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OCR for page 141
Keeping Pace with Science and Engineering. 1993.
Pp. 141-164. Washington, DC: National Academy Press.
Trihalomethanes and Other
By-Products Formed by Chlorination
of Drinking Water
Philip C. Singer
Chlorine has been used to disinfect drinking water in the United States
and in most of the world since 1908. Its widespread use has been credited
with the control of a number of waterborne diseases, most notably typhoid
fever and cholera. However, with the discovery in 1974 of trihalomethanes
(THMs) in chlorinated drinking water and, subsequently, other halogenated
disinfection by-products with potential adverse health impacts, the practice
of chlorination has been seriously questioned. Trihalomethanes in finished
drinking water have been regulated in the United States since 1979 and the
U.S. Environmental Protection Agency (EPA) is considering adopting more
stringent regulations for THMs; it may also establish maximum contaminant
levels or treatment techniques for several other disinfection by-products
(DBPs).
This paper reviews the scientific findings associated with the formation
of THMs and other disinfection by-products and discusses how these find-
ings have affected strategies for controlling these by-products in drinking
water, and the corresponding improvement in the protection of public health.
There are a number of confounding issues associated with the management
and regulatory strategies for controlling THMs and the other disinfection
by-products; these confounding factors are included in the discussion.
CHRONOLOGY OF SCIENTIFIC FINDINGS
Figure 1 presents a chronology of the more noteworthy scientific find-
ings involving the formation of THMs and other disinfection by-products
141
OCR for page 142
142
Science and Engineering
- THMs discovered in chlorinated drinking water
- First of many epidemiological studies linking cancer
and the consumption of chlorinated drinking water ~
1 970
- EPA conducts National Organics ~ 1975
Reconnaissance Survey
- National Cancer Institute identifies chloroform - ~ I
as a carcinogen I, in= .
- Natural organic material (humic substances)- ~
identified as principal precursors of THMs ~ 1 c 80
1
EPA conducts National Organics Monitoring
Survey
Di - and trichloroacetic acids identified from
chlorination of natural organic material and in
chlorinated drinking water. Other disinfection by-
products identified, as well. Evidence that THMs
comprise only a fraction of the total organic halides.
-
l
Di - and trichloroactic acids identified as animal
carcinogens
Metropolitan Water District of Southern
California conducts nationwide survey
of disinfection by-products in drinking
water
BrO3 discovered as a significant by-product I
of ozonation of Br-containing waters
I'
LeChevatiier discovers that Glardia !
occurrence may be greater than
initially believed
PHILIP C. SINGER
Policy and Regulation
- ~- Congress legislates Safe Drinking Water Act
- EPA issues advance notice of proposed rulemaking
for organic chemicals in water
- ~- EPA proposes maximum contaminant level for total THMs
- ~-THMs regulated
Utilities serving more than 75,000 people must comply
with THM rule
- Utilities serving from 10,000 to 75,000 must comply
with THM rule
- EPA proposes treatment techniques to comply with
maximum contaminant level for total THMs
_ _ _
1 985
- · - Congress legislates SDWA amendments
- EPA proposes Surface Water Treatment Rule
- EPA proposes Total Coliform Rule
,___ ~ - AWWARF survey determines impact of THM
,===- _ ~ regulation onwaterutilities
- EPA finalizes Surface Water Treatment Rule
' ,- 19 go - EPA finalizes Total Coliform Rule
l ~ - EPA initiates approach to balance microbial
risks and risks associated with disinfection
by-products
_ _ _
_ _
L EPA originally expected to establish maximum
contaminant levels for additional disinfection by-products
and more stringent maximum contaminant levels for THMs;
proposed rule delayed until 1993
EPA considers ~enhanced" Surface Water Treatment
Rule
- EPA requests negotiated rulemaking (RegNeg) for
disinfection by-products
FIGURE 1 Timeline of significant scientific and regulatory events for trihalo-
methanes and other chlorination by-products.
.
OCR for page 143
THMs AND OTHER DISINFECTION BY-PRODUCTS
143
generated during the treatment of drinking water. Trihalomethanes were
first identified in finished drinking water in 1974, both in the Netherlands
in Rotterdam (Rook, 1974) and in the United States in New Orleans, Louisi-
ana (Bellar et al., 1974~. Their presence was linked to the practice of
chlorinating water. In 1975 the U.S. Environmental Protection Agency con-
ducted the National Organics Reconnaissance Survey of 80 cities in the
United States and found that the four THMs chloroform (CHC13), bromo-
dichloromethane (CHBrCl2), dibromochloromethane (CHBr2Cl), and bromoform
(CHBr3) occurred widely in chlorinated drinking water and resulted from
the practice of chlorination (Symons et al., 1975~. Figure 2 shows that in
the 80-city survey, total trihalomethane concentrations in the finished drink-
ing water correlated with the nonpurgeable organic carbon concentrations in
the raw water. From March 1976 to January 1977 the EPA conducted the
National Organics Monitoring Survey, which verified the findings of the
earlier survey, and demonstrated that THMs continued to form to a signifi-
cant extent in the finished water distribution system (Brass et al., 1977~.
4.0
_ 3
o
-
~n
a)
a) 2.0
ct
. _
.0 _
co
0 1.0
.
.
· ~
.
