Chlorine Disinfection in Water Treatment

by Becky Cheadle and Mark Smith

Table of contents

Introduction to Chlorine
Advantages of Chlorine
Disadvantages of Chlorine
Regulations of Chlorine
Chemical Background of Chlorine
Chlorine Application in Water Treatment
Dechlorination in Treated Water

Introduction to Chlorine

Disinfection in water treatment is required by the Surface Water Treatment Rule which was created in June, 1990. This rule mandates two components for effective disinfection.

  • Removal by filtration
  • Inactivation of organisms (bacteria, viruses) by disinfectants (chlorine/ ozone/chlorine dioxide)

This page describes the process of inactivation by chlorine. The purpose of inactivation is to kill pathogens and microorganisms in the water. Chlorine is one of the more common methods for achieving this. Here is a picture of the chlorine unit at the Christiansburg-Blacksburg-VPI Water Authority.

Advantages of Chlorine

  • Chlorine provides a strong residual in the distribution system.
  • Chlorine can be easily converted to chloramines which also provide a strong residual and do not produce by-products.
  • Chlorine is easy to apply.
  • Chlorine is a relatively inexpensive disinfecting agent.
  • Chlorine is effective at low concentrations.

Disadvantages of Chlorine

  • When chlorine reacts with organic material its' concentration is reduced and trihalomethanes(THM's) and disinfection-by-products(DBP's) are formed. These compounds are a concern because they are carcinogenic.
  • Chlorine provides poor Cryptosporidium and Giardia control. Cryptosporidium and Giardia are protozoa that cause Cryptosporidiosis and Giardiasis, respectively. Both of these diseases cause gastrointestinal problems, which in severe instances can cause death. Filtration is one method that can be used in addition to chlorination to destroy protozoa in the water (Hoehn, 1997).

Dr. Marc Edwards, a professor in the Environmental Engineering Department at Virginia Tech, speaks about the advantages and disadvantages of chlorine application.

Regulations of Chlorine Use

Maximum contaminant levels (MCL's) limit the maximum concentration of many contaminants that are typically found in water. Primary MCL's (PMCL's) attempt to control substances that can be harmful when ingested, and are enforceable by the government. Secondary MCL's (SMCL's) set desired levels of pollutants to maintain aesthetic standards. SMCL's are not enforceable, but treatment facilities are encouraged to maintain these standards. Two SMCL's apply to chlorine: 4mg/l for Cl2, and 250mg/l for Cl-.

Drinking water regulations are constantly under review, including ammendments to the Safe Water Drinking Act that were adopted in 1996, and new MCL's for chlorine currently being considered. For more information on water quality regulations, the Surface Water treatment Rule, and the Safe Water Drinking Act, visit the EPA homepage.

Chemical Background of Chlorine

When chlorine is used in water treatment systems it is added in either a gaseous or liquid form of chlorine(Cl2). When it combines with water chlorine produces hypochlorous acid (HOCl)and hydrochloric acid (HCl). The chemical equation for this reaction is:

Cl2 + H2O -> HOCl + HCl

This animation demonstrates this reaction of chlorine with water; moving from chlorine entering the water, to the formation of HOCl and HCl, to the partial dissociation of hypochlorous acid into H+ and OCL-.

As pH varies in a system so does the concentration of hypochlorous acid versus the concentration of hypochlorite ion. Both HOCl and OCl are good disinfecting agents, but HOCl is more effective (Reynolds, 1996).

The hypochlorous acid will also react with ammonia in the water and form chloramines, which as mentioned earlier are a benefit due to their strong residuals and because when chlorine is converted to chloramines the formation of disinfection-by-products ceases. However, the chloramines are not very effective against viruses and are a weaker disinfectant.

Chlorine Application in Water Treatment

The amount of time that chlorine is present during treatment is called the contact time and is related to the 'Ct' value. Ct values are calculated to determine the amount of time that a disinfectant must be present in the system to achieve a specific kill of microorganisms, for a given disinfectant concentration. A large Ct value means that disinfection alone will not be sufficient treatment and additional methods will be necessary to eliminate the microorganisms. The contact time is directly related to the chemicals' efficiency of eliminating bacteria and viruses from the water. The chart below shows the amount of time for HOCl, OCl-, and NH2Cl need to be present in the treatment system in order to achieve a 99% kill of E. coli (Reynolds, 1996).

Adapted from Reynolds & Richards

However, increased contact time for chlorine increases the chlorine demand. Depending on the amount of organic material and free chlorine residual present in the water, the probability of producing disinfection-by-products, such as trihalomethanes, may increase.

Free chlorine residual refers to the combined concentration of chlorine gas, hypochlorous acid, and hypochlorite. The free chlorine residuals are what remains in the water after the demand is met, and these residuals react quickly and have a strong disinfecting capacity.

This graph shows the relationship between chlorine dosage, chlorine residual, and the ratio of Cl2 to NH3-N in the water. The portion of the residual plot to the left of the breakpoint is predominately chloramine residual, while that portion to the right of the breakpoint is predominately free chlorine residual. The breakpoint is the point at which most chloramines have been oxidized to nitrogen gas. This usually occurs when the ratio of chlorine to ammonia nitrogen is approximately 1.5 to 2 (Reynolds, 1996).

Dechlorination of Treated Water

High concentrations of chlorine in treated water can have significant effects on receiving waters, and therefore removal of chlorine residuals from effluent is becoming a common procedure. A common technique for achieving the removal of residuals is the addition of sulfur dioxide gas(SO2) to the effluent. Sulfur dioxide dissociates to sulfite ion(SO3-2) by the following reaction:

H2SO3 <-> H+ + HSO3-

HSO3- <-> H+ + SO3-2

The sulfite ion then combines with the forms of chlorine by the following reactions:

SO3-2 + HOCl -> SO4-2 + Cl- + H+

SO3-2 + NH2Cl + H2O -> Cl- + SO4-2 + NH4+

(Reynolds, 1996)


Hoehn, Robert C. CE 4104:Water and Wastewater Treatment Design Notes. Virginia Tech, Spring 1997. pp.98-111.

Reynolds, Tom D., Richards, Paul A. Unit Operations and Processes in Environmental Engineering. PWS Publishing Company; 1996. pp.740-748.

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Send Comments or suggestions to:
Student Authors: Becky Cheadle, mailto:bcheadle@vt.eduand Mark Smith,
Faculty Advisor: Daniel Gallagher,
Copyright © 1997 Daniel Gallagher
Last modified: 2-25-1998