1.3: Disinfection with chlorine
The possible presence of microbial contaminants in drinking water poses a greater risk to public health than the possible presence of disinfection by-products (DBP). Therefore, disinfection should not be compromised in order to control DBP.
Where the concentrations of chlorinated DBP consistently exceed associated health-based guideline values, the methods of water treatment, disinfection and distribution should be reviewed.
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, odours and some toxins produced by algae.
In water, chlorine reacts to form hypochlorous acid (HOCl) (see below), a very effective disinfectant which can dissociate to form the hypochlorite ion () 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.
HOCl then dissociates to the hypochlorite ion () in a pH dependent reaction:
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 into the drinking water distribution system;
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 also 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.
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 often used in preference to gaseous chlorine. 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 and some toxins produced by algae.
Performance validation
Table IS1.3.1 presents published C.t values for chlorine that have been demonstrated as achieving a two and four log reduction in the target microorganism. These values are supplied for illustrative purposes only. For chlorine C.t values that achieve a greater log reduction, the cited references should be consulted. The C.t value that is applied at a particular water treatment plant should be based on the microbial risk assessment for that particular water supply system.
Table IS1.3.1 Published C.t values for 99% (2 log) and 99.99% (4 log) inactivation of various microorganisms by chlorine 1,2
Escherichia coli
<1 (10-15°C)
<1 (10-15°C)
LeChevallier and Au 2004
CB5 virus
4 (10°C)
6 (10°C)
Keegan et al. 2012
Naegleria fowleri
30 (30°C)
NA
Robinson and Christy 1984
Giardia
60 (15°C)
120 (15°C)
USEPA 1991
Cryptosporidium
7200 (25°C)
NA
Korich et al. 1990
Notes:
pH is within the range of 6-9 for E.coli and pH 7 for the other organisms.
The values in the table are based on published values and should be viewed as the minimum values necessary to achieve effective disinfection.
The important conclusion to draw from Table IS1.3.1 is that, at the typical chlorine C.t values used in Australian drinking water supplies, which are usually based on the World Health Organization’s recommendation that effective disinfection can generally be achieved by applying a 30 minute contact time to a free chlorine concentration of 0.5 mg/L (WHO 2011) (i.e. equivalent to a chlorine C.t value of 15 mg/L.min), chlorine will inactivate bacteria and viruses, but will not inactivate Giardia or Cryptosporidium.
Chlorine is also effective against Naegleria fowleri, but the elevated C.t requirement means dosing must be adjusted to provide a sufficient residual throughout the distribution system. Naegleria can encyst and when in this state are more resistant to disinfection. Unless the chlorine residual is continuous, the cysts are also able to survive in tank sediments and pipe biofilm. A free chlorine residual at 0.5 mg/L or higher will control N. fowleri, provided the disinfectant residual persists throughout the water supply system at all times.
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 for all pathogens. Increasing the turbidity from <1 to 20 NTU increased the C.t for 4 log inactivation of CB5 from 6 to 25 mg.min/L at pH 7 (Keegan et al. 2012).
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 the chlorination process should be validated.
Relationships with other parameters, such as natural organic matter or colour, have not been well studied; however, it is known that these parameters adversely impact on the chlorine dose required to achieve a free chlorine residual and effective disinfection.
Where contact tanks or clear water storages are used to achieve the desired contact time, the T10 contact time needs to be taken into consideration (Church and Colton 2013; USEPA 2003). The T10 contact time is the minimum detention time experienced by 90 percent of the water passing through the tank, and is based on a baffling factor or tracer studies (Church and Colton 2013).
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 matter (and inorganic compounds, such as iron) in drinking water, and other factors such as temperature and sunlight (if the system incorporates open storages). The persistence of chlorine also makes it suitable for the control of themophilic Naegleria, including N. fowleri, particularly where a sufficient residual can be maintained throughout the distribution system.
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).
By-products
Chlorine, in reaction with natural organic matter present in source water, can form a wide range of halogenated disinfection by-products, 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 by-products 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).
Operational considerations
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 and critical limits for the chlorination process (section 3.4.2);
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; and
undertaking employee training (section 3.7.2) to ensure that the chlorination process operates to the established target criteria and critical limits.
Operational monitoring
The table below summarises the operational monitoring that should be undertaken for chlorine.
pH
Online monitoring
Turbidity
Online monitoring
Contact time
Calculated
Chlorine residual (total)
Online monitoring
References
Church J. and Colton J. (2013). Optimise chlorine contact Tank performance, WaterWorks, May 2013, 10-13, Water Industry Operators Association of Australia (WIOA), Shepparton, Australia.
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.
Keegan A, Wati S, Robinson B. (2012). Chlor(am)ine disinfection of human pathogenic viruses in recycled waters. Smart Water Fund Project SWF62M-2114, Smart Water Fund, Melbourne, Australia.
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, Switzerland.
Robinson BS and Christy PE. (1984). Disinfection of water for control of amoebae. Water 11: 21-24.
United States Environmental Protection Agency (US EPA) (1991). Guidance manual for compliance with the filtration and disinfection requirements for public water systems using surface water sources. US EPA Office of Water: Washington D.C.
United States Environmental Protection Agency (US EPA) (2003). Guidance Manual for Disinfection Profiling and Benchmarking US EPA Office of Water: Washington D.C.
World Health Organization (WHO) (2011). Guidelines for Drinking-water Quality, fourth edition, Geneva, Switzerland.
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