...W
..S~.
o
0 5
10 15 20
Nonpurgeable Organic Carbon (mg/L)
FIGURE 2 Relationship between trihalomethane formation in finished water and
nonpurgeable organic carbon in source water. SOURCE: Symons et al. (1981~.
-
OCR for page 144
44
PHILIP C. SINGER
A number of studies conducted in the late 1970s and early 1980s indi-
cated that many other halogenated by-products also formed in drinking wa-
ter as a result of chlorination, in addition to the TlIMs. These studies were
conducted by chlorinating raw drinking water and humic material, the prin-
cipal organic component of most natural waters, and by making measure-
ments in finished drinking water. The most frequently identified disinfec-
tion by-products, in addition to the THMs, were di- and trichloroacetic acid,
di- and trichloroacetonitrile, chlorinated ketones, chloral hydrate, and chlo-
ropicrin (e.g., Christman et al., 1983; Coleman et al., 1984; Miller and
Uden, 1983; Oliver, 1983; Quimby et al., 1980; Reckhow and Singer, 1984:
Rook, 1977; Trehy and Bieber, 1981~. Numerous other halogenated disin-
fection by-products have been identified, but with less frequency and at
trace levels (e.g., Stevens et al., 1989~. The formation of halogenated disin-
fection by-products from the reaction between chlorine and natural organic
material (NOM) is given by the following general equation:
C1' + NOM ~ CHC1~ + Other THMs + Other DBPs
-
Despite the fact that researchers have identified hundreds of haloge-
nated disinfection by-products in chlorinated water, the total concentration
of those compounds that have been quantified amounts to only about 50
percent of the total organic halide (TOX) content (e.g., Christman et al.,
1983; Reckhow and Singer, 1984; Singer and Chang, 1989~. By separately
measuring the total organic halide concentration in chlorinated water using
an adsorption/pyrolysis/coulometric detection procedure (Standard Methods,
1985), researchers have demonstrated that the sum of the measured THMs,
haloacetic acids (HAAs), haloacetonitriles, etc., when converted to chlo-
rine-equivalent concentrations, accounts for only about 50 percent of the
measured total organic halide concentration, also in chlorine-equivalent units.
This means that approximately half of the halogenated disinfection by-prod-
ucts consist of unidentified halogenated compounds.
In 1988-89 the Metropolitan Water District of Southern California and
James M. Montgomery Consulting Engineers, in a project jointly sponsored
by the EPA, the California Department of Health Services, and the Associa-
tion of Metropolitan Water Agencies, conducted the most comprehensive
survey of disinfection by-products in finished drinking water to date (Krasner
et al., 1989; McGuire et al., 19891. The investigators analyzed finished
drinking water in 35 utilities nationwide, 10 of which were in California,
for a variety of disinfection by-products for which analytical techniques
were available (see Table 1~. The study was directed at detecting disinfec-
tion by-products, seasonal patterns in their formation, the effects of raw
water quality on the levels and distribution of the by-products analyzed, and
effects of treatment modifications on by-product formation.
OCR for page 145
THMs AND OTHER DISINFECTION BY-PRODUCTS
TABLE 1 Targeted Compounds in Nationwide
Disinfection By-Product Survey
Trihalomethanes
Chloroform
Bromodichloromethane
Dibromochloromethane
Bromoform
Haloacetic acids
Chloroacetic acid
Dichloroacetic acid
Trichloroacetic acid
Bromoacetic acid
Dibromoacetic acid
Haloacetonitriles
Dichloroacetonitrile
Trichloroacetonitrile
B romochloro ace ton itril e
Dibromoacetonitrile
Haloketones
1, 1 -dichloropropanone
1,1, 1 -trichloropropanone
Miscellaneous chloro-organics
Chloropicrin
Chloral hydrate
Cyanogen chloride
2,4,6-trichlorophenol
Aldehydes
Formaldehyde
Acetaldehyde
SOURCE: McGuire et al. (1989).
145
The results of this survey indicated that THMs were the by-products
present in the highest concentrations in finished drinking water, with the
haloacetic acids present at approximately 50 percent of the total THM con-
centration. The mean and median total THM concentrations were 39 and 36
micrograms/liter (,ug/L), respectively, while the median total haloacetic acid
concentration was 17 ,ug/L. fRecent research by J.M. Montgomery Consult-
ing Engineers (1992) and Singer et al. (1992) suggests a higher ratio of
haloacetic acids to trihalomethanes.] A number of water samples had sig-
nificant concentrations of by-products containing bromine because the source
waters had high concentrations of bromide ion.
OCR for page 146
146
PHILIP C. SINGER
As a result of all of these findings, epidemiologists and toxicologists
have conducted numerous studies in an attempt to evaluate the impact of
chlorination on public health and, particularly, the health effects of the
specific halogenated disinfection by-products that have been identified. A
large number of epidemiological studies have been conducted in the United
States since 1974. These studies have repeatedly shown a weak association
between the chlorination of drinking water and an increased incidence of
cancer, but a causal relationship between exposure to chlorinated drinking
water and cancer has not be established (Craun, 1991~. Most of the histori-
cal studies have demonstrated weak relationships between bladder, colon,
and rectal cancers and the consumption of chlorinated drinking water, but
more recent studies have also shown evidence of relationships between pan-
creatic cancer (IJsselmuiden et al., 1992) and birth defects (Bove et al.,
1992a,b) with the consumption of chlorinated drinking water. Although
there is no single study that can be cited as a seminal study linking chlori-
nation and cancer, it is the sheer weight of evidence provided by the large
number of studies showing a positive relationship, albeit a weak one, that
underscores the concern, from a cancer risk perspective, about the safety of
drinking chlorinated water (Morris et al., 1992~. These epidemiological
studies have been extensively reviewed by Bull and Kopfler (1991), Craun
(1988, 1991), and the National Research Council (NRC, 1980, 1987~.
