FORMATION AND MEASUREMENT OF TRIHALOMETHANES IN DRINKING WATER
by
Alan A. Stevens and James M. Symons
From
Proceedings - Control of Organic Chemical Contaminants in Drinking Water
A Series of Seminars Sponsored by
The Office of Drinking Water
U. S. Environmental Protection Agency
Physical & Chemical Contaminant Removal Branch
Drinking Water Research Division
Municipal Environmental Research Laboratory
U. S. Environmental Protection Agency
Cincinnati, Ohio 45268
January 1980

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FORMATION AND MEASUREMENT OF TRIHALOMETHANES IN DRINKING WATER*
by
Alan A. Stevens and James M. Symons
INTRODUCTION
Because of findings concerning the carcinogenicity of chloroform and
the confirmation of the ubiquity of chloroform and other trihalomethanes in
chlorinated drinking water, many purveyors of potable water are interested
in sampling their product to determine the extent of their individual chloro-
form problems and resolve them when possible. Additionally, as a direct
result of the announced EPA policy regarding the initiation of a Voluntary
Nationwide Chloroform Reduction Program, other water utilities are antici-
pated to attempt to reduce, through modification of the treatment process,
concentrations of chloroform reaching the consumer.
In these treatment-modification and surveillance programs, difficulties
often arise concerning both the factors that must be considered in select-
ing modifications of treatment and what should be considered when selecting
sampling and analysis techniques to best evaluate the full extent of the
problem or the success or failure of the efforts to reduce that problem.
The physical and chemical factors controlling production of chloroform and
the influence of these factors on the concentrations of chloroform and other
trihalomethane (THM) species that are observed in a sample at the time of
analysis must be understood.
In addition to physical and chemical considerations, adequate bacterio-
logical monitoring of finished waters during chloroform-reduction programs
must be included. Any research program in which disinfection practice is a
variable requires careful attention on the part of the utility operators to
ensure that water of adequate microbiological quality reaches the consumer.
*This paper is a summary of information that is presented in considerably
more detail in the three publications listed under references, below.
Those three references include all appropriate supporting literature
citations, and the reader is referred to them for additional reading. For
that reason, individual literature citations do not appear in this paper.

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2
FACTORS INFLUENCING THM FORMATION THAT AFFECT TREATMENT STRATEGIES
General
The formation of trihalomethanes during chlorination of drinking water
now seems to be well accepted to result from a complicated mechanism of
attack by aqueous halogen species on natural aquatic humic substances (humic
and fulvic acids) and not usually significantly from the sources of indus-
trial water pollution. Thus chloroform results from the generalized reaction,
aqueous chlorine + "precursor" (humic Acids)	>
chloroform
This occurs to some extent in any water-treatment plant where chlorination
for disinfection is practiced. The reaction is not instantaneous and occurs
over a period of a few days until either chlorine or precursor is exhausted.
In the presence of natural bromide, the reaction products include some mixed
halogen trihalomethane species (bromodichloromethane, dibromochloromethane)
and bromoform. This occurs in most chlorinated drinking water, even where
bromide concentrations in the source water are small. Iodine-containing
species have also been observed, presumably because of the presence of
natural iodide. Because the chemical reactions for formation of these
bromine- and iodine-containing trihalomethanes are probably mechanistically
similar to those for formation of chloroform, the trihalomethanes includ-
ing chloroform, can be discussed as a group for treatment evaluations.
Design of the most effective treatment strategy depends on a good
knowledge of factors influencing trihalomethane formation. Two factors,
however, that have a strong influence on trihalomethane concentrations
over which the water treatment plant operator has little or no control
under most circumstances are temperature and Br or I concentrations.
Temperature
Figure 1 clearly demonstrates the positive effect of increasing
temperature on trihalomethane formation upon chlorination of Ohio River
water in the laboratory. A corresponding seasonal variation is noticed at
a water utility using that same source and has been shown to be largely a
temperature effect. Thus treatment problems become more acute during
seasons of higher ambient air temperature causing higher water temperature
during treatment and distribution.
Bromide and Iodide Concentration
Bromide and iodide ions are oxidized by aqueous chlorine to species
capable of participating in organic substitution reactions resulting in
the formation of pure- and mixed halogen trihalomethanes. Bunn, et al. of
USEPA, Kansas City, first confirmed one of the suspicions of Rook~Tn the
Netherlands that this could occur in aqueous systems when they chlorinated
Missouri River water in the presence of added fluoride, bromide, and iodide

