WHO SEMINAR PACK FOR DRINKING-WATER QUALITY

Disinfectants and Disinfection By-Products 

 Session Objectives 

• To describe the importance of disinfection in providing safe drinking water. 

• To describe the key disinfectants evaluated in the Guidelines and describe their principal 

characteristics and effectiveness. 

• To describe the key by-products formed by the principal disinfectants and describe the likely 

health risk from their presence in water. 

• To describe the balance between microbiological and chemical health risks and emphasise 

the need to prioritise microbiological quality.
1

Disinfectants and Disinfection By-Products 

Introduction

Disinfection of drinking-water is essential if we are to protect the public from outbreaks of 

waterborne infectious and parasitic diseases. The main disinfectants evaluated in the Guidelines 

are free chlorine, chloramines, chlorine dioxide and ozone. 

As much as the perfect indicator organism does not exist, each of the commonly used 

disinfectants has its advantages and disadvantages in terms of cost, efficacy, stability, ease of 

application and formation of by-products. 

Table 1 summarizes the Ct values for the four main disinfectant, 

where C = disinfectant concentration in mg/litre, and 

 t = the contact time in minutes required to inactivate a specified percentage of 

microorganisms. 

Table 1. Summary of C.t values (mg/L. min)for 99% inactivation at 5C (Clark et al, 

1993)

Organism Disinfectant 

 Free chlorine, 

pH 6 to 7 

Pre-formed 

chloramine, 

pH 8 to 9 

Chlorine 

dioxide, 

pH 6 to 7 

Ozone 

pH 6 to 7 

E. coli 0.034-0.05 95-180 0.4-0.75 0.02 

Polio virus 1 1.1-2.5 768-3740 0.2-6.7 0.1-0.2 

Rotavirus 0.01-0.05 3806-6476 0.2-2.1 0.006-0.06 

Bacteriophage f2 0.08-0.18 - - - 

G. lamblia cysts 47->150 - - 0.5-0.6 

G. muris cysts 30-630 - 7.2-18.5 1.8-2.0a

C. parvum 7200b 7200c 78b 5-10c

a Values for 99.9% inactivation at pH 6-9. 

b 99% inactivation at pH 7 and 25°C.

c 90% inactivation at pH 7 and 25°C.

From the Ct values, ozone is the most efficient and chloramine the least efficient, particularly 

for viral agents. Free chlorine is more effective than chlorine dioxide with regard to E. coli and 

rotavirus. Chlorine dioxide is more effective than free chlorine with regard to the protozoa 

Giardia lamblia and muris. Ozone is the most efficient disinfectant for cryptosporidium 

parvum. As the temperature increases, the Ct values decrease for all disinfectants. The effect of 

pH varies with the nature of the disinfectant and is most pronounced for chlorine. 
WHO SEMINAR PACK FOR DRINKING-WATER QUALITY 

2

Chlorine and its by-products

Chlorine is the most widely used drinking-water disinfectant. When added to water the 

following reaction occurs within a second or less: 

 Cl2 + H2O = HOCl + H+

 + ClThe magnitude of the equilibrium hydrolysis constant is such that hydrolysis to hypochlorous 

acid, HOCl, is virtually complete in fresh water at pH > 4 and at chlorine doses up to 100 

mg/litre (Morris, 1982). 

Hypochlorous acid is a weak acid that dissociates partially in water as follows: 

 HOCl = H+

 + OClThe value of the acid ionization constant is about 3 x 10-8. As shown in Figure 1, at 20°C and 

pH 7.5, there is an equal distribution of HOCl and OCl-

. At pH 8, about 30% of the free 

chlorine is present as HOCl, and at pH 6.5, 90% is present as HOCl (Morris, 1982). The term 

free chlorine refers to the sum of hypochlorous acid and hypochlorite ion. Since HOCl is a 

considerably more efficient disinfectant than OCl-

, and free chlorine, even as hypochlorite, is 

more effective than combined chlorine (e.g. chloramines), the Guidelines recommend that 

disinfection be carried out at pH less than 8 and at a free chlorine concentration ≥ 0.5 mg/litre. 