From a toxicological viewpoint, chloroform has been shown to induce
liver tumors in mice and kidney tumors in rats (Jorgenson et al., 1985;
National Cancer Institute, 1976~. Bromodichloromethane has been shown
to induce renal tumors in mice and rats, liver tumors in mice, and intestinal
tumors in rats (National Toxicology Program, 1986~. Bromoform produced
intestinal tumors in male and female rats (National Toxicology Program,
1989~. Dichloroacetic acid and trichloroacetic acid induced the formation
of hepatic tumors in mice (Bull et al., 1990; Herren-Freund et al., 1987~.
Accordingly, based on these animal studies, it can be concluded that a
number of halogenated by-products formed during the chlorination of drink-
ing water are probable human carcinogens. The National Research Council
(1987) and Bull and Kopfler (1991) recently reviewed and profiled the health
effects of a number of disinfectants and disinfection by-products. Accord-
ing to Bull and Kopfler (1991), the only halogenated by-products that ap-
pear to approach concentrations of regulatory concern in chlorinated drink-
ing water are the four THMs, di- and trichloroacetic acid, chloropicrin, and
trichlorophenol, although the EPA has generated a different list of candidate
compounds for regulation, as illustrated in Table 2 (Regli et al., 1992~.
Another class of halogenated by-products, the halogenated furanones,
exemplified by MX [3-chloro-4-(dichloromethyl)-5-hydroxy-2~5H)-furanone],
have been found in chlorinated drinking water (Kronberg et al., 1988) at
concentrations on the order of 50 nanograms/liter (0.050 ,ug/L). Despite
_~
OCR for page 147
THMs AND OTHER DISINFECTION BY-PRODUCTS
TABLE 2 Candidate Disinfection By-Products for Regulation
147
Possible Maximum
Health Effects and Contaminant Level
Compound Cancer Status Goal
Trihalomethanes
Chloroform Cancer, B2 0
Bromodichloromethane Cancer, B2 0
Dibromochloromethane Liver, C 60 ,ug/L
Bromoform Cancer, B2 0
Haloacetic Acids
Trichloroacetic Acid Liver, C 100 ,ug/L
Dichloroacetic Acid Cancer, B2 0
Other
Chloral Hydrate Liver, C 5 ,ug/L
Bromate Cancer, B2 0
Chlorine Blood, D 4 mg/L
Chloramines Blood, D 4 mg/L
Chlorine Dioxide Blood, Neurological, D 0.8 mg/L
Chlorite Blood, D 0.3 mg/L
CODE: B2 = Probable human carcinogen; C = Possible human carcinogen; D = Inadequate
or no evidence of human or animal carcinogenicity
SOURCE: Regli et al. (1992).
their low concentrations, they have been shown to be responsible for up to
50 percent of the mutagenicity of chlorinated drinking water. The signifi-
cance of this class of compounds to public health has been questioned,
however, because of the likelihood that they are detoxified in mammalian
systems following ingestion. The link between mutagenicity and human
carcinogenicity is a subject of scientific debate.
In fact, interpretations of the health effects studies overall have been
extensively criticized by scientists, engineers, and water utility managers.
In 1991 the International Agency for Research on Cancer concluded that
there was inadequate evidence for the carcinogenicity of chlorinated drink-
ing water in humans or laboratory animals and that chlorinated drinking
water was not classifiable as to its carcinogenicity (World Health Organiza-
tion, 1991~. The issues involve the weakness of the reported epidemiologi-
cal relationships between cancer and consumption of chlorinated drinking
water, the high dosages of test compounds administered to laboratory ani-
mals to induce tumors, and the validity of the models used to extrapolate
from these high dosage effects to the low concentrations at which these
compounds are found in drinking water. These are issues common to many
-
OCR for page 148
148
PHILIP C. SINGER
environmental contaminants. There is little question that estimation of the
public health risk involved in consuming chlorinated drinking water and in
establishing maximum contaminant levels for disinfection by-products is
fraught with uncertainty.
CHRONOLOGY OF REGULATORY ACTIONS
The chronology of regulatory actions taken in response to the scientific
discoveries concerning the formation of trihalomethanes in chlorinated drinking
water and the associated health concerns is also shown in Figure 1. Follow-
ing passage of the Safe Drinking Water Act by Congress in 1974 and the
findings of the National Organics Reconnaissance Survey, the Environmen-
tal Protection Agency published its advance notice of proposed rulemaking
on July 14, 1976 to address control options for organic chemical contami-
nants in drinking water (EPA, 1976~. Two approaches were considered:
establishing maximum contaminant levels for specific organic chemicals or
for surrogates (indicators) of these organic chemicals, and establishing des-
ignated treatment techniques to control specific organic contaminants or
their surrogates.