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3
TEMPERATURE
225 -
O) 200--
150--
100-
oC
50--
0
60
40
120
20
80
100
TIME (hrs)
FIG.l EFFECT OF TEMPERATURE ON CHLOROFORM FORMATION

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4
and observed the formation of all ten possible chlorine-, bromine-, and
iodine-containing pure- and mixed halogen trihalomethanes. On a theoreti-
cal basis, fluorine substitution was not expected and was not observed.
To date, at least six of these species have been found in finished drinking
water (chloroform, bromodichloromethane, dibromochloromethane, bromoform,
dichloriodomethane, and bromochloroiodomethane).
Figure 2 illustrates the results of work conducted in our laboratory
on the effect of added bromide on the ratio of trihalomethanes produced
during reaction of aqueous chlorine with humic acids. Note that bromine sub-
stitution is favored over chlorine even though chlorine is in large excess
compared with the initial bromide. Additionally, the total molar yield of
trihalomethanes increases with increasing bromine substitution. This was
also observed when pure aqueous bromine was reacted with the humic acid
under the same conditions as aqueous chlorine. Thus, bromine competes more
effectively than chlorine for active sites on the humic acid precursor
molecule, perhaps mechanistically by way of a faster substitution reaction
rate. This effect is so pronounced as to dramatically increase total halo-
form yields where bromide is present. Indeed, similar increases in total
haloform yield have been reported to occur on chlorination of a bromide
spiked natural water and more importantly at a water treatment plant in the
USA where sea water intrusion was temporarily responsible for increases in
bromide (Fig. 3, Lange, et al., Contra Costa, CA). Thus, much more complete
control of trihalomethane precursor, as one method of meeting proposed USEPA
drinking water standards, is necessary when significant concentrations of
bromide are present in the source water.
Effect of pH
Increasing the pH of the water being treated has been shown by numerous
workers to dramatically influence rates of formation of haloforms during
water treatment (Fig. 4). Control of pH during treatment before chlorina-
tion, such as recarbonation in a lime softening system, has been used in
attempts to control haloform formation.
The increase of trihalomethane formation rate with pH was expected
because the classical haloform reaction is base catalyzed; however, this
explanation is likely to be an oversimplification where rather complex
humic acid structures are involved. Simple methyl ketones, models for the
haloform reaction, have been shown to react too slowly to account for tri-
halomethane formation under most drinking water treatment conditions.
Figure 5 shows the results of attempts to react chlorine at pH 7 with simple
acetyl compounds (acetone, acetaldehyde, and acetophenone), when these
compounds were spiked at 5 umol/L into raw- and granular activated carbon
(GAC) filtered water. Trihalomethanes were not produced at rates signifi-
cantly higher than those for the blank samples.
Figure 6, however, shows that at higher pH values, the simplist
methylketone, acetone, reacts at a much higher rate, and this class of
compounds could become a significant source of precursor in those pH
ranges.

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® 10
282jUequtv. CI2 dose per liter
m—=
20
30
40 50
11 mols Br— added/liter
60
313
Figure 2 TRIHALOMETHANES FORMED BY REACTION OF HUMIC ACID WITH AQUEOUS
CHLORINE IN THE PRESENCE OF VARYING BROMIDE ION

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180
160
140
120
100
80
60
40
20
¦¦ •
LEGEND
• 3THM
o CHCI3
a CHCIBr2
~ CHCl2Br
¦ CHBr3
O N
100
50
250
150
200
CHLORIDE IN RAW WATER, mg/l
3 EFFECT OF SALT WATER INTRUSION ON
THM FORMATION POTENTIAL (FROM
LANGE, 1978)

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120 -
pH H-5
- 100 -
t 80 -
Z 60 -
u. 40 -
20
10
0
20
30
40
60
70
50
90
80
REACTION TIME (hrs)
Figure 4 EFFECT OF pH ON CHLOROFORM PRODUCTION,
SETTLED WATER 25°C, 10mg/l CHLORINE DOSE

