27

2. CHEMISTRY OF DISINFECTANTS AND

DISINFECTANT BY-PRODUCTS

2.1 Background

The use of chlorine (Cl2) as a water disinfectant has come under

scrutiny because of its potential to react with natural organic matter

(NOM) and form chlorinated disinfectant by-products(DBPs). Within

this context, NOM serves asthe organic DBP precursor, whereas bromide ion (Br–

) serves as the inorganic precursor. Treatment strategies

generallyavailable to water systems exceeding drinking-water standards

include removing DBP precursors and using alternative disinfectants

for primary and/or secondary (distribution system) disinfection.

Alternative disinfectant options that show promise are chloramines

(NH2Cl, monochloramine), chlorine dioxide (ClO2) and ozone (O3).

While ozone can serve as a primary disinfectant only and chloramines

as a secondarydisinfectant only, both chlorine andchlorine dioxide can

serve as either primary or secondary disinfectants.

Chloramine presents the significant advantage of virtually eliminating the formation of chlorination by-products and, unlike chlorine,

does not react with phenols to create taste- and odour-causing compounds. However, the required contact time for inactivation of viruses

and Giardia cysts is rarely obtainable by chloramine post-disinfection

at existing water treatment facilities (monochloramine is significantly

less biocidal than free chlorine). More recently, the presence of

nitrifying bacteria and nitrite (NO2

–

) and nitrate (NO3

–

) production in

chloraminated distribution systems as well asthe formation of organic

chloramines have raised concern. 

The use of chlorine dioxide, like chloramine, can reduce the

formation of chlorinated by-products during primary disinfection.

However, production of chlorine dioxide, its decomposition and

reaction with NOM lead to the formation of by-products such as

chlorite (ClO2

–

), a compound that is of health concern.

If used as a primarydisinfectant followed bya chloramine residual

in the distribution system, ozone can eliminate the need for contact

between DBP precursors and chlorine. Ozone is known to react both
EHC 216: Disinfectants and Disinfectant By-products

28

with NOM to produce organic DBPs such as aldehydes and increase

levels of assimilable organic carbon and with bromide ion to form

bromate.

A thorough understanding of the mechanisms of DBP formation

allows microbial inactivation goals and DBP control goals to be

successfullybalanced.This chapter examines a range ofissues affecting

DBP formationand control to provide guidance to utilities considering

the use of various disinfecting chemicals to achieve microbial

inactivation with DBP control.

2.2 Physical and chemical properties of common

disinfectants and inorganic disinfectant byproducts

The important physical andchemical properties of commonlyused

disinfectants and inorganic DBPs are summarized in Table 1.

2.2.1 Chlorine

Chlorine, a gas under normal pressure and temperature, can be

compressed to a liquid and stored in cylindrical containers. Because

chlorine gas is poisonous, it is dissolved in water under vacuum, and

this concentrated solution is applied to the water being treated. For

small plants, cylinders of about 70 kg are used; for medium to large

plants,tonne containers are common; andfor verylarge plants, chlorine

is delivered byrailwaytank cars or road (truck) tankers.Chlorine is also

available in granular or powdered form as calcium hypochlorite

(Ca(OCl)2) or in liquid form as sodium hypochlorite (NaOCl; bleach).

Chlorine is used in the form of gaseous chlorine or hypochlorite

(OCl–). In either form, it acts as a potent oxidizing agent and often

dissipates in side reactions so rapidly that little disinfection is

accomplished until amountsinexcess ofthe chlorine demandhavebeen

added. As an oxidizing agent, chlorine reacts with a wide variety of

compounds, in particular those that are considered reducing agents

(hydrogen sulfide [H2S], manganese(II), iron(II), sulfite [SO3

2–], Br–

, 
Table 1. Physical and chemical properties of commonly used disinfectants and inorganic disinfectant by-products

Chemicala E

o

 (V)b Oxidation number of Cl or Br 8max (nm)ce (mol–1 litre–1 cm–1)

d

p,

o e

pKa

f

HOCl/Cl– +1.49 +1 254

292 (OCl–)

60

419

+25.2 7.5

ClO2/ClO2

– +0.95 +4 359 1250 +16.1 –

NH2Cl – +1 245 416 – –

O3/O2 +2.07 – 254 3200 +35.0

HOBr/Br– +1.33 +1 330 50 +22.5 8.7

ClO2

–

/Cl– +0.76 +3 262 – +12.8 1.96

ClO3

–

/Cl– +0.62 +5 360 – +10.5 1.45

BrO3

–

/Br– +0.61 +5 195 – 0.72

a Half-cell reactants/products.

b Eo = standard electrode potential(redoxpotential) inwater at 25 °C. The oxidation–reduction state of anaqueous environment at equilibrium canbe stated

in terms of its redox potential. In the chemistryliterature,this is generally expressed in volts,E, or as the negative logarithm of the electronactivity, p,. When

p, is large, the electron activity is low and the system tends to be an oxidizing one: i.e., half-reactions tend to be driven to the left. When p, is small, the

system is reducing, and reactions tend to be driven to the right.

c 8max = maximum absorbance wavelength of that particular solution in nm.

d

e = molar absorptivity(molar extinctioncoefficient), in mol–1 litre–1 cm–1. This can be used for quantitative determinationof the various species of chemicals

and is the only direct physical measurement. There is often some background absorbance that may interfere with the measurement in natural waters that

should be considered.

e

p,

o

 = ! log {e–} where {e–} = electron activity.

f

pKa = negative logarithm of the acid ionization constant (e.g., at pH 7.5, the molar concentration of HOCl is same as that of OCl–). As this parameter is

dependent upon temperature, the values listed were determined at 25 °C.
EHC 216: Disinfectants and Disinfectant By-products

30

iodide [I–], nitrite). From the point of view of DBP formation and

disinfection, these reactionsmaybe important because theymaybe fast

and result in the consumption of chlorine.

Chlorine gas hydrolyses in water almost completely to form

hypochlorous acid (HOCl):

Cl2 + H2O 6 HOCl + H+ + Cl–

The hypochlorous acid dissociates into hydrogen ions (H+) and

hypochlorite ions in the reversible reaction:

HOCl : H+ + OCl–

Hypochlorous acid is a weak acidwith a pKa of approximately7.5

at 25 °C. Hypochlorous acid, the prime disinfecting agent, istherefore

dominant at a pH below 7.5 and is a more effective disinfectant than

hypochlorite ion, which dominates above pH 7.5.

The rates of the decomposition reactions of chlorine increase as

the solution becomes more alkaline, and these reactions can theoretically produce chlorite and chlorate (ClO3

–

); they occur during the

electrolysis of chloride (Cl–) solutions when the anodic and cathodic

compartments are not separated, in which case the chlorine formed at

the anode can react with the alkali formed at the cathode. On the other

hand, hypochlorous acid/hypochlorite (or hypobromous acid/hypobromite, HOBr/OBr–

) can be formed by the action of chlorine (or

bromine) in neutral or alkaline solutions. The decomposition of hypohalites(XO–

) isfavoured in alkaline solutions (2XO– 6 X

– + XO2

–

) and

is such that there is no longer any domain of thermodynamic stability

for the hypohalite ions. These oxyhalites are further converted to stable

oxyhalates as follows:

XO– + XO2

– 6 X–

 + XO3

–

Another reactionthat occursinwaters containing bromide ion and

hypochlorite is the production of hypobromous acid:

HOCl + Br– 6 HOBr + Cl–

This reaction is irreversible, and the product hypobromous acid is a

better halogenating agent than hypochlorous acid and interferes with
Chemistry of Disinfectants and Disinfectant By-products

31

common analytical procedures for free chlorine. The presence of

bromide in hypochlorite solutions can ultimatelylead to the formation

of bromate (BrO3

–

).

Hypobromous acid is a weak acid (pKa = 8.7); like hypochlorite,

hypobromite is metastable. In alkaline solution, it decomposesto give

bromate and bromide:

3OBr– 6 BrO3

–

 + 2Br–

Bromic acid (HBrO3) is a strong acid (pKa= 0.7).Bromic acidand

bromate can be obtained by the electrolytic oxidation of bromide

solutions or bromine water using chlorine. Bromic acid and bromate

are powerful oxidizing agents, but the speed of their oxidation reactions

is generally slow (Mel et al., 1953).

2.2.2 Chlorine dioxide

Chlorine dioxide is one of the few compounds that exists almost

entirely as monomeric free radicals. Concentrated chlorine dioxide

vapouris potentiallyexplosive, and attemptsto compress and store this

gas, either alone or in combination with other gases, have been

commercially unsuccessful. Because of this, chlorine dioxide, like

ozone, must be manufactured at the point of use. Chlorine dioxide in

water does not hydrolyse to any appreciable extent. Neutral or acidic

dilute aqueous solutions are quite stable if kept cool, well sealed and

protected from sunlight.

Chlorine dioxide represents an oxidation state (+4) intermediate

between those of chlorite (+3) and chlorate (+5). No acid or ion of the

same degree of oxidation is known. Chlorine dioxide is a powerful

oxidizing agent that can decompose to chlorite; in the absence of

oxidizable substances andinthe presence of alkali,it dissolvesin water,

decomposing with the slow formation of chlorite and chlorate:

2ClO2 + H2O 6 ClO2

–

 + ClO3

–

 + 2H+

Chlorine dioxide has an absorption spectrum with a maximumat

359 nm, with a molar absorptivity of 1250 mol–1litre–1 cm–1. This

extinction coefficient is independent of temperature, pH, chloride and

ionic strength. Chlorine dioxide is readily soluble in water, forming a
EHC 216: Disinfectants and Disinfectant By-products

32

greenish-yellow solution. It can be involved in a variety of redox

reactions, such as oxidation of iodide ion, sulfide ion, iron(II) and

manganese(II). When chlorine dioxide reacts with aqueous contaminants, it is usuallyreduced to chlorite ion. The corresponding electron

transfer reactions are comparable to those occurring when singlet

oxygen acts as an oxidant (Tratnyek & Hoigne, 1994).

Bromide (in the absence of sunlight) is not oxidized by chlorine

dioxide. Therefore, water treatment with chlorine dioxide will not

transform bromide ion into hypobromite and will not give rise to the

formation of bromoform (CHBr3) or bromate. This is an important

difference between the use of chlorine dioxide as anoxidant andthe use

of chlorine or ozone as an oxidant.

2.2.3 Ozone

Ozone is a strong oxidizing agent (E

o = 2.07 V). Oxidation

reactions initiated by ozone in water are generally rather complex; in

water, only part of the ozone reacts directly with dissolved solutes.

Another part may decompose before reaction. Such decomposition is

catalysed by hydroxide ions (OH–) and other solutes. Highly reactive

secondary oxidants, such as hydroxyl radicals (OH@), are thereby

formed. These radicals and their reaction products can additionally

accelerate the decomposition of ozone. Consequently, radical-type

chainreactionsmayoccur,which consume ozone concurrentlywiththe

direct reaction of ozone with dissolved organic material.

Many oxidative applications of ozone have been developed,

including disinfection, control of algae, removal of tastes and odours,

removal of colour,removal of iron and manganese,microflocculation,

removal of turbiditybyoxidative flocculation, removal of organics by

oxidation of phenols, detergents and some pesticides, partial oxidation

of dissolved organics and control of halogenated organic compounds.

For disinfection and for oxidation of many organic and inorganic

contaminants in drinking-water, the kinetics of ozone reactions are

favourable; on the other hand, for many difficult-to-oxidize organic

compounds, such as chloroform (CHCl3), the kinetics of ozone

oxidation are very slow (Hoigne et al., 1985).
Chemistry of Disinfectants and Disinfectant By-products

1 TheCT value isthe product of the disinfectant concentrationC in mg/litre and

the contact time T in minutes required to inactivate a specified percentage (e.g.,

99%) of microorganisms.

33

2.2.4 Chloramines

Monochloramine has much higher CT values1than free chlorine

and is therefore a poor primary disinfectant. Additionally, it is a poor

oxidant and is not effective for taste andodour control or for oxidation

of iron and manganese. However, because of its persistence, it is an

attractive secondarydisinfectant for the maintenance of a stable distribution system residual. The use of disinfectants such as ozone or

chlorine dioxide combined with chloramines as a secondary disinfectant

appearsto be attractive forminimizingDBP formation (Singer, 1994b).

Monochloramine is the only useful ammonia-chloramine disinfectant. Dichloramine (NHCl2

) and nitrogen trichloride (NCl3) are too

unstable to be useful and highly malodorous. Conditions practically

employed for chloramination are designed to produce only monochloramine.

2.3 Analytical methods for disinfectant by-products

and disinfectants

Analyticalmethodsfor variousDBPs and their detection limits are

summarized in Table 2. Methods for disinfectants are summarized in

APHA (1995).

2.3.1 Trihalomethanes, haloacetonitriles, chloral hydrate,

chloropicrin and haloacetic acids

Gas chromatographic (GC) techniques are generally employed

for organic DBPs. Detection and quantification of haloacetonitriles

(HANs) and chloral hydrate in chlorinated natural waters are complicated by (i) hydrolysis of dihaloacetonitriles and chloral hydrate to

dihaloacetic acids and chloroform, respectively; (ii) degradation of

HANs by dechlorinating agents such as sodium sulfite and sodium

thiosulfate;(iii) low purge efficiency for the HANs and chloral hydrate
Table 2. Summary of analytical methods for various DBPs and their minimum detection limits

DBPs Analytical method APHAa

method

Minimum detection

limit (:g/litre)

Major

interferences

References

THMs MTBE extraction

Pentane extraction

–

6232B

0.4

0.1

None AWWARF (1991)

HAAs Salted MTBE extraction and derivatization

with diazomethane

6233B 0.5–1.0 None AWWARF (1991)

HANs Pentane extraction 6232B 0.05 None Koch et al. (1988)

Cyanogen chloride MTBE extraction 6233A 0.5 None AWWARF (1991)

Chloramine Derivatization with 2-mercaptobenzothiazole – – None Lukasewycz et al. (1989)

Haloketonesb Pentane extraction 6232B 0.2 None Krasner et al. (1995)

AWWARF (1991)

Chloral hydrate MTBE extraction – 0.5 None AWWARF (1991)

Aldehydes Extraction with hexane and derivatization with

PFBHA

– 1.0 PFBHA sulfate Sclimenti et al. (1990)

Bromate Ion chromatography (H3BO3/NaOH) 4500 2.0c

0.2

Cl– Siddiqui et al. (1996a)

Krasner et al. (1993)

Weinberg et al. (1998)

Chlorate Ion chromatography (H3BO3/NaOH) 4500 5 Cl–, acetate Siddiqui (1996)

Chlorite Ion chromatography (NaHCO3/Na2CO3) 4500 10 Cl–, acetate AWWARF (1991)

TOC UV/persulfate or combustion 5310 200 Metals APHA (1995)

a American Public Health Association.

b Sum of 1,1-DCPN and 1,1,1-TCPN.

c

1.0 :g/litre with high-capacity column.
Chemistry of Disinfectants and Disinfectant By-products

35

in the purge-and-trap technique; and (iv) low extraction efficiency for

chloral hydrate with pentane in the liquid–liquid extraction normally

used. Although chloral hydrate is not efficiently extracted from water

with pentane, it can be extracted with an efficiency of approximately

36% when the ratio by volume of methyl tert-butyl ether (MTBE) to

water is 1 : 5 (Amy et al., 1998). MTBE quantitativelyextracts HANs,

trihalomethanes (THMs), chloral hydrate and chloropicrin, permitting

simultaneous analysis for all of these DBPs. Chloral hydrate decomposes on packed columns to trichloroacetaldehyde, resulting in considerable band broadening, although this does not appear to be a

significant problem with DB-1 and DB-5 columns.

The extraction of THMs can be accomplished using MTBE (EPA

Method 551) or pentane. Method 551 also permits simultaneous

extraction and measurement of chloral hydrate, HANs, THMs,

chloropicrin and haloketones. The pentane method can be used to

extract THMs, HANs, haloketones and chloropicrin but not chloral

hydrate in the same run (APHA, 1995).

The haloacetic acid (HAA) analytical method involves using an

acidic salted ether or acidicmethanol liquid–liquid extraction, requiring

esterification with diazomethane prior to analysis on a gas chromatograph equipped with an electron capture detector (ECD). THMs and

HANs can be analysed by extraction with pentane prior to analysis on

a capillary-column GC equipped with an ECD. The analysis of cyanogen compoundsinvolves extractionwith MTBE prior to injection into

GC–ECD. Aldehydes require derivatization with O-(2,3,4,5,6-pentafluorobenzyl)-hydroxylamine (PFBHA) (to formanoxime), extraction

with hexane and GC–ECD analysis [(C6F5)-CH2ONH2 + RCHO 6

(C6F5)–CH2ON=CHR+ H2O].It should be noted that PFBHA peaks are

very large relative to other peaks in the chromatogram from a purgeand-trap system, whereas the peaks are comparable to other peaksin a

GC–ECD chromatogram (Trehy et al., 1986).

2.3.2 Inorganic disinfectant by-products

An ion chromatography (IC) method (EPAMethod 300) has been

developed to determine inorganic by-products. The elution order is

fluoride, chlorite, bromate, chloride, nitrite, bromide, chlorate, nitrate

and sulfate ion. The eluent is a carbonate buffer. Ethylenediamine is

used to preserve chlorite samples and to minimize the potential for

chlorite ion reaction on the IC separating column. EPA Method 300
EHC 216: Disinfectants and Disinfectant By-products

36

involves measurement byan IC system using a separating column (e.g.,

Ion Pac AS9-SC) fitted with an anion micromembrane suppressor

column. An eluent containing 2.0 mmol ofsodiumcarbonate (Na2CO3)

per litre / 0.75 mmol of sodiumbicarbonate (NaHCO3) perlitre is used

for bromide determination, and an eluent containing 40 mmol of boric

acid(H3BO3) per litre / 20 mmol ofsodiumhydroxide (NaOH) perlitre

is used for bromate and chlorate determination. The analytical minimum detection limits for bromate and chlorate using a borate eluent

have been reported as 2 :g/litre and 5 :g/litre, respectively(Siddiqui,

1996; Siddiqui et al., 1996a). For samples with high chloride ion

content, a silver cartridge can be used to remove chloride prior to IC

analysis to minimize its interference with bromate measurement. It

should be noted that for natural sources and waters with high total

organic carbon (TOC) levels, detection limits will be slightlydifferent

because of the masking effect of NOM and high concentrations of

carbonate/bicarbonate ions that may interfere with bromate/chlorate

measurement.

2.3.3 Total organic carbon and UV absorbance at 254 nm

TOC is the primary surrogate parameter for the measurement of

NOM in water supplies. Several investigators have reported that the

ultraviolet (UV)/persulfate oxidationmethod underestimatesthe TOC

concentration in natural waters as compared with the combustion

method because of the inability of the persulfate method to oxidize

highly polymerized organic matter. It is generally assumed that the

calibration of a TOC analyser with a potassium hydrogen phthalate

standard is sufficient for the measurement of TOC in natural waters,

but potassiumhydrogen phthalate has a simplemolecularstructure and

is easy to oxidize. Dissolved organic carbon (DOC) is operationally

defined by a (0.45-:m) filtration step. UV absorbance at 254 nm

(UVA254) is used to describe the type and character of NOM, whereas

TOC describes just the amount of NOM.

2.3.4 Chloramines

Knowledge of the amine content of the water during water treatment processes involving chloramination is important to define more

adequately the content of a matrix described only as a combined

chlorine residual. The presence of organic nitrogen and the instability
Chemistry of Disinfectants and Disinfectant By-products

37

of many organic chloramines continue to challenge the analyst.

Lukasewycz et al. (1989) developed a technique for the analysis of

chloramines and organic chloramines present inwater using 2-mercaptobenzothiazole as a derivatizing agent. The resulting sulfanilamides are

stable and can be conveniently analysed by high-performance liquid

chromatography (HPLC) using UV or electrochemical detection. This

method appears to be superior to the use of diazotization or phenylarsine oxide as a method of detection. Organic chloramines are much

weaker disinfectants than inorganic monochloramine but are indistinguishable by the common analytical methods.