On February 9, 1978, the EPA published a proposed rule to amend the
National Interim Primary Drinking Water Regulations to include a maxi-
mum contaminant level and associated monitoring and reporting require-
ments for total trihalomethanes (EPA, 1978~. At the same time, the EPA
proposed a requirement for the use of granular activated carbon or equiva-
lent technology for application to drinking water that was presumed to be
vulnerable to contamination by synthetic organic chemicals of industrial
. .
Orlgln.
Following a period of public comment, on November 29, 1979, the EPA
adopted its final rule for the control of THMs in drinking water (EPA,
19791. The rule amended the National Interim Primary Drinking Water
Regulations by establishing a maximum contaminant level for total THMs
of 0.10 milligrams/liter (mg/L) (100 ,ug/L). For community water systems
serving 75,000 or more persons, the effective date of compliance with the
maximum contaminant level was November 29, 1981, and for community
water systems serving 10,000 to 75,000 persons, the effective date of com-
pliance with the maximum contaminant level was November 29, 1983. Com-
pliance for systems serving fewer than 10,000 customers was left to the
discretion of the individual states. The THM rule also established monitor-
ing and reporting requirements that revolved around the collection and analysis
of samples from representative locations in the water distribution system on
a quarterly basis. The rule stipulated that the running annual average of the
arithmetic sum of the concentrations of all four TlIM species had to be less
than or equal to 0.10 mg/L.
-
.
OCR for page 149
THMs AND OTHER DISINFECTION BY-PRODUCTS
149
The earlier proposed requirement for the use of granular activated car-
bon for vulnerable water supplies was not included in the final THM rule,
but was subsequently adopted as part of EPA's regulations controlling syn-
thetic organic chemicals in drinking water. tThe Safe Drinking Water Act
Amendments of 1986 declared granular activated carbon treatment to be the
best available technology (BAT) for the control of synthetic organic chemi-
cals.]
The EPA's selection of an interim maximum contaminant level of 0.10
mg/L was based on balancing public health considerations against the tech-
nological and economic feasibility of limiting total THM concentrations to
such levels in public water systems in the United States. In addition, the
limited data base available at that time, from the standpoint of both occur-
rence and health effects, prevented the EPA from establishing individual
maximum contaminant levels for each of the four THM species. Finally,
the EPA did not extend the maximum contaminant level to community water
systems serving fewer than 10,000 persons because of concerns about the
technical and economic feasibility of such systems being able to comply
with the rule without jeopardizing their disinfection practices and putting
their customers at an increased risk of waterborne diseases.
It was assumed that many water utilities would ultimately be able to
achieve total THM concentrations as low as 0.010 to 0.025 mg/L (10-25 ,ug/
L), and EPA suggested these values as future goals to be considered in the
Revised National Primary Drinking Water Regulations.
On February 28, 1983, in accordance with the stipulations of the Safe
Drinking Water Act, the EPA identified the best technology and treatment
techniques that community water systems could use to achieve compliance
with the maximum contaminant level for total THMs (EPA, 1983~. These
techniques were believed to be "generally available, taking costs into con-
sideration." The specific techniques identified were the use of chloramines
or chlorine dioxide as alternative or supplemental disinfectants and oxi-
dants, improved clarification for THM precursor removal, moving the point
of chlorination to reduce THM formation, and the use of powdered activated
carbon to remove THMs or THM precursors. Additional techniques not
determined to be "generally available" included the use of ozone as an
alternative or supplemental disinfectant or oxidant, aeration for THM re-
moval, off-line water storage, consideration of alternative sources of raw
water, and implementation of clarification if not currently practiced.
Following promulgation of the THM rule and the setting of a maximum
contaminant level for total THMs in finished drinking water, the water treat-
ment industry responded by adopting practices to limit THM formation.
The principal treatment modifications involved moving the point of chlori-
nation downstream in the water treatment plant, optimizing the coagulation
process to enhance the removal of THM precursors, and using chloramines
OCR for page 150
150
PHILIP C. SINGER
to supplement or replace the use of free chlorine. Young and Singer (1979)
had shown that moving the point of chlorine application from the raw water
to a location after clarification could reduce THM formation by approxi-
mately 40 percent. A number of other researchers (e.g., Babcock and Singer,
1979; Johnson end Randtke, 1983; Kavanaugh, 1978; Semmens end Field,
1980) had demonstrated that up to 75 percent of the THM precursors could
be removed by coagulation, sedimentation, and filtration if the coagulant
dose and pH were optimized. Other researchers (e.g., Brodtmann et al.,
1980; Duke et al., 1980; Lange and Kawczynski 1978; Norman et al., 1980)
had shown that the addition of ammonia to water containing chlorine essen-
tially stopped the subsequent formation of THMs. This made the use of
chloramines a very simple, inexpensive, and therefore attractive approach
for limiting THM formation. However, because chloramines are a much
weaker disinfectant than free chlorine, concern about compromising the
microbiological quality of drinking water became an important issue.
