Information Sheet 1.3
Disinfection with chlorine
Disinfection should not be compromised to control disinfection by-products
General description
Chlorine was introduced widely as a water disinfectant early in the 20th century and still remains
the most common drinking water disinfectant used around the world. It is a strong disinfectant
with excellent bactericidal and virucidal properties and is effective at short contact times.
Chlorine is less effective against protozoa and while it can inactivate Giardia at moderate doses
and contact times it has little effect against Cryptosporidium at doses that can be practically used
in drinking water. It is also a strong oxidising agent that can bleach colour compounds in water,
oxidise soluble iron, manganese and sulfides, and remove the tastes and odours produced by
some algae.
In water, chlorine reacts to form hypochlorous acid (HOCl) (see below), a very effective
disinfectant which can dissociate to form the hypochlorite ion (OCl–) in a pH dependent reaction
with no dissociation below pH 6.5 and complete dissociation above pH 8.5. From a disinfection
standpoint lower pHs are preferred as the hypochlorite ion is estimated to be 150 to 300 times
less effective as a disinfectant than hypochlorous acid.
Cl2 (gas) + H2O ⇄ H+ + Cl- + HOCl
NaOCl + H2O ⇄ NaOH + HOCl
Ca(OCl)2 + 2H2O ⇄ Ca(OH)2 + 2HOCl
HOCl then dissociates to the hypochlorite ion (OCl-) in a pH dependent reaction:
HOCl ⇄ H+ + OCl-
pKa = 7.5
Performance validation
Although chlorine has a long history of use there are some gaps in data on effectiveness
particularly in relation to the influence of turbidity on performance. Most studies on the
effectiveness of chlorine have been conducted in buffered water containing virtually no turbidity,
or in filtered water. It has been suggested that chlorine is only effective at turbidities below 1
NTU, but chlorine is often used to disinfect water containing higher turbidities and has been
shown to be effective in inactivating indicator bacteria such as E.coli at higher turbidities than 1
NTU. However, it is generally accepted that 99% inactivation of enteric bacteria can achieved at
Ct1 values <1 mg/L.min in drinking water at temperatures between 10 and 15OC, and pH 7.0-7.5.
Turbidity may have a greater impact on the effectiveness of chlorine against viruses and Giardia.
In 1991 the USEPA published Cts for inactivation of Giardia and viruses which are still being
applied. Inactivation of 99% Giardia cysts can be achieved at a Ct of 60 mg/L.min at 15OC and
1
Ct refers to the product of concentration measured as mg/L and time measured in minutes.
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pH 7.5 and 99% inactivation of viruses can be achieved at a Ct of 2 mg/L.min at 15OC and pH 69.
The inactivation rate for viruses is based on experiments with disaggregated Hepatitis A virus.
Later studies have suggested that the US EPA figures may underestimate the inactivation rates
for other viruses (Black et al 2009, Kahler et al 2010). Black et al (2009) showed that in buffered
water 99% inactivation of Coxsackie B5 virus could be achieved at a Ct of 5.4 mg/L.min at 5 OC
and pH 7.5 (the US EPA Ct value at the same pH and temperature is 4 mg/L.min).
Chlorine is practically ineffective against Cryptosporidium at doses that can be applied in
drinking water (LeChevallier and Au 2004).
Table IS1.3.1 provides a summary of Ct value ranges for the 99% inactivation of various
microorganisms by chlorine. The Ct values should be considered as indicative only, and suitable
safety factors should be applied to ensure adequate disinfection.
Table IS1.3.1 Examples of Ct values for 99% (2 log) inactivation of various
microorganisms by chlorine 1,2,3
Microorganism
Free chlorine Ct value
(mg/L.min)
Reference
Escherichia coli
<1 (10-15oC)
LeChevallier and
Au 2004
Enteric viruses
5.4 (15oC)
Black et al 2009
Giardia
60 (15oC)
USEPA 1991
Cryptosporidium
7200 (25 C)
Korich et al 1990
Notes: (1) Temperature is 5oC unless stated.
(2) pH is within range of 6-9 unless stated.
(3) The values in the table are based on published values and should be viewed as the
minimum values necessary to achieve effective disinfection
o
The important conclusion to draw from Table IS1.3.1 is that, at the typical chlorine Cts used in
Australian drinking water supplies, which are usually at least 15 mg/L.min, chlorine will
inactivate bacteria and viruses, but will not inactivate Giardia or Cryptosporidium.
Given that the chlorination process will be a critical control point (CCP), other important issues
that will need to be considered to ensure the effectiveness of the process are:

establishing target criteria (section 3.4.2) and critical limits for the chlorination process

preparing and implementing operational procedures (section 3.4.1) and operational
monitoring (section 3.4.2) for the process