2.4 Mechanisms involved in the formation of disinfectant

by-products

2.4.1 Chlorine reactions

Chlorine reacts with humic substances(dissolved organic matter)

present in most watersupplies, forming a varietyof halogenatedDBPs,

such as THMs, HAAs, HANs, chloral hydrate and chloropicrin, as

follows:

HOCl + DOC 6 DBPs

It is generally accepted that the reaction between chlorine and

humic substances, a major component of NOM, is responsible for the

production of organochlorine compounds during drinking-watertreatment. Humic and fulvic acids show a high reactivity towards chlorine

and constitute 50–90% of the total DOC in river and lake waters

(Thurman, 1985). Otherfractions oftheDOC comprise the hydrophilic

acids(up to 30%), carbohydrates(10%), simple carboxylic acids(5%)

and proteins/amino acids (5%). The reactivity of carbohydrates and

carboxylic acids towards chlorine is low, and they are not expected to

contribute to the production of organochlorine compounds. However,

hydrophilic acids such as citric acid and amino acids will react with

chlorine to produce chloroform and other products andmaycontribute

towardstotal organochlorine production (Larson & Rockwell, 1979).

Free chlorine reacts with water constituents by three general

pathways: oxidation, addition and substitution (Johnson & Jensen,

1986). Chlorine can undergo an addition reaction if the organic

compound has a double bond. For many compounds with double bonds,
EHC 216: Disinfectants and Disinfectant By-products

38

this reaction is too slow to be of importance in water treatment. The

oxidation reactions with water constituents such as carbohydrates or

fatty acids (e.g., oleic acid) are generally slow.

Most chlorine DBPs are formed through oxidation and substitution reactions. THMs have the general formula CHX3

, where X can be

Cl or Br. Chloroform may be produced through a series of reactions

with functional groups of humic substances. The major functional

groups of humic substances include acetyl, carboxyl, phenol, alcohol,

carbonyl and methoxyl. The reactions proceed much more rapidly at

high pH than at low pH.

Rook (1977) proposed resorcinol structures to be the major

precursor structure in humic material for chloroform formation. In

accordance with this hypothesis in the chlorination of terrestrial and

aquatic humic substances, a series of intermediates were detected that

contained a trichloromethyl group and that could be converted to

chloroform by further oxidation or substitution reactions (Stevens et

al., 1976).

However, the production of chlorinated compounds such as

dichloropropanedoic acid, 2,2-dichlorobutanedoic acid, cyanogen

chloride (CNCl), HANs or the cyano-substituted acids cannot be

explained onthe basis ofresorcinolstructures, and possible production

pathways require protein-type precursors (De Leer et al., 1986). The

reactionpathwayfor amino acidsinvolvesinitial rapidformationofthe

monochloramine and dichloramine, which can react further to form

aldehyde or HANs, respectively. Trehy et al. (1986) demonstrated the

formation of chloral hydrate along with HANs after chlorination of

amino acids by substitution reactions, and aldehydes were shown to be

the oxidationproducts.Luknitskii (1975) provided a detailed chemistry

of chloral hydrate formation.

Christman et al. (1983) also identified chloroform, chloral

hydrate,dichloroacetic acid(DCA),trichloroacetic acid (TCA) and 2,2-

dichlorobutanedoic acid asthe major products, accounting for 53% of

the total organic halogen (TOX). A number of other minor products

have been detected, including several chlorinated alkanoic acids and

non-chlorinated benzene carboxylic acids. De Leer et al. (1985)

extended these studies to incorporate chloroform intermediates,

chlorinated aromatic acids andcyano-compounds as potential products
Chemistry of Disinfectants and Disinfectant By-products

39

in drinking-water. The presence of unhalogenated aldehydes and HANs

in chlorinated natural waters can be attributed inpartto the presence of

amino acids or peptides in natural waters. Humic acids may also

contribute to the presence of amino acidsinnaturalwaters, astheyhave

amino acids associated with them either in a free or in a combined

form. Several studies regarding the chlorination of amino acids have

shown that the primary amino group on the amino acids can be

converted to either an aldehyde or a nitrile group (Morris et al., 1980;

Isaac & Morris, 1983). These studies indicate that with an equimolar

amount of halogenating agent, the major product is an aldehyde.

However, if an excess of halogenating agent is added, then the

corresponding nitrile canalso be formed, with the ratio of the aldehyde

to nitrile formed increasing with pH.

Many treated waters contain not only chlorinated but also

brominated compounds, such as bromoform. These compounds form

because aqueous chlorine converts bromide in the water to hypobromousacid.Thebromine can then react withthe organicmatterinthe

same way as hypochlorous acid to form various bromochlorinated

DBPs.However, comparedwithhypochlorousacid, hypobromousacid

is a weaker oxidant and stronger halogenating agent.

Chlorate, an inorganic by-product of chlorine, is formed in

concentrated hypochlorite solutions during their production and storage

through the following reactions (Gordon et al., 1997):

OCl– + OCl– 6 ClO2

–

 + Cl–

OCl– + ClO2

– 6 ClO3

–

 + Cl–

The first reaction proceeds at a much slower rate and is rate

limiting, hence the generally observed second-order kinetics. Sodium

hypochlorite is stored at pH greater than 12 to prevent rapid decomposition, and most of the sodium hypochlorite is present as hypochlorite ion. The average rate constant for the formation of chlorate is

85 × 10–5 mol–1 litre–1 d–1 (Gordon et al., 1995).

2.4.2 Chlorine dioxide reactions

The major chlorine dioxide by-products of concern are chlorite and

chlorate. Chlorine dioxide reacts generallyas an electron acceptor, and

hydrogenatoms present inactivated organicC–Hor N–Hstructures are
EHC 216: Disinfectants and Disinfectant By-products

40

therebynotsubstituted bychlorine (Hoigne&Bader, 1994). Moreover,

in contrast to chlorine, chlorine dioxide’s efficiency for disinfection

does not vary with pH or in the presence of ammonia, and it does not

oxidize bromide. As opposed to chlorine, which reacts via oxidation

andelectrophilic substitution, chlorine dioxide reacts only by oxidation;

this explains why it does not produce organochlorine compounds. In

addition to this, chlorine dioxide is more selective in typical water

treatment applications, as evidenced byitssomewhatlower disinfectant

demand as compared with chlorine.

Chlorine dioxide is generally produced by reacting aqueous

(sodium) chlorite with chlorine (Gordon & Rosenblatt, 1996):

2ClO2

–

 + HOCl + H+ 6 2ClO2(aq) + Cl– + H2O

However, under conditions of low initial reactant concentrations

orinthe presence of excess chlorine,the reactant produces chlorate ion:

ClO2

–

 + HOCl 6 ClO3

–

 + Cl– + H+

Thisreactionscenario is common in generators that overchlorinate

to achieve high reaction yields based on chlorite ion consumption.

An alternative approach to chlorine dioxide generation is with

hydrochloric acid (HCl), a process that results in less chlorate during

production:

5NaClO2 + 4HCl 6 4ClO2 + 5NaCl + 2H2O

Chlorite ion is also produced when chlorine dioxide reacts with

organics (Gordon & Rosenblatt, 1996):

ClO2 + NOM 6 Products + ClO2

–

Chlorine dioxide can also undergo a series of photochemically

initiated reactionsresulting in the formation of chlorate ion(Gordonet

al., 1995).

While bromide is not generally oxidized by chlorine dioxide,

bromate can be formed in the presence of sunlight over a wide range of
Chemistry of Disinfectants and Disinfectant By-products

41

pH values (Gordon & Emmert, 1996). Utilities need to be concerned

with bromate ion inthe chlorine dioxide treatment of drinking-water if

the water contains bromide and is exposed to sunlight. Practically, this

means minimizing exposure to sunlight when chlorine dioxide is

applied in the presence of bromide ion. There appears to be a problem

with chlorine dioxide producing odour-causing compounds at the tap.

This has been linked to chlorine dioxide reacting with volatile organic

compounds derived fromnewcarpets andoffice products(Hoehn et al.,

1990).

Hoigne&Bader(1994) described the kinetics ofreactionbetween

chlorine dioxide and awide range of organic and inorganic compounds

that are of concern in water treatment. Measured rate constants were

high for nitrite, hydrogen peroxide, ozone, iodide, iron(II), phenolic

compounds, tertiary amines and thiols. Bromide, ammonia,structures

containing olefinic double bonds, aromatic hydrocarbons, primary and

secondaryamines, aldehydes, ketones andcarbohydrates are unreactive

under the conditions of water treatment. Chlorine dioxide rapidly

oxidizes substituted phenoxide anions and many phenols, and secondorderrate constants havebeenmeasured (Rav-Acha &Choshen, 1987).

2.4.3 Chloramine reactions

Chloramination of drinking-water producesTHMs(if chloramine

is formed by chlorination followed by ammonia addition), HAAs,

chloral hydrate,hydrazine, cyanogen compounds, nitrate, nitrite, organic

chloramines and 1,1-dichloropropanone (1,1-DCPN) (Dlyamandoglu

& Selleck, 1992; Kirmeyer et al., 1993, 1995).

In the presence of even small quantities of organic nitrogen, it is

possible for chloramination to produce organic chloramines. Several

researchers have shown that monochloramine readily transfers its

chlorine at a comparatively rapid rate to organic amines to form

organohalogen amines (Isaac & Morris, 1983; Bercz & Bawa, 1986).

Monochloramine wasshown to cause binding ofradiolabelled halogen

to nutrients such as tyrosine and folic acid; the amount of binding

varied with pH but was generally less at neutral pH than at higher pH

(Bercz & Bawa, 1986). Organic chloramines are much weaker

disinfectantsthan inorganicmonochloramine but are indistinguishable

by common analytical methods. Organic chloramine formation may
EHC 216: Disinfectants and Disinfectant By-products

42

necessitate changing chloramination conditions (e.g., ammonia and

chlorine addition order, chlorine-to-ammonia ratios and contact time).

HANs and non-halogenated acetonitriles are produced when

chloramines are reacted with humic materials and amino acids (Trehy

et al., 1986). The reaction pathway for these products is quite

complicated and very similar to that for chlorine, with many

intermediates and by-products formed. In the case of aspartic acid, De

Leer et al. (1986) demonstrated the presence of at least 11 other

significant products.

2.4.4 Ozone reactions

Ozone has been shown to oxidize bromide to hypobromite and

bromate, and hypochlorite to chlorate (Glaze et al., 1993; Siddiqui et

al., 1995; Siddiqui, 1996).

Bromate generally forms through a combination of molecular

ozone attack and reaction of bromide with free radical species. The

molecular ozone mechanism does not account for hydroxyl radicals

always formed as secondary oxidants from decomposed ozone during

water treatment. Siddiqui et al. (1995) indicated that there is a radical

pathway that is influenced by both pH and alkalinity. The hydroxyl

radical and,to a lesser degree,the carbonate radical(CO3@) pathwaymay

be more important than themolecular ozone pathway. Oxidantssuch as

hydroxyl andcarbonate radicalsmayinteractwithintermediate bromine

species,leading to the formationof hypobromite radicals(BrO@),which

eventually undergo disproportionation to form hypobromite and

bromite (BrO2

–

). Bromate isthen formed through oxidationof bromite

byozone.The radical mechanismforthe formationof bromate includes

two decisive reaction steps still involving molecular ozone: the

formation of hypobromite and oxidation of bromite.

Bromate ion formed through reactions with molecular ozone

contributes in the range of 30–80% to the overall bromate ion

formationinNOM-containingwaters(von Gunten and Hoigne, 1994).

Siddiqui et al. (1995) reported up to 65% and 100% bromate ion

formation through the radical pathway in NOM-free and NOMcontainingwaters,respectively. DifferencesinNOM-containing waters

can be attributed to differences in the characteristics of the NOM

present.Achange inmechanismas a function of pHandthe competitive
Chemistry of Disinfectants and Disinfectant By-products

43

roles of the free radical (one electron transfer) mechanism above pH 7

versus oxygen atom (two electron transfer) mechanism help explain

both the large variations in bromate ion yield and the sensitivity to

reactor design, concentrationof organic precursors andozone/bromide

ion concentrations (Gordon, 1993).

The presence of bromide ionina sourcewaterfurther complicates

the reaction of ozone and leads to the formation of additional DBPs,

such as bromoform, dibromoacetonitrile (DBAN) anddibromoacetone

(DBAC) (Siddiqui, 1992; Amy et al., 1993, 1994).

2.5 Formation oforganohalogen disinfectant by-products

Table 3 summarizesthe DBPsidentified as being formed fromthe

use of chlorine, chlorine dioxide, chloramine and ozone.

The formation of organochlorine and organobromine compounds

during drinking-water treatment is a cause of health concern in many

countries. These compounds include THMs, HAAs, HANs, chloral

hydrate, chloropicrin, acetohalides, halogenated furanones and other

compounds.

2.5.1 Chlorine organohalogen by-products

Table 4 summarizes the range of concentrations of chlorinated

DBPs formed from the reaction of chlorine with NOM, from various

sources.

Themajor chlorinationDBPsidentified are THMs, HAAs,HANs,

haloketones, chloropicrin and chloral hydrate. HAAs represent amajor

portion of the non-THM halogenated organic compounds (Miller &

Uden, 1983; Reckhow& Singer, 1985). Manyresearchers have identified HANs and haloketones as other important DBPs(Trehy& Bieber,

1981; Miller & Uden, 1983; Oliver, 1983;Reckhow& Singer, 1985).

According to an AWWARF (1991) study, for all eight utilitiestested,

1,1,1-trichloropropanone (1,1,1-TCPN)wasthe more prevalent of the

two measured haloketone compounds. In addition, Kronberg et al.

(1988) identified the extremely mutagenic compound, MX.
EHC 216: Disinfectants and Disinfectant By-products

44

Table 3. Disinfectant by-products present in disinfected waters

Disinfectant Significant organohalogen productsSignificant

inorganic

products

Significant nonhalogenated

products

Chlorine/

hypochlorous

acid

THMs, HAAs,

HANs, chloral

hydrate,

chloropicrin,

chlorophenols, 

N-chloramines,

halofuranones,

bromohydrins

Chlorate (mostly

from hypochlorite use)

Aldehydes,

cyanoalkanoic

acids, alkanoic

acids, benzene,

carboxylic acids

Chlorine

dioxide

chlorite, chlorate unknown

Chloramine HANs, cyanogen

chloride, organic

chloramines,

chloramino acids,

chloral hydrate,

haloketones

nitrate, nitrite,

chlorate,

hydrazine

aldehydes,

ketones

Ozone bromoform, MBA,

DBA, DBAC,

cyanogen bromide

chlorate, iodate,

bromate,

hydrogen

peroxide,

hypobromous

acid, epoxides,

ozonates

aldehydes,

ketoacids,

ketones,

carboxylic acids

Despite the fact that HAA formation and THM formation have

very different pH dependencies, HAA formation correlates strongly

withTHMformationwhen treatment conditions are relativelyuniform

and when the water has a low bromide concentration (Singer, 1993).

DBP formation and requisite chlorine dosage for disinfection strongly

correspond to the concentration of TOC at the point of chlorine

addition, suggesting that optimized or enhanced removal of organic

carbon prior to chlorination will decrease the formation of DBPs.

HAA formation can be appreciable when drinking-water is

chlorinated under conditions of slightly acidic pH and low bromide

concentrations. The concentrations of DCAand TCAare similar to the

concentrations of chloroform, and the total HAA concentration can be

as much as 50% greater than the THM concentration in the finished

water on a weight basis.
Table 4. Concentration range of chlorinated disinfectant by-products in drinking-watera

DBPs Peters et al. (1990);

Peters (1991)

Krasner et al. (1989) Nieminski et al. (1993) Koch et al. (1991) Reckhow et al. (1990)

THMs 3.1–49.5 30.0–44.0 17.0–51.0 49.0–81.0 201–1280

HAAs <0.5–14.7 13.0–21.0 5.0–25.0 22.0–32.0 118–1230

HANs 0.04–1.05 2.5–4.0 0.5–5.0 2.0–2.6 3.0–12.0

Haloketones – 0.9–1.8 0.2–1.6 1.0–2.0 4.8–25.3

Chlorophenols – – 0.5–1.0 – –

Chloral hydrate – 1.7–3.0 – – –

Chloropicrin – 0.1–0.16 <0.1–0.6 – –

TOC 1.7–5.6 2.9–3.2 1.5–6.0 2.5–3.0 4.8–26.6

Bromide 100–500 70–100 – 170–420 –

a All values shown in :g/litre, except TOC (mg/litre).
EHC 216: Disinfectants and Disinfectant By-products

46

McGuire & Meadow (1988) reported that the national average

THM concentration in the USA was 42 :g/litre for drinking-water

utilities serving more than 100 000 persons, and only 3% of systems

were above the US maximum contaminant level of 100 :g/litre. Amy

et al. (1993) estimated that the national average THMconcentration in

the USA was 40 :g/litre, with an average TOC concentration of

3.0 mg/litre. The median annual average THMconcentrationfound for

utilities among the American Water Works Association’s (AWWA)

WaterIndustryDatabase was 35 :g/litre, as compared with 50 :g/litre

for the non-database utilities (Montgomery Watson, Inc., 1993). 

In Germany, 10% of the utilities produced disinfected drinkingwater with a THM concentration above 10 :g/litre; the median annual

average concentrationwas between 1 and 4 :g/litre, depending on raw

water quality and size of facility (Haberer, 1994).

Total THM levels in treated drinking-water were reported in one

survey in the United Kingdom (Water Research Centre, 1980):

chlorinated water derived from a lowland river contained a mean level

of 89.2 :g/litre, and that from an upland reservoir, 18.7 :g/litre. The

study also showed that chlorinated groundwater was contaminated by

THMs to a significantly lesser extent than chlorinated surface waters.

In a national survey of the water supplies of 70 communities

serving about 38% of the population in Canada, conducted in the

winter of 1976–1977, chloroform concentrations in treated water of

the distribution system 0.8 km from the treatment plant, determined by

the gas sparge technique, averaged 22.7 :g/litre. Levels of the other

THMs were considerably lower, averaging 2.9 :g/litre for bromodichloromethane (BDCM), 0.4 :g/litre for dibromochloromethane

(DBCM) and 0.1 :g/litre for bromoform. Using direct aqueous

injectiontechniques, average concentrations ofmost oftheTHMs were

higher (Health Canada, 1993).

Samples collected from the distribution systems of eight major

cities in Saudi Arabia showed that THMs occurred in all the water

supplies, at concentrations ranging between 0.03 and 41.7 :g/litre.

Median total THM concentrations in several cities were higher during

the summer than during the winter. In addition, THM concentrations

were low in citiesthat did not mix groundwater and desalinated water.
Chemistry of Disinfectants and Disinfectant By-products

47

BrominatedTHMs dominated (withbromoform the most abundant) and

existed at the highest concentration levels, whereas chloroformwasthe

least prevalent compound.Thisisthe opposite ofthe occurrence pattern

found in almost all water distribution systems worldwide (Fayad,

1993).

The concentrations of chloral hydrate in drinking-water in the USA

were summarized byIARC(1995) andvaried from0.01 to 28 :g/litre.

The highest values were found indrinking-water prepared fromsurface

water.

Chlorination of water aswell asthe combinationof ozonationand

chlorination can lead to the formation of chloropicrin (Merlet et al.,

1985). In a study conducted for over 25 utilities, very low levels of

chloropicrin were observed, and chlorination produced maximum

concentrations of lessthan 2 :g/litre (AWWARF, 1991). The chloropicrin appeared to form slowly during the incubation period, with

concentrations tending to level off at approximately 40 h.