McGuire and Meadow (1988) surveyed 727 water utilities for the American
Water Works Association Research Foundation (AWWARF) to determine
the extent to which utilities were in compliance with the maximum contami-
nant level for total THMs, and the cost of achieving such compliance. The
results showed that enactment of the THM regulation resulted in a 40-50
percent average reduction in total THM concentrations for the larger utili-
ties surveyed. The median total THM concentration among all the respon-
dents was 38 ~g/L, a value not much different than the results of EPA's
National Organics Reconnaissance Survey (Symons et al., 1975) or the Na-
tional Organics Monitoring Survey (Brass et al., 1977~. The principal dif-
ferences were that the utilities with high THM levels were able to reduce
their THM concentrations substantially, as shown in Figure 3. Of those
systems that implemented THM control measures, the majority did one or
more of the following things: (1) modified their pointers) of chlorine applica-
tion, (2) changed their chlorine dosages, and (3) adopted the use of chloramines
to ensure compliance with the maximum contaminant level for total THMs.
A large number of utilities changed from free chlorine to chloramines as
their primary disinfectant. Table 3 summarizes the treatment modifications
made by the utilities included in the survey. Although compliance with the
0.10 mg/L (100 ,ug/L) maximum contaminant level was found to be not
particularly costly, it was concluded that reducing the maximum contami-
nant level significantly below 50 ~g/L would cause a large number of utili-
ties to exceed the maximum contaminant level and would require extensive
capital expenditures to bring these utilities into compliance with such a
more stringent value.
In summary, the scientific findings on THMs and other halogenated
disinfection by-products in chlorinated drinking water led to health effects
studies that showed that chloroform and other disinfection by-products were
OCR for page 151
THMs AND OTHER DISINFECTION BY-PRODUCTS
1 ,000
500
u, 100
a)
o
. _
50
10
5
O
151
NOMS all ~ .fK~iNiORS
phases avert
1/
l
10 30 50 70 90 95 99 99.9 99.99
1
AWWARF
utility survey
Percentage Less Than or Equal to Given Concentration
FIGURE 3 Frequency distribution of survey data from American Water Works
Association Research Foundation (AWWARF) Utility Survey compared with the
National Organics Reconnaissance Survey (NORS) and the National Organics Mon-
itoring Survey (NOMS). SOURCE: McGuire and Meadow (1988~. Reprinted from
the Journal of the American Water Works Association by permission. Copyright @)
1988, American Water Works Association.
carcinogenic in laboratory animals and that people drinking chlorinated wa-
ter might be at somewhat greater risk in developing bladder, colon, and
rectal cancer than those consuming unchlorinated drinking water. Addi-
tional scientific findings involving the formation and behavior of THMs and
the other disinfection by-products led to the development of water treatment
strategies to limit the formation of halogenated disinfection by-products.
The enactment of the THM regulation resulted in treatment modifications
that subsequently reduced the extent of THM formation and, most likely, of
many of the other disinfection by-products as well, thereby lowering the
public health risk associated with THMs and the consumption of chlori-
nated drinking water. Implementation of some of these modifications, how-
ever, may have compromised the microbiological quality of drinking water
(see later discussion).
;'
OCR for page 154
54
PHILIP C. SINGER
At the heart of the Surface Water Treatment Rule is the CT concept,
which specifies that a sufficient concentration (C) of disinfectant must per-
sist in the water for a satisfactory contact time (T) in order to ensure an
adequate degree of inactivation. CT values were developed from experi-
mental results for a variety of disinfectants, specifically free chlorine,
chloramines, chlorine dioxide, and ozone, for a variety of solution condi-
tions (e.g., pH and temperature), and for various degrees of inactivation of
Giardia and viruses. CT credit of 2- to 2.5-log removal (99 to 99.7 percent)
for Giardia and 1- to 2-log removal (90 to 99 percent) for viruses were
allocated to surface water treatment systems practicing filtration, with the
remaining degrees of inactivation of the two groups of organisms to be
achieved by chemical disinfection.
Another key aspect of the Surface Water Treatment Rule that affected
the disinfection by-product issue was the requirement that a disinfectant
residual of at least 0.2 mg/L must be maintained at all times in the water
entering the distribution system, and measurable disinfectant residuals must
be achieved in more than 95 percent of the distribution system samples
analyzed.
The Total Coliform Rule was also initially proposed in 1987 (EPA,
1987b) and finalized in 1989 (EPA, 1989b). This rule also mandated greater
attention to disinfection practices, particularly with regard to maintaining
the biological quality of treated water in the distribution system.
A significant impact of the disinfection requirements of both the Sur-
face Water Treatment Rule and the Total Coliform Rule was that many of
the water utilities, such as those surveyed by McGuire and Meadow (1988),
which had previously modified their disinfection practices to comply with
the 1979 THM regulation might not be in compliance with the CT or re-
sidual disinfectant requirements of the Surface Water Treatment Rule and
the various stipulations of the Total Coliform Rule. This was especially
true for those utilities that adopted chloramination for primary disinfection.
Accordingly, the population served by these utilities was potentially being
exposed to a greater microbial risk as a result of modified disinfection
practices. Furthermore, even without the Surface Water Treatment Rule and
Total Coliform Rule, most of the utilities surveyed by McGuire and Meadow
indicated they would have difficulty complying with a maximum contami-
nant level for total THMs significantly below 50 ,ug/L. Clearly, given the
stringent provisions of the two new rules, reduction of the maximum con-
taminant level to less than 50 ,ug/L would be expected to have an even
greater economic impact than that projected by the McGuire and Meadow
survey.