preparing corrective action procedures (section 3.4.3) in the event that there are excursions in
the operational parameters
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
undertaking employee training (section 3.7.2) to ensure that the chlorination process operates
to the established target criteria and critical limits
Water quality considerations
Evidence from various studies indicates that pH influences disinfection, with lower pHs being
optimal as the hypochlorous acid is far more effective than the hypochlorite ion. Temperature
also influences efficacy with disinfection times reduced at higher temperatures. Although it has
been suggested that particles may act as a protective shield for micro-organisms, and that
turbidity should be kept below 1 NTU for effective disinfection, the relationship between
turbidity and the effectiveness of chlorine has not been established.
Whilst many water suppliers often achieve satisfactory inactivation of bacteria at turbidities that
are greater than 1 NTU, generally, the lower the turbidity of the water at the time of chlorination
the more effective chlorination will be. Where chlorination is routinely occurring at turbidities
that are greater than 1 NTU, the effectiveness of chlorination process should be validated.
Relationships with other parameters such as colour have also not been determined.
Practical considerations
Chlorine can be applied as a gas, liquid (sodium hypochlorite) or solid (calcium hypochlorite).
Due to the strict safety requirements associated with the use of gaseous chlorine, liquid chlorine,
which is easier to use, is becoming increasingly popular. The disadvantages of sodium
hypochlorite are that concentrations degrade over time, chlorate can be formed during storage
and it is a corrosive solution. Calcium hypochlorite needs to be stored in a cool dry environment
and kept away from moisture and heat. Chlorine residuals and chlorination by-products can
produce distinctive tastes and odours (see Taste and Odour Fact Sheet).
Advantages of chlorination include its common and long-standing use and the availability of
reliable dosing and monitoring equipment. Reliable and robust field kits for measuring chlorine
residuals within the distribution system are also available. In addition to being a proven
disinfectant against most enteric pathogens (excluding Cryptosporidium) chlorine is also a strong
oxidising agent that can bleach colour compounds in water, oxidise soluble iron, manganese and
sulfides, and remove the tastes and odours produced by some algae.
Persistence
A major advantage of chlorination is that it produces a residual disinfectant that is moderately
persistent with longevity limited by chlorine being a highly reactive oxidant. Chlorine can be
used to provide a residual disinfectant in distribution systems with persistence dependent on the
chlorine demand imparted by natural organic materials in drinking water, and other factors such
as temperature and sunlight (if the system incorporates open storages).
Chlorine will not persist in long distribution systems, particularly those incorporating long
above-ground pipelines, because of the elevated water temperature that occurs in these pipelines.
Such distribution systems lend themselves to chloramination (see Disinfection with chloramine
Information Sheet)
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By-products
Chlorine can form a wide range of halogenated disinfection by-products in reactions with natural
organic matter in source water, with over 600 identified to date (Hrudey 2009, Itoh et al 2011).
These include trihalomethanes, haloacetic acids, haloacetonitriles and trichloroacetaldehyde
(chloral hydrate). The chemistry of the reactions is complex and not fully understood. Factors
that influence the formation of disinfection by-products include the chlorine dose, the
concentrations and types of natural organic matter that are present, temperature, pH and
detention time. Chlorate can be produced in association with degradation of concentrated sodium
hypochlorite solutions.
Guideline values have been developed for a number of disinfection by-products (see Chapter 10
and associated Fact Sheets). While every effort should be taken to minimise the formation and
concentration of chemical disinfection byproducts this should never be done in a manner that
compromises disinfection as poor microbiological quality represents a greater and more
immediate risk to human health than short term exposure to disinfection by-products (Hrudey
2009).
Application
Chlorine is the most versatile of disinfectants used to treat drinking water supplies. It can be
applied:
 as a primary disinfectant at the point of entry of source water into drinking water systems
 as a secondary disinfectant within distribution systems to boost concentrations of
chlorine residuals in the system as a barrier against regrowth of opportunistic free-living
pathogens and ingress of faecal contamination
 to disinfect new and repaired water mains and
 to disinfect storage tanks as part of cleaning and maintenance or following the detection
of contamination.
Chlorine can be applied alone or in combination with other disinfectants. For example, it can be
used in combination with UV light disinfection as joint primary disinfectants where UV light is
used primarily to inactivate Cryptosporidium and Giardia, and chlorine is used to inactivate
viruses and bacteria. In this combination chlorination also provides a residual disinfectant to
provide protection of distribution systems against regrowth and recontamination. Chlorine can
also be used for this purpose in conjunction with ozone and chlorine dioxide.
Chlorine can be used in combination with chloramines either as a primary disinfectant before
production of persistent chloramine residuals or as a secondary disinfectant in subsections of
water distribution systems
Operational monitoring
There is a variety of equipment available to continuously monitor concentrations of chlorine in
drinking water. It is common practice to undertake continuous monitoring of free and/or total
chlorine residual, with remote monitoring and alarm systems being applied at drinking water
treatment plants.
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Operational monitoring is more heavily focussed on monitoring chlorine concentrations/residuals
to indicate effectiveness rather than Cts. The latter can be more challenging, as contact times
need to take into account variable flows. A solution has been to calculate minimum Cts, based on
maximum flow rates. A more accurate alternative is to include side-stream monitoring using
contact channels/basins and fixed speed pumps.
Due to the influence of pH on the effectiveness of chlorine disinfection, where possible, the pH
of the water at the point of disinfection should also be monitored on a continuous basis.
References
Black S, Thurston JA and Gerba CP (2009) Determination of Ct values for chlorine of
enteroviruses. J.Environ.Science. Health. A 44: 336-339
Hrudey S. (2009) Chlorination disinfection by-products, public health risk tradeoffs and me.
Water Research 43: 2057-2092.
Itoh S, Gordon BA, Callan P and Bartram J (2011) Regulations and perspectives on disinfection
by-products: Importance of estimating overall toxicity. J. Water Supply: Research and
Technology-Aqua 60, 261-274.
Kahler AM, Cromeans TL, Roberts JM and Hill VR (2010) Effects of source water quality on
chlorine inactivation of Adenovirus, Coxsackievirus, Echovirus and Murine Norovirus. Applied
and Environ. Micro 76: 5159-5164
Korich DG, Mead JR, Madore MS, Sinclair NA and Sterling CR (1990) Effects of ozone,
chlorine dioxide, chlorine, and monochloramine on Cryptosporidium parvum oocyst viability.
Applied and Environ Micro. 56, 1423-1428
LeChevallier MW and Au K-K (2004) Water treatment and pathogen control. World Health
Organization, Geneva.
USEPA (1991) Guidance manual for compliance with the filtration and disinfection
requirements for public water systems using surface water sources. USEPA Washington DC
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