Dichloroacetonitrile (DCAN) is by far the most predominant HAN

species detected in watersources with bromide levels of 20 :g/litre or

less. Forsources with higher bromide levels(50–80 :g/litre), bromochloroacetonitrile (BCAN) wasthe second most prevalent compound.

However, none of these sources had a DBAN concentration exceeding

0.5 :g/litre, including one source water that had a much higher bromide level, 170 :g/litre.Thus,it appearsthat ambient bromide concentration is not the only factor influencing the speciation of HAN

compounds.

Chlorine can react with phenols to produce mono-, di- or

trichlorophenols, which can impart tastes and odours to waters. The

control of chlorophenolic tastes and odours produced when phenolladenwateristreatedwithchlorine is essential.The sources of phenolic

compounds in water supplies are reported to be industrial wastes.

In natural waters, one of the most important sources of organic

nitrogen is proteins and their hydrolysis products. The reaction of

aqueous chlorine or monochloramine with organic nitrogen mayform

complex organic chloramines(Feng, 1966;Morris et al., 1980; Snyder
EHC 216: Disinfectants and Disinfectant By-products

48

& Margerum, 1982). The formation of N-chloramines resulting from

the reaction of amines and chlorine has been reported (Weil & Morris,

1949;Grayet al., 1979; Morris et al., 1980). Likewise,the chlorination

of amides has been reported (Morris et al., 1980).

Nieminski et al. (1993) reported the occurrence of DBPsfor Utah

(USA) water treatment plants. All plants used chlorine for primary and

secondarydisinfectionpurposes.Overall,THMs andHAAsrepresented

75% of the total specific DBPs analysed for the survey; however, total

DBPs represented only 25–50% of the TOX concentration. THMs

constituted 64% of the total DBPs by weight;HAAs were 30% of the

total DBPs by weight and approximately one-half of the total THM

concentrations. (However, in some waters, HAA concentrations may

approach or possiblyexceedTHMconcentrations.) HANs, haloketones,

chlorophenols andchloropicrinrepresented 3%,1.5%,1.0% and 0.5%,

respectively, of the total surveyed DBPs.

The occurrence of DBPs in drinking-waters in the USA was

evaluated at 35 water treatment facilities that had a broad range of

source water qualities and treatment processes (Krasner et al., 1989).

THMs were the largest class of DBPs, and HAAs were the next most

significant class. Aldehydes, by-products of ozonation, were also produced by chlorination. Over four quarterly sampling periods, median

total THMconcentrationsranged from 30 to 44 :g/litre, with chloroform, BDCM, DBCM and bromoform ranges of 9.6–15, 4.1–10,

2.6–4.5 and 0.33–0.88 :g/litre, respectively. Median total HAA

concentrations ranged from 13 to 21 :g/litre, withTCA, DCA, monochloroacetic acid (MCA), dibromoacetic acid(DBA) andmonobromoacetic acid (MBA) ranges of 4.0–6.0, 5.0–7.3, <1–1.2, 0.9–1.5 and

<0.5 :g/litre, respectively.

Concentrations of DCA and TCA measured in various water

sources have been summarized by IARC (1995): in Japan, chlorinated

drinking-water contained 4.5 and 7.5 :g of DCA and TCA per litre,

respectively; rainwater in Germanycontained 1.35 :g of DCAper litre

and 0.1–20 :g of TCA per litre, whereas groundwater contained

0.05 :g of TCA per litre; in Australia, a maximum concentration of

200 :g/litre wasfound for DCAand TCAin chlorinated treatedwater;

andchlorinatedwaterinSwitzerland contained 3.0 :g of TCAper litre.
Chemistry of Disinfectants and Disinfectant By-products

49

In a survey of 20 drinking-waters prepared from different source

waters in the Netherlands, HAAs were found in all drinking-waters

prepared from surface water, whereas they could not be detected in

drinking-waters prepared from groundwaters. Brominated acetic acids

accounted for 65% of the total acid concentration (Peters et al., 1991).

In another survey of Dutch drinking waters, the average concentration

of dihaloacetonitrileswas about 5% ofthe averageTHMconcentration

(Peters, 1990).

2.5.2 Chloramine organohalogen by-products

Chloramine treatment practice involves three potential approaches:

free chlorine followed by ammonia addition, ammonia addition

followed bychlorine addition(in situ production) andpre-formed (offline formation) chloramines. Generally, the objective is

monochloramine formation. Chlorine followed by ammonia is a

commonapproach, and,during the free-chlorine period, DBP formation

may mimic that of chlorine. Chloramination results in the production

of THMs(predominantlyformed bychlorinationfollowed byammonia

addition), HAAs, chloral hydrate, hydrazine, cyanogen compounds,

organic chloramines and 1,1-DCPN (Dlyamandoglu & Selleck, 1992;

Singer, 1993; Kirmeyer et al., 1993, 1995). Chloramination

significantlyreduces but does not eliminateTHMformation; cyanogen

chloride and TOX represent the major DBP issues with respect to

chloramines.

Scullyet al. (1990) identified chloramino acidssuch as N-chloroglycine, N-chloroleucine and N-chlorophenylalanine as by-products

after chlorination of water containing nitrogen or after chloramination.

2.5.3 Chlorine dioxide organohalogen by-products

Chlorine-free chlorine dioxide does not form THMs (Noack &

Doerr, 1978; Symons et al., 1981). Severalstudiesshow that the TOX

formed with chlorine dioxide is 1–15% of the TOX formed with

chlorine under the same reaction conditions (Chow & Roberts, 1981;

Symons et al., 1981; Fleischacker & Randtke, 1983).

Treatment of phenol-laden source waters with chlorine dioxide

does not produce the typical chlorophenolic taste and odour compounds
EHC 216: Disinfectants and Disinfectant By-products

50

that are produced when the water is treated using chlorine and is

effective in removing existing tastes and odours of this type.

2.5.4 Ozone organohalogen by-products

Ozonation of drinking-water containing bromide ion has been

shown to produce hypobromous acid/hypobromite, with hypobromite

ion serving as an intermediate to bromate formation. In the presence of

NOM, hypobromous acid produces a host of brominated organic

compounds, such as bromoform, MBA, DBA, DBAN, cyanogen

bromide and DBAC (Glaze et al., 1993; Siddiqui & Amy, 1993).

Cavanagh et al. (1992) and Glaze et al. (1993) reported the identification of bromohydrins, a new group of labile brominated organic

compounds from the ozonation of a natural water in the presence of

enhanced levels of bromide. However, results by Kristiansen et al.

(1994) strongly suggest that the bromohydrins, such as 3-bromo-2-

methyl-2-butanol, in extracts of unquenched disinfected water are

artefactsformed fromthe reactionof excessive hypobromousacidwith

traces of olefins in the extraction solvents and not novel DBPs.

Table 5 comparesthemedian concentrations of variousDBPs after

ozonation and chlorination.

2.6 Formation of inorganic disinfectant by-products

Although organic DBPs have been the subject of study over a

longer time frame, the formation of many inorganic by-products is

coming under increasing scrutiny. 

2.6.1 Chlorine inorganic by-products

Chlorite and chlorate are inorganic by-products formed in some

chlorine solutions. This is of interest because many small drinkingwater utilities use hypochlorite solutions as a source of free chlorine

for disinfection. Bolyard & Fair (1992) examined the occurrence of

chlorate in samples of untreated source water, drinking-water and

hypochlorite solutions from 14 sites that use hypochlorite solutions.

The hypochlorite solutions used were found to contain significant

levels of chlorate. Chlorite and bromate were also found in

hypochlorite solutions from these same water utilities. Chlorate was

present indrinking-water, either as amanufacturing by-product orfrom
Chemistry of Disinfectants and Disinfectant By-products

51

Table 5. Median concentrations of organic disinfectant by-products in drinking-water

DBPs Median concentration

(:g/litre): chlorinationa

Median concentration

(:g/litre): ozonationb

THMs

Chloroform

BDCM

DBCM

Bromoform

40

15

10

4.5

0.57

<1.0

–

–

–

<1.0

HANs

TCAN

DCAN

BCAN

DBAN

2.5

<0.012

1.1

0.58

0.48

<1.0

–

–

–

<1.0

Haloketones

DCPN

TCPN

0.94

0.46

0.35

–

–

–

HAAs

MCA

DCA

TCA

MBA

DBA

20

1.2

6.8

5.8

<0.5

1.5

<5.0

–

–

–

<1.0

<5.0

Aldehydes

Formaldehyde

Acetaldehyde

Glyoxal

Methylglyoxal

7.8

5.1

2.7

–

–

45

20

11

9

5

Chloral hydrate 3.0 –

Ketoacids – 75

Trichlorophenol <0.4 –

a Krasner et al. (1989).

b Siddiqui et al. (1993).

decomposition reactions occurring during storage. Approximately

0.2 mg of chlorate per litre was observed in water following the

addition of chlorine as sodium hypochlorite at a dose sufficient to

maintain a residual of 0.45 mg/litre (Andrews &Ferguson, 1995). The

concentration of chlorite in commercial bleach solutions typically

ranges from 0.002 to 0.0046 mol/litre;similarly, the chlorate concentration rangesfrom about 0.02 to 0.08 mol/litre (Gordon et al., 1995).
EHC 216: Disinfectants and Disinfectant By-products

52

A detailed study by Bolyard & Fair (1992) demonstrated that

hypochlorite solutions used to disinfect drinking-water contain

significant levels of chlorite andchlorate.The concentrationof chlorite

ranged from <2 to 130 mg/litre for free available chlorine concentrationsranging from3 to 110 g/litre.The concentrationof chlorate varied

over the range 0.19–50 g/litre, with a median of 12 g/litre. These

solutions also contained bromate levelsranging from<2 to 51 mg/litre.

The concentrations of chlorate in treated sourcewatersranged from 11

to 660 :g/litre. In another study involving 25 samples from plants

using gaseouschlorine, no chloratewas detected,indicating thatthe use

of gaseous chlorine does not produce chlorate (Bolyard&Fair, 1992).

Nieminski et al. (1993) measured chlorate and chlorite for six water

treatment plants that use liquid chlorine (i.e., hypochlorite) and found

chlorate concentrations ranging from 40 to 700 :g/litre, with no

chlorite or bromate detected in finished waters. These chlorate concentrations may be attributed to high concentrations of chlorate, ranging

from 1000 to 8000 mg/litre, detected in a bleach used for disinfection

and resulting from the decomposition of hypochlorite stock solution.

However, no chlorite or chlorate was detected in anyof the samples of

finished water of the treatment plants that apply gaseous chlorine.

Chlorate formation is expected to be minimal in low-strength hypochlorite solutionsfreshlyprepared from calciumhypochlorite, because

of the low hypochlorite concentration and only mildly alkaline pH.

2.6.2 Chloramine inorganic by-products

Inorganic by-products of chloramination include nitrate, nitrite,

hydrazine and, to some extent, chlorate (Dlyamandoglu & Selleck,

1992; Kirmeyer et al., 1995). 

2.6.3 Chlorine dioxide inorganic by-products

The major inorganic by-products of chlorine dioxide disinfection

have been identified as chlorite and chlorate. Andrews & Ferguson

(1995) measured a chlorate concentration of 0.38 mg/litre when a

chlorine dioxide residual of 0.33 mg/litre was maintained. The application of chlorine dioxide produces about 0.5–0.7 mg of chlorite and

0.3 mg of chlorate per mg of chlorine dioxide consumed or applied

(Andrews & Ferguson, 1995).
Chemistry of Disinfectants and Disinfectant By-products

53

2.6.4 Ozone inorganic by-products

When bromide or iodide ions are present in waters, some of the

halogen-containing oxidants that can be produced during ozonation

include free bromine, hypobromous acid, hypobromite ion, bromate

ion, free iodine, hypoiodous acid and iodate ion.

During the oxidation or chemical disinfection of natural waters

containing bromide ion with ozone, bromate ion can be formed at

concentrations ranging from 0 to 150 :g/litre under normal water

treatment conditions (Siddiqui, 1992). Chlorate formation with an

initial total chlorine concentration of 0.6 mg/litre was evaluated at pH

levels of 8.0, 7.0 and 6.0, and chlorate concentrationsranging from 10

to 106 :g/litre were formed after ozonation (Siddiqui et al., 1996a).

It has been reported that ozone reacts with many metal ions and

with cyanide ion (Hoigne et al., 1985; Yang & Neely, 1986). Bailey

(1978) discussed the formation of ozonates, compounds of metal

cations having the generalformulaM+O3

–

.Hydrogen peroxide has been

identified as a by-product of ozonation of organic unsaturated

compounds (Bailey, 1978).

Table 6 provides the range of bromate concentrations normally

encountered in drinking-waters with a variety of source water

characteristics after ozonation.

2.7 Formation of non-halogenated organic disinfectant

by-products

2.7.1 Chlorine organic by-products

Lykins & Clark (1988) conducted a 1-year pilot plant study of the

effects of ozone and chlorine and determined that the concentration of

aldehydes increased by 144% upon ozonation. In the chlorinated

stream, the concentration of these aldehydes increased by 56%. This

study indicates that aldehyde formation, although greater with ozone,

is not uniqueto ozonation, but is associatedwithchlorinationandother

oxidants as well.
Table 6. Summary of bromate ion formation potentials in different source waters under different conditions following ozonation

N

a Bromide (:g/litre) Ozone (mg/litre) pH Alkalinity (mg/litre) DOC (mg/litre) Bromate (:g/litre) Reference

18 10–800 1–9.3 5.6–9.4 20–132 2.2–8.2 <5–60 Krasner et al. (1992)

4 60–340 3–12 6.5–8.5 90–230 3–7 <5–40 Siddiqui & Amy (1993)

28 10–100 2–4 6.8–8.8 20–120 0.3–11 <5–100 Amy et al. (1993, 1994)

4 12–37 0–3.97 7.8 N/A N/A <7–35 Hautman & Bolyard (1993)

1 500 2.3–9.5 7.2–8.3 N/A N/A 13–293 Yamada (1993)

23 12–207 0.3–4.3 5.7–8.2 14–246 0.5–6.8 <2–16 Legube et al. (1993)

8 107–237 1–5 6.8–8.0 N/A 2–5 <5–50 Kruithof & Meijers (1993)

a N = number of sources studied.
Chemistry of Disinfectants and Disinfectant By-products

55

2.7.2 Chloramine organic by-products

When Suwannee River (USA) fulvic acid was reacted with

aqueoussolutions of 15N-labelled chloramine and 15N-labelled ammonia, lyophilized products exhibiting nuclear magnetic resonances

between 90 and 120 ppm were observed, denoting the formation of

amides, enaminones and aminoquinones (Ginwalla & Mikita, 1992).

This represents evidence for the formation of nitrogen-containing

compounds from the chloramination of NOM in natural waters.

Amino acids, peptides and amino sugars were chlorinated under

various chlorine/nitrogen ratios (Bruchet et al., 1992). Six natural

amino acids (alanine, methionine, valine, phenylalanine, leucine and

isoleucine) were shown to induce tastes and odours at concentrations

in the range of 10–20 :g/litre. Detectable odours were consistently

induced in a multicomponent mixture containing each of these amino

acids after a 2-h contact time with chlorine. Investigation of the byproductsindicated that the odours generatedwere systematicallylinked

to the aliphatic aldehydesformed.Thepeptidesinvestigated hadvarying

degrees of odour formation potential, while the amino sugars did not

impart any odour. Chlorinous odours occasionally detected during

these experiments were found to be due to organic chloramines and

other oxidation by-products.

2.7.3 Chlorine dioxide organic by-products

Gilli (1990) showed the formation of carbonyl compounds

(34 :g/litre) such as n-valeraldehyde (7–15 :g/litre), formaldehyde

(3.4–9 :g/litre), acetaldehyde (4.5 :g/litre) and acetone (3.2 :g/litre)

after using chlorine dioxide.

2.7.4 Ozone organic by-products

Ozone aliphatic oxidation products from organic impurities in

water are usually acids, ketones, aldehydes and alcohols. So-called

ultimate oxidation products of organic materials are carbon dioxide,

water, oxalic acid and acetic acid. However, ozonation conditions

generally employed in treating drinking-water are rarely sufficient to

form high concentrations of these ultimate products.
EHC 216: Disinfectants and Disinfectant By-products

56

When source waters containing NOM and unsaturated organic

compounds are ozonated, ozonides, peroxides, diperoxides, triperoxides

and peroxy acids, for example, can be produced. The limited research

that has been conducted in aqueous solutions indicates that these

intermediates decompose readily in water to form products such as

aldehydes, ketones, carboxylic acids and ketoacids.

Coleman et al. (1992) identified numerous compounds in addition

to the following in ozonated humic samples: monocarboxylic acids up

to C-24, dicarboxylic acids up to C-10, ketoacids, furan carboxylic

acids, and benzene mono-, di- and tricarboxylic acids. Among the

various aldehydes,Paode et al.(1997) found four (formaldehyde, acetaldehyde, glyoxal and methylglyoxal) to be dominant. Table 7 provides

a range of concentrationsfor aldehydesfrom the ozonation of a variety

of source waters.

2.8 Influence of source water characteristics on the

amount and type of by-products produced

The extensive literature pertaining to DBP levels in disinfected

source waters and control of DBPs by various treatment processes

atteststo thewide varietyof factorsinfluencingDBP formationandthe

complex interrelationships between these factors. Variables including

the concentrationandcharacteristics of precursor material, pH, chlorine

concentration, bromide level, presence of chlorine-demanding

substances such as ammonia, temperature and contact time all play a

role in DBP formation reactions.

2.8.1 Effect of natural organic matter and UV absorbance at 254 nm

NOMconsists of amixture of humic substances(humic andfulvic

acids) and non-humic (hydrophilic) material. Both the amount (as

indicated by TOC or UVA254) and the character (as described by

UVA254) of NOM can affect DBP formation. NOM provides the

precursormaterialfromwhich organicDBPs are formed; consequently,

increasing concentrations of NOMlead to increasing concentrations of

by-products. This relationship has led to the use of TOC and UVA254

measurements assurrogate parameters for estimating the extent of DBP

formation. 
Table 7. Effect of ozone dose and TOC on non-halogenated organic by-products

Ozone dose

(mg/litre)

TOC

(mg/litre)

Formal

(:g/litre)

Acetal

(:g/litre)

Glyoxal

(:g/litre)

Methyl-glyoxal

(:g/litre)

Reference

1.2–4.4 2.66 8–24 2–4 4–11 4–15 Miltner et al. (1992)

1.0–9.2 1.0–25.9 3–30 7–65 3–15 3–35 Weinberg et al. (1993)

5.5–28.5 5.4–17.4 58–567 6–28 15–166 17–54 Schechter & Singer (1995)
EHC 216: Disinfectants and Disinfectant By-products

58

The removal of NOM is strongly influenced by those properties

embodying the size, structure and functionality of this heterogeneous

mixture. The humic acids are more reactive than fulvic acids with

chlorine (Reckhow et al., 1990) and ozone, in terms of both oxidant/

disinfectant demand and DBP formation. Processes such as coagulation, adsorption and membrane filtration are separation processesthat

remove NOMintact,while ozonation transforms part of the NOMinto

biodegradable organic matter, potentially removable by biofiltration.

Coagulation preferentially removes humic/higher molecular weight

NOM; the selectivity of membranes for NOM removal is largely

dictated by the molecular weight cutoff of the membrane; the use of

granular activated carbon (GAC) requires a significant empty bed

contact time; biofiltration can remove only the rapidly biodegradable

NOM fraction.

Inaninvestigation of the nature of humic and fulvic acidsisolated

from a variety of natural waters, Reckhow et al. (1990) found that the

fulvic fractions had a lower aromatic content and smaller molecular

size than the humic fractions. UV absorbance was correspondingly

higher for the humic fractions, owing to the higher aromatic content

and larger size. These researchers also found that for all of the organic

material investigated, the production of chloroform, TCA, DCA and

DCAN was higher upon chlorination of the humic fractionsthan upon

chlorination of the corresponding fulvic fractions. These findings

support the findings of other researchers and show that the UV absorbance measurement is an indicator of the nature of the precursor

material present ina sample.Thismeasurement,inconjunctionwiththe

TOC(or DOC)measurement, canbe employed inthe evaluationof data

to provide an indication of the reactivity of NOM towards forming

DBPs.

