27 

### 2. CHEMISTRY OF DISINFECTANTS AND

DISINFECTANT BY-PRODUCTS

2.1 Background

The use of chlorine (Cl

2 ) 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 as the organic DBP precursor, whereas bromide ion (Br

– ) serves as the inorganic precursor. Treatment strategies

generally available to water systems exceeding drinking-water standards

include removing DBP precursors and using alternative disinfectants

for primary and/or secondary (distribution sys t em) disinfection.

Alternative disinfectant options that show promise are chloramines

(NH

2 Cl, monochloramine), chlor ine dioxide (ClO

2 ) and ozone (O

3 ).

While ozone can serve as a primary disinfectant only and chloramines

as a secondary disinfectant only, both chlorine and chlorine 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 reac t 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 (NO

2 – ) and nitrate (NO

3 – ) production in

chloraminated distribution systems as well as the formation of organic

chloramines have raised concern.

Th e 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 t o the formation of by-products such as

chlorite (ClO

2

– ), a compound that is of health concern.

If used as a primary disinfectant followed by a chloramine residual

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

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

28 with NOM to produce organic DBPs s uch 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 inactivatio n goals and DBP control goals to be

successfully balanced. This chapter examines a range of issues affecting

DBP formation and control to provide guidance to utilities considering

the use of various disinfecting chemicals to achiev e microbial

inactivation with DBP control.



### 2.2 Physical and chemical properties of common

disinfectants and inorganic disinfectant byproducts

The important physical and c h emical properties of commonly used

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 liq

uid 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 wate r being treated. For

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

plants, tonne containers are common; and for very large plants, chlorine

is delivered by railway tank cars or road (truck) tankers. Chlorine is also

available in granular o r 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 r apidly that little disinfection is

accomplished until amounts in excess of the chlorine demand have been

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

compounds, in particular those that are considered reducing agents

(hydrogen sulfide [H

2 S], manganese(II), iron(II), sulfite [SO

3 2– ], Br

–

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

Chemical a E o (V) b Oxidation number of Cl or Br 8

max (nm) c e (mol –1 litre –1 cm –1 ) d p

, o e p K

a f

HOCl/Cl

– +1.49 +1 254

292 (OCl

– ) 60

419 +25.2 7.5

ClO

2 /ClO

2 – +0.95 +4 359 1250 +16.1

–

NH

2 Cl

– +1 245 416

– –

O

3 /O

2 +2.07

– 254 3200 +35.0

HOBr/Br

– +1.33 +1 330 50 +22.5 8.7

ClO

2 – /Cl

– +0.76 +3 262

– +12.8 1.96

ClO

3 – /Cl

– +0.62 +5 360

– +10.5 1.45

BrO

3 – /Br

– +0.61 +5 195

– 0.72

a Half-cell reactants/products.

b E o = standard electrode potential (redox potential) in water at 25 °C. The oxidation–reduction state of an aqueous environment at equilibrium can be stated

in terms of its redox potential. In the chemistry literature, this is generally expressed in volts, E

, or as the negative logarithm of the electron activity, 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 8

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

d e = molar absorptivity (molar extinction coefficient), in mol –1 litre –1 cm –1 . This can be used for quantitative determination of 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 p K

a = 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 reactions may be important because they may be fast

and result in the consumption of chlorine.

Chlorine ga s hydrolyses in water almost completely to form

hypochlorous acid (HOCl):

Cl

2 + H

2 O 

### 6 HOCl + H + + Cl

–

The hypochlorous acid dissoc iates into hydrogen ions (H + ) and

hypochlorite ions in the reversible reaction:

HOCl 

### : H + + OCl

–

Hypochlorous acid is a weak acid with a p K

a of approximately 7.5

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

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 an d chlorate (ClO

3 – ); 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 f

ormed by the action of chlorine (or

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

– ) is favoured in alkaline solutions (2XO

– 

### 6 X

– + XO

2

– ) 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

– + XO

2

– 

### 6 X

– + XO

3 –

Another reaction that occurs in waters containing bromide ion and

hypochlorite is the production of hypobromous acid:

HOCl + Br

– 

### 6 HOBr + Cl

–

This reaction is irreversible, and the product hypobrom ous acid is a

better halo genating agent than hypochlorous acid and interferes withChemistry of Disinfectants and Disinfectant By-products

31 common analytical procedures for free chlorine. The presence of

bromide in hypochlorite solutions can ultimately lead to the formation

of bromate (BrO

3 – ).

Hypobromous acid is a weak acid (p K

a = 8.7); like hypochlorite,

hypobromite is metastable. In alkaline solution, it decomposes to give

bromate and bromide:

3OBr

– 

### 6 BrO

3 – + 2Br

–

Bromic acid (HBrO

3 ) is a strong acid (p K

a = 0.7). Bromic acid and

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

entirel y as monomeric free radicals. Concentrated chlorine dioxide

vapour is potentially explosive, and attempts to 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 k n own. Chlorine dioxide is a powerful

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

oxidizable substances and in the presence of alkali, it dissolves in water,

decomposing with the slow formation of chlorite and chlorate:

2ClO

2 + H

2 O 

### 6 ClO

2 – + ClO

3 – + 2H +

Chlorine dioxide has an absorption spectrum with a maximum at

3 59 nm, with a molar absorptivity of 1250 mol

– 1 litre –1 cm –1

. This

extinction coefficient is independent of temperature, pH, chloride and

ionic strength. Chlorine dioxide is readily soluble in water, forming aEHC 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 usually reduced 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

d ioxide. Therefore, water treatment with chlorine dioxide wil l not

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

formation of bromoform (CHBr

3 ) or bromate. This is an important

difference between the use of chlorine dioxide as an oxidant and the 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 diss olved solutes.

Another part may decompose before reaction. Such decomposition is

catalysed by hydroxide ions (OH

– ) and other solutes. Highly reactive

secondary oxida nts, such as hydroxyl radicals (OH 

### @ ), are thereby

formed. These radicals and their reaction products can additionally

accelerate the decomposition of ozone. Conse q uently, radical-type

chain reactions may occur, which consume ozone concurrently with the

direct reaction of ozone with dissolved organic material.

Many ox idative 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 turbidity by oxidative 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

co ntaminants in drinking-water, the kinetics of ozone reactions are

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

compounds, such as chloroform ( CHCl

3 ), the kinetics of ozone

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

1 The CT value is the product of the disinfectant concentration C 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 values 1 than free chlorine

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

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

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

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

chlorine dioxide comb ined with chloramines as a secondary disinfectant

appears to be attractive for minimizing DBP f ormation (Singer, 1994b).

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

2 ) and nitrogen trichloride (NCl

3 ) are too

unstable to be useful and highly malodorous. C onditions practically

employed for chlorami nation are designed to produce only monochloramine.



### 2.3 Analytical methods for disinfectant by-products

and disinfectants

Analytical methods for various DBPs and their detection limits are

summarized in Table 2. Methods for disinfectants are summarized in

APHA (1995).