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8
200
X ACETONE
• ACETALDEHYDE
o ACETOPHENONE
¦ BLANK
180
160
140 -
RAW
120 -
O 100
O 80-
60
40--
20
CARBON FILTERED
20
10
30
REACTION TIME (hrs.)
40
50
70
80
60
(hrs,
RAW AND CARBON FILTERED WATER SPIKED AT 5juM WITH
LOW MOLECULAR WEIGHT ACETYL COMPOUNDS, pH 7,
10mg/l CHLORINE
FIG. 5

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*
.7 -
5
x
H
H
50
70
80
100
40
60
90
30
10
20
TIME (hrs)
FIG. 6 EFFECT OF pH ON TRIHALOMETHANE PRODUCTION FROM ACETONE, 1mg/l,
25°C, CHLORINE DOSE 10mg/l

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10
The complex humic structure therefore must have more active groups than
the simple methyl ketones for chlorine substitution to account for reactivity
at pH 7 with the possibility of less active (actyl) groups becoming more
significant at higher pH, increasing reaction rate and yield.
An alternate explanation for the effect of pH on rate and yield with
humic acid precursor has been suggested, by Christman, however (personal
communication). The macromolecule may simply be "opening up" by mutual
repulsion of the negative changes at high pH increasing the availability of
additional reactive sites on that molecule.
Characteristics and Concentration of Precursors
In artifical systems, increasing the concentration of humic acid pre-
cursor in the presence of excess chlorine with otherwise constant reaction
conditions causes haloform yields to increase in direct proporation to the
humic acid dose (Fig. 7). Although at similar TOC concentrations, humic
acids and natural water have been shown to result in similar THH yields
(Fig. 8), from supply to supply, only crude relationships have been found
between organic carbon concentrations and trihalomethane yields. Similarly
the relationship is not perfect when water at various stages of treatment
are examined. Further, rate curves seem to take on distinctly different
shapes depending on the source of precursor substances. The work of Rook
shows the reaction of fulvic acid solutions to be characteristic of m-di-
hydroxyphenyl moieties (e.g. resorcinol) as that reaction is nearly complete
at near neutral pH in less than two hours (Fig. 9). Quite a different
characteristic curve is observed with Ohio River water precursor and a
different source of humic acid under similar conditions where the reaction
takes place relatively slowly over a period of many days (Fig. 8). The
probable differences in precursors at different locations has been further
demonstrated in work at the EPA Cincinnati laboratory where, as expected,
treatment with permanganate at low dosages was nearly 100 percent effective
in preventing the formation of trihalomethanes on chlorination of resorcinol
and m-dihydroxybenzoic acid solutions, yet permanganage was only marginally
(10-20%) effective in reducing the ability of Ohio River precursors to form
trihalomethanes upon subsequent chlorination.
Chlorine Dose and Type
Additionally, work at the Cincinnati laboratory has shown only a slight
influence on trihalomethane formation rate (or yield) occurred with increas-
ing free chlorine dose (beyond demand) where "precursor" is kept constant
(Fig. 10). Both similar (Fig. 11) and contrary results have been reported
by others while working with different sources of precursor. Combined
chlorine (chloramines) do not cause formation of THM's (Fig. 12).
The above serves only to indicate that although precursor materials
from various supplies may be of largely natural origin, the composition
of that material is likely to be different, depending on the type of
supply involved and the source of precursors in the watershed. Consider-
ably more work is needed, therefore, to understand the complex mechanisms

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1.0mg/l HA
0.3- -
0.2 -
o
E
3
0.5mg/l HA
5
x
0.1
0.1mg/l HA
20
10
30
0
40
60
70
50
80
90
100
TIME (hrs)
Figure 7 EFFECT OF HUMIC ACID CONCENTRATION ON
TRIHALOMETHANE PRODUCTION, pH 6.7, 25°C, 10mg/l CHLORINE DOSE

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12
180-r
160 +
140+
120+
o 100+
O 80 +
60 +
404-
20+;
70
30
40
60
50
80
10
20
0
REACTION TIME (hrs)
Figure 8 COMPARISON OF HUMIC ACID, RAW WATER REACTION RATES
AT SIMILAR NPTOC CONCENTRATIONS, 10 mg/l CHLORINE DOSE