The reaction of ozone with NOM can occur directlyor byradical

processes. The disappearance of disinfecting chemical is influenced by

the type and concentration of NOM present in natural waters. Direct

consumption of these chemicals is greater when the UV absorbance

(due to electrophilic and nucleophilic sites of NOM) of the source

water is significant, resulting in decreased DBP formation potential.

It appearsthat the nature of the organic material in a source water

may have some impact on the relative concentrations of THMs and

HAAs formed upon chlorination. Treatment techniques that lower the
Chemistry of Disinfectants and Disinfectant By-products

59

levels of DOC without affecting bromide levels have been implicated

in a shift from chlorinated to brominated THM compounds. This is of

concernbecause the theoretical risk to humans variesforthe individual

THMs, with the brominated species generally being of more concern

(Bull & Kopfler, 1991).

2.8.2 Effect of pH

The impact of pH on THM concentrations has been reported by a

number of researchers since THMs in drinking-water first came to the

attention of the water industry (Stevens et al., 1976; Lange &

Kawczynski, 1978; Trussell & Umphres, 1978). More recently, the

impact of pH on a number of other chlorination by-products has been

reported (Miller & Uden, 1983;Reckhow&Singer, 1985). The rate of

THM formation increases with the pH (Stevens et al., 1976).

Kavanaugh et al. (1980) reported a 3-fold increase in the reaction rate

per unit pH.

In general, increasing pH has been associated with increasing

concentrations of THMs and decreasing concentrations of HAAs (pH

primarilyimpactingTCA),HANs and haloketones. The concentrations

of TCAtend to be higher in waters with pH levels less than 8.0 than in

waters with pH levels greater than 8.0; a less marked trend is observed

for DCA. Other researchers have reported similar findings with respect

to the pH dependency of HAA concentrations. For example, Stevens et

al. (1976) found that TCA concentrations were significantly lower at

a pH of 9.4 than at pH levels of approximately5 and 7. TCAwas byfar

the most predominant of the measured HAA species atsix of the eight

utilities surveyed. Carlson & Hardy (1998) reported that at pH levels

greater than 9.0, THM formation decreased with increasing pH. It is

possible that the shift in chlorine species from hypochlorous acid to

hypochlorite affects THM formation during short reaction times.

AWWARF (1991) observed no relationship between pH and the

concentrations of THMs at eight utilities over time, suggesting that

although THM concentrations for a particular water are known to be

pH dependent, factors other than pH influence THM concentrations

over a variety of source waters. Nieminski et al. (1993) reported that

treatment plants with a pH of about 5.5 in finished water produced

equal amounts of THMs and HAAs, whereas plants with pHs greater
EHC 216: Disinfectants and Disinfectant By-products

60

than 7.0 in finished water produced higher amounts of THMs as

compared with HAAs.

No strong relationship has been observed between HAN

concentration and pH over time. Within the approximate pH range

7–8.5, HAN concentrations increased slightly over time. In general, a

trend of decreasing HAN concentrations with increasing pH would be

expected,since these compounds are known to undergo base-catalysed

hydrolysis andhavebeenidentified asintermediatesin the formationof

chloroform (Reckhow & Singer, 1985). Therefore, these compounds

may be unstable in the presence of free chlorine or under basic

conditions. In general, after an initial formation period, HAN and

haloketone concentrations level off or begin to decline over the

remainder of the reaction period. This indicates that base-catalysed

hydrolysis may not be a significant mechanism of reaction for the

relatively low pH sources.

Stevens et al.(1989) evaluated the effects of pH and reaction time

(4, 48 and 144 h) on the formation of chloral hydrate. Chloral hydrate

formation increased over time at pH 5 and 7, whereas chloral hydrate

that had formed within 4 h at pH 9.4 decayed over time at the elevated

pH.

The pH of the source water can also affect the formation of byproducts after chloramine addition. The disproportionation of monochloramine, which is an important reaction leading to an oxidant loss,

has been shown byseveral researchersto be catalysed by hydrogen ion,

phosphate, carbonate and silicate (Valentine & Solomon, 1987).

Humic acids have shown reaction rates with chlorine dioxide that

increased by a factor of 3 per pH unit (pH 4–8) (Hoigne & Bader,

1994).

Inadditionto the impact of pH onTHMandHAAformationnoted

above, overall TOX formation decreases with increasing pH. Many of

the halogenated DBPs tend to hydrolyse at alkaline pH levels (>8.0)

(Singer, 1994a). This has significant implications, for example, for

precipitative softening facilities.

pH has a strong effect onaldehyde formation(Schechter & Singer,

1995). Higher ozonation pH values produced lower amounts of
Chemistry of Disinfectants and Disinfectant By-products

61

aldehydes, supporting the theorythat these DBPs are formed primarily

through the direct molecular ozone reactionpathway, as opposed to the

radical pathway. These results may also reflect greater destruction of

aldehydes by hydroxyl radicals at elevated pH levels.

2.8.3 Effect of bromide

The presence of bromide ion during water treatment disinfection

can lead to the formation of DBPs such as brominated organics and

bromate ion. Low but significant levels of bromide, the ultimate

precursor to bromate and other brominated compounds, may occur in

drinking-water sources as a result of pollution and saltwater intrusion

in addition to bromide from natural sources. An understanding of the

sources and levels of bromide ion in different source waters is crucial

for an understanding of the bromate ion formation potential in

drinking-waters.There are no known treatment techniques available for

economically removing bromide ion present in source waters during

drinking-water treatment.

The impact of bromide on the speciation of DBPs within a class

of compounds such as THMs or HAAs has been discussed by Cooper

et al. (1983, 1985) and Amy et al. (1998). Rook et al. (1978) reported

that bromine is more effective than chlorine in participating in

substitution reactions with organic molecules;furthermore, precursor

materials may differ in their susceptibility to bromination versus

chlorination reactions. Hypobromous acid formed from bromide may

also react with ammonia to form bromamines (Galal-Gorchev &

Morris, 1965).

2.8.4 Effect of reaction rates

After chlorine addition, there is a period of rapidTHMformation

for the initial few hours (e.g., 4 h), followed bya decline in the rate of

THM formation, suggesting fast and slow NOM reactive sites. Many

authors have indicated that the concentration of chloroform appearsto

increase slowly even after 96 h, suggesting that as long as low

concentrations of free chlorine are present, chloroform continues to

form. Bromochlorinated THM species have been found to form more

rapidly than chloroform. Further data from manysourcesindicate that

bromoform formation slows at approximately 7–8 h and levels off

almost completely after 20 h (AWWARF, 1991; Koch et al., 1991). 
EHC 216: Disinfectants and Disinfectant By-products

62

The same general kinetic trend observed forTHMs also appearsto apply

to HAAs. A period of rapid formation occurs during the first 4–8 h,

followed by a reduction in the formation rate. In general, for most

sources, concentrations of chlorinated HAAs appear to slowlyincrease

even after 96 h, while the formation of DBA levels off after about

18–20 h.

Miller & Uden (1983) observed that nearly 90% of the final

concentrations of THMs, TCA and DCA form within the first 24 h of

chlorine addition to waters containing NOM. Reckhow et al. (1990)

found that althoughwaters containing precursor materials isolated from

sixdifferent watersources differed intheir yields of chlorinated organic

by-products, the formation curves for chloroform, TCA and DCAhad

the same general shapes for all six precursor materials. Some

researchers have suggested that DCA may be an intermediate in TCA

formation; however, for all eight source watersstudied, both DCAand

TCAconcentrationsincreased or remained stable throughout the 96-h

reaction period,suggesting that DCAwas an end-product(AWWARF,

1991). Carlson & Hardy (1998) indicated that HAA formation followed a pattern similar to that of THMformation. As with the THMs,

HAA formation rate appeared to be rapid for the first 30 min; after

30 min, HAAs formed at nearly a constant rate in four of the source

waters studied.

Different trends were observed in the HAN concentrations of

different source waters. For two source waters, HAN levels formed

rapidlyfor the first 8 h and continued to increase slowlyor levelled off

after 96 h (AWWARF, 1991). DBAN levelsremained relativelystable

overthe 96 h, as didBCAN and DCAN levels.For othersources, levels

of HANs consisting mostlyof DCAN increased rapidlyupto 4–8 hand

began to decline by the end of the 96-h period. For these sources,

BCAN appeared to be slightly more stable than DCAN.

Verylow levels of chloropicrin formation have been observed by

many researchers (AWWARF, 1991). The highest concentration

observed was 4.0 :g/litre. Chloropicrin appearsto form slowlyduring

the incubation period, with concentrations tending to level off at

approximately 40 h.
Chemistry of Disinfectants and Disinfectant By-products

63

2.8.5 Effect of temperature

The formationrates of THMs, HAAs, bromate ionandHANshave

been shown to increase with temperature (AWWARF, 1991; Siddiqui

& Amy, 1993). Both haloketone and chloropicrin levels were found to

be higher at a lower temperature, while the concentrations of other

DBP species were similar or not significantly different. These results

suggest that a higher temperature allowsfor more rapid progression of

the transformation of haloketones to other by-products. In studies on

the effect of temperature on THMs, Peters et al. (1980) found an

Arrhenius dependency between the rate constant and temperature with

an activation energy of 10–20 kJ/mol.

The impact of temperature on THMs was strongest at longer

contact times (Carlson & Hardy, 1998). On a conceptual basis, it may

be that rapidly forming compounds are more reactive and form DBPs

regardless of temperature. On the other hand, slowly forming

compounds require higher activation energy, and an increase in the

temperature supplies the energy. In addition to reaction kinetics, the

temperature of a source water can also affect disinfection efficiency.

The biocidal effectiveness ofmonochloramine issignificantlylessthan

that of free chlorine and is dependent on temperature, pH and residual

concentration.

2.8.6 Effect of alkalinity

Although pHis a veryinfluential variable andalkalinity affects pH,

alkalinity itself does not appear to directly affect the formation of

THMs and HAAs (by chlorination) and has only a slight effect on

aldehydes and other organic by-products following ozonation (Andrews

et al., 1996). However, the majority of studies on the effect of

alkalinity on the formation of bromate during ozonation indicate that

increased alkalinityincreases bromate formation (Siddiqui et al., 1995).

Thequantity of aldehydes produced remains approximately constant for

similar changes in alkalinity and pH; however, deviation from

equivalent changes in pH and alkalinity results in increased aldehyde

concentrations. Therefore, conditions of high alkalinityand low pH or

lowalkalinityand high pH produce greater quantities of aldehydesthan

do intermediate values of these parameters (Andrews et al., 1996).
EHC 216: Disinfectants and Disinfectant By-products

64

2.9 Influence of water treatment variables on the

amount and type of by-products produced

Since DBPs are formed byall of the above chemical disinfectants,

the adoption of alternative disinfectants for DBP control often means

only a trade-off between one group of DBPs and another. The most

effective DBP control strategy is organic precursor (NOM) removal

through enhanced coagulation, biofiltration, GAC or membrane

filtration. There has been little success with bromide removal. Other

DBP control options include water quality modifications — for

example, acid or ammonia addition for bromate minimization.

2.9.1 Effect of ammonia

Thepresence of ammonia insourcewaters during disinfection can

cause chlorine and ozone demand and participation in the formation of

by-products such as nitrate, cyanogen chloride and other nitrogenous

compounds.

Ammonia also does not consume chlorine dioxide. In contrast to

chlorine, chlorine dioxide can therefore be considered as a virucide

when ammonia is present. This might be one of the historical reasons

why chlorine dioxide has been adopted as a disinfectant by some

treatment plants using well oxidized waters but containing changing

ammonia concentrations. The addition of ammonia has been shown to

reduce the formationof bromate after ozonation(Siddiqui et al., 1995),

and the ammonia has been shown to participate in the formation of

HANs and cyanogen bromide (CNBr) (Siddiqui & Amy, 1993).

The growth of nitrifying bacteria is a potential problem in

chloraminated water supplies or chlorination of sources containing

nitrogen. In a study conducted by Cunliffe (1991), nitrifying bacteria

were detected in 64% of samples collected from five chloraminated

watersuppliesinSouthAustralia and in 21% of samplesthat contained

more than 5 mg of monochloramine per litre. Increased numbers of the

bacteria were associated with monochloramine decay within the

distribution systems.
Chemistry of Disinfectants and Disinfectant By-products

65

2.9.2 Effect of disinfectant dose

Chlorine dose is a factor affecting the type and concentration of

DBPs formed. The THM level rises with increasing chlorine dose

(Kavanaugh et al., 1980). However, there is some disagreement

regarding the quantitative relations between chlorine concentrationand

THMlevels(or the rate of THMproduction).Most investigatorsfound

a linear relationship between chlorine consumption and THM

production, with an order of reaction greater than or equal to unity

(Trussell & Umphres, 1978; Kavanaugh et al., 1980). However, it is

also possible that the order of reaction changesduring the course of the

reaction.

Reckhow & Singer (1985) found that the concentration of DBP

intermediates such as DCAN and 1,1,1-TCPN formed after 72 h of

reaction time was dependent on chlorine dose. DCAN, which was

measured at a concentration of approximately 5 :g/litre at a chlorine

dose of 10 mg/litre, was not detected in samples dosed with 50 mg of

chlorine per litre. The concentration of chloroform was about

150 :g/litre in a sample dosedwith 10 mg of chlorine per litre but was

approximately 200 :g/litre in a sample dosed with 20 mg of chlorine

per litre. Thus, it is imperative to have uniform chlorine doses for

performing DBP formation kinetic measurements.

Since chloramine residuals are longer-lasting than free chlorine

residuals, the doses for each set of chlorinated and chloraminated

samples will be different in order to achieve the prescribed target

residual. The disappearance of chloramines can be explained approximately by a second-order reaction. However, as the chlorine dose

increased, the observed rate constant was found to decrease, then

increased after reaching a minimum value (Dlyamandoglu & Selleck,

1992). Below the chlorine dose at the minimum value of the observed

rate constant, the rate constant was proportional to the 1.4 power ofthe

chlorine dose, regardless ofthe ammonia concentration (Yamamoto et

al., 1988).

2.9.3 Effect of advanced oxidation processes

Water utilities can add treatment processes that remove DBP

precursors or DBPs. Many utilities will be using both approaches. The
EHC 216: Disinfectants and Disinfectant By-products

66

hydrogen peroxide/UV process, an advanced oxidation process, offers

small water utilities a treatment process with the potential to provide

primarydisinfection and a method of DBP control (Symons & Worley,

1995). This process has been shown to oxidize dissolved organic

halogens and decrease TOC. TOC removal as a function of UV dose

has also been demonstrated by Worley (1994), withTOC removals of

between 0% and 80%. Andrews et al. (1996) evaluated the effect ofthe

hydrogen peroxide/UV process on THMformation and concluded that

this process is only slightly effective in reducing the formation of

DBPs.However, using hydrogen peroxide at 1 mg/litre incombination

with UV effectively reduced or prevented the formation of aldehydes.

Other advanced oxidation processes (e.g., hydrogen peroxide/ozone,

ozone/UV) involving hydroxyl (and hydroperoxyl) radical formation

may provide similar opportunities.

2.9.4 Effect of chemical coagulation

Enhanced coagulationand softening will remove TOC. Enhanced

coagulation is characterized by coagulant doses greater than those

required for optimum turbidity removal; as an alternative to higher

doses, a combination of acid (pH depression) and coagulant addition

can be practised.

All organicDBPswere reduced bythe addition of commonlyused

coagulants. Iron-based coagulants, such as ferric chloride, were

consistentlymore effective than alumin removing NOM(Crozes et al.,

1995). Alumcoagulation removed all DBP precursorsto a significant

extent. The percentage removals showed the same trends as, but were

not identical to, the percentage removals of TOC and UV absorbance.

UV absorbance was removed to a somewhat greater extent than TOC.

Hence, TOC and UV absorbance can serve assurrogate parametersfor

DBP formation potential. A fairly good correlation was observed

between the ratio of HAAs to THMs and the ratio of UV absorbance to

TOC,indicating thatthe relative concentrations of HAAs and THMs do

to some extent depend on the nature of the precursor material.

However, more data from waters of different qualities would be

required to evaluate the validity of this relationship.

The effectiveness of coagulants in removing DBP precursors is

dependent upon the molecular size of the dissolved organic matter.

Normally, higher molecular weight fractions are effectively removed
Chemistry of Disinfectants and Disinfectant By-products

67

through coagulation. In a study conducted byTeng & Veenstra (1995),

water containing dissolved organic matter with molecular weights in

the range 1000–10 000 daltons generallyproduced the largest amounts

of THMs andHAAs under conditions offree chlorination. Coagulation

and ozonation shift a proportionately greater amount of the THM and

HAA formation potential to the smallest molecular weight range

(<1000 daltons).

Coagulation and filtration remove NOM but not bromide, hence

increasing the ratio of bromide to TOC. As a result, the subsequent use

of chlorine generally favours the formation of brominated organic

DBPs.

2.9.5 Effect of pre-ozonation

Several studies of ozone oxidation followed by chlorination

showed increased, rather than decreased, levels of THMs (Trussell &

Umphres, 1978). This is attributed, at least partially, to the formation

of aldehydes by ozonation. Another possibility is hydroxylation of

aromatic compounds to produce m-dihydroxy aromatic derivatives,

which are known THM precursors (Lykins & Clark, 1988). Although

the aldehydes produced contain polar groupings, they are nevertheless

not easily removed during the flocculation step by complexation with

aluminiumorironsalts.Aconvenient andmore appropriatemethodfor

the removal of the aldehydes formed during ozonation is the incorporation of a biological treatment step (biofiltration) in the water

treatment process following ozone oxidation.

Pre-ozonationcanhavebothpositive and negative effects onDBP

formation. Pre-ozonation with typical water treatment dosages and

bicarbonate levels has been shown to remove TCA and DCAN

precursors. However,such treatment can result in no net change in the

DCAprecursors and maylead to an increase in1,1,1-TCPN precursors

(Reckhow&Singer, 1985). According to Teng&Veenstra (1995), preozonation resulted in enhanced formation of DCAduring chlorination

and chloramination in the presence of precursors in the <1000 dalton

molecular weight range. They also indicated that pre-ozonation plus

chloramination controlled the overall production of THMs and HAAs.

However, the use of pre-ozonation coupled with free chlorination
EHC 216: Disinfectants and Disinfectant By-products

68

increased the yield of DCA for both the hydrophilic and hydrophobic

fractions of NOM as compared with free chlorination alone.

With ozone–chlorine treatment, chloral hydrate formation can be

enhanced. This behaviour, which has also been observed for DCA,

suggests that the reaction that produces chloral hydrate is accelerated

underthe conditions of ozonation in combinationwithprechlorination

and warm water temperatures (LeBel et al., 1995).

Ozonation in the presence of traces of hypochlorite ion can form

inorganic by-productssuch as chlorate.Siddiqui et al. (1996a) showed

that if there is any residual chlorine present, ozone can potentially

oxidize hypochlorite ion to chlorate.

Coleman et al. (1992) suggested that brominated MX analogues

and other mixed bromochlorinated by-productsformed after ozonation

and chlorination can possibly increase mutagenic activity.

2.9.6 Effect of biofiltration

Biofiltration (ozone–sand filtration or ozone–GAC) can potentially reduce TOC, organic by-products and the formation of halogenated DBPs.

Passage of ozonated water samples through a rapid sand filter

reduced the concentration of aldehydes by 62% (Lykins & Clark, 1988).