2.3.1 Trih alomethanes, 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) hydro lysis of dihaloacetonitriles and chloral hydrate to

dihaloacetic acids and chloroform, respectively; (ii) degradati on of

HAN s by dechlorinating agents such as sodium sulfite and sodium

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

DBPs Analytical method APHA a

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)

Haloketones b 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 (H

3 BO

3 /NaOH) 4500 2.0 c

0.2 Cl

– Siddiqui et al. (1996a)

Krasner et al. (1993)

Weinberg et al. (1998)

Chlorate Ion chromatography (H

3 BO

3 /NaOH) 4500 5 Cl

– , acetate Siddiqui (1996)

Chlorite Ion chromatography (NaHCO

3 /Na

2 CO

3 ) 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 quantitatively extracts 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 c hloral 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 acidic metha nol liquid–liquid extraction, requiring

esterification with diazomethane prior to analysis on a gas chromatograph equipped with an electron capture detector (E CD). 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 compounds involves extraction with MTBE prior to injection into

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

with hexane and GC–ECD analysis [(C

6 F

5 )-CH

2 ONH

2 + RCHO 

### 6

(C

6 F

5 )–CH

2 ON=CHR + H

2 O]. 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 peaks in a

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

2.3.2 Inorganic disinfectant by-products

An ion chromatography (IC) method (EPA Method 300) has been

developed to determine inorganic by-product s. The elution order is

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

and sulfate ion. Th e eluent is a carbonate buffer. Ethylenediamine is

used to preserve chlorit e samples and to minimize the potential for

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

36 involves measurement by an IC system using a separating column (e.g.,

Ion Pac AS9-SC) fit ted with an anion micromembrane suppressor

column. An eluent containing 2.0 mmol of sodium carbonate (Na

2 CO

3 )

per litre / 0.75 mmol of sodium bicarbonate (NaHCO

3 ) per litre is used

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

acid (H

3 BO

3 ) per litre / 20 mmol of sodium hydroxide (NaOH) per litre

is used for bromate and chlorate determination. The ana lytical 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). Fo r samples with high chloride ion

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

a nalysis 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 slightly different

because of the masking effect of NOM and high concentrations of

carbonate/bicar

b onate 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. Se veral investigators have reported that the

ultraviolet (UV)/persulfate oxidation method underestimates the TOC

concentration in na

tural 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 hydr ogen phthalate

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

but potassium hydrogen phthalate has a simple molecular structure and

is easy to oxidi ze. Dissolved organic carbon (DOC) is operationally

defined by a (0.45- 

### : m) filtration step. UV absorbance at 254 nm

(UVA

254 ) 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

a dequately the content of a matrix described only as a combined

chlorine residual. The presence of organic nitrogen and the instabilityChemistry 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 in water using 2-mercaptobenzot hiazole 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 i nvolved in the formation of disinfectant

by-products

2.4.1 Chlorine reactions

Chlorine reacts with humic substances (dissolved organic matter)

present in most water supplies, forming a variety of halogenated DBPs,

su ch 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-water treatment. Humic and fulvic acids show a high reactivity towards chlorine

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

(Thurman, 1985). Other fractions of the DOC comprise the hydrophilic

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

and proteins/ami no 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 and may contribute

towards total organochlorine production (Larson & Rockwell, 1979).

Free chlorine reacts with water constituents by th ree general

pathways: oxidation, addition and s ubstitution (Johnson & Jensen,

1986). Chlorine can undergo an addition reacti on if the organic

compound has a dou ble 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

oxi d ation 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 CHX

3 , where X can be

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

w ith functional groups of humic substances. The major functiona l

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

carbonyl and methoxyl. The reacti ons proceed much more rapidly at

high pH than at low pH.

Rook (1977) proposed resorcin ol structures to be the major

precursor structure in humic material for chlorof orm formation. In

accordance wi th this hypothesis in the chlorination of terrestrial and

aquatic humic substances, a series of intermediates were detected that

contained a trichlorome t hyl group and that could be converted to

chloroform by further oxidation or substitution reactions (Stevens et

al., 1976).

However, the production of chlo rinated compounds such as

dichloropropanedoic acid, 2,2-dich lorobutanedoic acid, cyanogen

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

explained on the basis of resorcinol structures, and possible production

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

reaction pathway for amino acids involves initial rapid formation of the

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 oxidation products. Luk nitskii (1975) provided a detailed chemistry

of chloral hydrate formation.

Christman et al. (1983) also identified chlo roform, chloral

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

dichlorobutanedoic acid as the major products, accounting for 53% of

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

have been detected, including several chlorinated a lkanoic acids and

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

extended these studies to incorporate chloroform intermediates,

chlorinated aromatic acids and cyano-compounds as potential productsChemistry of Disinfectants and Disinfectant By-products

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

in chlorinated natural waters can be attributed in part to the presence of

amino acids or peptides in natu ral waters. Humic acids may also

contribute to the presence of amino acids in natural waters, as they have

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 a m ino 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 ad ded, then the

corresponding nitrile can also 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 hypobromous acid. The bromine can then react with the organic matter in the

same way as hypochlorous acid to form various bromochlo rinated

DBPs. However, compared with hypochlorous a c id, hypobromous acid

is a weaker oxidant and stronger halogenating agent.

Chlorate, an inorganic by-pr oduct of chlorine, is formed in

concentrated

hypochlorite solutions during their production and storage

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

OCl

– + OCl

– 

### 6

ClO

2 – + Cl

–

OCl

– + ClO

2

– 

### 6 ClO

3 – + 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, an d 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

T he major chlorine dioxide by-products of concern are chlorite and

chlorate. Chlorine dioxide reacts generally as an electron acceptor, and

hydrogen atoms present in activated organic C–H or N–H structures areEHC 216: Disinfectants and Disinfectant By-products

40 thereby not substituted by chlorine (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 oxid a tion

and elect rophilic 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 by its somewhat lower disinfectant

demand as compared with chlorine.

Chlorine dioxide is generally produced by reacting aqueous

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

2ClO

2 – + HOCl + H + 

### 6

2ClO

2 (aq) + Cl

– + H

2 O

However, under conditions of low initial reactant concentrations

or in the presence of excess chlorine, the reactant produces chlorate ion:

ClO

2 – + HOCl 

### 6

ClO

3 – + Cl

– + H +

This reaction scena

rio is common in generators that overchlorinate

to achieve high reaction yields based on chlorite ion consumption.

An alternative approach to chlorine dio xide generation is with

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

production:

5NaClO

2

+ 4HCl 

### 6

4ClO

2 + 5NaCl + 2H

2 O

Chlorite ion is also produced when ch lorine dioxide reacts with

organics (Gordon & Rosenblatt, 1996):

ClO

2 + NOM 

### 6

Products + ClO

2 –

Chlorine dioxide can also undergo a series of photochemically

initiated reactions resulting in the formation of chlorate ion (Gordon et

al., 1995).

While bromide is not generally oxidized by chlorine dioxide,

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

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

with bromate ion in the 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 from new carpets and office products (Hoehn et al.,

1990).

Hoigne & Bader (1994) described the kinetics of reaction between

chlorine dioxide and a wide 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

secondary amines, aldehydes, ketones and carbohydrates are unreactive

under the conditions of water treatment. C hlorine dioxide rapidly

oxidizes substituted phenoxide anions and many phenols, and secondorder rate constants have been measured (Rav-Acha & Choshen, 1987).

2.4.3 Chloramine reactions

Chloramination of drinking-water produces THMs (if chloramine

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

chloral hydrate, hydrazin

e, 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 r ate to organic amines to form

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

Monochloramine was shown to cause binding of radiolabelled halogen

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

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

(Bercz & Bawa, 1986). Organic c h loramines are much weaker

disinfectants than inorganic monochloramine but are indistinguishable

b y common analytical methods. Organic chloramine formation mayEHC 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 a l

. (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 reac t ion of bromide with free radical species. The

molecular ozone mechanism does n ot 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

pa thway that is influenced by both pH and alkalinity. The hydroxyl

radical and, to a lesser degree, the carbonate radical (CO

3 

### @ ) pathway may

be more important than the molecular ozone pathway. Oxidants such as

hydroxyl and carbonate radicals may interact with intermediate bromi n e

spe c ies, leading to the formation of hypobromite radicals (BrO 

### @ ), which

eventually undergo disproportionat i on to form hypobromite and

bromite (BrO

2

– ). Bromate is then formed through oxidation of bromite

by ozone. The radical mechanism for the formation of bromate includes

two decisive reaction steps still involv ing molecular ozone: the

formation of hypobromite and oxidation of bromite.

Bromate ion formed through reactions w ith molecular ozone

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

formation in NOM-containing waters (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 NOMcontaining waters, respectively. Differences in NOM-c ontaining waters

can be attribu ted to differences in the characteristics of the NOM

present. A change in mechanism as a function of pH and the competitiveChemistry 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) mecha nism help explain

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

reactor design, concentration of organic precursors and ozone/bromide

ion concentrations (Gordon, 1993).

The presence of bromide ion in a source water further complicates

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

such as bromoform, dibromoacetonitr i le (DBAN) and dibromoacetone

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



### 2.5 Formation of organohalogen dis infectant by-products

Table 3 summarizes the DBPs identified as being formed from the

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, haloge nated 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.