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O 150
co 100-
resorcinol
—i f~
-o
o
FULV1CACTO		 ^
T-
20
i
30
T~
40
r
50

T
"O
40 «
o
>
30 O
O
I-
o
E
¦20 ^
O
X
a
¦10 |
110 120 130
Figure 9 REACTION OF MODEL PRECURSORS WITH AQUEOUS CHLORINE

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Chlorine Dose:
6.0 mg/L
4.0 mg/L
3.0 mg/L
0.6-
-5.0 <
0.5-
Total THM's
-4.0 S
E 0.4-
¦3.0
2 0.3J
.o
-2.0 JS
Chlorine Residuals
-1.0 £
0.1-
-a
80
90
70
60
40
30
50
Time (Hrs)
10
20
Figure 10 EFFECT OF CHLORINE DOSE ON TRIHALOMETHANE FORMATION

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450
Ln
Chlorine Dose, mg/l
FIG. 11 CHLOROFORM FORMATION
COMPARED WITH CHLORINE RESIDUAL

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16
120
110
100
90
80
70
CONC-
M9/1 60
50
40
30
20
10


. COMBINED CHLORINE RESIDUAL
X
JL
0 10 20 30 40 50 60 70
REACTION TIME, HOURS
FIG. 12 CHLOROFORM FORMATION BY
FREE AND COMBINED RESIDUAL.

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17
of trihalomethane formation during water chlorination and to determine
whether water treatment strategies for control of THM's could vary signifi-
cantly among these various supplies. These various strategies are described
elsewhere in these proceedings.
FACTORS INFLUENCING THM MEASUREMENT DURING TREATMENT EVALUATIONS
Introduction of Trihalomethane Formation Potential Concept
Because their formation is not instantaneous, THM concentrations
increase in the water as it flows through a water-treatment plant (unless
removed during treatment) to reach some value higher than that which would
be observed if an analysis for THM species were performed immediately after
sampling at the first point of chlorination. Further, the consumer is
likely to receive water with THM concentrations higher than, those leaving
the plant because the reaction proceeds in the distribution system. This
also occurs during sample storage, and in each case the concentration is
time dependent. The formation rates vary according to all of the reaction
conditions described in the previous section. These factors will be dis-
cussed again in the context of the THM measurement. To evaluate treatment
success, four definitions are important:
1.	Instantaneous THM (InstTHM) concentration-the concentration of THM
in the water at the moment of sampling. This may be expressed in terms of
the individual species or their sum as total trihalomethane (TTHM).
2.	Terminal THM (TermTHM) concentration-the concentration of THM that
occurs at the termination of the measurement of this parameter. To measure
TermTHM concentration, chlorine-precursor reaction conditions are selected
according to the treatment practiced at the particular plant being evaluated
In general, a sample of water is chlorinated under these conditions, and
chloroform and other THM species are measured after a specified time period.
TermTHM concentration is equally important as a parameter for evalu-
ating consumer risk as is the InstTHM concentration, but because this para-
meter Is a measure of the sum of the amounts of THM species already present
(instantaneous) and those formed during the reaction time, a third parameter
must be defined that is useful for evaluating unit-process performance for
removal of unreacted precursor.
3.	THM formation potential (THMFP)-measured as the increase in THM
concentration that occurs during the storage period in the determination
of the TermTHM concentration. The THMFP is obtained by subtraction of the
InstTHM concentration from the TermTHM concentration either when TTHM or
when the individual species data are used. THMFP is a measure of the portion
of the total precursor material (of most concern to the consumer) remaining
in the water at a given point in the treatment train. This parameter, when
measured on unit-process influent and effluent samples, can be used to
determine the efficiency of that process for removal of that pertinent
fraction of precursor material.