Chlorinated samples experienced a 26% reduction in aldehyde

concentrations underthe same conditions.These reductionsinaldehyde

levels are attributable to biological activity in the sand filters. If GAC

filtration follows sand filtration, ozone oxidation can be expected to

promote more bioactivity in the GAC filter, because a better

colonization environment is provided for microorganisms on GAC

particles than on sand. Thus, the biological conversion of oxidized

water impurities to carbon dioxide and water will be greater during

passage through GAC media. Similar aldehyde removals have been

observed byseveral researchers(Van Hoof et al., 1985;Sketchell et al.,

1995).

Drinking-watertreatment techniquesthat remove organic contaminants without affecting bromide concentrations cause a shift in the

formation of DBPs towards brominated DBPs. Sketchell et al. (1995)
Chemistry of Disinfectants and Disinfectant By-products

69

studied three sources containing three different DOC levels and ambient

bromides, which were filtered through biologicallyactive GAC filters.

Analysis of treated waters showed no removal of bromide ion and a

shift towards more brominated organo-DBPs. THM levels after

treatment withGAC with no addedozone decreased from900–1700 to

100–700 :g/litre. These watersources contained DOC levels ranging

from 10 to 25 mg/litre and high concentrations of biodegradable DOC

(DOC removals ranged from 60% to 80% after GAC treatment).

Table 8 summarizes the effects of ozonation and biofiltration on

the formation of DBPs from various sources.

2.10 Comparative assessment of disinfectants

A comparative assessment (Table 9) of various disinfecting

chemicalsfor pre-disinfection (or oxidation) andpost-disinfection and

maintaining a residual for 5 days to simulate concentrations in the

distribution system showed that the use of free chlorine produces the

largest concentration of halogenated DBPs (Clark et al., 1994). The

concentration of DBPs may be reduced by adding ozone or chlorine

dioxide as a preoxidant, although enhanced formation has been

observed.

Table 10 summarizes the effects of water quality and treatment

variables on the formation of DBPs.

2.11 Alternative strategies for disinfectant by-product

control

The concern about chlorite, bromate, chlorate and other DBPsin

drinking-water following treatment with disinfectants has stimulated

research into ways to eliminate the production or enhance the removal

of DBPs. Strategiesfor DBP control include source control, precursor

removal, use of alternative disinfectants and removal of DBPs by

technologies such as air stripping, activated carbon, UV light and

advanced oxidation technologies. For DBPs that can arise in

hypochlorite solutions(e.g., chlorate),the purityandstorage conditions

of the solutions are important concerns.
Table 8. Effects of ozonation and biofiltration on chlorine organic by-products

DBPs Ozonation

(% change)

Biofiltration

(% change)

Ozonation + biofiltration

(% change)

Reference

THMs !20 !20 !40 (chlorine) Speitel et al. (1993)

HAAs !10 !13 !25 (chlorine) Speitel et al. (1993)

Chloropicrin +50 to +250 !50 to !100 (chlorine) Miltner et al. (1992)

Aldehydes +425 to +1300 !40 to !50 !92 to !98 Miltner et al. (1992)

TOX !30 !51 (chlorine) Miltner et al. (1992)

TOX !10 !38 !47 (chlorine) Shukairy & Summers (1992)

TOX +32 !69 !60 (monochloramine) Shukairy & Summers (1992)
Table 9. Comparative assessment of organic disinfectant by-products (:g/litre) in distribution systemsa,b

DBPs Sand–Cl2 Cl2–Sand–Cl2 O3–Sand–Cl2 NH2Cl–Sand–NH2Cl O3–Sand–NH2Cl ClO2–Sand–Cl2

THMs 236.0 225.0 154.0 9.0 3.2 138.0

HAAs 60.0 146.0 82.0 14.0 9.0 44.0

HANs 3.1 2.9 2.7 <0.1 <0.1 <0.1

Haloketones 2.1 2.6 2.6 <0.1 <0.1 4.2

Chloropicrin 1.3 1.3 7.7 <0.1 <0.1 1.4

Chloral hydrate 79.0 75.0 55.0 <0.1 <0.1 45.0

TOX 557.0 540.0 339.0 59.0 27.0 379.0

a Clark et al. (1994). 

b TOC = 3.0 mg/litre; pH = 7.6.
Table 10. Summary of impact of water quality and treatment variables on disinfectant by-product formation

Variable Impact on THMs Impact on HAAs Impact on aldehydes Impact on chlorate/chlorite Impact on bromate

Contact time Curvilinear increase

with increasing contact

time

Rapid formation <5 h

90% formation in 24 h

Levels off at 96 h

Curvilinear increase with

increasing contact time

Rapid formation <5 h

90% formation in 24 h

Levels off at 150 h

Linear increase as long

as residual chemical

present

Secondary reactions

between disinfectants

and aldehydes possible

Linear increase in bleach

solutions

No discernible effects in

dilute solutions

If oxidation of

hypochlorite, contact time

has a positive effect

Curvilinear increase with

most bromate forming in

<5 min

Formation is a function of

ozone residual and

bromide

Disinfectant

dose

Rapid and curvilinear

increase after TOC

demand with dose,

levelling off at 2.0

mg/litre for TOC of 2.0

mg/litre

Curvilinear increase

after TOC demand with

increasing dose,

levelling off at 2.0

mg/litre

Curvilinear with

increasing ozone dose

or chlorine dose

No appreciable effect

after ozone/DOC = 2 : 1

Concentrations related to

hypochlorite doses

applied

Ozone oxidation of

hypochlorite increases

with dose

Linear increase after

TOC demand and then

levelling off after ozone

residual disappearance

pH Curvilinear increase

with increasing pH to

pH 7.0 and possible

pH maximum

No positive effect at pH

> 9.5

Mixed, possible pH

maximum for DCAA and

DBAA

TCAA decreases up to

pH > 9

DCAA maximum at pH

7–7.5

Negative effect (forms

mostly through molecular

ozone)

25% decrease for pH

7–8.5

Positive effect

Decomposition of hypochlorite increases with pH

Oxidation of hypochlorite

by ozone increases

Strong linear positive

effect

Hydroxyl radical

generation efficiency

increases
Table 10 (Contd).

Variable Impact on THMs Impact on HAAs Impact on aldehydes Impact on chlorate/chlorite Impact on bromate

91

Temperature Linear increase with

increasing temperature

(10–30 °C; 15–25%

increase)

Linear increase with

increasing temperature

(10–30 °C; 20–30%

increase)

Terminal products such

as carbon dioxide

increase and total

aldehydes slightly

decrease

Positive effect

Decomposition of

hypochlorite increases

Curvilinear increase

20–30% increase for

15–25 °C

TOC Increase with

increasing TOC;

precursor content

important

Humic acids more

reactive than fulvic

acids

Increase with increasing

TOC; precursor content

important

Humic acids more

reactive than fulvic acids

Positive effect

(hydrophobic fraction

mostly responsible)

Doubles for every 2

mg/litre 

Negative effect if ozone is

used for hypochlorite

oxidation

Most likely no effect with

hypochlorite

Decreases with

increasing TOC;

precursor content

important

Non-humic acid being

less reactive with ozone

UVA254 Increase with

increasing UV

absorbance; precursor

content important

Aromaticity of TOC

being more important

Increase with increasing

UV absorbance;

precursor content

important

Aromaticity of TOC

being more important

Positive effect

Ozone demand

increases with UV (UV

absorbance is mostly

due to aromaticity and

hydrophobic fraction)

Negative effect if ozone is

used for hypochlorite

oxidation

Probable negative effect

with hypochlorous acid

Decreases with

increasing UV

absorbance; precursor

content important

Humic acid being more

reactive with ozone

Bromide Shift towards

brominated species

Shift towards

brominated species

Independent of bromide

at <0.25 mg/litre

At >0.25 mg/litre,

aldehydes can decrease

due to ozone–bromide

oxidation

Shift towards more toxic

bromate in hypochlorite

solutions

Bromide threshold

Curvilinear increase and

dependent upon ozone

residual

Alkalinity No discernible effect No discernible effect Slight positive effect Unknown Positive effect
Table 10 (Contd).

Variable Impact on THMs Impact on HAAs Impact on aldehydes Impact on chlorate/chlorite Impact on bromate

Minimization

strategies

TOC removal,

minimizing chlorine

residual, alternative

disinfectants, pH

control, minimizing

contact time

TOC removal,

minimizing chlorine

residual, alternative

disinfectants, pH control,

minimizing contact time

pH control, TOC removal

by coagulation, GAC,

optimizing doses,

contact time

Avoid hypochlorite dosing

solution Minimize storage

Properly tune generators

Use freshly made

solutions

pH depression, ammonia

addition, radical

scavengers, minimizing

and optimizing ozone

residual

Removal

strategies

GAC, electron beam,

air stripping

GAC, electron beam Biofiltration, advanced

oxidation, GAC,

nanofilters

Ferrous sulfate, GAC,

electron beam, UV

irradiation, nanofilters

Ferrous sulfate, UV

irradiation, high-energy

electron beam, GAC
Chemistry of Disinfectants and Disinfectant By-products

75

2.11.1 Source control

Source control options involve controlling nutrient inputs to

waters(e.g., algae growth control) (Hoehn et al., 1990)that are used as

drinking-water sources, watershed management (e.g., constructing

stormwater detentionbasins),saltwaterintrusioncontrol (e.g., development of structural or hydrodynamic barriers to control TOC and

bromide), and using the concept known as aquiferstorage andrecovery

(e.g., drawing water during seasons when the quality of the water is

best) (Singer, 1994a).

2.11.2 Organohalogen by-products

Strategies for control of organohalogen by-products include

removal of DBPsthat are formed using technologiessuch as oxidation,

aeration and carbon adsorption (Clark et al., 1994); and removal of

precursors using treatment techniquessuch as conventional treatment,

oxidation, membrane processes, carbon adsorption and biological

degradation. For manyorganic compoundsthat are difficult to oxidize,

such as chloroform, the kinetics of ozone oxidation are generally very

slow but are faster if used in combination with UV irradiation. GAC

adsorption and membrane filtration are relativelyexpensive processes;

moreover, NOM removal by GAC cannot be accomplished to any

significant degree in a filter/adsorber (i.e., GAC filter cap) mode but

requires a separate post-filtration adsorber bed. The use of membranes

requires pretreatment to prevent fouling, as well as processing of waste

brine. The use of ozone in combination with biologically active GAC

filters is a promising alternative to reduce DBP precursors.

2.11.3 Inorganic by-products

Properly designed and operated chlorine and chlorine dioxide

generatorsystems canminimize some ofthe productionof chlorate ion.

Removal of chlorite and chlorate has been reported using reduction by

Fe2+ or sulfite or by GAC (Voudrias et al., 1983; Lykins & Clark,

1988). GAC isseen as problematic because of chlorate productionand

a short bed life. A chemical process using an appropriate agent such as

reduced iron (e.g., ferrous sulfate) appears to be a more promising

approach (Kraft & van Eldick, 1989; Gordon et al., 1990).
EHC 216: Disinfectants and Disinfectant By-products

76

If bromate is present in treated water entering the coagulation

process(i.e.,formed during pre-ozonation),several options existforits

removal. An aqueous-phase reducing agent (e.g., Fe2+) can be added at

the rapidmix step. Powdered activated carbon canlikewise be added as

a solid-phase reductant to remove bromate andDBP precursors.Not all

utilities contemplating ozone application intend to employ preozonation. Rather, they may use intermediate ozonation prior to the

filtration process; in this situation, removal of bromate by activated

carbon is possible.This approach has potential relevance to integration

of GAC columns into a process train or, more realistically, to retrofitting of rapid sand filters with GAC filters. For groundwaters that

require no coagulation, bromate can be removed after ozonation using

a GAC filter, UV irradiation or high-energy electron beam irradiation

(Siddiqui et al., 1994, 1996a,b,c).

Brominated or bromochlorinated amines formed during the oxidation step of the processtrain using chlorine can potentiallybe removed

using a suitable activated carbon before terminal chlorination. However, carbonthat has an accumulation of surface oxides, which develop

through reaction of amines, will have a diminished capacity to reduce

halogenated amines to nitrogen. Organic amines can potentially be

removed by activated carbon adsorption.

2.11.4 Organic by-products

There are some technologies for removing organic contaminants

formed after chlorination and chloramination, a less viable option than

minimizing their formation through DBP precursor removal or use of

alternative disinfectants. Studies of ozone oxidation have shown that

aromatic compounds, alkenes and certain pesticides (some of which

have structuralsimilaritiesto certain organic DBPs) are removed well

byozone treatment, but that alkanes are poorlyremoved. Also,removal

efficiency improves for the alkenes and aromatic compounds with

increasing ozone dosage and for some alkanes with increasing pH. For

most compounds, the efficacy of ozone is not affected by the background water matrix if the ozone is used after coagulation. Andrews et

al. (1996) showed that using hydrogen peroxide at 1.0 mg/litre in

combination with UV effectively reduced or prevented the formation

of aldehydes.
Chemistry of Disinfectants and Disinfectant By-products

77

2.12 Models for predicting disinfectant by-product

formation

The regulation of THMs and other halogenated DBPs has been

complicated by findings that alternative disinfectants to free chlorine

may also form by-products that are of potential health concern.

Additional complicating factorsimpacting the regulationof DBPs have

been the emergence of Giardia and Cryptosporidium as major waterborne pathogens.

In view of the finding that water chlorination produces DBPs,

some of which are carcinogenic, mutagenic or possibly teratogenic,

several countries have recently laid down standards for various DBP

levels. Thisstimulated the search for mathematical modelsto describe

or predict DBP formation in disinfected water and to evaluate the

effectiveness of water treatment technologies designed to reduce DBP

levels so as to comply with the standards. Most of these models are

based on fitting mathematical equationsto various empirical observations,ratherthanmechanistic andkinetic considerations. Thisis mainly

due to the complexityof the reactions between organic precursors and

disinfecting chemicals,which usuallyinvolve several parallel pathways

leading to a great variety of products. The complexity of the DBP

formation reactions also makes it difficult to develop universally

applicable models for simulating DBP formation potential associated

with disinfection of a diverse array of naturalsource waters. However,

the analysis presented by many models suggests that many waters

exhibit comparable general responses to changes in a given parameter

(i.e., responses lending themselves to simulation by a particular

mathematical functionality), although specific responses associated

with individual waters may vary. The multiple regression models

developed by many researchers represent a rational framework for

modelling DBP formation in many sources. Another potential application is the modelling of DBP mixtures, e.g., predicting HAA levels

from THM and water quality data.

2.12.1 Factors affecting disinfectant by-product formation and

variables of interest in disinfectant by-product modelling

The information on the factors controllingDBP formation, which

is available in the literature, is briefly summarized below.
EHC 216: Disinfectants and Disinfectant By-products

78

The extent of formation of DBPs is dependent on several water

quality parameters, such as TOC concentration, UVA254, bromide

concentration and temperature. It is also dependent on chlorination

conditions, such as chlorine dose, pH, ammonia concentration and

contact time. After the various statistically significant factors were

identified, mathematical equations were developed to describe the

formation of various DBPs. A least squares method was used to

determine the optimum equation coefficients that best describe the

experimental data.Theoptimumcoefficients have been defined as those

that produce a minimum residual error between the mathematical

predictions and the experimental data.

2.12.2 Empirical models for disinfectant by-product formation

Numerousmodelsfor predicting THMformation through chlorinationhavebeenreported (Moore et al., 1979a;Kavanaugh et al., 1980;

Engerholm & Amy, 1983; Urano et al., 1983;Amyet al., 1987, 1998;

Morrow& Minear, 1987; AWWARF,1991; Hutton & Chung, 1992).

Of these, modelsreported byAWWARF (1991) and Amyet al. (1998)

are more recent and were derived from a variety of natural source

waters and more realistic treatment conditions. Not much information

has been reported on the formation of other chlorination DBPs. Only

Amyet al. (1998) summarized empirical modelsfor THMs, HAAs and

chloral hydrate. These chlorination by-product models can be used to

assess both in-plant and distributionsystemformationof THMs,HAAs

and chloral hydrate. Water quality conditions such as DOC, pH,

temperature and bromide are needed asinputsto the models; such data

then allow assessment of chlorination DBP formation as a function of

reaction time:

DBP concentration (total THMs or THM species, total HAAs or

HAA species, or chloral hydrate) =

f(TOC, bromide, chlorine, pH, temperature, time)

Relatively little is known about the kinetics of the formation of

bromate and other DBPs during ozonation and the quantitative effects

of water quality factors (temperature, pH, etc.);such an understanding

is crucialfor evaluating variousbromate controlstrategies. Siddiqui &

Amy (1993) and Amy et al. (1998) developed statistical relations to

predict the concentrations of various ozone DBPs, including bromate,

as a function of water treatment variables. Correlation matrix analysis
Chemistry of Disinfectants and Disinfectant By-products

79

has shown that ozone dose, dissolved ozone concentration, bromide

concentration, pH and reaction time all have a positive influence on

bromate formation. Von Gunten & Hoigne (1994) developed kinetic

models for bromate formation.

Ozone, as a result ofitsstrong oxidizing power, produces a variety

of organic by-products, such as aldehydes and ketoacids, when used to

treat natural source waters. These by-products — especially aldehydes

— are highly biodegradable, and there is concern for regrowth of

microorganisms following ozone treatment. They are also potentially

hazardous and may produce increased amounts of chlorinated byproducts upon chlorination. Siddiqui et al. (1997) developed a model

to estimate the potential for total aldehyde formation in source waters

upon ozonation.

2.12.3 Models for predicting disinfectant by-product precursor

removal

It is recognized that chlorination will continue to be the most

common disinfection process; hence, enhanced removal of DBP

precursors present in raw sources represents a valuable option for

reducing the potential for by-product formation. The removal of NOM

can be achieved either by providing additional processes,such as GAC

and nanofiltration, or by enhancing the existing coagulation, flocculation and sedimentation processes. Predictive models have been developed for assessing coagulation efficiency in removing NOM and

reducing DBP precursor levels (AWWARF, 1991; Amy et al., 1998).

Coagulation can reduce DOC and DBP precursors but not

bromide levels; hence, a greater proportionof brominatedDBP species

can potentially be produced in the finished water.

The effects of precursor removal bychemical coagulation can be

assessed through the use oftreatedwatermodels.One caneither predict

DBPs formed under a given degree of precursor removal or define the

degree of precursor removal required to meet DBP regulations. The

impact of bromide ion on meeting regulations can also be assessed. If

one makes the assumption that precursor reactivity (i.e., DBP/DOC)

does not change,one canalso assess other precursorremoval processes,

such as GAC or membrane processes,through use of the raw/untreated
EHC 216: Disinfectants and Disinfectant By-products

80

water models. Care should be exercised when using models to

approximate post-chlorination DBPs following an ozonation step.

2.13 Summary

• The primary and most important role of drinking-water treatment

is to remove or inactivate harmful microorganisms. Another role

is to minimize the concentrations of disinfectants and DBPs

without compromising in any way the removal or inactivation of

pathogens.

• Drinking-water utility managers must be more knowledgeable

about optionsto meet regulations.It is oftenmore practical to use

treatment methods that control the concentration of several

contaminants than to modify treatment practices for each new

standard that is promulgated. 

• A thorough understanding of DBP formation would help the

successful balancing of appropriate microbial inactivation with the

minimization of DBPs. Water quality variables affect DBP

formation and must be considered when developing a strategy to

control DBPs with various disinfectants.

• The chemistry of chlorine and its by-products has been well

studied, and ozone and its by-products have recently received

much attention. Studies of chlorine dioxide and chloramines and

their by-products are relativelyfew, although more work in these

areas is now being undertaken.

• One of the simplest processes to minimize halogenated DBP

formation is limiting the free chlorine contact time by using

monochloramine to maintain a distribution system residual

following primarydisinfectionbychlorine or ozone. Chloramines

are an effective means of controlling DBPs. However,the growth

of nitrifying bacteria (and related production of nitrite) is a

potential problem in chloraminated water supplies.

• Various nitrogen-containing organic compounds may be present

in source waters after chlorination and chloramination. Because

of analytical complexities, very few detailed studies have been

undertaken to 
Chemistry of Disinfectants and Disinfectant By-products

81

determine the individual compounds present and their concentrations.