Hence, in developing national regulations for the control of disinfec-
tants and disinfection by-products, the EPA must ensure that the maximum
contaminant levels established for the disinfectants and disinfection by
~.
OCR for page 155
THMs AND OTHER DISINFECTION BY-PRODUCTS
155
products are consistent with the requirements of the Surface Water Treat-
ment Rule, the impending Groundwater Disinfection Rule (EPA, 1992a),
and the Total Coliform Rule in that they do not cause changes in water
treatment practices that result in significant increases in risk from waterborne
pathogens or from other disinfection by-products that do not get regulated
at that time. Furthermore, as stated above, the new regulations would have
to apply to all public water systems using disinfection, not just to those
serving more than 10,000 persons.
In developing the maximum contaminant levels for disinfection by-
products, EPA's approach has been to consider various THM concentrations
as maximum contaminant levels, evaluate different treatment options that
could meet these maximum contaminant levels, determine the costs associ-
ated with these options, and establish an appropriate maximum contaminant
level for the THMs with corresponding treatment technologies to achieve
compliance. The focus has continued to be directed at THMs because of the
greater availability of data on occurrence, toxicological effects, and treat-
ment control methodologies, although increasing amounts of data are being
generated for the haloacetic acids in response to their perceived significance
from an occurrence and health effects point of view.
Because chloroform, bromodichloromethane, bromoform, and dichloro-
acetic acid are tentatively classified as probable human carcinogens based
on the latest toxicological information (Regli et al., 1992), the Safe Drink-
ing Water Act essentially requires the EPA to set the maximum contaminant
level goals for these compounds at zero. The Safe Drinking Water Act also
requires the EPA to set the actual maximum contaminant levels as close to
the maximum contaminant level goal as is technologically and economically
feasible to achieve. For carcinogenic compounds that have a maximum
contaminant level goal of zero, the EPA attempts to establish maximum
contaminant levels at concentrations which ensure that the average indi-
vidual lifetime risk of acquiring cancer through exposure to these com-
pounds in drinking water is no more than one in ten thousand to one in a
million (equivalent to a 10-4 to 10-6 risk).
For those chlorinated disinfection by-products tentatively classified as
probable human carcinogens, Table 4 lists the drinking water concentrations
corresponding to 10-4, 1O-5, and 10-6 increased lifetime cancer risks (Regli
et al., 1992), along with the reported range of occurrence. Given the mea-
sured concentrations of these disinfection by-products in drinking water, it
is clear that utilities would have great difficulty complying with maximum
contaminant levels for the THMs at anything more stringent than a 10-5 risk
level, and a maximum contaminant level for dichloroacetic acid (DCAA) at
anything more stringent than a 10-4 risk level. While it might be possible to
achieve maximum contaminant levels corresponding to these risk levels by
reducing the dose of chlorine for drinking water treatment, the Surface
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156
PHILIP C. SINGER
TABLE 4 Drinking Water Concentrations Associated with Various Levels
of Increased Lifetime Cancer Risk
Disinfection Concentration (,ug/L) Range of
By-Product 10-4 10-5 10-6 Occurrence (pg/L)
.
Bromodichloromethane 100 10 1 0-100
Bromoform 400 40 4 0-50
Chloroform 600 60 6 0-340
Dichloroacetic acid 10 1 0.1 0-80
SOURCE: Regli et al. (1992).
Water Treatment Rule prevents this from happening by requiring strict ad-
herence to CT values and disinfectant residuals that would ensure compli-
ance with the specified levels of pathogen inactivation and the correspond-
ing reduction in microbial risk.
Alternatively, other disinfectants could be used in place of free chlorine
to meet the requirements of the Surface Water Treatment Rule, but these
alternatives are not without their own health risks. For example, the use of
chlorine dioxide results in the presence of chlorite (ClO2-) and chlorate
(ClO3-) in the treated water (Rav-Acha et al., 1984; Werdehoff and Singer,
1987~. Chlorite is formed as a by-product from the reaction of chlorine
dioxide, and both chlorite and chlorate are frequently found as contaminants
in chlorine dioxide feed streams (Griese et al., 19911. Chlorite has been
found to be toxic in animals (Court et al., 1982), and a maximum contami-
nant level goal of 0.3 mg/L has been suggested (Regli et al., 1992~.
In the case of ozone, the principal by-products appear to be aldehydes
such as formaldehyde, acetaldehyde, glyoxal, and methyl glyoxal (Glaze et
al., 1991~. There is not enough information at this time to ascertain the
health risks of exposure to these chemicals in drinking water. However, in
water containing bromide, ozonation can lead to the formation of undesir-
able levels of bromate (BrO3~) and brominated organic by-products such as
bromoform, dibromoacetic acid, etc. (Amy and Siddiqui, 1991; Krasner et
al., 1993~. Bromate is also tentatively classified as a probable human car-
cinogen, with a 5 ,ug/L concentration in drinking water corresponding to a
10-4 individual lifetime cancer risk, and 0.5 ,ug/L (below most analytical
detection limits) for a 10-5 risk (Regli et al., 1992~. Therefore, while ozone
is a more potent disinfectant than free chlorine, its widespread applicability
may be limited by the bromide content of the source water and the corre-
sponding formation of bromate. In addition, because ozone is relatively
unstable in water, a secondary disinfectant is required in order to maintain a
disinfectant residual in the distribution system.