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18
4. Total precursor - A distinction between THMFP and a total precursor
is important. Total precursor concentration is the concentration of all
organic THM precursor materials present in the water that could react with
halogen species under conditions that maximize the yield of trihalo-
methane s.
Because the identities of these organic compounds are not known precise-
ly at this time, total THM precursor concentration also could be expressed as
concentrations of THM or concentration of TTHM obtained from that reaction.
No standardized procedure for measuring this parameter exists, however, and
considerable research will be required to establish the optimum conditions
to ensure the complete reaction of all precursors to yield maximum trihalo-
methane concentrations.
Because the chlorination conditions for TermTHM concentration measure-
ment are somewhat less that optimum for THM formation, the TermTHM concen-
tration obtained in that determination will be somewhat less than the
theoretical maximum THM concentration. Thus, the value obtained for THMFP
under these conditions, obtained by subtraction of the InstTHM concentra-
tion from TermTHM, will be smaller than the theoretical "total precursor"
parameter. Although the value obtained (THMFP) is not the total precursor
concentration, it is an index of the concentration of materials of most con-
cern relative to THM formation at a particular water-treatment plant. Also,
because controlling parameters (under treatment-plant conditions) are
measured easily at the operating plant, TermTHM concentration (and, there-
fore, THMFP) is a practical measurement. Figure 13 presents graphically
the four parameters discussed.
Measurement of Instantaneous THM Concentrations
For InstTHM concentration measurement, the reaction of chlorine with
precursor materials must be halted at the time of sampling with the goal
being to measure only trihalomethanes present at the time of sampling. A
small amount of reducing agent is added to the sample to react with the
chlorine and, thus, render the chlorine unavailable for oxidation or
substitution reactions. A small increase in trihalomethane concentrations
upon storage after addition of reducing agent usually is observed; this
is probably caused by a slow hydrolysis of certain trihalo-intermediates.
The hydrolysis step does not require the presence of chlorine. The dis-
tinction should be made between this minor effect on the InstTHM concentra-
tion and the continued THM formation reaction when no reducing agent is
added. The increase in THM concentration during storage after the addition
of a reducing agent has amounted to only a few percent of the total value.
Measurement of Terminal THM Concentration and THM FormaHnn Potential
These two parameters are discussed together because the measurement for
TermTHM concentration together with the InstTHM concentration yields the
THMFP by subtraction. The TermTHM concentration is measured by reacting
chlorine with THM precursors in a given sample under certain controlled con-
ditions that affect yield and rate of formation of the trihalomethane and
subsequently measuring the concentrations of THM species produced. Because

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T
T rihalo -
methane
Cone.
Approxi-
mation of
Cone, in
Tap Water
i
Cone.
at Time
Sampling
{
Terminal
THM Cone.
(CI2 Sample
Stored
Appropriately)
Inst.
THM
Cone.
Jul
A B
Remainder
of Total
Precursor
(Little
Consequence)
{
THM
Formation
Potential
(B-A)
(Important
Portion of
Precursor)
Total
FIG. 13 FOUR TRIHALOMETHANE MEASUREMENT PARAMETERS

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20
this is not a total-precursor-concentration measurement, the selected con-
ditions for this measurement must be selected as appropriate according the
conditions under study and be reproducible from sample to sample. As with
treatment strategy selection, critical conditions to be considered are time
of reaction (time elapsed before halting the halogenation reaction with a
reducing agent), maintenance of a free chlorine residual, temperature, and
pH.
Effect of Time —
Although a single measurement of THM concentrations after a storage
period of several days in a bottle under appropriate conditions can give a
useful determination of TermTHM concentration for that specified time, much
more information can be gained from the reaction-rate curves obtained by
plotting THM concentration vs time. The rate curves obtained by periodic
measurement of THM concentrations of properly stored water can be used to
estimate the future THM concentrations at any given time after water is
taken from a sampling point within the plant, when the purpose is to use
the concentration obtained to calculate the THMFP at that point in treat-
ment for evaluation of unit-process effectiveness.
In any system, the generation of a rate curve is preferable, at least
initially, so that the nature of the reaction that occurs at a location can
be determined. For example, Figure 14 shows two hypothetical curves describ-
ing the rate of chloroform formation that might be expected for finished
waters of distinctly different quality after leaving typical water-treatment
plants.
Curves A and B in Fig. 14 represent two extreme situtations that might
occur. Although at time T, the chloroform concentrations are the same for
both waters, the short term chloroform concentration is greater in Plant A,
and long term chloroform concentration is greater in Plant B. A Plant A
curve would be expected where chloroform-formation potential is relatively
low but the precursor present is of the type that reacts quickly under the
given conditons i.e., the final concentration of chloroform is reached early.
A Plant B curve would be expected where chloroform-formation potential is
high but the reaction with chlorine is slow because of the nature of pre-
cursor or reaction conditions. Thus, these curves are more informative than
a single chloroform determination performed at time T, and the single measure-
ment at each plant easily could be misinterpreted to mean that the plant
situations were the same.
Good approximations of both curves are obtained by the selection of
three or four points beyond time 0 (instantaneous value) such as 1/2T, T,
2T as shown, where 2T is equal to or slightly longer than the maxiumum
distribution-system residence time. These added analyses could cause the
generation of rate curves to be time-consuming, especially if conditions
are such that reactions are slow and the distribution-system residence
time is long. If the development of the rate curve is beyond the capa-
bility of a utility, the time for the determination of TermTHM concentra-
tion should be the longest residence time in the distribution system, as
this represents the most stringent conditon for that utility.

