet this bacteriological standard at all wastewater treatment plants.  In this case experiments must be made to determine the residual chlorine value that must be obtained to comply with the standard if it is applicable.  This reaction is then used to control the chlorine application. 

    Disinfection when required must be a continuous process as it would be hazardous to discharge untreated effluent even for a short period of time.  Proper contacting of the microbes with chlorine is important and the point of chlorine application must be at a place where the chlorine feed can be rapidly mixed with the entire flow of wastewater and where the mixture of chlorine and wastewater can be held for a minimum of 15 minutes before discharge into the receiving water. 

    Where the outfall pipe is long enough to provide at least 15 minutes for the effluent to flow from the plant to the stream, chlorination of the effluent as it leaves the plant can be used.  Control, in this case, should be by measurement of the residual chlorine in the wastewater at the end of the outfall.  Many times the end of the outfall is under water or at an inconvenient distance from the plant.  It is advisable under such conditions to collect a sample of chlorinated wastewater making sure it is taken at a place where the chlorine is completely mixed with the wastewater, and hold the sample for 15 minutes before measuring the residual chlorine. 

    If the desired residual is 0.5 mg/L and actual chlorine residual is greater or less than 0.5 mg/L, the chlorine feed is decreased or increased until the proper residual is obtained.  Since the chlorine demand of wastewater varies during the day, the chlorine feed required to maintain a 0.5 mg/L residual will vary.  In a small wastewater plant (less than 1 mgd), the operator may not have time to check the residual chlorine repeatedly and adjust the rate of chlorine application.  In this case, the chlorine feed is adjusted once daily to give the required residual at the time of maximum wastewater flow, which generally coincides with the time of maximum chlorine demand and in most plants occurs about 10:00 a.m.  Then at all other times during the day the chlorine residual should be greater than 0.5 mg/L.  This means that chlorine is being wasted, but the operator is sure that disinfection is being accomplished. 

    The amount of chlorine required to produce 0.5 mg/L residual in most secondary effluents will be between 40 and 50 lbs. per million gallons.  By frequent adjustment of the chlorine feed it might be possible to save about five to ten lbs. of chlorine per million gallons.  This can represent an economic factor in wastewater treatment plant operation and will vary with plant size.  In a larger plant (10 mgd) the waste of chlorine might be significant and is worth trying to save.  Therefore, residual chlorine values are measured possibly three or four times a day and the chlorinator adjusted each time.  In still larger plants, it pays to make measurements frequently and it is often the practice to adjust the chlorine feed rate hourly. 

    Where the outlet sewer does not provide 15 minutes holding time at peak hourly flow, or 30 minutes holding at average rate of flow, a chlorine contact tank is built and so designed as to give the required 15 or 30 minutes contact time at maximum or average flows, respectively.  In this case, chlorine is applied to the influent of the contact tank and the residual measured in the effluent. 

    The object of disinfection is the destruction of pathogenic bacteria and the ultimate measure of effectiveness is in the bacteriological result.  The measurement of residual chlorine does supply a tool for practical control.  The 0.5 mg/L residual chlorine, while generally effective, is not a rigid standard but a guide that may be changed to meet local requirements.  One special case would be the use of chlorine in the effluent from a plant serving a tuberculosis hospital.  Studies have indicated that a residual of at least 2.0 mg/L should be maintained in the effluent from this type institution and that the detention period should be at least two hours at the average rate of flow instead of the 30 minutes which is normally used for basis of design. 

Fish Toxicity

Chlorine as well as chloramines are generally toxic to fish as well as harmful to aquatic biota even at low concentrations.  The toxicity to aquatic life in a receiving water will depend upon the concentration of the residual chlorine, the relative amounts of chloramines if they are present, the amount of free chlorine, as well as the dilutions that take place in the receiving waters.  Fifty percent of all rainbow trout and minnows, for example, have been reported killed by levels of about 0.2 mg/L of residual chlorine in 96 hours.  Trout have been shown to "avoid" free chlorine levels of 0.001 mg/L.  However, it should also be noted that this would represent chlorine levels measured after mixing and dilution in the receiving waters rather than in the effluent.  It should also be noted that residual chlorine concentrations diminish with time and mixing as well as by temperature elevations. 