Of all the disinfectants, the chemistry and toxicity of the reaction by-products of chlorine have 

been the most extensively studied. 

Figure 1: Distribution of hypochlorous acid and hypochlorite ion in water at different pH 

values and temperatures (Morris,1951) 
2: THE GUIDELINES (DISINFECTANTS AND DISINFECTION BY-PRODUCTS)

3

Since Rook's discovery of the formation of haloforms during chlorination of Rotterdam water 

supply (Rook, 1974), numerous halogenated compounds have been identified in chlorinated 

drinking-water and their toxicity assessed. Precursors of these halogenated compounds include 

natural humic and fulvic compounds and algal material. The most commonly found chlorine 

disinfection by-products are the trihalomethanes (THM), halogenated acetic acids, halogenated 

acetonitriles, chloral hydrate and the chlorinated phenols. Others include chlorinated furanone 

MX, halopicrins, cyanogen halides, haloketones and haloaldehydes. The halogenated 

disinfection by-products identified account for only about half of the total formed. 

Based on animal toxicological studies, Guideline Values (GVs) have been recommended for a 

number of these compounds. Undoubtedly, the third edition of the Guidelines, planned for the 

year 2002, will include additional chlorination by-products. 

The following chemicals resulting from chlorination of water supplies have been evaluated in 

the Guidelines:

• free chlorine (HOCl + OCl-

)

• trihalomethanes

• chlorinated acetic acids 

• halogenated acetonitriles 

• chloral hydrate (trichloroacetaldehyde) 

• chlorophenols

• MX (3-chloro-4-dichloromethyl-5-hydroxy-2(5H)-furanone) 

For countries wishing to control DBP, it may not be necessary to set standards for all of the 

DBP for which guideline values have been proposed. The trihalomethanes, of which 

chloroform is the major component, are likely to be the main DBP, together with the chlorinated 

acetic acids in some instances. In many cases, control of chloroform levels and, where 

appropriate, trichloroacetic acid will also provide an adequate measure of control over other 

chlorination by-products. 

(a) Chlorine 

Free chlorine in drinking-water is not particularly toxic to humans. The major source of 

exposure to chlorine is drinking-water. Therefore, 100% of the TDI was allocated to drinkingwater giving a health-based GV of 5 mg/litre for the sum of hypochlorous acid and hypochlorite 

ion. Based on the taste and odour threshold of free chlorine, it is doubtful however that 

consumers would tolerate such a high level of chlorine. Most individuals are able to taste 

chlorine at concentrations below 5mg/litre, and some at levels as low as 0.3 mg/litre. The 

health-based GV for chlorine should not be interpreted as a desirable level of chlorination.

(b) Trihalomethanes

The predominant chlorine disinfection by-products are the THMs. Nevertheless, they account 

for only about 10% of the total organic halogen compounds formed by water chlorination. 

THMs are formed by the aqueous chlorination of humic substances, of soluble compounds 

secreted from algae and of naturally occurring nitrogenous compounds (Morris, 1982). THMs 

consist primarily of chloroform, bromodichloromethane, dibromochloromethane and 

bromoform.
WHO SEMINAR PACK FOR DRINKING-WATER QUALITY 

4

When bromide is present in drinking-water, it is oxidized to hypobromous acid by chlorine: 

 HOCl + Br- = HOBr + ClHOBr reacts with natural organic compounds to form brominated halomethanes. Similarly, the 

presence of iodide may lead to the formation of mixed chlorobromoiodo-methanes. 

Some generalized statements can be made with regard to THMs in chlorinated drinking-water 

(IARC, 1991; Morris, 1982; Canada, 1993): 

• Concentration of THMs in drinking-water varies widely and ranges from not detectable to 1 

mg/litre or more; 

• THM levels are higher in chlorinated surface water than in chlorinated groundwater; 

• Concentrations of THMs tend to increase with increasing temperature, pH and chlorine 

dosage; 

• Concentrations of THMs increase upon storage even after exhaustion of residual chlorine or 

after dechlorination. This indicates the formation of intermediates products leading to the 

slow production of THMs; 

• Chloroform is usually the most abundant THM often accounting for greater than 90% of the 

total THM concentration; 

• If there is a significant amount of bromide in the raw water, the brominated THMs, 

including bromoform, may be dominant; 

• Formation of THMs can be minimized by avoiding pre-chlorination and by effective 

coagulation, sedimentation and filtration to remove organic precursors prior to final 

disinfection;

• Removal of THMs after their formation is difficult and involves resource-intensive 

processes such as activated carbon adsorption or air stripping. 