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THMs AND OTHER DISINFECTION BY-PROD UCTS
157
The use of chloramines in place of free chlorine to minimize disinfec-
tion by-product formation is limited because of their poor virucidal and
cysticidal properties, that is, the high CT requirements make chloramines
impractical for primary disinfection in most cases. Chloramines still re-
main an attractive option for secondary disinfection, that is, for maintaining
a persistent disinfectant residual in the distribution system.
Also, the application of alternative primary disinfectants may enhance
the formation of some by-products during secondary disinfection. For ex-
ample, ozonation leads to high concentrations of chloral hydrate when the
secondary disinfectant is free chlorine (Logsdon et al., 1992; McKnight and
Reckhow, 1992~.
Some additional confounding factors involve the emergence of crypto-
sporidiosis as a major waterborne disease; several outbreaks have occurred
within the past five years (Rose, 1988~. Although they were not considered
in the development of the Surface Water Treatment Rule, Cryptosporidium
cysts have been found to be appreciably more resistant to conventional
disinfectants than Giardia, and higher CT values may be required to inacti-
vate this organism. Such CT criteria are still in the developmental stage. In
addition, LeChevallier et al. (1991) have recently reported that concentra-
tions of Giardia and Cryptosporidium cysts in raw drinking water may be
orders of magnitude higher than originally expected. Although the infectiv-
ity of the cysts was not verified, these findings suggest that the specifica-
tions for 3-log and 4-log inactivation or removal of Giardia and viruses,
respectively, in the Surface Water Treatment Rule may not be restrictive
enough to reduce the risk of exposure to these waterborne diseases to levels
that EPA deems acceptable. Accordingly, the EPA is considering adopting
an "enhanced" Surface Water Treatment Rule. If this were done, increased
contact times with chlorine or higher doses of chlorine would be required to
provide enhanced disinfection. The result would almost certainly be an
increase in the formation of halogenated by-products.
These considerations draw attention to the conundrum facing water utilities
and regulators, that is, while a great deal of attention has been directed at
formation of disinfection by-products and reducing the chemical risks asso-
ciated with these by-products, insufficient attention has been focused on the
microbial risks associated with modified disinfection practices and the pres-
ence of disease-causing and disinfection-resistant organisms in raw water
supplies. From the standpoint of reducing microbial risk from these infec-
tious agents, greater reliance on chlorine and other chemical disinfectants
might be desirable despite the increased formation of disinfection by-prod-
ucts and their attendant chemical risks.
All of the above confounding factors are considered, to some degree, in
EPA's Disinfection By-Product Risk Assessment Model (Regli et al., 1992~.
The EPA first proposed this model in November 1990 as an innovative
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158
PHILIP C. SINGER
approach to balance the risk between exposure to microbial hazards and
exposure to disinfectants and disinfection by-products. The approach in-
volves (1) identification of candidate best available technologies for con-
trolling both disinfection by-products and pathogens; (2) determination of
the degree to which these technologies are consistent with the criteria of the
Surface Water Treatment Rule and the Total Coliform Rule, and with the
potential criteria of an enhanced Surface Water Treatment Rule; (3) evalua-
tion of the predicted performance of these candidate best available tech-
nologies in a variety of source water qualities (e.g., total organic carbon,
bromide concentrations); (4) consideration of candidate maximum contami-
nant levels for THMs and haloacetic acids based upon predicted levels that
can be achieved by the candidate best available technologies for a reason-
able number of water systems; (5) consideration of potential net changes in
risk from exposure to both disinfection by-products and pathogens; and (6)
the costs to implement such changes.
EPA is still developing their risk assessment model for disinfection by-
products. Initial predictions indicated that there would be a dramatic in-
crease in the incidence of waterborne disease if more stringent maximum
contaminant levels for THMs and other disinfection by-products were es-
tablished, but there is a vast amount of uncertainty associated with the
model, deriving primarily from inadequate scientific (primarily toxicologi-
cal and microbiological), technological, and cost information. While the
risk assessment modeling approach is a laudable one, it represents a rather
ambitious undertaking, given the lack of supporting documentation that is
needed for incorporation into the model.
In fact, it is the lack of such information in the face of the confounding
factors enumerated above that, in large part, has led the EPA to recommend
that the disinfection by-product regulations be developed through the pro-
cess of negotiated rulemaking (RegNeg). This process is an alternative to
traditional rulemaking, in which representatives of all interested parties,
including the EPA, meet and collectively develop the proposed rule by con-
sensus. The process is managed by a third party that convenes the meetings
and oversees the deliberations. On September 15, 1992, the EPA issued a
notice of intent to proceed with a negotiated rulemaking (EPA, 1992b). The
RegNeg process for disinfection by-products is under way and is scheduled
to be completed by mid-1993.
DISCUSSION AND CONCLUSIONS
There can be no questioning the fact that the regulation of disinfection
by-products is a difficult issue. In some ways, the regulatory process for
THMs worked effectively, at least in the early stages. Scientific evidence
supported the need to control THMs. THMs were found in chlorinated
_
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THMs AND OTHER DISINFECTION BY-PRODUCTS
159
water and were produced as a result of the chlorination process. They
induced cancers in laboratory animals. Epidemiological studies suggested
that people drinking chlorinated water appeared to be subjected to a some-
what increased incidence of cancers of the urinary and digestive tracts.