The major chlorination DBPs identified are THMs, HAAs, HANs,

haloketones, chloropicrin and chloral hydrate. HAAs represent a major

portion of the non-THM halogenated or ganic compounds (Miller &

Uden, 1983; Reckhow & Singer, 1985). Many researchers 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 utilities tested,

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

two measured haloketone compound s. 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 products Significant

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 TH M formation have

very different pH dependencies, HAA formation correlates strongly

with THM formation when treatment conditions are relatively uniform

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 p oint 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 DCA and TCA are 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-water a

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 THM concentration in

the USA was 40 

### : g/litre, with an average TOC concentra tion of

3.0 mg/litre. The median annual average THM concentration found for

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

Water Industry Database 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 concentration was 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 surve y of the water supplies of 70 communities

serving about 38% of the population in Cana da, 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 con s iderably lower, averaging 2.9 

### : g/litre for bromodichloromethane (BDC M), 0.4 

### : g/litre for dibromochloromethane

(DBCM) and 0.1 

### : g/litre for bro moform. Using direct aqueous

injection techniques, average concentrations of most of the THMs were

higher (Health Canada, 1993).

Samples collected from the distribution systems of eigh t major

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

supplies, at concentrations ranging between 0.03 a nd 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 cities that did not mix groundwater and desalinated water.Chemistry of Disinfectants and Disinfectant By-products

47 Brominated THMs dominated (with bro moform the most abundant) and

existed at the highest concentration levels, whereas chloroform was the

least prevalent compound. This is the opposite of the occurrence pa t tern

found in almost all water distribution syst ems worldwide (Fayad,

1993).

The concentrations of chl oral hydrate in drinking-water in the USA

were summarized by IARC (1995) and varied from 0.01 to 28 

### : g/litre.

The highest values were found in drinking-water prepared from surface

water.

Chlorination of water as well as the combination of ozonation and

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

1985). In a stud

y conducted for over 25 utilities, very low levels of

chloropicrin were observed, and chlori n ation produced maximum

concentrations of less than 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.

Dichloroacetonitr ile (DCAN) is by far the most predominant HAN

species detected in water sources with bromide levels of 20 

### : g/litre or

less. For sources with higher bromide levels (50–80 

### : g/litre), bromochloroacetonitrile (BCAN) was the 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 appears that 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 phenolladen water is treated with chlorine is essential. The sources of phenolic

compounds in water supplies are reported to be industrial wastes.

In n a tural 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 may form

complex organic chloramines (Feng, 1966; Morris et al., 1980; SnyderEHC 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; Gray et 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 DBPs for Utah

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

secondary disinfection p u rposes. Overall, THMs and HAAs represented

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

D BPs represented only 25–50% of the TOX concentration. THM s

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

total DBPs by weight and approx imately one-half of the total THM

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

approach or possibly exceed THM co ncentrations.) HANs, haloketones,

chlorophenols and chloropicrin represented 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

eva luated 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 THM concentrations ranged from 30 to 44 

### : g/litre, with chloroform, BDCM, DBC M 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, with TCA, DCA, monochloroacetic acid (MCA), dibromoacetic acid (DBA) and monobromoacetic 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 Germany contained 1.35 

### : g of DCA per litre

and 0.1–20 

### : g of TCA per litre, whereas gr oundwater contained

0.05 

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

200 

### : g/litre was found for DCA and TCA in chlorinated treated water;

and chlorinated water in Switzerland contained 3.0 

### : g of TCA per 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 al l drinking-waters

prepared from surface water, whereas they could not be detect ed 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 dihaloacetonitriles was about 5% of the average THM concentration

(Peters, 1990).

2.5.2 Chloramine organohalogen by-products

Chloramine tr eatment practice involves three potential approaches:

free chlorine followed by ammonia addition, ammonia addition

followed by chlorine addition ( in situ production) and pre-formed (offline format ion) chloramines. Generally, the objective is

monochloramine formation. Chlorine followed by ammonia is a

common approach, and, during the fr ee-chlorine period, DBP formation

may mimic that of chlorine. Chloramination results in the production

of THMs (predominantly formed by chlorination followed by ammonia

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

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

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

significantly reduces but does not eliminate THM formation; cyanogen

chloride and TOX represent the major DBP issues with respect to

chloramines.

Scully et al. (1990) identified chloramino acids such as N-chloroglycine, N

-chloroleuci ne 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). Several studies show 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).

Trea tment of phenol-laden source waters with chlorine dioxide

does not produce th e typical chlorophenolic taste and odour compoundsEHC 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 h ost 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) report ed the identification of bromohydrins, a new group of labile brominated organic

compounds from the ozonation of a natural water i n the presence of

enhance d 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

artefacts formed from the reaction of excessive hypobromous acid with

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

Table 5 compares the median concentrations of various D BPs after

ozonation and chlorination.



### 2.6 Formation of inorganic disinfectant by-products

Although organic DBPs have been t he subject of study over a

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

coming under increasing scrutiny.

2.6.1 Chlorine inorganic by-products

Chlorite and chlorate are inorganic by-products formed in some

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

for disinfection. Bolyard & F air (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 signi ficant

levels of chlorate. Chlorite and bromate were also found in

hypochlorite solutions from these same water utilities. Chlorate was

present in drinking-water, either as a manufacturing by-product or fromChemistry of Disinfectants and Disinfectant By-products

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

DBPs Median concentration

( : g/litre): chlorination a Median concentration

( : g/litre): ozonation b

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 th e

addition of chl orine as sodium hypochlorite at a dose sufficient to

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

concentration of chlo r ite in commercial bleach solutions typically

ranges from 0.002 to 0.0046 mol/litre; similarly, the chlorate concentration ranges from 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

h ypochlorite solutions used to disinfect drinking-water co ntain

significant levels of chlorite and chlorate. The concentration of chlorite

ranged from <2 to 130 mg/litre for free available chlorine concentrations ranging from 3 to 110 g/litre. The concentration of chlorate varied

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

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

The concentrations of chlorate in treated source waters ranged from 11

t o 660 

### : g/litre. In another study involving 25 samples from p lants

using gaseous chlorine, no chlorate was detected, indicating that the 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 concen trations 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 any of the samples of

finished water of the treatment plants that apply gaseous chlorine.

Chlorate formation

is expected to be minimal in low-strength hypochlorite solutions freshly prepared from calcium hypochlorite, because

of the low hypochlorite concentration and only mildly alkaline pH.

2.6.2 Chloramine inorganic by-products

Inorganic by-products of chloramination include nit rate, 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 tha t can be produced during ozonation

inc lude free bromine, hypobromous acid, hypobromite ion, bromate

ion, free iodine, hypoiodous acid and iodate ion.

During the oxidation or c h emical disinfection of natural waters

containing bromide ion with ozone, bromate ion ca n be formed at

concentrations ranging from 0 to 150 

### : g/litre under normal water

trea tment 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 concentrations ranging 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

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

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

cations having the general formula M + O

3 –

. Hydrogen peroxide has been

identified as a b y-product of ozonation of organic unsaturated

compounds (Bailey, 1978).

Table 6 provides the range of bromate concentra tions normally

encountere d 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 c hlorinated

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

study indicates that aldehyde formation, although greater with ozone,

is not unique to ozonation, but is associated with chlorination and other

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) fulvi c acid was reacted with

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

between 90 and 120 ppm were observed, de noting 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., 1 992). Six natural

amino a cids (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

aci ds after a 2-h contact time with chlorine. Investigation of the byproducts indicated that the odours generated were systematically linked

to the aliphatic aldehydes formed. The peptides investigated had varying

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

imp art any odour. Chlorinous odours occasionally detected during

these ex periments 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 carbon yl 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 usua

l ly acids, ketones, aldehydes and alcohols. So-called

ultimate oxidat i on products of organic materials are carbon dioxide,

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

generally employe d 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 unsat urated organic

compounds are ozonated, ozonides, peroxides, dipero xides, triperoxides

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

that has been condu cted 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 concentrations for aldehydes from 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

attests to the wide variety of factors influencing DBP formation and the

complex interrelationships between these factors. Variables including

the concentration and characteristics of precu rsor 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

NOM consists of a mixture of humic substances (humic and fulvic

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

indicated by TO C or UVA

2 54 ) and the character (as described by

UVA

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

precursor material from which organic DBPs are formed; consequently,

increasing concentrations of NOM lead to increasing concentrations of

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

254

measurements as surrogate 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 processes that

remove NOM intact, while ozonation transforms part of the NOM into

biodegradable organic matter, potentially removable by biofiltratio n

. Coagulation preferentially removes humic/higher molecular weight

NOM; the selectivity of membranes for NOM r emoval is largely

dictated by the molecular wei ght 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.