Chlorine can be effectively eliminated by the addition of dechlorinating chemicals.  Because the chlorine concentrations of concern in the receiving waters are usually below the level of detection by the orthotolidine method, a more sensitive analytical method is now required.  It has also been shown that small amounts of chlorine can greatly increase the toxicity of various industrial effluents. 

Generally the National Academy of Sciences indicates that aquatic life will be protected as long as the concentration of residual chlorine does not exceed 0.003 mg/L at any time or place.  They further not that aquatic organisms will tolerate short-term exposure to relatively high levels of chlorine and recommend that total residual chlorine should not exceed 0.05 mg/L for a period up to 30 minutes in any 24 hour period. 


At times it is necessary to dechlorinate, or remove chlorine from a wastewater by the addition of dechlorinating agents.  Generally this must be done to counteract the reactive effects of chlorine in effluent samples, for example, in determining coliforms or BOD's.  At present only a small number of treatment plants in New York State dechlorinate, however, in the future it may become a more common practice to dechlorinate treated wastewater. 

Generally, the most common chemicals used for dechlorination are sulfur dioxide, sodium bisulfate, sodium sulfite, sodium thiosulfate and activated carbon.  The chemical equivalents required for dechlorination can be calculated, however, laboratory experiments should be used to help to define the required dose.  For laboratory samples Standard Methods give the recommended dosages for dechlorinating chemicals. 

Chlorine Hazards

Chlorine is a yellow green gas that is extremely toxic as well as corrosive in moist atmospheres.  Chlorine is about two-and-a-half times as heavy as air.  Chlorine is not flammable or explosive and will not freeze, even at the lowest temperatures.  Chlorine will react readily with water, moisture or moist tissues.  While dry chlorine gas will not attack iron, copper, lead and some other metals and alloys, moist chlorine readily attacks most metals.  Thus, with moisture, chlorine must be handled in corrosion resisting materials such as silver, glass, rubber and certain plastics.  Chlorine can be detected at very low levels and has a characteristic sharp odor.  At moderately low levels chlorine can be penetrating and very irritating to mucous membranes.  A very small percentage in air causes severe coughing.  Heavy exposure can be fatal. 

Physiological Effect of Breathing Air Chlorine Mixtures

Effect of Exposure
Parts of Chlorine Gas Per Million of Air 
By Volume (ppm)
Slight symptoms after several hours exposure
Irritates throat
10 - 15 
Causes Coughing
Dangerous in 30 minutes
40 - 60 
Fatal in a few breaths
1000 ppm 

Mild exposure to chlorine produces no cumulative effects and complete recovery usually occurs.  Inhalation of chlorine gas will cause an initial restlessness, anxiety, a severe irritation of the throat, and the production of excessive saliva.  These symptoms are followed by coughing, retching, vomiting and difficulty in breathing.  Individuals suffering from asthma and certain types of chronic bronchitis are particularly affected.  Exposure of the skin to liquid chlorine will result in severe irritation and blisters. 

Wastewater treatment plant operators should be constantly alert for any chlorine leaks as well as thoroughly familiar with the properties of chlorine, the proper ways to handle it and protective as well as first aid measures associated with emergencies.  Individuals working with chlorine should be trained in the use of self-contained breathing apparatus.  While several types of gas masks should be available at the plant, it should be noted that the usual industrial canister type gas mask is not effective when chlorine in the air exceeds 1%.  Hence, they are not recommended in dealing with chlorine gas.  The plant should be supplied with a chlorine gas mask of a design approved by the Bureau of Mines.  When the oxygen content is limited (below 16%) a self-contained "supplied air" or oxygen supply type breathing apparatus is recommended.  The masks should be located in readily accessible points, away from any areas that are likely to be contaminated by chlorine gas.  Masks should be checked regularly.  However, it is to be emphasized that whenever a room must be entered that may contain chlorine gas, great care must be taken.  When approaching this situation the door must be carefully opened and left ajar to check for the smell of chlorine gas.  An individual should never enter a room containing harmful levels of chlorine without: 