Because trihalomethanes usually occur together, it has been the practice to consider total 

trihalomethanes as a group, and a number of countries have set guidelines or standards on this 

basis, ranging from 0.025 to 0.25 mg/litre. 

In the 1993 WHO Guidelines, individual GV have been recommended for the four 

trihalomethanes. With an underlying assumption that the THMs may exert potential toxic 

effects through similar biological mechanisms, authorities may want to establish standards for 

total THMs that would account for possible additive effects and not simply add up the guideline 

values for the individual compounds in order to arrive at a standard. Instead, the following 

approach is recommended: 
2: THE GUIDELINES (DISINFECTANTS AND DISINFECTION BY-PRODUCTS)

5

where C = concentration, and 

 GV = guideline value 

Epidemiological studies of carcinogenicity of chlorine and DBP

In 1991, WHO International Agency for Research on Cancer (IARC) published an evaluation of 

the carcinogenic risks to humans of chlorinated drinking-water based on a number of animal 

toxicological and epidemiological studies. IARC concluded that because of one or more 

methodological weaknesses, the epidemiological studies reviewed cannot constitute the basis of 

valid risk assessment. 

The epidemiological investigation of the relation between exposure to chlorinated drinkingwater and cancer occurrence was considered problematic because any increase in relative risk 

over that in people drinking unchlorinated water is likely to be small and therefore difficult to 

detect in epidemiological studies. In all of the studies evaluated, estimates of exposure were 

imprecise and surrogates (e.g surface versus groundwater) do not reflect exposure during the 

relevant time periods for the etiology of the cancers in question. Many variables, such as 

smoking habits, dietary practices, use of alcohol, socio-economic status, and ethnicity are 

known to affect cancer incidence and were not taken into account in most of the studies (IARC, 

1991).

In its overall evaluation, IARC concluded that there is inadequate evidence for the 

carcinogenicity of chlorinated drinking-water in humans as well as in experimental animals 

(IARC, 1991). 

Chloramine and its by-products

Chloramine generally produces by-products similar to those observed with chlorine but at much 

lower concentrations. An exception to this is the formation of cyanogen chloride, CNCl (Bull 

and Kopfler, 1991). The use of chloramine as a disinfectant has increased in recent years 

because of limited formation of THMs, however, little is known about the nature of other byproducts.

Monochloramine is about 2000 and 100 000 times less effective than free chlorine for the 

inactivation of E. coli and rotaviruses, respectively. Monochloramine cannot therefore be relied 

upon as primary disinfectant. It is useful for maintaining a residual disinfectant in distribution 

systems. The shift to monochloramine to control THM formation may thus compromise 

disinfection and the Guidelines caution against such procedure. Organic chloramines are even 

less effective disinfectants than monochloramine. 

bromoform

bromoform

DBCM

DBCM

BDCM

BDCM

chloroform

chloroform

C

GV

 + C

GV

 + C

GV

 + 

C

GV

 1 ≤
WHO SEMINAR PACK FOR DRINKING-WATER QUALITY 

6

Chlorine dioxide and its by-products

Because of its explosive hazard, chlorine dioxide is manufactured at the point of use. CLO2 is 

generated through the reaction of sodium chlorite and chlorine. Chlorine dioxide reactions with 

humic substances do not form significant levels of THMs. In addition, it does not react with 

ammonia to form chloramines. The main disinfection by-products of chlorine dioxide are 

chloride, chlorate and chlorite. 