In an investigation of the nature of humic and fulvic acids isolated

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

fulvic fractions had a lower aromatic content and smaller molec ular

size than the humic fractions. UV absorbance was correspondingly

higher for the humic fracti o ns, 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 fractions than upon

chlorination of the corre sponding 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 in a sample. This measurement, in conjunction with the

TOC (or DOC) measurement, can be employed in the evaluation of data

to provide an indication of the re activity of NOM towards forming

DBPs.

The reaction of ozone with NOM can occur directly or by radical

processes. The disappearance of disinfecting chemical is influenced by

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

consump tion of these chemicals is greater when the UV absorbance

(due to electrophilic and nucleophilic sites of N OM) of the source

water is significant, resulting in decreased DBP formation potential.

It appears that the nature of the organic material in a source water

may have some impact on t he relative concentrations of THMs and

HAAs formed upon chlorination. Treatment techniques that lower theChemistry 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

concern because the theoretical risk to humans varies for the 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

primarily impacting TCA), HANs and haloketones. The concentrations

of TCA tend 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 approximately 5 and 7. TCA was by far

the most predominant of the measured HAA species at six of the eight

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

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

possible t hat 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

t r eatment plants with a pH of about 5.5 in finished water produced

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

60 t han 7.0 in finished water produced higher amounts of TH Ms 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 and have been identified as intermediates in the formation of

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

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

c onditions. 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 by several researchers to 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).

In addition to the impact of pH on THM and HAA formation noted

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 s ignificant implications, for example, for

precipitative softening facilities.

pH has a strong effect on aldehyde formation (Schechter & Singer,

1995). Higher ozonation pH values p roduced lower amounts ofChemistry of Disinfectants and Disinfectant By-products

61 aldehydes, supporting the theory that these DBPs are formed primarily

through the direct molecular ozone reaction pathway, 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 forma tion 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 remov

i ng 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 par ticipating in

substitution reactions with organic molecules; furthermore, precursor

materials may differ in their susceptibility to bromination v e rsus

chlorination reactions. Hypobromous acid formed from bromide may

also r eact with ammonia to form bromamines (Galal-Gorchev &

Morris, 1965).

2.8.4 Effect of reaction rates

After chlorine addition, there is a period of rapid THM formation

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

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

authors have indicated that the concentration of chloroform appears to

incr ease slowly even after 96 h, suggesting that as long as low

concentrations of free chlorine are pre sent, chloroform continues to

form. Bromochlorinated THM species have been found to form more

rapidly than chloroform. Further data from many sources indicate that

bromoform formation slows at approxim ately 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 for THMs also appears to apply

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

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

sources, concentrations of chlorinated HAAs appear to slowly increase

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

18–20 h.

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

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

chlorine addition to waters conta i ning NOM. Reckhow et al. (1990)

found that although waters containing precur sor materials isolated from

six different wa t

er sources differed in their yields of chlorinated organic

by-products, the formation curves for chloroform, TCA and DCA had

th e 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 waters studied, both DCA and

TCA concentrations increased or remained stable throughout the 96-h

reaction period, suggesting that DCA was an end-product (AWWARF,

1991). Carlson & Hardy (1 998) indicated that HAA formation followed a pattern similar to that of THM formation. As with the THMs,

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

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

waters studied.

Diffe rent trends were observed in the HAN concentrations of

different source waters. Fo r two source waters, HAN levels formed

rapidly for the first 8 h and continued to increase slowly or levelled off

after 96 h (AWWARF, 1991). DBAN levels remained relatively stable

over the 96 h, as did BCAN and DCAN levels. For other sources, levels

of HANs consisting mostly of DCAN increased rapidly up to 4–8 h and

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

BCAN appeared to be slightly more stable than DCAN.

Very low levels of chloropicrin formation have been observed by

many researchers (AWWARF, 1991). Th e highest concentration

observed was 4.0 

### : g/litre. Chloropicrin appears to form slowly during

the inc ubation 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 formation rates of THMs, HAAs, bromate ion and HANs have

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 concentrati ons of other

DBP species were similar or not significantly different. These results

suggest that a higher temperature allows for more rapid progression of

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

t he effect of temperature on THMs, Peters et al. (1980) fou nd 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 han d, slowly forming

compounds require higher activation energy, and an increase in the

temperature supplies the energy. In addition to rea ction kinetics, the

temperature of a source water can also affect disinfection efficiency.

The biocidal effectiveness of monochloramine is significantly less than

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

concentration.

2.8.6 Effect of alkalinity

Although pH is a very influential variable and alkal inity affects pH,

alkalin ity itself does not appear to directly affect the formation of

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

aldehyd es 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 alkalinity increases bromate formatio n (Siddiqui et al., 1995).

The quantity

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 alkalinity and low pH or

low alkalinity and high pH produce greater quantities of aldehydes than

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 by all of the above chemical disinfectants,

the adoption of alternative disinfectants for DBP control often means

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

effective DBP control strategy is organic precursor (NOM) removal

thr ough enhanced coagulation, biofiltration, GAC or membrane

filtration. There has been lit t le success with bromide removal. Other

DBP control options include water qu ality modifications — for

example, acid or ammonia addition for bromate minimization.

2.9.1 Effect of ammonia

The presence of ammonia in source waters 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 plan ts using well oxidized waters but containing changing

ammonia concentrations. The addition of ammonia has been shown to

reduce the formation of 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 n itrifying bacteria is a potential problem in

chl oraminated water supplies or chlorination of sources containing

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

were det ected in 64% of samples collected from five chloraminated

water supplies in South Australia and in 21% of samples that 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 concentration and

THM levels (or the rate of THM production). Most investigators found

a linear relationship between chlorin e consumption and THM

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

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

also possible that the order of reaction changes during 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

r eaction 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 dosed with 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 long er-lasting than free chlorine

residuals, the dose s for each set of chlorinated and chloraminated

samples will be differe nt 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

incre ased, 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 of the

chlorine dose, regardless of the 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. TheEHC 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

primary disinfection and a method of DBP control (Symons & Worley,

1995). This process has been shown to oxidi z e dissolved organic

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

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

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

hydrogen peroxide/UV process on THM formation and concluded that

this process is only slightly effec tive in reducing the formation of

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

with UV effectively reduced or prevented the formation of aldehydes.

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

ozone/UV) involving hydroxyl (and hydrop eroxyl) radical formation

may provide similar opportunities.

2.9.4 Effect of chemical coagulation

Enhanced coagulation and softening will remove TOC. Enhanced

coagulation is characterized by coagu lant doses greater than those

required for optimum turbidity removal; as a n alternative to higher

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

can be practised.

All organic DBPs were reduced by the addition of commonly used

coagulants. Iron-based

coagulants, such as ferric chloride, were

consistently more effective than alum in removing NOM (Crozes et al.,

1995). Alum coagulation removed all DBP precursors to 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 as surrogate parameters for

DBP formation potentia l. A fairly good correlation was observed

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

TOC, indicating that the relative concentrations of HAAs and THMs do

to some extent depend on the nature of the precursor ma t erial.

However, more dat a from waters of different qualities would be

required to evaluate the validity of this relationship.

The effectiveness of coagulants in removing DBP precu rsors is

dependent upo n the molecular size of the dissolved organic matter.