• Many factors between the source and the tap can influence the

DBPs to which consumers are exposed. Although THMs and

HAAs continue to form with increasing contact time, some other

halogenated DBPs, such as HANs and haloketones, form rapidly

but then decayin the distribution system as a result of hydrolysis.

This has major implications regarding exposure to these DBPs,

dependingupon their proximityto the treatment plant. For treated

source waters, median levels of HAAs are often approximately

one-half of the median THM levels.

• For low-bromide source waters, chloroform is normally the

dominant THM species; DCA and TCA are the most prevalent

HAA species; DCAN is the most prevalent HAN species; and

1,1,1-TCPN is the most prevalent of the two measured haloketones. Very low levels of chloropicrin have been observed by

variousresearchers;this compound appearsto formslowlyduring

the incubation period, with concentrations tending to level off at

40 h. For high-bromide waters, increased levels of brominated

DBPs are observed.

• Chlorine dioxide is a strong oxidant that under certain conditions

surpasses chlorine in its ability to destroy pathogenic organisms.

When chlorine dioxide is prepared and administered without

excessfree chlorine, THMs and other chlorinated by-products are

not produced, but inorganic by-products are formed. 

• TOC levels have been found to be correlated with halogenated

DBP formation. The nature of this relationship varies with the

source. TOC removal can be used as a surrogate for the reduction

of DBP formation.

• Although the presence of chloral hydrate and HANsin chlorinated

samples may be attributed to precursors other than amino acids,

the potential for amino acids to be present in natural sources is

well documented. Surface waters, but not groundwaters, tend to

contain amino acids. However, the removal of these precursors

during conventional water treatment is not well understood.
EHC 216: Disinfectants and Disinfectant By-products

82

• The amount of chlorate that is present in delivered hypochlorite

solutions depends on many factors. Freshly made hypochlorite

solutions will containless chlorate than hypochlorite thatisstored

without concern for temperature and pH. If a utility is using a

single tank to store hypochlorite, it is likely that the level of

chlorate is increasing in the tank. Thus, storage tanks should be

periodicallyflushed and cleaned, and, if possible, the storage time

should be reduced.

• Models have been developed that can be used to simulate the fate

and movement of DBP precursors in distribution systems. The

models can be designed as a planning tool for evaluating the

impacts of source water management strategies and estimating

DBP exposures. Some limitations of existing models include

calibration with a limited database, application to only a specific

watersource or group of related sources, lack oftermsto simulate

important parameters, such as reaction time, and inadequate

validation.
83

3. TOXICOLOGY OF DISINFECTANTS

In assessing the hazards associated with drinking-water disinfection, it is important not to neglect the disinfectants themselves.

Adding disinfectant in excess of the demand has several practical

benefits. First, it ensures that reaction of the disinfectant with DBP

precursors (largely organic material and ammonia) does not shorten

contact time to the point of ineffective disinfection. Second, residual

disinfectant helps to prevent regrowth of organisms in the remaining

portions of the treatment and distribution systems.

The result of this practice, however, is that one of the chemicals

that is present in the finished water at the highest concentration isthe

disinfectant. In the present regulatory climate in many countries,

chemicals that are introduced as direct additives to food would be

subjected to a significant amount of toxicological screening before

they could be used. Since the major disinfectants were introduced

almost 100 years ago, they were subjected to much less thorough

toxicological evaluations than would be required today. However,

many of these data gaps have been addressed in the past decade.

3.1 Chlorine and hypochlorite

3.1.1 General toxicological properties and information on dose–

response in animals

Chlorine gas has long been recognized as a lung irritant. This

topic will not be reviewed in the present document, as it appears to be

largely irrelevant to the small amounts of chlorine that are volatilized

from chlorinated water in showers or other points of use in the household. In water treatment plants, however, there is a possibilityof occupational exposures that could have severe sequelae. For information

on these higher-level exposures, the interested reader is referred to a

recent review by Das & Blanc (1993). The effects of chlorine gas that

have been observed in humans will be discussed in section 3.1.3.

Sodium hypochlorite (NaOCl) or calcium hypochlorite

(Ca(OCl)2) solutions have also been utilized extensively in the

disinfection of drinking-water. The stock solutions used for this
EHC 216: Disinfectants and Disinfectant By-products

84

purpose are highly caustic and are a clear concern for occupational

exposures. The concentration required to produce irritation and

decreased basal cell viability in the skin of guinea-pigs after an

application period of 2 weeks was 0.5% sodium hypochlorite (Cotter

et al., 1985). Reducing the concentration to 0.1% resulted in no effect

on basal cell viability relative to control animals. Yarington (1970)

demonstrated that instillation of bleach into the oesophagus of dogs

produced irritation.Theminimal exposure that produced oesophageal

burns was 10 ml of commercial bleach with a 5-min exposure. It

should be noted that the highly alkaline pH (about pH 11) of sodium

hypochlorite is not likely to be encountered in drinking-water.

There have been relatively few evaluations of the effects of

chlorine or hypochlorite in drinking-water. The present review will

focus on studies with treatment periods longer than 4 weeks where

drinking-water wasthe primaryrouteof exposure.Reference toearlier

studies of shorter duration and less general applicability to a safety

evaluation can be found in previous reviews (Bull, 1980, 1982a,b,

1992; Bull & Kopfler, 1991).

Daniel et al. (1990a) evaluated the toxicity of solutions of

chlorine prepared by bubbling chlorine gas into distilled water and

adjusting the pH to 9.4. The nominal concentrations of chlorine used

were 0, 25, 100, 175 or 250 mg/litre in distilled water (approximately

0, 3, 10, 16 or 21 mg/kg of body weight per day). These solutions

were provided as drinking-water to both male and female SpragueDawley rats (10 per sex per dose) for 90 days. No deaths occurred in

any treatment group. However, there were statistically significant

decreases in drinking-water consumption in femalestreated with 100

mg/litre and higher, probably due to decreased palatability. There

were no consistent effects of chlorine treatment on organ to body

weight ratios or clinical chemistry parameters. A no-observed-effect

level (NOEL) of 10 mg/kg of body weight per day was identified by

the authors based on reduced body weight gain. However, since this

was associated with reduced palatability of the drinking-water, it is

not considered to be a true toxicological end-point.

The study in rats was followed up with another study in B6C3F1

mice (Daniel et al., 1991a). Male and femaleB6C3F1mice (10 per sex

per group) were administered 12.5, 25, 50, 100 or 200 mg of chlorine

per litre of drinking-water for 90 days (calculated mean daily doses
Toxicology of Disinfectants

85

were 2.7, 5.1, 10.3, 19.8 or 34.4 mg/kg of body weight in males and

2.8, 5.8, 11.7, 21.2 or 39.2 mg/kg of body weight in females). Spleen

and liver weights were depressed in males, but not in females, at the

highest dose rates (100 and 200 mg/litre). There were no other consistent indications of target organ effects based on serum enzyme

concentrations. No gross or microscopic lesions could be related to

treatment with chlorine.

Several of the following studies utilized solutions of sodium

hypochlorite as the treatment chemical. It is now known that such

solutions can contain very high concentrations of chlorate within a

short time of their preparation (Bolyard et al., 1993). The extent of

this contamination has not been reported.

Hasegawa et al. (1986) examined the effects of much higher

concentrations(0.025, 0.05, 0.1, 0.2 or 0.4%) ofsodium hypochlorite

(equivalent to 7, 14, 28, 55 and 111 mg/kg of body weight per day)

administered in drinking-water to male and female F344 rats for

13 weeks. Twenty rats of each sex were assigned to each experimental

group. Significantsuppression of bodyweight(as a result ofdecreased

consumption of water and food) occurred at 0.2% and above. The

authors noted slight damage to the liver as indicated by increased

levels of serum enzymes (not specified) at 0.2% and 0.4% sodium

hypochlorite in both sexes. No evidence of treatment-related

pathology was observed in this study or in a 2-year study in which

males were subjected to 0.05% or 0.1% (13.5 or 27.7 mg/kg of body

weight per day) and females to 0.1% or 0.2% (34 or 63 mg/kg of body

weight per day) sodium hypochlorite. The extended exposures were

conducted with 50 animals of each sex per treatment group. Analysis

of dosing solutions was not reported.

In a 2-year bioassay, the National Toxicology Program (NTP)

examined chlorine at 0, 70, 140 or 275 mg/litre (expressed as atomic

chlorine, Cl) in drinking-water of F344 rats and B6C3F1 mice (70 per

sex per group) (NTP, 1992). These solutions were prepared from

gaseous chlorine and neutralized to pH 9 by the addition of sodium

hydroxide. Stability studies indicated that 85% of the initial target

concentration remained after 3 days of preparation. Stock solutions

(concentrations not specified) were prepared once weekly, and solutions for drinking were prepared 4 times weekly. Based on body

weight and water consumption, doses in these studies were
EHC 216: Disinfectants and Disinfectant By-products

86

approximately 0, 4, 7 or 14 mg/kg of body weight per day for male

rats; 0, 4, 8 or 14 mg/kg of body weight per day for female rats; 0, 7,

14 or 24 mg/kg of body weight per day for male mice; and 0, 8, 14 or

24 mg/kg of bodyweight per day for female mice. The onlytreatmentrelated non-tumour pathology was found to be a dilatation of renal

tubules in male mice receiving 275 mg/litre for more than 66 weeks.

No non-neoplastic lesionswereobserved in either male or female rats.

A number of immunological changes have been associated with

the treatment of rodents with sodium hypochlorite in drinking-water.

Water containing 25–30 mg of sodium hypochlorite per litre was

found to reduce the mean number of peritoneal exudate cells

recovered from female C57BL/6N mice after 2 weeks of treatment.

This was, in turn, associated with a significant decrease in

macrophage-mediated cytotoxicitytomelanoma and fibrosarcoma cell

lines (Fidler, 1977). The treatment period was increased to 4 weeks

in a subsequent study, which demonstrated that 25 mg of sodium

hypochlorite per litre decreased the ability of peritoneal macrophages

to phagocytose 51Cr-labelled sheep red blood cells. Macrophages

obtained from themice treated with hypochloritewere found to be less

effective in destroying B16-BL6 melanoma cells in vitro. Mice so

treated were also found to have increased pulmonary metastasis of

B16-BL6 cells when they were introduced by subcutaneous injection

(Fidler et al., 1982).

Exon et al. (1987) examined the immunotoxicological effects of

sodium hypochlorite at 5, 15 or 30 mg/litre (0.7, 2.1 or 4.2 mg/kg of

body weight per day) in the drinking-water of male Sprague-Dawley

rats (12 per dose) for 9 weeks. Delayed hypersensitivity reaction to

bovine serum albumin was observed at the highest dose administered.

Oxidative metabolism by adherent resident peritoneal cells was

decreased at 15 and 30 mg/litre, and the prostaglandin E2levels of

these cells were found to be significantly elevated. No effects on

natural killer cell cytotoxicity, antibody responses, interleukin 2

production or phagocytic activity were observed. The effects on

macrophage activity suggest that some impairment does occur at

relatively low levels of sodium hypochlorite. As pointed out by the

authors, these were relatively mild effects, the significance of which

was unknown. It is not clear that these effectswould be translated into

a significant impairment of the immune response to a particular

infectious agent. However, modification of macrophage function

appears to be one of the most sensitive responses identified in studies
Toxicology of Disinfectants

87

of chlorine or hypochlorite in experimental animals. A studyin which

female C57BL/6 mice were administered hypochlorite in their drinking-water (7.5, 15 or 30 mg of hypochlorite per litre) for 2 weeks

showed no effects on the immune system as measured by spleen and

thymusweight, plaque-formingcellresponse, haemagglutination titre

and lymphocyte proliferation (French et al., 1998).

Altered liver lipid composition has been observed as a result of

acute intragastric administration of sodium hypochlorite (5 ml of a

1% solution) to rats (Chang et al., 1981). These data do not provide

a clear indication of whether these effects might give rise to

pathology. The concentrations of hypochlorite utilized were much

greater than those that would be encountered in drinking-water.

The effects of hypochlorous acid and hypochlorite on the skin

have received relatively little attention despite the current interest in

bathing as a significant source of chemical exposure from drinkingwater. Robinson et al. (1986) examined the effects of both

hypochlorite and hypochlorous acid solutions applied to the skin of

the entire body of female Sencar mice except for the head. Exposures

were to 1, 100, 300 or 1000 mg/litre as hypochlorous acid at pH 6.5

for 10 min on 4 consecutive days. Hypochlorite (formed by raising the

pH to 8.5) was studied only at 1000 mg/litre. Significant increases in

epidermal thickness and cell counts within the epidermal layer were

observed at concentrations of hypochlorous acid (pH 6.5) of 300

mg/litre and above, but the thickness of the skin was not significantly

different from that in animals at 100 mg/litre. The increases in skin

thickness were associated with an epidermis whose thickness was

increased to 4–6 cells as compared with the normal 1–2 cells seen in

mice. The effects of hypochlorite were much less marked. Following

a single application, the increased thickness of the skin observed in

mice exposed to hypochlorous acid (i.e., pH 6.5) did not appear until

4 days after the treatment. This differed from hypochlorite, other

disinfectants and the positive control, 12-O-tetradecanoylphorbol-13-

acetate (TPA). In the latter cases, the maximal response was observed

within 24–48 h after treatment. The hyperplastic response to

hypochlorous acid required 12 days to return to normal. This study

suggests a considerable margin of safety between the concentrations

of chlorine required to produce hyperplasia and those that are found

in drinking-water.
EHC 216: Disinfectants and Disinfectant By-products

88

The reactive nature of chlorine always raises questions of

whether it is chlorine or a by-product that isresponsible for any effect.

Several studies have examined the formation of by-products in the

gastrointestinal tract following the administration of chlorine.

Invariably, these studies have involved the administration of chlorine

or hypochlorite by gavage at very high concentrations relative to the

amountsthat would be encountered in chlorinated drinking-water. As

a consequence, the by-products formed following gavage dosing of

high concentrations may not be representative of the by-productsthat

would be seen following the consumption of modest to moderate

levels of chlorine in larger volumes of water. A particular issue is that

the high organic carbon concentration relative to chlorine that would

be encountered in the gastrointestinal tract when water is consumed

at low concentrations should dissipate disinfectant before sufficient

oxidative power would be present to break down substrates to small

molecules. Despite these design flaws, the data do indicate that byproducts are formed. The bulk of them remain as higher molecular

weight products, which may have little toxicological importance.

Vogt et al. (1979) reported that chloroform could bemeasured in

the blood, brain, liver, kidneys and fat of rats to which sodium hypochlorite was administered by gavage at doses of 20, 50 or 80 mg in

5 ml of water. Thus, the by-product chloroform can be formed by the

reaction of chlorine with stomach contents.

Mink and co-workers(1983) pursued this observation and found

that other by-products could be detected in the stomach contents and

plasma of rats that had been administered sodium hypochlorite

solutions neutralized to pH 7.9. In addition to chloroform, DCAN,

DCA and TCA were detected in the stomach contents analysis. DCA

and TCA were also detected in blood plasma.

The third group of compounds identified as by-products of

chlorination in stomach contents of the rat are the organic Nchloramines (Scully et al., 1990). N-Chloroglycine, N-chloroleucine

or N-chloroisoleucine and N-chlorophenylalanine were confirmed

products of reactions with normal amino acids that would ordinarily

be found in the gastrointestinal tract. N-Chlorovaline and Nchloroserinewerealso tentativelyidentified. Organic chloramines are

reactive and could be responsible for toxic effects that may be

attributed to chlorine in toxicological studies. The chlorine demand
Toxicology of Disinfectants

89

of free amino acids in stomach contents was found to be only about

4% of the total. Consequently, this process may be substrate-limited

at concentrations of chlorine found as residuals in drinking-water.

However, use of higher concentrations of chlorine would also lead to

breakdown of proteins present in the stomach fluid. Thus, as

concentrations are increased to levels that would be used in animal

studies, these products would be formed at a much higher

concentration, similar to the phenomena noted with THM and HAA

by-products.

3.1.2 Reproductive and developmental toxicity

In general, animal studies have demonstrated no reproductive or

teratogenic effects of chlorine. Druckrey (1968) examined the effects

of water chlorinated to a level of 100 mg/litre (approximately 10

mg/kg of body weight per day) in BDII rats for seven generations. No

effects were observed on fertility, growth or survival. 

A number of subsequent studies have studied the effects of

chlorine or hypochlorite on more specific aspects of reproduction or

development. Meier et al. (1985b) reported that oral administration

of sodium hypochlorite (pH 8.5) prepared from chlorine gas and

administered at 4 or 8 mg/kg of body weight per day for 5 weeks

increased the incidence ofsperm head abnormalities in B6C3F1 mice

(10 animals per group). The effect was not observed when the

solutions were administered at pHs at which hypochlorous acid was

the predominant species (pH 6.5). However, other studies have not

been able to associate adverse reproductive outcomes with the

administration of chlorine or sodium hypochlorite.

Carlton et al. (1986) found no evidence of sperm head abnormalities or adverse reproductive outcomes in Long-Evans rats. Male

rats were treated for 56 days prior to mating and female rats from

14 days prior to mating through gestation. Each experimental group

consisted of 11–12 males and 23–24 females. Solutions of chlorine

were prepared at pH 8.5, so the study evaluated hypochlorite as the

dominant form in the drinking-water. Doses were as high as 5 mg/kg

of body weight per day. 
EHC 216: Disinfectants and Disinfectant By-products

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3.1.3 Toxicity in humans

There have been significant human exposures to chlorine and

hypochlorite solutions. Much of that experience is with inhalation of

chlorine gas, which is known to be a strong respiratory irritant.

Chlorine gas is also the largest single component involved in toxic

release incidents. A third major source of exposure is solutions of

sodium hypochlorite, usuallymarketed as bleach.Bleach isfrequently

involved in human poisonings. These exposures are not particularly

relevant to exposures to chlorine or hypochlorite in drinking-water.

Therefore, only a few case reports are identified that illustrate the

types of problems that have been encountered. There was no attempt

to make this review comprehensive.

The irritating effects of chlorine gas have been well documented

because of its use as a chemical warfare agent during World War I

(Das & Blanc, 1993). In a follow-up of survivors of gassing, it was

concluded that there was no evidence of permanent lung damage;

however, these studies clearly indicated that survivors had breath

soundsthat suggested bronchitis and limited chest and diaphragmatic

movement, even emphysema. Most studies suggested that there were

high incidences of acute respiratory disease and a lesser prevalence of

chronic sequelae. Similar sequelae have been identified following

exposure of humans to accidental releases of chlorine gas. In these

more modern characterizations, the acute signs and symptoms

included a high incidence of pulmonary oedema and severe

bronchitis. These signs and symptoms are of generally short duration

and resolve themselves over the course of about 1–4 weeks. However,

chronic sequelae are observed in some individuals, depending in part

upon the severity of the exposure. In such cases, a decrease in the

forced expiratory volume is the most consistently reported clinical

sign. 

Two recent reports suggest that chronic sequelae to acute exposures to chlorine gas may be more prevalent than previously

appreciated. Moore& Sherman (1991) reported on an individual who

was previously asymptomatic and who developed chronic, recurrent

asthma after exposure to chlorine gas in an enclosed place. Schwartz

et al. (1990) followed 20 individuals who had been exposed to

chlorine gas in a 1975 incident. The prevalence of low residual lung

volume was increased during the follow-up period. Sixty-seven per

cent of those tested were found to have residual volumes below 80%
Toxicology of Disinfectants

91

of their predicted values. Five of 13 subjects tested for airway

reactivity to methacholine were found to have a greater than 15%

decline in forced expiratory volume.