Chlorine dioxide is more effective towards inactivation of Giardia cysts than free chlorine, but 

less effective towards rotavirus and E. coli. Unlike chlorine, the disinfection efficiency of 

chlorine dioxide is independent of pH and the presence of ammonia. 

A provisional GV was recommended for chlorite while no adequate data were available to 

recommend a GV for chlorate. No GV has been recommended for chlorine dioxide per se

because of its rapid breakdown in aqueous solutions and the chlorite GV is adequately 

protective for potential toxicity from chlorine dioxide. Furthermore, the taste and odour 

threshold for chlorine dioxide in water is 0.4 mg/litre which constitutes a limiting factor and a 

signal for its presence at higher concentrations in drinking-water. 

Other reaction by-products of chlorine dioxide with organics in drinking-water have not been 

well characterized but include aldehydes, carboxylic acids, haloacids, chlorophenols, quinones 

and benzoquinone (Bull and Kopfler, 1991). In a recent article, more than 40 organic 

disinfection by-products were identified in a pilot plant in Indiana which uses chlorine dioxide 

as a primary disinfectant. The toxicity of these by-products is largely unknown (Richardson et 

al. 1994). 

Ozone and its by-products 

Ozone decomposes rapidly following application, and for this reason no GV has been proposed 

for ozone. 

By products of ozonation that have been identified include formaldehyde and other aldehydes, 

carboxylic acids, hydrogen peroxide, bromate, bromomethanes, brominated acetic acids, 

brominated acetonitriles and ketones. Guideline values have been recommended for bromate 

and formaldehyde. 

Ozone is the most efficient disinfectant for all types of microorganisms. Disadvantages include 

lack of disinfectant residual, biological regrowth problems in distribution systems, high cost, 

and limited information on the nature and toxicity of its by-products. 
2: THE GUIDELINES (DISINFECTANTS AND DISINFECTION BY-PRODUCTS)

7

Balancing Chemical and Microbial Risks

Quantitative assessments of risks associated with the microbial contamination of drinking-water 

are scarce. Although there are gaps in our knowledge, we cannot afford to postpone action until 

rigorous quantitative assessment of chemical versus microbial risks are available and every 

answer is known. 

A semi-quantitative presentation of risks associated with disinfection was first attempted by 

Morris (1978) and is given in Figure 2. The following is more or less a quote of his work: The 

risk of waterborne infectious disease is very high when no chlorination is used, and drops 

sharply to a low value when even minimal levels of chlorination are maintained. We know this 

on the basis of a century's experience, Morris stated. As the level of chlorination is increased 

the risk continues to drop slightly, but never quite reaches zero, for no system is perfect. At 

very high levels of chlorine the microbial risk increases as taste and odour may cause the use of 

unsafe supplies. 

Figure 2: Risks and benefits of water chlorination (Morris, 1978) 
WHO SEMINAR PACK FOR DRINKING-WATER QUALITY 

8

The chemical risk does not start at zero for there is some hazard connected with the organic 

matter before chlorination. The chemical risk decreases initially because destruction of 

chemicals by oxidation more than compensate for the formation of new chemicals at low levels 

of chlorination. Because of the formation of by-products, the chemical risk increases with 

increasing level of chlorination. Intuitively, he depicted the chemical risk from chlorination as 

being considerably lower than the microbial risk from a non-disinfected supply. 

In developed countries, since filtration and chlorination became common for community water 

supplies, morbidity and mortality due to waterborne intestinal diseases, particularly typhoid 

fever and cholera, have declined to negligible levels. Almost all of the waterborne outbreaks 

that still occur are associated with the use of untreated water or water from systems in which 

chlorination was inadequate. 

Other health impact studies concern the beneficial effects on health of safe and sufficient water 

supplies and adequate sanitation, three factors that are so intertwined that it is often not feasible 

to draw definite lines of demarcations between them. Together, they constitute the pillars of 

public health protection. Projected reduction in morbidity achievable through the provision of 

safe and sufficient water supplies and adequate sanitation are estimated to be (WHO, 1992): 

 Projected reduction in morbidity (%)

 Cholera, typhoid 80 

 Diarrhoeal diseases 40 

 Dracunculiasis 100 

 Schistosomiasis 60 

When applying these percentage reductions to the global morbidity and mortality rates for these 

diseases, the benefits of saving millions of lives through these interventions are immediately 

apparent.