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

67 through coagulation. In a study conducted by Teng & Veenstra (1995),

water containing dissolved organic matter with molecular weights in

the range 1000–10 000 daltons generally produced the largest amounts

of THMs and HAAs under conditions of free chlorination. Coagulation

and ozonation shift a proportionately greater amount of the THM and

HAA formation potential to the smallest molecular wei ght 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 c hlorine generally favours the formation of brominated organic

DBPs.

2.9.5 Effect of pre-ozonation

Several studies of ozone oxidation fo l lowed 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

aluminium or iron salts. A convenient and more appropriate method for

the removal of the aldehydes formed during ozonation is the i n corporation of a biological treatment step (biofil tration) in the water

treatment process following ozone oxidation.

Pre-ozonation can have both positive and negative effects on DBP

forma t ion. Pre-ozonation with typical water treatment dosages and

bic arbonate levels has been shown to remove TCA and DCAN

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

DCA precursors and may lead to an increase in 1,1,1-TCPN precursors

(Reckhow & Singer, 1985). According to Teng & Veenstra (1 9 95), preozonation resulted in enhanced formation of DCA during 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 chlorinationEHC 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

under the conditions of ozonation in combination with prechlorination

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

Ozonation in the presence of traces of hypochlorite ion can form

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

that if there is any resi dual chlorine present, ozone can potentially

oxidize hypochlorite ion to chlorate.

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

and other mixed bromochlorinated by-products formed 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 wa ter samples through a rapid sand filter

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

Chlorinated samples experienced a 26% reduction in al dehyde

concentrations under the same conditions. The s e reductions in aldehyde

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 micr oorganisms on GAC

particles than on san d. Thus, the biological conversion of oxidized

water impurities to carbon dioxide and water will be greater during

passage through GAC media. Similar aldehyde removals ha ve been

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

1995).

Drinking-water treatment techniques tha t 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 contain ing three different DOC levels and ambient

bromides, which were filtered through biologically active GAC filters.

Analysi s of treated waters showed no removal of bromide ion and a

shift towards more brominated organo-DBPs. T HM levels after

treatment with GAC with no added ozone decreased from 900–1700 to

100–700 

### : g/litre. These water sources 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

chemicals for pre-disinfection (or oxidation) and post-disinfection and

maintaining a residual for 5 days to simulate concentra tions 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

dio x ide as a preoxidant, although enhanced formation has been

observed.

Table 10 summarizes the effects of water quality and tr eatment

variables on the formation of DBPs.



### 2.11 Alternative strategies for disinfectant by-product

control

The concern about chlorite, bromate, chlorate and other DBPs in

drinking-water following treatment with disinfectants has stimulated

research into ways to eliminate the production or enhance the removal

of DBPs. Strategies for 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 purity and stora ge 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 systems a,b

DBPs Sand–Cl

2 Cl

2 –Sand–Cl

2 O

3 –Sand–Cl

2 NH

2 Cl–Sand–NH

2 Cl O

3 –Sand–NH

2 Cl ClO

2 –Sand–Cl

2

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

increasesTable 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

UVA 254 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 effectTable 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, GACChemistry of Disinfectants and Disinfectant By-products

75 2.11.1 Source control

Source control options involve controlling nutrient inp uts to

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

dri nking-water sources, watershed management (e.g., constructing

stormwater detention basins), saltwater intrusion contro l (e.g., development of structural or hydrodynamic barriers to control TOC and

bromide), and using the concept known as aquifer storage and recovery

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

best) (Singer, 1994a).

2.11.2 Organohalogen by-products

Strategies for control of organohalogen by-products include

removal of DBPs that are formed using technologies such as oxidation,

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

precursors using treatment techniques such as conventional treatment,

oxidation, me mbrane processes, carbon adsorption and biological

degradation. For many organic compounds that 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 relatively expensive processes;

moreover, NOM removal by GAC cannot be accomplish ed 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

generator systems can minimize some of the production of chlorate ion.

Removal of chlorite and chlorate has been reported using reduction by

Fe 2+ or sulfite or by GAC (Voudrias et al., 1983; Lykins & C lark,

1988). GAC is seen as problematic because of chlorate production and

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 w ater entering the coagulation

process (i.e., formed during pre-ozonation), several options exist for its

removal. An aqueous-phase reducing agent (e.g., Fe 2+ ) can be added at

the rapid mix step. Powdered activated carbon can likewise be added as

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

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

filtration process; in this situation, removal of broma te by activated

carbon is possible. This approach has potential relevance to integration

of GAC columns into a process train or, more r ealistically, to retrof itting 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 am ines formed during the oxidation step of the process train using chlorine can potentially be removed

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

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

halogenated amines to nitrogen. Organ ic 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 o f which

have structural similarities to certain organic DBPs) are removed well

by ozone treatment, but that alkanes are poorly removed. Also, removal

efficiency improves for the alkenes and aromat ic 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 hy drogen 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 reg ulation 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 factors impacting the regulation of DBP s 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 car c inogenic, mutagenic or possibly teratogenic,

several countries have recently laid down standards for various DBP

levels. This stimulated the search for mathematical models to describe

or predict DBP formation in disinfected wat er and to evaluate the

effectiveness of water treatment technologies designed to reduce DBP

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

based on fitting mathematical equations to various empirical observations, rather than mechanistic and kinetic considerations. This is mainly

due to the complexity of the reactions between organic precursors and

disinfecting chemicals, which usually involve several parallel pathways

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

formation reactions a lso makes it difficult to develop universally

applicable models for simulating DBP formation potential associated

with disinfection of a diverse array of natural source waters. However,

the analysis presented b y many models suggests that many waters

exhibit comparable general responses to changes in a given parameter

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

mathematical functionality), although specific responses associated

w ith 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 ap p lication 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 controlling DBP formation, which

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

78 The extent of forma t ion of DBPs is dependent on several water

quality parameters, such as TOC concentration, UVA

254 , bromide

concentration and temperature. It is also dependent on chlorination

conditions, such as chlorine dose, pH, ammonia concentr ation and

contact time. After the various statistically significant factors were

identified, mathematical e q uations were developed to describe the

formation of various DBPs. A least square s method was used to

determine the optimum equation coefficients that best describe the

experimental data. The optimum co efficients 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

Numerous models for predicting THM formation through chlorination have been reported (Moore et al., 1979a; Kavanaugh et al., 1980;

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

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

Of these, models reported by AWWARF (1991) and Amy et 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

Amy et al. (1998) summarized empirical models for THMs, HAAs and

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

assess both in-plant and distribution system formation of THMs, HAAs

and chloral hydrate. Water quality conditions suc h as DOC, pH,

temperature and bromide are needed as inputs to 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 crucial for evaluating various bromate control strategies. 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 analysisChemistry of Disinfectants and Disinfectant By-products

79 has shown that ozone dos e, 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 of its strong 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 the re is concern for regrowth of

microorganisms following ozone treatment. They are also potentially

hazardous and may produce increased amounts of ch lorinated 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- p roduct precursor

removal

It is recognized that chlorination wi ll continue to be the most

common disinfec tion process; hence, enhanced removal of DBP

precursors present in raw sourc es 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 asses sing coagulation efficiency in removing NOM and

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

Coagulation can reduce DOC and DB P precursors but not

bromide levels; hence, a greater proportion of brominated DBP species

can potentially be produced in the finished water.

The effects of precursor removal by chemical coagulation can be

assessed through the use of treated water models. One can either 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 rea c tivity (i.e., DBP/DOC)

does not change, one can also assess other precursor removal processes,

such as GAC or membrane processes, through use of the raw/untreatedEHC 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 options to meet regulations. It is often more 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 understa nding 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-product s has been well

studie d, and ozone and its by-products have recently received

much attention. Studies of chlorine dioxide and chloramines and

their by-products are relatively few, although more work in these

areas is now being undertaken.

• One of the simplest processes to minimize halogenated DBP

formation is limiting the fr ee chlorine contact time by using

monochloramine to maintain a distr ibution system residual

following primary disinfection by chlorine 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 anal ytical complexities, very few detailed studies have been

undertaken toChemistry of Disinfectants and Disinfectant By-products

81 determine the individual compounds present and their concentrations.