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21
Plant B
Plant A
Inst.
CHCI
2T
3
2
Time (Days)
Cone.
Formation of Chloroform Under Widely Different
Treatment Plant Conditions
FIG. 14

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22
Maintenance of Chlorine Residual —
In conventional U.S. water-treatment practice, maintenance of a free
chlorine residual through the distribution system often is recommended or
required. As mentioned above, the continued reaction of precursor with
chlorine to yield trihalomethanes depends on the maintenance of a free
chlorine residual. Again, with chloroform as an example, the raw water curve
presented in Figure 15 shows the abrupt cessation of chloroform production as
the chlorine became depleted. The 24-hr and all later samples gave the same
chloroform concentration, and chlorine-residual determinations confirmed the
lack of chlorine. Thus, the 24-hr and later chloroform concentrations could
be misleading, assuming one of the conditions in the water utility under
investiation was maintenance of a chlorine residual throughout the distribu-
tion system. Thus, for evalaution of systems where free chlorination is
practiced, to ensure that misleading results are not obtained, a chlorine-
residual measurement always must be performed at the time of THM analysis
to ensure that a free residual is present.
Work at the DWRD laboratory indicates that TermTHM concentrations are
not affected significantly by the amount of free chlorine present. This
may be only because the concentrations usually are limited by the amounts
of precursors present. Because some uncertainty (see above) exists about
the effect of chlorine concentration on reaction rate, the dose used in the
TermTHM determination should be nearly the same as that used at the treat-
ment plant; because that dose is adequate for maintenance of a distribution-
system residual, it should be adequate to supply the required residual for
the duration of the test.
Effect of Temperature —
Because temperature has a dramatic effect on rate of formation of THM
and therefore yield at any given time, a need for close temperature control
during the determmination of TermTHM concentration, therefore, is indicated.
Temperature largely is controlled seasonally at a water works, and selec-
tion of a sample storage temperature will depend on the experimental objec-
tive. For example, if the objective is to estimate consumer exposure through-
out a year, a logical choice is an estimated average distribution system
temperature that will vary with the time of the year. When the objective is
to evaluate "precursor" removal efficiencies of a unit process, the measure-
ment should not be influenced by temperature changes and the temperature
must be mainatained at a single value for all of the tests regardless of the
time of the year. The investigator may even choose to run multiple tests
at each sampling time including more than one storage temperature reflect-
ing the annual range.
Effect of pH —
The selection of the pH for the controlled reaction during the TermTHM
concentration determination is less straight-forward than that for reaction
time and temperature. The variation of pH through an operating water-
treatment plant can be quite wide, and the variation is controlled opera-
tionally.