As shown in Figure 3 overleaf, provision of safe drinking-water can result in a 20% reduction in 

infant mortality (Regli et al., 1993). 

In their pioneering work on comparison of estimated risk from known pathogens in untreated 

surface water and chlorination by products in drinking-water, Regli et al. (1993) concluded that: 

• the risk of death from pathogens is at least 100 to 1000 times greater than the risk of cancer 

from disinfection by-products (DBPs); 

• the risk of illness from pathogens is at least 10 000 to 1 million times greater than the risk of 

cancer from DBPs; 

• morbidity and mortality rates from pathogens compared with those from DBPs, may be 

considerably higher in developing countries where the sanitary and health status is not as 

good; 
2: THE GUIDELINES (DISINFECTANTS AND DISINFECTION BY-PRODUCTS)

9

• in societies where infant mortality and life expectancy is low, many people would not be 

expected to live long enough to incur cancer, which also causes much higher differences in 

risk resulting from exposure to pathogens versus DBPs cited above. 

While this last statement seems cynical, it does reflect the true situation in many developing 

countries.

Figure 3. Infant mortality versus access to safe water (Regli et. al., 1993)

Conclusion

Adequate disinfection of drinking-water is the most important priority to assure a safe water 

supply. Recent cholera outbreaks in Latin America and Rwanda provide dramatic evidence of 

the importance of adequate water disinfection. There is some limited evidence of possible 

health effects from disinfectant by-products, particularly possible cancer risks from chloroform 

and the other trihalomethanes and by-products. This evidence is based on high-dose animal 

studies.

Epidemiological studies conducted to date do not provide any evidence that disinfectants and 

their by-products affect human health at the concentrations found in drinking-water. The 

International Agency for Research on Cancer has concluded that there is inadequate evidence

for the carcinogenicity of chlorinated drinking-water in humans and experimental animals. 

Although stated in qualitative way, the message of the Guidelines is clear: 

The estimated risks to health from disinfectants and their by-products are extremely small 

in comparison to the real risks associated with inadequate disinfection, and it is important 

that disinfection should not be compromised in attempting to control such by-products. 

The destruction of microbial pathogens through the use of disinfectants is essential for the 

protection of public health. 

All disinfectants by necessity are reactive substances and produce by-products. Little is known 

about the nature and toxicity of the by-products of ozone, chlorine dioxide or chloramines. The 
WHO SEMINAR PACK FOR DRINKING-WATER QUALITY 

10

by-products of chlorination are the ones that have been most extensively identified and their 

toxicity assessed. Disinfection with chlorine should not be penalized for this reason. In 

addition, in many countries, if disinfection can be practised at all, it will be through the use of 

chlorine.

There are now more and more indication that the estimated risks to health from disinfectants 

and their by-products are several order of magnitude lower than the real risks associated with 

inadequate disinfection. So while there is great scientific certainty that inadequately disinfected 

water results in devastating microbial disease epidemics, there is relatively great uncertainty 

regarding the possible health risks from DDBPs. In establishing standards for disinfectants by 

products, it is emphasized that "Where local circumstances require that a choice must be 

made between meeting either microbiological guidelines or guidelines for disinfectants or 

disinfectant by-products, the microbiological quality must always take precedence, and 

where necessary, a chemical guideline value can be adopted at a higher level of risk. 

Efficient disinfection must never be compromised." (1993 Guidelines)

References

Bull R.J., Kopfler F.C. (1991). Health Effects of Disinfectants and Disinfection By-products, 

American Water Works Association, Denver. 

Canada Health and Welfare (1993). Water treatment principles and applications. Canadian 

water and wastewater association, Ottawa. 

Clark RM, Hurst CJ, Regli S (1993). Costs and benefits of pathogen control in drinking-water. 

In: Safety of water disinfection: balancing chemical and microbial risks. Craun G.F. ed. ILSI 

Press, Washington, D.C. 