• Many factors between the source and the tap can influen ce the

DBPs to which consumers are exposed. Altho ugh THMs and

HAAs continue to form with increasing contact time, some other

halogenated DBPs, such as HANs and haloketones, form rapidly

but then decay in the distribution system as a result of hydrolysis.

This has major implications regar ding exposure to these DBPs,

depending upon their proximity to the treatment plant. For treated

source waters, median levels of HAAs are often approximately

one-half of the median THM levels.

• F or low-bromide source waters, chloroform is normally the

dominant THM species; DCA an d TCA are the most prevalent

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

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

various researchers; this compound appears to form slowly during

the incubation period, with concentrations tending to level off at

40 h. For high-b

r omide 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

excess free 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 HANs in chlorinated

samples may be attributed to precursors other than amino acids,

the potential for amino acids to be present in natural sourc es 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 de pends on many factors. Freshly made hypochlorite

solutions will contain less chlorate than hypochlorite that is stored

without concern for t emperature and pH. If a utility is using a

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

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

periodically flushed and cleaned, and, if possible, the storage time

should be reduced.

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

and m o vement of DBP precursors in distribution systems. The

models can b e designed as a planning tool for evaluating the

impacts of source water management s trategies and estimating

DBP exposures. Some limitations of e xisting models include

calibration with a limited database, application to only a specific

water source or group of related sources, lack of terms to simulate

important parameters, such as r eaction time, and inadequate

validation.83 

### 3. TOXICOLOGY OF DISINFECTANTS

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

Adding disinfectant in excess of the demand has seve ral 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 is the

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

th ey could be used. Since the major disinfectants were introduced

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

toxicologi c al 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 lun g 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 possibility of 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 ex tensively in the

disinfection of drinking-water. The stock solutions used for thisEHC 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 guin ea-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

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

demonstrated that instillation of bleach into the oesophagus of dogs

produced irritation. The minimal exposure that produced oesophageal

burns wa s 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 f ew evaluations of the effects of

chlorine or hypochlorite in drinking-wate r. The present review will

focus on studies with t reatment periods longer than 4 weeks where

drinking-water was the primary route of exposure. Reference to earlier

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 e t 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-wate r 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 stati stically significant

decreases in drinking-water consumption in females treated with 100

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

were no consistent effects of chlorine trea tment 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 B6C3F

1

mice (Daniel et al., 1991a). Male and female B6C3F

1 mice (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 dosesToxicology 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 les ions could be related to

treatment with chlorine.

Several of the following studies utili zed solutions of sodium

hypochlorite as the treatment che mical. It is now known that such

solutions can contain ver y high concentrations of chlorate within a

short time of the ir 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%) of sodium 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. Significant suppression of body weight (as a result of decreased

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

authors no ted 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 evidenc e of treatment-related

pathology was observed in th is 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

we ight 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-ye ar 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 B6C3F

1 mice (70 per

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

gaseous chlorine and neutralized to pH 9 by the ad dition of sodium

hydroxide. Stability stud ies 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 stu dies wereEHC 216: Disinfectants and Disinfectant By-products

86 approximately 0, 4, 7 or 14 mg/kg of b ody 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 body weight per day for female mice. The only treatmentrelated 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 lesions were observed 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 hypochlori te per litre was

found to red u ce 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 cytotoxicity to melanoma and fibrosa rcoma cell

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

in a subsequent study, which demonst rated that 25 mg of sodium

hypochlorite per litre decreased the ability of peritoneal macrophages

to phagocytose 51 Cr-labelled sheep re d blood cells. Macrophages

obtained from the mice treated with hypochlorite were found to be less

effective in destroying B16-BL6 mela noma cells i n vitro

. Mice so

treated were also found to have increased pulmon ary 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 c ells was

dec reased at 15 and 30 mg/litre, and the prostaglandin E

2 levels of

these cells were found to be sig nificantly elevated. No effects on

natural killer cell cytotoxicity, antibody responses, interleukin 2

production or phag o cytic activity were observed. The effects on

macrophage activity suggest that some impairment does oc c ur at

relatively low levels of sodium hypochlorite. As poi nted out by the

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

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

a significant im pairment of the immune response to a particular

infectious agent. However, modification of macrophage func t ion

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

87 of chlorine or hypochlorite in experimental animals. A study in which

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

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

thymus weight, plaque-forming cell r esponse, 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 mi ght give rise to

pathology. The concentrations of h y pochlorite 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 hypoch lorous 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 concentrat ions 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 wh ose 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 hy perplastic 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 natur e of chlorine always raises questions of

whether it is chlorine or a by-product that is responsible 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

amounts that 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-products that

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 disinfect a nt before sufficient

oxidative power would be present to break down substrates to small

molecules. Despite these design flaws, thedata 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 be measured in

the blood, brain, liver, kidneys and fat of rats to which sodium hypochlorite w as 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 ra ts 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-p roducts of

chlorination in st omach contents of the rat are the organic N

chloramines (Scully et al., 1990). N -Chloroglycine, N

-chloroleucine

or N -chloroisoleucine and N

-chlorophenylalanine were con firmed

products of reactions with normal amino acids that would ordinarily

be found in the gastroin testinal tract. N -Chlorovaline and N

chloroserine were also tentatively identified. Organic chloramines are

reactive and could be responsible for toxic effects that may be

a ttributed to chlorine in toxicological studies. The chlorine demandToxicology of Disinfectants

89 of free amino ac ids in stomach contents was found to be only about

4% of the total. Consequently, this process may be substrate-limited

at concentrations of chlori ne found as residuals in drinking-water.

However, use of higher concentrations of chlorine would also lead to

breakdown of proteins present in the stomach flu id. 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 studie d the effects of

chlorine or hypochlorite on more specific aspects of reproduction or

development. Meier et al. (1985b) reported that or al administration

of sodium hypochlorite (pH 8.5) prepared from chlorine gas and

administered at 4 or 8 mg/kg of body weigh t per day for 5 weeks

increased the incidence of sperm head abnormalities in B6C3F

1 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 a ble 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 d ays 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

90 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 sing le component involved in toxic

release incidents. A third major source of exposure is solutions of

sodium hypochlorite, usually marketed as bleach. Bleach is frequently

involved in human poisonings. These exposures are not particularly

relevant to exposures to chlorine or hypochlorite in drinking-water.

Therefore, only a few case rep orts 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

be c ause 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 the

re was no evidence of permanent lung damage;

however, these studies clearly indicated that survivors had breath

sounds that 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 s equelae have been identified following

exposure of humans to accidental releases of chlorine gas. In these

more mo dern characterizations, the acute signs and symptoms

included a high incidenc e 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 exposu re. In such cases, a decrease in the

forced expirat o ry volume is the most consistently reported clinical

sign.

Two recent reports suggest that chronic sequelae to acute exposures to chlorine gas may be more p revalent 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 increa sed during the follow-up period. Sixty-seven per

cent of those tested were found to have residual volumes below 80%Toxicology of Disinfectants

91 o f their predicted values. Five of 13 subjects tested for ai rway

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/m 3 (0.5 and 1.0 ppm)

for 4 or 8 h (reviewed in Das & Blanc, 1993). Four hours of exposure

to 2.9 mg/m 3 (1.0 ppm) produced significant decreases in the forced

expiratory volume. One individual who was found to be experiencing

more difficulty than other subjects at this dose and who was later

exposed to 1.5 mg/m 3 (0.5 ppm) experienced a significant decrease in

forced expiratory volume. While 1.5 mg/m 3 (0.5 ppm) appears protective for most people, some more 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, b ut the trend

was not statistically significant (Das & Blanc, 1993). Other studies

failed to identify chronic sequelae.

There are frequent reports of h uman 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 d ichloramine, both of which are

effective respiratory irritants (MMWR, 1991).

Any potential effects of chlorine or hypoc hlorite in drinkingwater are obscured by the fact that by-products inevitably coexist with

the residual chlorine. One series of studies in which by-produc t s

formed were minimized by dissolving chlorine in distilled water

attempted to identify effects of chlorine in drinking-water on humans.