        1. A self contained air supply 
        2. Protection to the eyes 
        3. Protective clothing 
        4. Help standing by, and 
        5. Notifying proper authorities 

    First Aid Measures for Exposure to Chlorine

        1. Be sure you know the location of breathing apparatus, first aid kits, and other safety equipment at all times. 
        2. Remove clothing contaminated with liquid chlorine at once.  Carry patient away from gas area -- if possible to a room with a temperature of 70°F.  Keep patient warm, with blankets if necessary.  Keep him quiet. 
        3. Place patient on his back with his head higher than the rest of his body. 
        4. Call a doctor and fire department immediatelyImmediately begin appropriate treatment
        5. Eyes.  If even small quantities of chlorine have entered the eyes, hold the eyelids apart and flush copiously with lukewarm running water.  Continue flushing for about fifteen minutes.  Do not attempt any medication except under specific instructions from a physician. 


    Chlorine Leaks

    In general, daily inspection of all chlorine cylinders will avoid major problems.  Small leaks, detected in early stages can usually be corrected.  Before any new system is put into service it should be cleaned, dried and tested for leaks by pressurizing with 150 psi dry air and testing with soapy water applications.  Prompt measures are necessary since chlorine leaks become progressively greater.  Small leaks around valve stems can usually be corrected by tightening the packing nut or closing the valve.  A leak can also be reduced by removing the chlorine as rapidly as possible.  If it cannot be added to the process there are several chemicals which can be used to absorb the chlorine gas.  For example, chlorine can be absorbed by using 1  1/4 pounds of caustic soda or hydrated line, or 3 pounds of soda ash per pound of chlorine.  Therefore, to absorb 100 pounds of chlorine use 125 pounds of solid or flake caustic soda dissolved in 40 gallons of water.  A 55 gallon drum may be used.  The chlorine line should be well below the surface and mixing improves removal of chlorine. 

    If the leaking container can be moved, it should be transported to an outdoors area where minimal harm will occur.  Keep the leaking part the most elevated so that gaseous chlorine will leak rather than liquid chlorine. 

    If the leak is large, all persons in the adjacent area must be warned and evacuated.  Only authorized persons equipped with the proper breathing apparatus, and protective measures to the eyes and body should investigate.  As noted, help should be standing by and all other persons should be cleared from the affected area.  The following generalizations can serve as guidelines. 

        1. The leak may be located by using a rag or brush on a stick soaked in a strong ammonia solution (about 5% ammonia).  When the rag is held close to the leak a white gas will be formed. 
        2. Never apply water to a leak, nor consider submerging a chlorine cylinder (for example, in a pond or tank), since it will probably float and water is not an efficient absorbent for chlorine. 
        3. Remember to keep windward of the leak. 
        4. Remember that chlorine gas is heavier than air and will accumulate in the lower parts of a room or building. 
        5. Remember that the fusible plug melts at 158°F. 
        6. Keep chlorine cylinder or container emergency repair kits available.  Be familiar with their use and location. 
        7. Leaks around valve stems and discharge outlets can usually be stopped. 
        8. Leaks at fusible plugs and cylinder valves requires special handling and emergency equipment.  The chlorine supplier must be notified immediately. 
        9. Pin hole leaks in cylinder walls or ton tanks can usually be stopped by mechanical pressure applications (clamps, turnbuckles, etc.).  This only temporary and may require your ingenuity. 
        10. Leaking containers cannot be shipped.