International Agency for Research on Cancer (1991). IARC monographs on the evaluation of 

carcinogenic risks to humans, Volume 52 Chlorinated drinking-water; chlorination by-products; 

some other halogenated compounds; cobalt and cobalt compounds, Lyon. 

Morris J.C. (1951), unpublished research, Harvard University, 1951 

Morris J.C. (1978). Conference summary. In: Water chlorination environmental impact and 

health effects. Volume 2. Jolley RL, Gorchev H., Hamilton D.H. eds., Ann Arbor Science, 

Ann Arbor, Michigan. 

Morris J.C. (1982). Health perspective in the oxidative treatment of water for potable supply. 

Part 2 Health assessment of current oxidant-disinfectants. National Institute for Water Supply. 

Leidschendam, Netherlands. 
2: THE GUIDELINES (DISINFECTANTS AND DISINFECTION BY-PRODUCTS)

11

Regli S., Berger P., Macler B., Haas C. (1993). Proposed decision tree for management of risks 

in drinking-water: consideration for health and socioeconomic factors. In: Safety of water 

disinfection: balancing chemical and microbial risks. Craun G.F. ed. ILSI Press, Washington, 

D.C.

Richardson S.D., Thruston A.D., Collette T.W. (1994). Multispectral identification of chlorine 

dioxide disinfection byproducts in drinking-water. Environ. Sci. Technol., 28: 592-599. 

Rook J.J. (1974). Formation of haloforms during chlorination of natural waters. J. Soc. for 

water treatment and examination 23: 234-243. 

World Health Organization (1992). Our planet, our health. Report of the WHO Commission 

on health and environment. Geneva. 
WHO SEMINAR PACK FOR DRINKING-WATER QUALITY 

12

 Annex 1

 BACTERIOLOGICAL QUALITY OF DRINKING-WATER 

 Organisms Guideline value 

All water intended for drinking

E. coli or thermotolerant coliform bacteria Must not be detectable in any 100-ml 

sample 

Treated water entering the distribution 

system 

E. coli or thermotolerant coliform bacteria Must not be detectable in any 100-ml 

sample 

Total coliform bacteria Must not be detectable in any 100-ml 

sample 

Treated water in the distribution system 

E. Coli or thermotolerant coliform bacteria Must not be detectable in any 100-ml 

sample 

Total coliform bacteria Must not be detectable in any 100-ml 

sample. In the case of large supplies, 

where sufficient sample are examined, 

must not be present in 95% of samples 

taken throughout any 12-month period. 
Disinfectants and Disinfectant By-Products 

Table 1: Summary of C.t values (mg/L. min) for 99% inactivation at 5o C (Clark et al, 1993) 

Organism Disinfectant 

 Free chlorine, 

pH 6 to 7 

Pre-formed 

chloramine, 

pH 8 to 9 

Chlorine dioxide, 

pH 6 to 7 

Ozone 

pH 6 to 7 

E. coli 0.034-0.05 95-180 0.4-0.75 0.02 Polio virus 1 1.1-2.5 768-3740 0.2-6.7 0.1-0.2 Rotavirus 0.01-0.05 3806-6476 0.2-2.1 0.006-0.06 Bacteriophage f2 0.08-0.18 - - - G. lamblia cysts 47->150 - - 0.5-0.6 G. muris cysts 30-630 - 7.2-18.5 1.8-2.0aC. parvum 7200b 7200c 78b 5-10c

a Values for 99.9% inactivation at pH 6-9. b 99% inactivation at pH 7 and 25C.c 90% inactivation at pH 7 and 25C.
WHO SEMINAR PACK FOR DRINKING-WATER QUALITY