Chlorine in drinking-water w as administered in a rising-dose

tolerance study be ginning 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 & Bian chine, 1984). No clinically

important changes were observed. No findings of clinical importanceEHC 216: Disinfectants and Disinfectant By-products

92 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

epidemiological study that associated small increases in cholesterol of

women with residence in communities having chlor i nated 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 compos ed 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

re ceived distilled water. Half the subjects were assigned to a group

that consumed 1.5 litres of water containing 20 mg of chlori ne 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

sign ificant effects. There was a slight trend towards lower thyroid

hormone levels in men consuming chlorine, but this was not clinically

significant (Wones et al., 1993a). These data suggest that observations

obtained p reviously 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 subsequent study 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 fro m studies in humans on theirToxicology of Disinfectants

93 carcinogenicity and inadequate evidence for their carcinogenicity in

experimental animals. Hypochlorite salts were assigned to Group 3:

the comp ounds are not classifiable as to their carcinogenicity to

humans (IARC, 1991).

Several studies have shown that sodium hypochlorite produces

muta genic responses in bacterial systems and mammalian cells i n

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 inves tigators 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 of sodium hypochlorite. This response

was unaffected by the addition of catalase, sugg esting 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 expr essed about whether responses observed with

such high and clearly cytotoxic concentrations in an in vitro system

rep resent 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 detoxif

ying 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, i t is difficult to use these data in interpreting the effects of

chlorine or hypochlorite in vivo

.EHC 216: Disinfectants and Disinfectant By-products

94 Ishidate (1987) studied the induction of chromosomal aberrations

in cultures of Chinese hamster CHL cells at sodium hypochlorite

concentrations ran ging 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 weakly positive

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 mar row 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 f or 5 consecutive days. There was no evidence of increased

micronuclei or chromosomal abnormali ties in bone marrow cells.

Signific ant 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 m utagenicity 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 hypoch lorite 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 drinkingwate r 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

and B6C3F

1 mice. 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 perToxicology 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–respons e, 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 of skin tumours following initiation with 4-nitroquinoline1-oxid e in female ddN mice (Hayatsu et al., 1971). A solution of

sodium hypochlorite that contained 10% effective concent rations 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 hypo chlorite 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 eit her 750 or 1500 

### : g of benzo[ a ]pyrene. Sodium

hypochlorite u sed in a more traditional initiation/promotion study

(i.e., sodium hypochlorite treatment following initiat ion 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 us e d, or the smaller experiment of

Hayatsu et al. (1971) may simply be a false result.

As pointed out in section 3.1.1, application of solutions of hypochlorous acid to the skin of Sencar mice results in 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 hypochloriteEHC 216: Disinfectants and Disinfectant By-products

96was 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 t han 300

mg/litre would suggest that this is of no concern.

3.1.5 Comparative pharmacokinetics and metabolism

A series of pharmacok inetic studies using 3 6 Cl-labelled hypochlorous acid w ere conducted by Abdel-Rahman and co-workers

(1 983). These studies are of limited value because the form of 3 6 Cl

could not be determined in various body compartments.

3.1.6 Mode of action

There are no spe cific 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 by virtue of its reaction with organic compounds that are

found in the stomach. Such react ions 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 few evaluations of the 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 pr imarily 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

potent respiratory irritants, because the formation of these compounds

when household bleach and ammonia are mixed results in a numberToxicology of Disinfectants

97 of poisoning cas e s each year (MMWR, 1991). In spite of this, there

has been no attempt to quantif y dose–response relationships in

animals.

Eaton et al. (19 73) investigated concerns about chloramineinduced methaemoglobin formation in kidney patients dialysed with

chloramine-containing water. This was done by examining the ability

o f 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 i n

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 tap water containing 1 mg of chloramine per litre or above. This

e ffect was not produced by comparable concentrations of sodi u m

hypochlorite. The ability to induce methae moglobin formation was

eliminated by treating the water by reverse osmosis followed by

c arbon filtration. Clearly, chloramine is capable of inducing

methaemoglobinaemia at low concentrat ions when there is a large

reservoir of chloramine. This is a decidedly different exposure pattern

from that of normal humans a nd 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 monochloramine per litre in carbonate/bica rbonate-buffered (pH 8.9) drinking-water. Twelve animals were ass igned to each group, and

treatments were maintained over a 30-day period. Consistent with the

interpretation provided above, there were no treatment-related effects

on osmotic fragility, me thaemoglobin levels, haemoglobin concentrations, reticulocyte counts or a number of other derived parameters.

Haematocrits of mice treated w i th 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 body weight

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 haematolo gical and clinical chemistryEHC 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 counts were

slightly depressed at 100 and 200 mg/litre. The authors concluded

that mon ochloramine 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 the no-observed-ad verse-effect level (NOAEL)

was 100 mg/litre, equivalent to 5.8 mg/kg of body weight per day.

The work in rats was followed up with a second study in B6C3F

1

mice (Daniel et al., 1991a). Male and female B6C3F

1 mice (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 d ecreased at 100 and 200 mg/litre

in males and at 25–200 mg/litre in females. Weight gain was

si gnificantly depressed in both sexes at 100 and 200 mg/litre.

Neutrophil c oncentrations 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 t o 8.6 mg/kg of body

weight per day.

A 13-week study in which g r oups of 10 Sprague-Dawley rats

were given drinking-water containing 200 mg of monochloramine per

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 outsta nding toxicological questions. The results of this

study indicated that the reduced body weight gain and the minor

biochemical, haematological, immunological and histopathologicalToxicology 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 re duced water and food consumption

(Poon et al., 1997).

In a 9-wee k study, Exon et al. (1987) examined the ability of

m onochloramine 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 E

2 synthesis by adherent resident peritoneal cells (which include macrop hages) in response to lipopolysaccharide stimulation. No attempt w a s

made to relate this finding to other indices of modified macrophage

function. A small depression in spleen weights was obser ved at the

highest dose. The implications of these data for immune function are

not clear. Other measures of immune function did not reve al statistically significant changes with treatment. These included a decrease

in antibody formation at the lowest and middle doses in response to

key hole limpet haemocyanin injection or delayed-type hypersensitivity

reactions to bovine serum albumin injected into the footpad.

The ef fect 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 m onths prior to breeding

and through gestation. Only six animals were assigned to each

treatment group. Reproductive performance was comparable between

groups, and fetal weights wer e not adversely affected by treatment.

Between 50 and 60 fetuses were available (male and female

combined) for evaluation. There was no evidence of treatment-related

skeletal or soft tissue anomalies.EHC 216: Disinfectants and Disinfectant By-products

100 Carlton et al. (1986) examin ed the effects of monochloramine

administered by gavage at doses of 0, 2.5, 5 o r 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 eff ects on

sperm morphology, concentration or motility were observed, 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 ef fects of chloramine have been

documented in humans p oisoned by chloramine formed when

household bleach was mixed with ammonia for use as a cleaning

solution. Chloramine is a strong res piratory irritant. These effects

were discussed in section 3.1.3.

Forty-eight men completed an 8-week protocol during which diet

and other factors known to affect lipid metabolism were controlled.

Durin g 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 m g/litre, no

significant changes were observed in total, HDL or LDL cholesterol,

triglycerides or apolipoproteins A1, A2 or B. Parameters of thyroid

function were unc hanged. 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 su btilis 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

of B. subtilis

, rec3, recA and polyA5, were consistently more sensitive

to the cytotoxic effects of chloramine, while the uvr and recB mutants

w e re 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 repa ir of DNA

lesions produced by both chemicals. Thus, it is possible that aToxicology of Disinfectants

101 comm on intermediate or mechanism is involved in the mutagenic

effects of hypochlorite and chloramine.

A broader list of chloramines was tested by Thomas et al. (1987)

in Salmonella typhimurium tester strains TA97a, TA100 and TA102.

The chloramines tested inclu ded 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 wer e 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). Hypochloro us 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 m ouse bone marrow micronucleus assay when

chloramine was administered orally. They suggested that the in vitro

clastogenic effects were probably attributable to non-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 micr onuclei or

chromoso mal 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 li mited to a

single set of 2-year experiments conduct ed by the NTP (1992).