Controlled studies have been conducted in healthy, non-smoking

men exposed to chlorine gas at 1.5 and 2.9 mg/m3(0.5 and 1.0 ppm)

for 4 or 8 h (reviewed in Das & Blanc, 1993). Four hours of exposure

to 2.9 mg/m3 (1.0 ppm) produced significant decreases in the forced

expiratory volume. One individual who wasfound to be experiencing

more difficulty than other subjects at this dose and who was later

exposed to 1.5 mg/m3(0.5 ppm) experienced a significant decrease in

forced expiratoryvolume. While 1.5mg/m3(0.5 ppm) appears protective for most people,somemore sensitive individuals may in fact have

more significant responses to chlorine gas. 

The effects of chronic exposure to chlorine gas have received

only limited study. In one study of paper mill workers, a more rapid

age-related decrease in lung volumes of workers exposed to chlorine

relative to those exposed to sulfur dioxide was noted, but the trend

was not statistically significant (Das & Blanc, 1993). Other studies

failed to identify chronic sequelae.

There are frequent reports of human poisonings from bleach.

Most often these exposures result from the mixing of bleach with

acidic products or ammonia. Acidification converts hypochlorite to

hypochlorous acid, which dissociates to chlorine gas, offgasses very

rapidly from the solutions and presents an inhalation exposure

(MMWR, 1991). Mixing bleach with ammonia results in the

formation of monochloramine and dichloramine, both of which are

effective respiratory irritants (MMWR, 1991).

Any potential effects of chlorine or hypochlorite in drinkingwater are obscured by the fact that by-productsinevitably coexist with

the residual chlorine. One series of studies in which by-products

formed were minimized by dissolving chlorine in distilled water

attempted to identify effects of chlorine in drinking-water on humans.

Chlorine in drinking-water was administered in a rising-dose

tolerance study beginning with 0.1 mg/litre in two 500-ml portions

and rising to a concentration of 24 mg/litre, equivalent to 0.34 mg/kg

of body weight per day (Lubbers & Bianchine, 1984). No clinically

important changes were observed. No findings of clinical importance
EHC 216: Disinfectants and Disinfectant By-products

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were identified in a follow-up treatment with repeated dosing with

500-ml portions of a solution containing 5 mg of chlorine per litre for

a 12-week period (Lubbers et al., 1984a).

Another study attempted to determine whether consumption of

chlorinated drinking-water affected blood cholesterol levels (Wones

et al., 1993a). The impetus for this study was a toxicological study in

pigeons that suggested that chlorine raised blood cholesterol levels

and modified serum thyroid levels (Revis et al., 1986a,b) and an

epidemiologicalstudythat associated small increasesin cholesterol of

women with residence in communities having chlorinated water

(Zeighami et al., 1990a,b; described in detail in section 5.2.2). A prior

study (quoted in Wones et al., 1993a) was conducted that examined

men who consumed water containing 2, 5 or 10 mg of chlorine per

litre and found a small increase in serum cholesterol levels at the

highest dose group. However, no control group was studied, so the

changes could have been attributed to the change in diet imposed as

part of the study protocol (Wones & Glueck, 1986). The longer-term

study was composed of 30 men and 30 women who received a

controlled diet for the duration of the study. The first 4 weeks

represented an acclimatization period during which all subjects

received distilled water. Half the subjects were assigned to a group

that consumed 1.5 litres of water containing 20 mg of chlorine per

litre for the following 4 weeks. At the end of each 4-week period,

blood was analysed for cholesterol, triglycerides, high-density

lipoprotein (HDL) cholesterol, low-density lipoprotein (LDL)

cholesterol or apolipoproteins A1, A2 and B. There were no

significant effects. There was a slight trend towards lower thyroid

hormone levelsin men consuming chlorine, but this was not clinically

significant(Wones et al., 1993a).These data suggest that observations

obtained previously in pigeons could not be repeated under

comparable conditions of chlorine consumption. It is notable that the

animals utilized in the original pigeon study had consumed a

modified diet (Revis et al., 1986a) that was deficient in calcium and

other trace metals. A subsequentstudy failed to replicate the previous

results in pigeons (Penn et al., 1990).

3.1.4 Carcinogenicity and mutagenicity

The International Agency for Research on Cancer (IARC) has

evaluated the carcinogenicity of hypochlorite salts and concluded that

there were no data available from studies in humans on their
Toxicology of Disinfectants

93

carcinogenicity and inadequate evidence for their carcinogenicity in

experimental animals. Hypochlorite salts were assigned to Group 3:

the compounds are not classifiable as to their carcinogenicity to

humans (IARC, 1991).

Several studies have shown that sodium hypochlorite produces

mutagenic responses in bacterial systems and mammalian cells in

vitro. However, there is no evidence of activity in mammalian test

systems in vivo. It is not clear to what extent this is influenced by the

formation of mutagenic by-products as a result of reactions with

components of the incubation media. Wlodkowski & Rosenkranz

(1975) used short-term exposures of Salmonella typhimurium strain

TA1530 followed by ascorbic acid-induced decomposition to reduce

the cytotoxic effects of hypochlorite. The investigators applied 0.14

:mol per tube and added ascorbic acid after intervals of 5, 10 and 15

min. At 5 min, a clear positive response was observed with minimal

cytotoxicity. Significant responses were also observed in strain

TA1535, but not in strain TA1538.

Rosenkranz (1973) and Rosenkranz et al. (1976) also demonstrated a positive mutagenic response in DNA polymerase A deficient

Escherichia coli to 0.006:mol ofsodium hypochlorite. Thisresponse

was unaffected by the addition of catalase, suggesting that the

response was not related to the generation of hydrogen peroxide.

Matsuoka et al. (1979) reported that sodium hypochlorite at a

concentration of 6.7 mmol/litre (0.5 mg/ml) produced chromosomal

aberrations in Chinese hamster ovary (CHO) cells in the presence of

S9 mix. This concentration was cytotoxic in the absence of S9. Some

concern must be expressed about whether responses observed with

such high and clearly cytotoxic concentrations in an in vitro system

represent specific clastogenic effects. The authors report only one

concentration tested with and without S9. It is probable that the

positive response in the presence of S9, if it is a specific response, was

a result of detoxifying hypochlorite. This protection could be nonspecific as well, in that it may not have depended upon any catalytic

activities present in the S9 fraction (i.e., the added protein may have

acted as a reactive sink to dissipate excess hypochlorite). Consequently, it is difficult to use these data in interpreting the effects of

chlorine or hypochlorite in vivo.
EHC 216: Disinfectants and Disinfectant By-products

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Ishidate (1987) studied the induction of chromosomal aberrations

in cultures of Chinese hamster CHL cells at sodium hypochlorite

concentrations ranging from 125 to 500 :g/ml without exogenous

metabolic activation and from 31 to 125 :g/ml with and without rat

liver S9 mix. A clear increase in the number of cells with structural

chromosomal aberrations was observed at 500 :g/ml without S9 mix,

while the results obtained in the other series, showing weaklypositive

responses, were considered inconclusive.

Meier et al. (1985b) evaluated the ability of hypochlorite and

hypochlorous acid to induce chromosomal damage or micronuclei in

the bone marrow of CD-1 mice. The samples to be tested were

generated by bubbling chlorine gas into water and then adjusting the

pH to 6.5 (predominantly hypochlorous acid) or 8.5 (predominantly

hypochlorite). The doses administered were 1.6, 4 or 8 mg/kg of body

weight for 5 consecutive days. There was no evidence of increased

micronuclei or chromosomal abnormalities in bone marrow cells.

Significant positive responses were observed with positive control

chemicals in both assays. As reported in section 3.1.2, these authors

detected a positive response in the sperm head abnormality assay in

mice treated at these same doses of hypochlorite in two separate

experiments. This assay is used primarily as a mutagenicity assay

rather than as an assay for reproductive toxicities. Hypochlorous acid

had no effect in the sperm head abnormality assay.

Tests of the ability of hypochlorite to induce cancer in rodents

were conducted in F344 rats by Hasegawa et al. (1986). Sodium

hypochlorite concentrations of 0, 500 or 1000 mg/litre (males) and 0,

1000 or 2000 mg/litre (females) were administered in the drinkingwater for 104 weeks (equivalent to 13.5 and 27.7 mg/kg of body

weight per day for males and 34.3 and 63.2 mg/kg of body weight per

day for females). There were 50 male and 50 female rats assigned to

each experimental group. No tumours could be attributed to sodium

hypochlorite administration.

NTP (1992) conducted a 2-year bioassay of chlorine in F344 rats

andB6C3F1mice. The concentrations administered in drinking-water

were 0, 70, 140 or 275 mg/litre, and there were 70 animals of each

sex assigned to each group (approximately 0, 4, 8 or 14 mg/kg of body

weight per day for rats and 0, 7, 14 or 24 mg/kg of body weight per
Toxicology of Disinfectants

95

day for mice). There was an apparent positive trend in the induction

of stromal polyps of the uterus of female mice treated with chlorine,

but this was considered unlikely to be treatment-related because the

incidence was below those observed in historical controls. In female

rats, there was an increase in mononuclear cell leukaemia at both 140

and 275 mg/litre (8 and 14 mg/kg of body weight per day). However,

the response was not considered treatment-related because it fell

within the range of historical controls, there was no apparent

dose–response, and there was no evidence for such an increase in

male F344 rats.

A single study suggested that sodium hypochlorite could act as

a promoter ofskin tumoursfollowinginitiation with 4-nitroquinoline1-oxide in female ddN mice (Hayatsu et al., 1971). A solution of

sodium hypochlorite that contained 10% effective concentrations of

chlorine was utilized. Skin tumours were produced in 9 of 32 mice

given 45 applications of sodium hypochlorite following initiation.

Sodium hydroxide solutions were utilized as a control for the alkaline

pH of sodium hypochlorite and produced no tumours. No tumours

were observed with 60 applications of sodium hypochlorite solution

in non-initiated mice. Pfeiffer (1978) conducted a much larger

experiment that utilized 100 mice per group. This author found that

a 1% sodium hypochlorite solution applied alternately with

benzo[a]pyrene for 128 weeks was ineffective in producing skin

tumours in female NMRI mice above those that had been initiated

with benzo[a]pyrene alone at doses of 750 or 1500 :g. Pretreatment

with the sodium hypochlorite solution before application of the

benzo[a]pyrene actually reduced tumour yields at 128 weeks with

doses of either 750 or 1500 :g of benzo[a]pyrene. Sodium

hypochlorite used in a more traditional initiation/promotion study

(i.e., sodium hypochlorite treatment following initiation with

benzo[a]pyrene) produced a decrease in the tumour yield with the 750

:g dose of benzo[a]pyrene, but had no effect following 1500 :g.

Thus, the ability of sodium hypochlorite to act as a tumour promoter

may depend upon the initiator used, or the smaller experiment of

Hayatsu et al. (1971) may simply be a false result.

As pointed out in section 3.1.1, application ofsolutions of hypochlorous acid to the skin of Sencar mice resultsin the development of

hyperplasia. The concentrations required are considerably lower

(300 mg/litre) (Robinson et al., 1986) than those used in the studies

of either Hayatsu et al. (1971) or Pfeiffer (1978). Sodium hypochlorite
EHC 216: Disinfectants and Disinfectant By-products

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was also effective at lower doses, but less so than equivalent concentrations of hypochlorous acid. These results suggest that these

prior evaluations may have been conducted at too high a dose. There

appear to be no reports on the effectiveness of hypochlorous acid as a

tumour promoter, but the lack of activity at doses of less than 300

mg/litre would suggest that this is of no concern.

3.1.5 Comparative pharmacokinetics and metabolism

A series of pharmacokinetic studies using 36Cl-labelled hypochlorous acid were conducted by Abdel-Rahman and co-workers

(1983). These studies are of limited value because the form of 36Cl

could not be determined in various body compartments. 

3.1.6 Mode of action

There are no specific toxicities of chlorine for which a

mechanism needs to be proposed. It is a strong oxidizing agent, and

it must be presumed that damage induced at high doses by either

gaseous chlorine or solutions of hypochlorite is at least partially

related to this property. In studies in which sodium hypochlorite is

used without neutralization, a strong alkaline pH can also contribute

to its effects. There is always the possibility that chlorine is inducing

subtle effects byvirtue of itsreaction with organic compoundsthat are

found in the stomach. Such reactions have been demonstrated, but

there is no convincing evidence to date that any specific toxicity can

be attributed to these by-products.

3.2 Chloramine

3.2.1 General toxicological properties and information on dose–

response in animals

There have been relatively fewevaluations ofthe toxic properties

of chloramine in experimental animals. In large part this is because

it is not marketed as a product but is created for disinfection purposes

on-site and in situ. Chloramine is primarily used as a residual

disinfectant in the distribution system. The final solution consists of

mostly monochloramine, with traces of other chloramines, such as

dichloramine. Chloramines, as a group, are generally recognized as

potentrespiratoryirritants, because the formation ofthese compounds

when household bleach and ammonia are mixed results in a number
Toxicology of Disinfectants

97

of poisoning cases each year (MMWR, 1991). In spite of this, there

has been no attempt to quantify dose–response relationships in

animals.

Eaton et al. (1973) investigated concerns about chloramineinduced methaemoglobin formation in kidney patients dialysed with

chloramine-containing water.This was done by examining the ability

of relatively large volumes of tapwater to oxidize haemoglobin in

dilute suspensions of red blood cells. This circumstance is reflective

of dialysis, but not of the concentrated suspension of these cells in

vivo. Nevertheless, the authors were able to show that

methaemoglobin formation occurred in a dose-related manner when

1 volume of red blood cells (human) was suspended in 100 volumes

of tapwater containing 1 mg of chloramine per litre or above. This

effect was not produced by comparable concentrations of sodium

hypochlorite. The ability to induce methaemoglobin formation was

eliminated by treating the water by reverse osmosis followed by

carbon filtration. Clearly, chloramine is capable of inducing

methaemoglobinaemia at low concentrations when there is a large

reservoir of chloramine. Thisis a decidedlydifferent exposurepattern

from that of normal humans and other mammals, as they consume

small volumes of water relative to the volume of red blood cells that

are exposed.

Moore et al. (1980a) studied alterations of blood parameters in

male A/J mice treated with 0, 2.5, 25, 50, 100 or 200 mg of monochloramineper litre in carbonate/bicarbonate-buffered (pH 8.9) drinking-water. Twelve animals were assigned to each group, and

treatments weremaintained over a 30-day period. Consistent with the

interpretation provided above, there were no treatment-related effects

on osmotic fragility, methaemoglobin levels, haemoglobin concentrations, reticulocyte counts or a number of other derived parameters.

Haematocrits of mice treated with 50, 100 or 200 mg/litre were

actually higher than those observed in control mice. White blood cell

counts were not altered in these animals.

Daniel et al. (1990a) conducted a more traditional 90-day study

of monochloramine in Sprague-Dawley rats (10 animals per sex per

dose). Treatment concentrations were 0, 25, 50, 100 or 200 mg/litre,

corresponding to doses of 0, 1.8, 3.4, 5.8 or 9.0 mg/kg of bodyweight

per day in males and 0, 2.6, 4.3, 7.7 or 12.1 mg/kg of body weight per

day in females. Controls received carbonated, pH-adjusted drinkingwater. A large number of haematological and clinical chemistry
EHC 216: Disinfectants and Disinfectant By-products

98

measures were included in the evaluation. Body weights were significantly depressed in both sexes at treatment concentrations in the

50–200 mg/litre range, but this appeared to be related to depressed

water and food consumption. There were minor changes in organ to

body weight ratios at the highest dose, but no evidence of treatmentrelated pathology was observed. Male rats were found to have

decreased haematocrits at 100 mg/litre, and red blood cell countswere

slightly depressed at 100 and 200 mg/litre. The authors concluded

that monochloramine was more toxic than chlorine or chlorine

dioxide. However, it must be noted that the changes in blood

parameters were small and in themselves of no clinical significance.

Other measures were not related to specific toxic reactions. Based on

the decrease in organ and body weights observed in both sexes, the

authors concluded that theno-observed-adverse-effect level (NOAEL)

was 100 mg/litre, equivalent to 5.8 mg/kg of body weight per day.

The work in rats wasfollowed up with a second study in B6C3F1

mice (Daniel et al., 1991a). Male and femaleB6C3F1mice (10 per sex

per group) were administered 0, 12.5, 25, 50, 100 or 200 mg of chloramine per litre of drinking-water for 90 days (calculated mean daily

dose was 0, 2.5, 5.0, 8.6, 11.1 or 15.6 mg/kg of body weight for males

and 0, 2.8, 5.3, 9.2, 12.9 or 15.8 mg/kg of body weight for females).

Water consumption significantly decreased at 100 and 200 mg/litre

in males and at 25–200 mg/litre in females. Weight gain was

significantly depressed in both sexes at 100 and 200 mg/litre.

Neutrophil concentrations in blood were significantly depressed in

both male and female mice at the two highest doses, but other white

blood cell counts were unaltered. Absolute and relative spleen and

liver weights were depressed at both 100 and 200 mg/litre. No gross

or microscopic evidence of target organ toxicity was observed that

could be related to treatment. Based on decreased organ weights,

weight gain, and food and water consumption, the authors concluded

that the NOAEL was 50 mg/litre, equivalent to 8.6 mg/kg of body

weight per day.

A 13-week study in which groups of 10 Sprague-Dawley rats

weregiven drinking-water containing 200mg ofmonochloramineper

litre or buffered water as a control, ad libitum or restricted to the same

consumption as the monochloramine group, was designed to resolve

some of the outstanding toxicological questions. The results of this

study indicated that the reduced body weight gain and the minor

biochemical, haematological, immunological and histopathological 
Toxicology of Disinfectants

99

changes associated with exposure to 200 mg of monochloramine per

litre (equivalent to 21.6 mg/kg of body weight per day) in drinkingwater were largely related to reduced water and food consumption

(Poon et al., 1997).

In a 9-week study, Exon et al. (1987) examined the ability of

monochloramine to modify immunological parameters in male

Sprague-Dawley rats (12 per dose) exposed to concentrations of 0, 9,

19 or 38 mg of monochloramine per litre, equivalent to 0, 1.3, 2.6 or

5.3 mg/kg of body weight per day. At the middle and highest dose,

chloramine treatment was observed to increase prostaglandin E2 synthesis by adherent resident peritoneal cells (which include macrophages)in response tolipopolysaccharide stimulation.Noattempt was

made to relate this finding to other indices of modified macrophage

function. A small depression in spleen weights was observed at the

highest dose. The implications of these data for immune function are

not clear. Other measures of immune function did not reveal statistically significant changes with treatment. These included a decrease

in antibody formation at the lowest and middle doses in response to

keyhole limpet haemocyanin injection or delayed-type hypersensitivity

reactions to bovine serum albumin injected into the footpad.

The effect of monochloramine on skin irritation was tested by

immersing Sencar mice into water containing chloramine at concentrations ranging from 1 to 1000 mg/litre for 10 min a day (Robinson

et al., 1986). Unlike hypochlorous acid (pH 6.5) or hypochlorite (pH

8.5), chloramine did not produce hyperplasia of the skin.

3.2.2 Reproductive and developmental toxicity

Studies in laboratory animals have indicated no reproductive or

developmental effects associated with chloramine. Abdel-Rahman et

al. (1982a) administered monochloramine to female Sprague-Dawley

rats at concentrations of 0, 1, 10 or 100 mg/litre (0, 0.15, 1.5 or

15 mg/kg of body weight per day) for 2.5 months prior to breeding

and through gestation. Only six animals were assigned to each

treatment group. Reproductive performance was comparable between

groups, and fetal weights were not adversely affected by treatment.

Between 50 and 60 fetuses were available (male and female

combined) for evaluation. There was no evidence oftreatment-related

skeletal or soft tissue anomalies.
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Carlton et al. (1986) examined the effects of monochloramine

administered by gavage at doses of 0, 2.5, 5 or 10 mg/kg of body

weight per day on the reproductive performance of Long-Evans rats.