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23
200t
175- -
150--
O)
2 125
<
oc
Cl2 EXHAUSTED
z
Ui
O 100-f
z
o
u
s
oc
O
u.
o
oc
o
/
/
75-]- I
I
I
I
50 -M
I
25
£/
/
/
SETTLED
FRESH GAC FILTERED
I	I =	1		
30	60	90
STORAGE TIME, HRS.
FIG. 15 EFFECT OF TREATMENTS ON CHLOROFORM
FORMATION 8mg/l CHLORINE DOSE, 25°C, pH-7
H
120

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24
If the determination of the TermTHM concentration and the THMFP for the
finished water only is desired, pH selection is not a problem. The samples
should be stored at the finished water pH. If however, a comparison of the
THMFP of the finished water with that of the source, or raw water, or with
water at any stage of treatment to evaluate success of a unit process in
reducing THMFP is desired, the selection of pH is more difficult.
The analyst must be sure that the same portion of the total precursor
concentration (pH dependent) is reacting at each point and that the reaction
rate of chlorine with that material (also pH dependent) is the same at each
point. Thus, all of the samples from each of the various sampling points
must be chlorinated and stored buffered at a single selected pH value.
Therefore, because the THMFP test is designed to measure the portion of the
total precursor that is significant in a given water as it leaves the treat-
ment plant, the logical selection of the single reaction pH value is still
that of the finished water entering the distribution system, as with the
choice of temperature.
Summary of Procedures for InstTHM, TermTHM, THMFP
Procedures for measurement of these parameters have been discussed in
terms of general concepts. The specific procedures, particularly the
quantatative measurement of the trihalomethanes themselves are covered
in detail and a more complete way elsewhere and will not be described
herein. The reader is referred to the references and EPA trihalomethane
analytical procedures for detail.
In summary and review, InstTHM is the measured THM concentration when
the chlorine-precursor reaction was stopped by the addition of a chemical
reducing agent at the time of sampling.
TermTHM is the measured THM concentration after the reaction between
precursors and chlorine has been allowed to continue in a sealed container
under defined specified conditions for a given time period.
THMFP is the arithmetic difference between TermTHM and InstTHM and
represents the concentration of organic precursor of concern to the analyst
that is unreacted and is present in the sample at the time of the original
sampling.
Examples of the Use of Methods and Interpretation of Results
Some hypothetical examples will help to demonstrate the use of the two
experimental determinations and the calculated THMFP to estimate both con-
sumer exposure to trihalomethanes resulting from the chlorination process and
the efficiences of the various unit processes within the plant for removing
precursor compounds during treatment. The efficiency of unit processes for
removing chloroform or other trihalomethanes can also be estimated.

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25
Simple Chlorination —
The first example, Fig. 16 represents the simplest case - a water-r
treatment plant with chlorination only. Figure 16 depicts the relative
values for the parameters that might be obtained if analyses were conducted
for the InstTHM concentration and TermTHM concentration for source water A,
plant clear well, B, and a theoretical point at the maximum residence time
in the distribution system, C. For simplification the triahlomethanes are
being discussed here as a group. Each bar could represent the single group
index (TTHM), any one of the individual species, or be subdivided hori-
zontally into four bars of different heights to represent all four commonly
found trihalomethanes.
According to the bar graph, trihalomethanes were absent in the un-
treated source water (InstTHM was not found on analysis of source water),
but the full THMFP was present and equal to the TermTHM concentration
obtained experimentally. At the clear well, some of the precursor measured
as THMFP has reacted to form trihalomethanes (measured as InstTHM in
finished water) leaving a smaller remaining THMFP. The remaining THMFP,
plus InstTHM concentration equals the TermTHM concentration determined
originally on the source water. At point C the entire original THMFP had
reacted to give an InstTHM concentration identical to the TermTHM concen-
tration.
No unit process exists at this plant that was effective for reduction
of either TermTHM or InstTHM concentrations. The practice of chlorination
itself converted THMFP to InstTHM, thereby causing a reduction in.the THMFP.
In assessing the THMFP removal by any unit process, care must be taken to
separate removal of THMFP by conversion to InstTHM by chlorination,. and
removal of THMFP by the unit process itself. Only at a point closer to the
treatment plant than the maximum length in the distribution system is
consumer exposure to THM lower than the TermTHM concentration shown in
Fig. 16.
Conventional Treatment —
Shown in Fig. 17, during conventional treatment with raw water chlori-
nation, some THM is formed during rapid mixing and throughout the following
treatment stages in the presence of chlorine. Thus, the InstTHM concentra-
tion increases as the water passes through rapid mixing, settling, and
filtration, points B, C, and D. Coagulation and settling do reduce THMFP
(i.e., precursor removal) so that parameter as well as TermTHM concentration
declines from point B to C. Filtration removes a little more precursor
material that is associated with the carryover floe; therefore, the THMFP
declines slightly again from point C to D. The remaining THMFP is converted
by chlorination to THM from point D to E, and therefore, the InstTHM
concentration determined for a sample taken at that point in distribution
equals the TermTHM concentration of the sample from point D.