Disinfectants and Disinfectant By-Products 

Presentation Plan

Section Key points OHP 

Introduction • disinfection of all waters supplied for drinking is recommended 

by WHO to protect public health 

• main disinfectants evaluated in the Guidelines are: free 

chlorine, chloramines, chlorine dioxide and ozone 

• overall ozone is the most effective disinfectant, although 

chlorine is also effective and efficient 

• all disinfectants have advantages and disadvantages and all 

produce by-products 

• a number of disinfection by-products were evaluated in the 

GDWQ 

1,2

Table 

1

Chlorine and 

its byproducts 

• chlorine is most common disinfectant 

• when chlorine is added to water it forms hypochlorous acid, 

hydrogen ion and a chlorine ion 

• because of greater efficiency, the Guidelines recommend 

disinfection with chlorine is done at pH less than 8 and a free 

chlorine concentration of greater than 0.5 mg/l 

• the use of chlorine leads to the formation of halogenated byproducts, including the THMs 

• precursors to THMs are natural humic and fulvic acids and 

algal material 

• numerous other by-products may be formed (see paper or 

Guidelines for examples 

• impurities in gaseous and liquid chlorine of relevance to the 

nature of by-products are carbon tetrachloride and bromide 

• GVs set for a number of chlorination by-products 

• very difficult to estimate exposure to halogenated organic 

compounds in drinking-water 

• may not need to set standards for all by-products included in 

Guidelines, it is better to concentrate on the major groups 

(e.g. THMs) 

• microbiological quality of water should never be 

compromised by concerns about disinfection by-products 

3,4,5
WHO SEMINAR PACK FOR DRINKING-WATER QUALITY

Section Key points OHP 

Chlorine • free chlorine in drinking-water is not particularly toxic and 

health-based GV is 5 mg/l 

• very unlikely consumers would accept such levels of chlorine as 

taste is noted as low as 0.3 mg/l 

• do not use GV as desirable level of chlorination 

Trihalomethanes 

• these are principal by-products of chlorination, but only form 10 

per cent of total organic compounds in drinking-water 

• THMs more likely to occur in chlorinated surface water than 

groundwater 

• THM concentrations vary widely; increasing with increasing 

temperature, pH, chlorine dosage and on storage after 

exhaustion of free chlorine or dechlorination 

• chloroform is most common THM (usually >90% of total 

THMs)

• when bromine present, brominated THMs likely to be dominant 

• THM formation can be minimised by avoiding prechlorination 

and by optimising treatment 

• THM removal is expensive and difficult 

6

Chloramine 

and byproducts 

• chloramines formed by reaction of chlorine and ammonia or 

organic amines 

• can get mono-, di- and trichloramines depending on pH and 

temperature 

• chloramine by-products similar to free chlorine, with exception 

of cyanogen chloride 

• monochloramine about 2000 to 100, 000 times less effective 

than free chlorine for inactivation of E.coli and rotaviruses 

7

Chlorine 

dioxide and 

by-products 

• chlorine dioxide made at point of use because of its explosive 

hazard 

• chlorine dioxide does not form THMs or chloramines 

• main by-products are chlorite, chlorate and chloride 

• chlorine dioxide more effective than free chlorine in inactivation 

of Giardia cysts but less effective against E.coli and rotaviruses 

• no GV for chlorine dioxide in water as it rapidly disassociates 

• GVs set for chlorite but not for chlorate 

8

Ozone and 

by-products 

• ozone decomposes rapidly following application and thus no 

GV has been proposed 

• by-products include formaldehyde, other aldehydes, hydrogen 

peroxide and bromomethanes (see paper/Guidelines for further 

examples) 

9
WHO SEMINAR PACK FOR DRINKING-WATER QUALITY

Section Key points OHP

Ozone and 

by-products 

(continued)