Males (12 per group) were treated from 56 days prior to and through

mating, and females (24 per group) from 14 days prior to mating and

throughout the mating period. No statistically significant effects on

spermmorphology, concentration or motilitywereobserved, nor were

there any effects on fertility, viability, litter size, pup weights, day of

eye opening or day of vaginal patency.

3.2.3 Toxicity in humans

The primary harmful effects of chloramine have been

documented in humans poisoned by chloramine formed when

household bleach was mixed with ammonia for use as a cleaning

solution. Chloramine is a strong respiratory irritant. These effects

were discussed in section 3.1.3.

Forty-eightmen completed an 8-week protocol during which diet

and other factors known to affect lipid metabolism were controlled.

During the first 4 weeks of the protocol, all subjects consumed

distilled water. During the second 4 weeks, one-third of the subjects

were assigned randomly to drink 1.5 litres of water containing 0, 2 or

15 mg of monochloramine per litre each day. At 2 mg/litre, no

significant changes were observed in total, HDL or LDL cholesterol,

triglycerides or apolipoproteins A1, A2 or B. Parameters of thyroid

function were unchanged. However, an increase in the level of

apolipoprotein B was observed at 15 mg/litre (Wones et al., 1993b).

3.2.4 Carcinogenicity and mutagenicity

Shih & Lederberg (1976) first demonstrated that

monochloramine induced mutation in a Bacillus subtilis reversion

assay. The concentration range studied extended from 18 to 74

:mol/litre. A positive dose–response was observed through 56

:mol/litre, but 74 :mol/litre was cytotoxic. Repair-deficient mutants

ofB.subtilis, rec3, recA and polyA5, were consistentlymore sensitive

to the cytotoxic effects of chloramine, while the uvr and recB mutants

were not. The sensitivity of the polyA5 mutants parallels the

observations of Rosenkranz (1973) with sodium hypochlorite and

suggests that DNA polymerase A is involved in the repair of DNA

lesions produced by both chemicals. Thus, it is possible that a
Toxicology of Disinfectants

101

common intermediate or mechanism is involved in the mutagenic

effects of hypochlorite and chloramine.

A broader list of chloramines wastested by Thomas et al. (1987)

in Salmonella typhimurium tester strains TA97a, TA100 and TA102.

The chloramines tested included those that could be formed at low

levels from natural substrates in drinking-water or in the stomach.

TA100 was found to consistently be the most sensitive strain. The

most potent mutagens were the lipophilic dichloramines formed with

histamine, ethanolamine and putrescine. The corresponding monochloramines were less potent. The more hydrophilic chloramines,

such as taurine-chloramine, had little activity. Monochloramine was

active in the 50 :mol/litre range, remarkably consistent with the data

of Shih & Lederberg (1976). Hypochlorous acid was inactive at all

concentrations that were tested, up to and including concentrations

that induced cytotoxicity.

Ashby and co-workers(1987) were unable to induce clastogenic

effects in the mouse bone marrow micronucleus assay when

chloramine was administered orally. They suggested that the in vitro

clastogenic effectswereprobablyattributable tonon-specific cytotoxic

effects that are secondary to the release of hypochlorite to the media.

Meier et al. (1985b) found that intraperitoneal administration of

monochloramine to CD-1 mice at doses of up to 8 mg/kg of body

weight was without significant effect on either micronuclei or

chromosomal aberrations in the bone marrow. These data would

appear to be consistent with the findings of Ashby et al. (1987).

Studies on the carcinogenicity of chloramine are limited to a

single set of 2-year experiments conducted by the NTP (1992).

Drinking-water containing 0, 50, 100 or 200 mg of chloramine per

litre was provided to F344 rats and B6C3F1 mice. Seventy animals of

each species and of each sex within a species were assigned to each

experimental and control group. Doses in rats were 0, 2.1, 4.8 or

8.7 mg/kg of body weight per day in males and 0, 2.8, 5.3 or 9.5

mg/kg of body weight per day in females; doses in mice were 0, 5.0,

8.9 or 15.9 mg/kg of body weight per day in males and 0, 4.9, 9.0 or

17.2 mg/kg of body weight per day in females. Of some interest was

the finding that two renal cell adenomas were found in male B6C3F1
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mice treated with the high dose of chloramine. In addition, one renal

adenoma wasfound in one male mouse treated with 100 mg/litre and

in one female mouse treated with 200 mg/litre. While thistumour site

is rare in both species, there was no real dose–response trend, nor

were the differences between the control and treatment groups

statistically significant. A second finding of some concern was an

increase in the incidence of mononuclear cell leukaemia in F344 rats.

This pathology was increased in rats treated with chloramine or

hypochlorite, although the effects were not clearly dose-dependent.

The incidence of mononuclear cell leukaemia was significantly

greater than in concurrent controls and was elevated above the

historical incidence as well. Nevertheless, these increases were not

considered to be treatment-related. In part, this conclusion arose from

the lack of a clear dose–response. It was also based on the fact that

there was no comparable trend in male rats.

3.2.5 Comparative pharmacokinetics and metabolism

Thepharmacokinetics of 36Cl derived from monochloramine have

been examined in male Sprague-Dawley rats (Abdel-Rahman et al.,

1983). These data are difficult to interpret because the specific form

of the label is not known.

3.3 Chlorine dioxide

3.3.1 General toxicological properties and information on dose–

response in animals

Despite its use as a disinfectant, there have been veryfewgeneral

toxicological evaluations of chlorine dioxide, because most studies

have focused on its major by-product, chlorite, which is considered in

section 4.6. The present review will first focus on the limited characterization of chlorine dioxide’s general toxicology, then follow up

with a discussion of its haematological and thyroid effects.

Some very cursory investigations of chlorine dioxide’s effects as

a respiratory irritant were published by Haller & Northgraves (1955)

in an article dealing with the general chemical properties of the

compound. In essence, these data suggested that exposure to chlorine

dioxide in air at a concentration of more than 420 mg/m3(150 ppm)

for 
Toxicology of Disinfectants

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longer than 15 min was fatal to guinea-pigs. The total study involved

six guinea-pigs.

Rats(3–5 per group) were exposed to chlorine dioxide in various

concentrations (0.28–9520 mg/m3 [0.1–3400 ppm]) and for various

periods (3 min–10 weeks). All the rats exposed daily to chlorine

dioxide at 28 mg/m3(10 ppm) died in less than 14 days. Purulent

bronchitis and disseminated bronchopneumonia were found at necroscopy. No such changes were demonstrable in rats exposed to approximately 0.28 mg/m3

(0.1 ppm) for about 10 weeks (Dalhamn, 1957).

The LC50 of chlorine dioxide in rats (5 per sex per group)

exposed by inhalation for 4 h was 90 mg/m3(32 ppm) (Ineris, 1996).

Robinson et al. (1986) studied the ability of chlorine and alternative disinfectants to induce epidermal hyperplasia in the skin of

Sencar mice. The thickness of the interfollicular epidermis wassignificantly increased by 10-min daily exposures to water containing up

to 1000 mg of chlorine dioxide per litre for 4 days. The thickness of

the epidermis was similar to that induced by an equivalent dose of

hypochlorous acid. Unlike hypochlorous acid, however, there was no

significantincrease in skin thickness at concentrations of300mg/litre

or less.

The study of Daniel et al. (1990a) wasthe first subchronic study

that adhered to modern expectations of toxicological studies. These

authors provided male and female Sprague-Dawley rats (10 per sex

per treatment group) with 0, 25, 50, 100 or 200 mg of chlorine

dioxide per litre of drinking-water for 90 days, equivalent to 0, 2, 4,

6 or 12 and 0, 2, 5, 8 or 15 mg/kg of body weight per day for males

and females, respectively. Conventional measures of body weight,

organ weights, a broad battery of clinical chemistry parameters and

histopathological examinations were all included in the study design.

Body and organ weights were significantly depressed at 200 mg/litre

in both sexes. This appeared to be secondary to depressed water

consumption, which is known to be tightly coupled to food

consumption in rats. The only significant histopathological damage

found was goblet cell hyperplasia and inflammation. This was

observed at all doses of chlorine dioxide in both male and female rats.

Presumably this inflammation occurs as a result of volatilization of
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104

chlorine dioxide from the water bottle. The amount of chlorine

dioxide actuallyinhaled as a result of volatilization from thedrinkingwater containing the lowest dose of chlorine dioxide (25 mg/litre)

must be extremely low. This suggests that there might be some

concern for sensitive individuals showering with water containing

chlorine dioxide.

The ability of chlorine dioxide to induce methaemoglobinaemia

and haemolytic anaemia hasreceived extensive study. Abdel-Rahman

et al. (1980) found decreased red blood cell glutathione (GSH)

concentrations and decreased osmotic fragility in Sprague-Dawley rats

and white leghorn chickens given drinking-water containing chlorine

dioxide concentrations of 1, 10, 100 and 1000 mg/litre for up to

4 months, but the changes were not consistently dose-related.

However, the authors found that the morphology of red blood cells

was modified (codocytes and echinocytes) in all dose groups, the

severity increasing with increased treatment concentration.

Methaemoglobin wasnot detected throughout these studies. However,

there was no formal statistical evaluation of these results, and only

four rats were assigned to each experimental group. Administration

of acute doses of as little as 1 mg/kg of body weight by gavage

decreased red blood cell GSH concentrations. This response was not

increased as dose was increased to 4 mg/kg of body weight.

Abdel-Rahman and co-workers extended these observations to

longer treatment periodsin a subsequent publication (Abdel-Rahman

et al., 1984a). Again, only four animals were assigned to each

treatment group. At 7 and 9 months of treatment, red blood cells

appeared to become resistant to osmotic shock at all treatment

concentrations (1–1000 mg/litre). These data did not display a clear

dose–response despite the large variation in the dose administered.

The study of Abdel-Rahman et al. (1984a) also reported changes

in the incorporation of 3H-thymidine into the DNA of various organs.

Incorporation was significantly inhibited in testes and apparently

increased in the intestinal mucosa. The effect on apparent DNA

synthesis was particularly marked in the testes at 100 mg/litre,

amounting to about 60% inhibition. These data are difficult to

interpret for several reasons. First, rats were sacrificed 8 h after being

injected with 3H-thymidine. Ordinarily, sacrifices are made 30–60

min after injection because the blood is essentially depleted of 3Hthymidine in an hour. Thus, it is not possible to determine if the

lowered amount of label is related to decreased synthesis or to
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105

increased turnover of DNA. Second, the result was based on total

counts in DNA, which makes it impossible to determine what cell

type is affected or whether the change was associated with replicative

or repair synthesis. Third, only four animals were used per

experimental group.

In a rising-dose protocol study, Bercz et al. (1982) evaluated the

effects of chlorine dioxide on African green monkeys. These animals

wereprovided chlorine dioxide in drinking-water at concentrations of

0, 30, 100 or 200 mg/litre, corresponding to doses of 0, 3.5, 9.5 or

11 mg/kg of body weight per day. Each dose was maintained for

30–60 days. Animalsshowed signs of dehydration at the highest dose

(11 mg/kg of body weight per day), so exposure at that dose was discontinued. No effect was observed on any haematological parameter,

including methaemoglobinaemia. However, statistically significant

depressions in serum thyroxine levels were observed when animals

were dosed with chlorine dioxide at a concentration of 100 mg/litre.

No effect had been observed in a prior exposure of the same animals

to 30 mg/litre for 30 days. The NOAEL in this study was 3.5 mg/kg

of body weight per day.

The effects of chlorine dioxide on thyroid function were followed

up by Harrington et al. (1986). Thyroxine levels in African green

monkeys administered drinking-water containing 100 mg of chlorine

dioxide per litre (4.6 mg/kg of bodyweight per day) were again found

to be depressed at 4 weeks of treatment, but rebounded to abovenormal levels after 8 weeks of treatment. These investigators also

found significantlydepressed thyroxine levelsin ratstreated with 100

or 200 mg of chlorine dioxide per litre in drinking-water (equivalent

to 14 and 28 mg/kg of body weight per day) for 8 weeks. This change

was dose-related. Lower doses were not examined in the rat study.

The authors indicated that the results were based on 12

determinations; it was not clear if these measurements were made on

individual animals. 

A set of in vivo and in vitro experiments was conducted in an

attempt to explain the effects of chlorine dioxide on serum thyroid

hormone concentrations. The authors demonstrated that chlorine

dioxide oxidizes iodide to reactive iodine species that would bind to

the stomach and oesophageal epithelium. Rat chow that was treated

with chlorine dioxide at approximately 80 mg/litre was found to

increase the binding of iodine to chow constituents. This activation of

iodine resulted in retention of labelled iodine in the ileum and colon
EHC 216: Disinfectants and Disinfectant By-products

106

and reduced uptake bythe thyroid gland. Previous work demonstrated

that chlorine dioxide was more effective than chlorine in activating

iodide to a form that would covalently bind with a variety of natural

foodstuffs (Bercz et al., 1986). Based on these observations, the

authors concluded that the effects of chlorine dioxide were probably

due toaltered gastrointestinal absorption ofiodide and reduced uptake

into the thyroid gland.

3.3.2 Reproductive and developmental toxicity

A number of reproductive effects have been reported in studies

with laboratory animals, but the relevance for humans of these

findings remains uncertain. The reproductive effects of chlorine

dioxide in Long-Evans rats were studied by Carlton et al. (1991).

Chlorine dioxide was administered by gavage at doses of 2.5, 5 or 10

mg/kg of body weight per day to male rats (12 per group) for 56 days

prior to and through mating and to female rats(24 per group) from 14

days prior to mating and through pregnancy. Fertility measures were

not significantly different among the dose groups. There were no

dose-related changes in sperm parameters (i.e., concentration,

motility, progressive movement or morphology). Thyroid hormone

levels were altered significantly, but not in a consistent pattern. The

onlysignificant difference was significantly depressed vaginal weights

in female pups whose dams had been treated with 10 mg/kg of body

weight per day.

An evaluation of the effects of chlorine dioxide on the fetal

development of Sprague-Dawley rats was conducted by Suh et al.

(1983). Chlorine dioxide was administered at 0, 1, 10 or 100 mg/litre

(0, 0.1, 1 or 10 mg/kg of body weight per day) for 2.5 months prior to

mating and throughout gestation. The total number of implants per

dam wassignificantlyreduced at the highest concentration of chlorine

dioxide. The percentageof anomalousfetuses wasincreased in a doserelated manner, but the response was not statistically significant.

These anomalies arose primarily as the percentage of abnormal or

incomplete sternebrae in treated rats relative to controls. The lack of

statistical significance was undoubtedly related to the relatively few

female ratsthat were included in the study (6–8 females per treatment

group). As a consequence, the results of thisstudymust be considered

inconclusive.
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Orme et al. (1985) found that chlorine dioxide administered in

the drinking-water of female Sprague-Dawley rats (13–16 per dose)

at concentrations of 0, 2, 20 or 100 mg/litre (0, 1, 3 or 14 mg/kg of

body weight per day) throughout pregnancy and through weaning

decreased thyroxine levels in the serum of the pups at 100 mg/litre.

This was associated with delayed development of exploratory

behaviour in the pups away from their dams, and this, in turn, was

probably due to an indirect effect on iodine uptake. In a second

experiment, pups given 14 mg of chlorine dioxide per kg of body

weight per day directly by gavage on postnatal days 5–20 showed

significantly depressed activity and a decrease in serum thyroxine

levels. Studies by the same group (Taylor & Pfohl, 1985) indicated

that cerebellar and forebrain cell counts (based on DNA

measurements) were depressed in 11-day-old pups that had been

treated with chlorine dioxide at 14 mg/kg of body weight per day by

gavage from 5 days of age. Cerebellar cell countsremained depressed

in rats at 21 days, but forebrain counts were essentially the same as in

controls. At 50–60 days of age, the locomotor activity (measured by

wheel-running) of these animals was depressed relative to control

animals.

The effects of chlorine dioxide on brain development were

examined further by Toth et al. (1990). These authors administered

chlorine dioxide by gavage at 14 mg/kg of body weight per day from

postnatal day 1 to 20. Body weight wasreduced, but cerebellar weight

was unaltered at any age. Forebrain weight and protein content were

reduced on postnatal days 21 and 35. DNA content was depressed on

postnatal day 35, and the number of dendritic spines on cerebral

cortical pyramidal cells was significantly reduced. No histopathological changes in the forebrain, cerebellum or brain stem were

observed. There were no consistent changes in serum thyroxine or

triiodothyronine levels in treated animals.

Collectively, these data suggest some effects of chlorine dioxide

on brain development. In most studies, there are suggestions of

modified thyroid function associated with these effects. It must be

pointed out that the changes in thyroid hormone levels are modest,

much less than are produced with classical antithyroid drugs such as

propylthiouracil (Toth et al., 1990).
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3.3.3 Toxicity in humans

The effects of chlorine dioxide were assessed in a two-phase

study in 10 healthymale volunteers. The first study was a rising-dose

tolerance study (Lubbers & Bianchine, 1984) in which doses of

chlorine dioxide were increased from 0.1 to 24 mg/litre, administered

in two 500-ml portions. The maximum dose for a 70-kg person was

0.34 mg/kg of body weight. The details of this study were described

in section 3.1.3. Some small changesin a variety of clinical chemistry

parameters were observed, but none was found to be outside the

accepted range of normal. The second phase of the experiment

involved the daily administration of a 500-ml portion of a solution

containing 5 mg/litre to 10 healthyvolunteersfor a period of 12 weeks

(Lubbers et al., 1984a). Again, measurement of a large battery of

clinical chemistryparameters and routine physical examination failed

to identify any effects of chlorine dioxide that fell outside of the

normal range. Parameters yielding significant differences appeared to

be primarily a result of parallel drift of values with the control group.

As with other disinfectants, it is important to recognize that

chlorine dioxide is a potent respiratory irritant. No quantitative data

can be used to construct a dose–response relationship for this effect.

3.3.4 Carcinogenicity and mutagenicity

The mutagenic or clastogenic effects of chlorine dioxide have

received little attention. Ishidate et al. (1984) found chlorine dioxide

to be positive in Salmonella typhimurium tester strain TA100. A

linear dose–response was observed at concentrations between 2 and

20 :g per plate. Chlorine dioxide was ineffective as a clastogenic

agent in a CHO system.

Meier et al. (1985b) evaluated the ability of chlorine dioxide to

induce chromosomal aberrations and micronuclei in bone marrow of

CD-1 mice or sperm head anomalies. Chlorine dioxide failed to

produce such damage following gavage doses of up to 16 mg/kg of

body weight for 5 days.

With the exception of a 1949 study by Haag (cited in TERA,

1998), which has serious limitations, no tests of the carcinogenic
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activity of chlorine dioxide in experimental animals were identified

in the scientific literature.

3.3.5 Comparative pharmacokinetics and metabolism

There are significant differences in the pharmacokinetics of 36Cl

obtained from different disinfectants. The absorption rate for 36Cllabelled chlorine dioxide was at least 10 times that observed with

chlorine, chloramine or chloride. The relative amount of 36Cl that is

eliminated in the urine and faeces at 24 h has a distinct pattern from

that observed with other disinfectants and sodium chloride. However,

the terminal half-life of the 36Cl appears similar for all disinfectants.

These data suggest that the form of 36Cl that is being absorbed differs

chemically with the different disinfectants. In the case of chlorine

dioxide, thisissupported by the observation that measurable amounts

of chlorite (about 3% of the original dose) are eliminated in the urine

during the first 24 h, and chlorite comprises about 20% of the label

present in blood 72 h after administration of the test dose (AbdelRahman et al., 1980). However, this higher absorption rate is not

explained by the absorption rates of chlorite and chlorate, which are

about one-tenth asrapid (Abdel-Rahman et al., 1982b). Thissuggests

that some of the absorption could be as chlorine dioxide itself. On the

surface, this hypothesis would seem to be incompatible with the high

reactivityofthis disinfectant. Aswith other disinfectants, the terminal

elimination phases observed for 36Cl from chlorine dioxide seem

compatible with the hypothesis that the bulk of the elimination is as

chloride ion.