Accordingly, THMs were regulated. The result of the regulation was a
lowering of THM levels in drinking water at a relatively modest cost. Sci-
entific findings led to a regulatory decision that produced a favorable out-
come at little cost to society, although there is some question whether the
effectiveness of disinfection was adversely affected. All of the above was
accomplished over a period of about 10 years.
It has been the attempt to fine-tune and expand the original objective,
and subsequent recognition that control of the chemical risks associated
with disinfection by-products may increase risks associated with disease-
causing microorganisms, that has led to the difficulties described in this
paper.
New analytical techniques have allowed many more contaminants in
drinking water to be identified. Some of these contaminants are present in
the raw water, and a number are produced by chemicals added to purify the
water. Some, like the THMs and haloacetic acids, are present in concentra-
tions as high as 50-100 ,ug/L. Most are present at what would be called
trace concentrations, that is, less than 10 ,ug/L. Despite the survey by
McGuire et al. (1989), there is no comprehensive data base profiling the
nationwide occurrence of disinfection by-products in chlorinated drinking
water.
Among the disinfection by-products, halogenated by-products have been
the easiest to identify and quantify analytically, although only about 50
percent of the mass of these halogenated by-products (on a halogen-equiva-
lent basis) has been accounted for. Little information is available on the
nature of the remaining 50 percent, or the health effects associated with this
material. Identification of by-products resulting from the chlorination of
water continues to be an active area of analytical research.
In addition, the focus of analytical activity on by-products of chlorina-
tion does not mean that alternative oxidants and disinfectants do not gener-
ate by-products too. Analytical techniques have not advanced to the point
where the polar compounds that are invariably produced by these alterna-
tive chemical additives such as ozone and chlorine dioxide can be reliably
measured. Identification of some of these other by-products is just begin-
ning to occur, but it is likely to be a long time before a mass balance on the
organic carbon content of finished drinking water can be attempted.
Assessment of the health effects of these trace contaminants is probably
the major source of scientific uncertainty. Epidemiologically, it cannot yet
be concluded that there is a significant cancer risk in drinking chlorinated
water. Confident quantification of this risk requires additional study. Toxi
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160
PHILIP C. SINGER
cologically, it can be concluded that some of these disinfection by-products
cause cancer in laboratory animals when administered in large doses, but
questions arise when attempts are made to extrapolate these results to the
low exposures associated with drinking water. The scientific basis for mak-
ing such low dose extrapolations is still debatable, yet the need is great if
reliable risk estimates are to be made.
Technologically, it is widely accepted that the formation of THMs and
other chlorinated by-products can be lowered by decreasing the use of free
chlorine. However, requirements for reliable disinfection of drinking water
to control waterborne pathogens are deservedly more stringent than ever
before, and likely to become even more stringent. There is a fear among
many that lower maximum contaminant levels for THMs and other disinfec-
tion by-products may compromise one of the fundamental objectives of
water treatment, that is, disinfection. If disinfectants other than free chlo-
rine were to be used, there are uncertainties with regard to the public health
impact of some of these alternatives, particularly in bromide-containing
waters. From a regulatory standpoint, it would not be desirable to establish
regulations that would essentially limit the use of free chlorine and force
utilities to use alternative disinfectants whose public health risk is more
uncertain at this time because of analytical deficiencies.
Alternative control options involve the use of technologies that would
remove disinfection by-product precursors (natural organic matter) from the
water before a disinfectant is added. Technologies for achieving this objec-
tive consist of enhanced coagulation, granular activated carbon adsorption,
and membrane filtration. The first of these is not effective in all waters, for
reasons that are not yet known. Research on this subject is continuing.
Using granular activated carbon adsorption to control disinfection by-prod-
ucts has been demonstrated to be a relatively expensive process for most
waters, and adoption of maximum contaminant levels for disinfection by-
products that are based on the use of granular activated carbon as the best
available technology is likely to have a major economic impact on society.
Opponents of such a requirement cite the high cost of this technology for a
questionable societal benefit. Membrane filtration is a relatively new de-
velopment in water treatment technology. A number of technical issues
must be addressed before this technology can be implemented on a large
scale. These include control of membrane fouling, ultimate disposal of the
concentrated waste from the process, and efficiency of product recovery. In
addition, like granular activated carbon, membrane filtration has a relatively
high price tag.
EPA's proposal to use a Disinfection By-Product Risk Assessment Model
to aid them in the rulemaking process is a logical approach in view of the
complexities involved in the disinfection/disinfection by-products issue.
Unfortunately, the scientific data base needed to support such an approach
-
.
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THMs AND OTHER DISINFECTION BY-PRODUCTS
161
at this time is weak. Either more research is needed to provide the missing
pieces, and there are a significant number of missing pieces as discussed in
this paper, or a compromise position will have to be adopted, as was the case
in 1979. The negotiated rulemaking process that is under way is likely to lead
to such a compromise position for the short term. The long-term regulation
of disinfection by-products must await additional scientific findings.
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-
Representative terms from entire chapter:
maximum contaminant