• ozone is the most efficient disinfectant with regard to 

microorganisms 

• disadvantages include: lack of residual, biological regrowth 

problems in distribution systems, high cost and limited 

information on nature and toxicity of by-products 

• when ozonation followed by chlorination, concentrations of 

brominated THMs may increase 

Balancing 

chemical and 

microbial 

risks

• currently a scarcity of quantitative assessment done of relative 

risks of microbial and chemical contamination of drinking-water 

• semi-quantitative presentation has been done by Morris: this 

showed that risk of infectious water-borne disease is high where 

chlorination not practised and this decreases sharply with even 

minimal levels of chlorination, though can never reach zero risk 

• at very high chlorine concentrations, microbial risk increases as 

taste and odour cause the use of unsafe supplies 

• chemical risks do not start at zero as always some hazard from 

organic matter prior to chlorination 

• chemical risks are low initially but increase with increasing 

chlorine dosages 

• risk of death from pathogens is at least 100 to 1000 times greater 

than risk of cancer from disinfected by-products and risk of 

illness from pathogens at least 10,000 to 1 million times greater 

• morbidity and mortality rates from pathogens compared to 

cancer risk from by-products may be much higher in developing 

countries where sanitary and health status poor 

10

Conclusions • disinfection is important to assure a safe drinking-water supply 

• limited information is available concerning health risk from 

disinfection by-products 

• disinfection by-product formation may be reduced if treatment 

process are optimised and prechlorination is avoided 

• inadequate evidence exists concerning the carcinogenicity of 

chlorinated drinking-water 

• more information is available concerning chlorine because it has 

been studied in more detail and this should not penalise the use 

of chlorine 

• as microbiological quality is of paramount importance, 

disinfection should not be compromised 
1

Disinfectants Evaluated

O Chlorine

O Chloramine

O Chlorine dioxide

O Ozone

O Iodine
2

Disinfectants and 

Disinfectant by-products

O Overall ozone is the most effective disinfectant, although 

chlorine is effective and efficient

O All disinfectants have advantages and disadvantages and 

all produce by-products

O A number of disinfectant by-products were evaluated in the 

Guidelines

Microbiological quality of water should never be compromised 

by concerns about disinfection by-products
3

Distribution of Hypochlorous Acid and 

Hypochlorite Ion in Water at Different pH Values 

and Temperatures
4

Relationship between Measured Free Residual 

Available Chlorine (HOCl+, OCl-

) and 

Bactericidally Active (HOCl)
5

Chlorine

O Chlorine is the most common disinfectant

O Chlorine by-products

» Free chlorine

» Trihalomethanes (THMs)

» Chlorinated acetics acids

» Halogenated acetonitriles

» Chloral hydrate (trichloroacetaldehyde)

» Chlorophenols

» MX 

(3-chloro-dichlormethyl-5-hydroxy-2(5H)-furanone)

May not need to set standards for all by-products

included in the Guidelines, it is better to concentrate on

the major groups (e.g. THMs)
6

Trihalomethanes

O The principal by-product of chlorination

O Formed by the aqueous chlorination of humic 

substances

O More likely to occur in chlorinated surface water 

than groundwater

O Concentrations of THMs tend to increase with 

increaseing temperature, pH and chlroine dosage

O THMs consist primarily of:

» Chlroform

» Bromodichloromethane

» Dibromochloromethane

» Bromoform

O Formation of THMs can be minimised by avoiding 

prechlorination and optimising treatment
7

Chloramine and its By-products

O Chloramines formed by reaction of chlorine and 

ammonia or organic amines

O Mono-, di- and trichloramines may be formed 

depending upon pH and temperature

O Chloramine by-products similar to free chlorine with 

the exception of cyanogen chloride

O Mono-chloramine is a less effective disinfectant than 

free chlorine and cannot be relied upon as a primary 

disinfectant; though useful for maintaining a 

residual.
8

Chlorine dioxide and its By-products

O Chlorine dioxide made at point of use because of its 

explosive hazard

O Reactions with humic substances do not form 

significant levels of THMs or chloramines

O Main by-products are:

» chlorite

» chlorate

» chloride

O More effective than free chlorine in inactivation of 

Giardia cysts but less effective against E.coli and

rotaviruses

O No GV for chlorine dioxide in water as it dissociates 

rapidly. GVs set for chlorite but not chlorate
9

Ozone and its By-products

O Most efficient disinfectant for all types of microorganisims

O Decomposes rapidly following application thus no 

GV has been proposed for ozone

O By-products include:

» formaldehyde

» aldehydes

» hydrogen peroxide

» bromomethanes

O Disdavantges include:

» lack of residual

» biological regrowth in distribution systems

» high cost

» limited information on toxicity of its by-products
10

Balancing chemical and microbiological 

risks