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V777\ InstTHM CONCENTRATION
~ THM FORMATION POTENTIAL
0+^ TermTHM CONCENTRATION
o
H
<
QC
H
Z
HI
O
z
o
o
ON
I
K
SOURCE
CLEAR
WELL
END OF DISTRIBUTION
SYSTEM
\
CHLORINE
FIG. 16 TRIHALOMETHANES FORMED DURING WATER
TREATMENT BY CHLORINATION ONLY

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E
InstTHM CONCENTRATION
I I THM FORMATION POTENTIAL
~11 + ^ TermTHM CONCENTRATION

SOURCE

CHLORINE
COAGULANT-J
RAPID
MIX
SETTLINGhWILTERS
END OF DISTRIBUTION
SYSTEM
FIG. 17 TRIHALOMETHANES FORMED DURING CONVENTIONAL
TREATMENT WITH RAW WATER CHLORINATION

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28
Summary of Examples —
These two hypothetical examples should not be considered to be pre-
dictions of the success or failure of certain unit processes in a treatment
train or to be indications of the relative effectiveness of those processes.
The examples do serve to indicate tfie kinds of results that might be obtained
when a plant is sampled for measurement of InstTHM and TermTHM concentrations
and THMFP and when the results of these measurements are compiled for unit
processes or whole plant evaluations.
Summary of Formation Potential Concept
Instantaneous trihalomethane concentrations in chlorinated drinking
water may be measured in samples where chlorination reactions were stopped
by addition of a suitable reducing agent at the time of sampling. The
trihalomethanes then are separated from the aqueous phase and subjected
to an acceptable form of measurement.
Terminal trihalomethane concentration is a measure of trihalomethanes
formed as a result of sample storage under conditions that closely approxi-
mate those of the distribution system corresponding to the plant under
study. The parameter can be used to estimate consumer exposure to trihalo-
methanes as well as provide a route to the calculation of trihalomethane-
formation potential remaining at any stage of treatment.
Trihalomethane-formation potential is a useful measure of pertinenent
unreacted precursor material. The value is obtained by subtraction of the
instantaneous trihalomethane concentration from the terminal trihalomethane
concentration in a given sample.
Total precursor concentration measured as maximum trihalomethane pro-*
duced on chlorination is not a viable parameter because establishing
completeness of the reaction is rather difficult and the measurement invari-
ably would give trihalomethane concentrations higher than those actually
reaching t^e consumer.
Generation of the trihalomethane-formation-rate curve, although not
always necessary, provides useful background information for plant and unit-
process evaluations. The curve, .when generated for finished water samples,
provides a useful estimate of trihalomethane concentrations for any given
time after the water leaves the treatment plant.
The proper measurements of instantaneous trihalomethane and terminal
trihalomethane concentrations and calculation of trihalomethane-formation
potential in fconjunction with a carefully planned sampling program can be
used to determine success or failure of efforts to reduce trihalomethane
concentrations reaching the consumer in modification of water-treatment
practice.

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29
REFERENCES
Stevens, A. A., J. M. Symons, "Measurement of Trihalomethane and
Precursor Concentration Changes", Jour. AWWA, 69, No. 10, 546
(Oct. 1977).
Stevens, A. A., C. J. Slocum, D. R. Seeger, G. G. Robeck, "Chlorina-
tlon of Organics in Drinking Water", Jour. AWWA, 68, No. 11, 615
(Nov. 1976).
Stevens, A. A., "Formation of Non-Polar Organochloro Compounds as
By-Products of Chlorination", Proceedings of International Conference
on Oxidation Techniques in Drinking Water Treatment, Sept. 9-12, 1978,
Karlsruhe, FRG., EPA 570/9-79-200.
U.». IWEBMmniWWGOWCE: mo-657-146/5588

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