Drinking-w ater containing 0, 50, 100 or 200 mg of chloramine per

litre was provided to F344 rats and B6C3F

1 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 B6C3F

1EHC 216: Disinfectants and Disinfectant By-products

102 mice treated with the high dose of chloramine. In addition, one renal

adenoma was found in one male mouse treated with 100 mg/litre and

in one female mouse treated with 200 mg/litre. While this tumour site

is rare in both species, there was no real dose–response trend, nor

were the differences between the control and treatment groups

st atistically 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 chlo ramine or

hypochlorite, although the effects were not clearly dose-de pendent.

The incidence of mononuclea r cell leukaemia was significantly

greater than in concurrent controls and was el evated above the

historical incidence as well. Nevertheless, th ese increases were not

considered to be treatment-related. In part, this conclusion arose from

the lack of a clear dose–response. It was also b ased on the fact that

there was no comparable trend in male rats.

3.2.5 Comparative pharmacokinetics and metabolism

The pharmacokinetics of 36 Cl derived from mo nochloramine 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 very few general

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 properti es of the

compound. In essence, these data suggested that exposure to chlorine

dioxide in air at a concentration of more than 420 mg/m 3 (150 ppm)

forToxicology of Disinfectants

103 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/m 3 [0.1–3400 ppm]) and for various

periods (3 min–10 weeks). All the rats exposed daily t o chlorine

dioxide at 28 mg/m 3 (10 ppm) died in les s than 14 days. Purulent

bronchitis and disseminated bronchop neumonia were found at necroscopy. No such changes were demonstrable in rats exposed to approximately 0.28 mg/m 3 (0.1 ppm) for about 10 weeks (Dalhamn, 1957).

The LC

50 of chlorine dioxide in rats (5 per se x per group)

exposed by inhalation for 4 h was 90 mg/m 3 (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 was significantly 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

significant increase in skin thickness at concentrations of 300 mg/litre

or less.

The study of Daniel et al. (1990a) was the 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, respectivel y. Conventional measures of body weight,

org a n 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. Thi s appeared to be secondary to depressed water

consumption, which is know n to be tightly coupled to food

consumption in rats. The only significant histopathological damage

found was goblet c ell hyperplasia and inflammation. This was

observed at all doses of chlorine dioxide in both male and female rats.

Presumably this in flammation occurs as a result of volatilization ofEHC 216: Disinfectants and Disinfectant By-products

104 chlorine dioxide from the water bottle. Th e amount of chlorine

dioxide actually inhaled as a result of volatilization from the drinkingwater containing the lowest dose of chlorine dioxi de (25 mg/litre)

must be extremely low. This sugg ests 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 has received 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 concentratio ns of 1, 10, 100 and 1000 mg/litre for up to

4 months, but the changes were not consisten t ly dose-related.

However, the authors fo und that the morphology of red blood cells

was modified (codocytes and echinocytes) i n all dose groups, the

severity increasing with i n creased treatment concentration.

Methaemoglobin was not detected throughout these studies. However,

there was no formal st atistical evaluation of these results, and only

four rats were assigned to each experimental group. Administration

of acute do ses 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 periods in a subsequent publication (Abdel-Rahman

et al., 1984a). Again, only four animals were assigned to each

tr eatment group. At 7 and 9 months of treatment, red blood cells

appeared to beco m e 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 3 H-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

synt h esis 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 3 H-thymidine. Ordinarily , sacrifices are made 30–60

min after injection because the blood is essentially depleted of 3 Hthymidine in an hour . Thus, it is not possible to determine if the

lowered amount of label is related to de creased synthesis or toToxicology of Disinfectants

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. Thir d, 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

were provided 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. Animals showed 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 methaemoglobi naemia. 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). T hyroxine levels in African green

monkeys administered drinking-water containing 100 mg of chlorine

dioxide per litre (4.6 mg/kg of body weight per day) were again found

to be depressed at 4 weeks oftreatment, but rebounded to abovenormal levels after 8 weeks of treatment. These investigators also

found significantly depressed thyroxine levels in rats treated 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 base d 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 auth ors demonstrated that chlorine

dioxide oxidizes iodide to reactive iodine species that would bind to

the stomach and oesophageal epithelium. Rat chow that was treated

wit h 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 colonEHC 216: Disinfectants and Disinfectant By-products

106 and reduced uptake by the 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 observatio ns, the

authors concluded that the effects of chlorine dioxide were probably

due to altered gastrointestinal absorption of iodide 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 remai ns 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., concentr ation,

motility, progressive movement or morphology ). Thyroid hormone

levels were altered significantly, but not in a consistent pattern. The

only s ignificant 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 was significantly reduced at the highest concentration of chlorine

dioxide. The percentage of anomalous fetuses was increased in a doserelated manner, but the respon se was not statistically significant.

These anomalies arose prim arily as the percentage of abnormal or

incomplete sternebrae in treated rats relative to controls. The lack of

statistical significance was undoubtedly r e lated to the relatively few

female rats that were included in the study (6–8 females per treatment

group). As a consequence, the results of this study must be considered

inconclusive.Toxicology of Disinfectants

107 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 se r um of the pups at 100 mg/litre.

This wa s associated with delayed development of exploratory

behaviour in the pups away from their dams, and this , in turn, was

probably due to an indirec t effect on iodine uptake. In a second

experiment, pups given 14 mg of chlorine dioxide per kg of body

weight per day directly by g avage on postnatal days 5–20 showed

significantly depressed activity and a decrease in ser um thyroxine

levels. Studies by the same group (Tay l or & Pfohl, 1985) indicated

that cerebellar and forebrain cel l counts (based on DNA

measurements) were depressed in 11-day-old p ups 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 counts remained 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 admini s tered

chlorine dioxide by gavage at 14 mg/kg of body weight per day from

postnatal day 1 to 20. Body weight was reduced, 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

po stnatal day 35, and the number of dendritic spines on cerebral

c o rtical 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 developme nt. In most studies, there are suggestions of

mo dified thyroid function associated with these effects. It must be

pointed out that the changes in th y roid hormone levels are modest,

much less than are produced with classical antithyroid drugs such as

propylthiouracil (Toth et al., 1990).EHC 216: Disinfectants and Disinfectant By-products

108 3.3.3 Toxicity in humans

Th e effects of chlorine dioxide were assessed in a two-phase

study in 10 healthy male 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 changes in a variety of clinical chemistry

parameters were observed, but none was found to be o utside the

accepted range of normal. The second phase of the experiment

in volved the daily administration of a 500-ml portion of a solution

containing 5 mg/litre to 10 healthy volunteers for a period of 12 weeks

(Lubbers et al., 1984a). Again, measurement of a large battery of

clinical chemistry parameters 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 recogniz e 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 clast ogenic effects of chlorine dioxide have

received little attention. Ishidate et al. (1984) found chlorine dioxide

to be positive in Salmonella typhi murium t ester 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 d i oxide failed to

produce such damage following gavage doses of up to 16 m g/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 carcinogenicToxicology of Disinfectants

109 activity of chlorine dioxide in experime n tal animals were identified

in the scientific literature.

3.3.5 Comparative pharmacokinetics and metabolism

There are significant differences in the pharmacokinetics of 36 Cl

obtained from different disinfectants. The absorption rate for 36 Cllabelled chlorine dioxide was at least 10 times that observed with

chlorine, chloramine or chloride. The relative amount of 36 Cl 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 36 Cl appears similar for all disinfectants.

These data suggest that the form of 36 Cl that is being absorbed differs

chem ically with the different disinfectants. In the case of chlorine

dioxide, this is supported by the observation that measurable amounts

of chlorite (about 3% of the original dose) are eliminated in the urine

during the first 2 4 h, and chlorite comprises about 20% of the label

present in blood 72 h after admini stration 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 as rapid (Abdel-Rahman et al., 1982b). This suggests

that some of the absorption could be as chlorine dioxide itself. On the

surface, this hypothesis would seem to be incompatible with the high

reactivity of this disinfectant. As with other disinfectants, the terminal

elimination phas es observed for 3 6 Cl from chlorine dioxide seem

compatible with the hypothesis that the bulk of the elimination is as

chloride ion.