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Trihalomethane formation in drinking water and production within a polyvinyl chloride pipe environment

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This project used pipe-incubated water samples collected from a pilot plant in North Vancouver, B.C., and the material-specific simulated distribution system (MS-SDS) test (Brereton 1998) to investigate the interaction between chlorine residuals in drinking water and the interior surface of polyvinyl chloride (PVC) distribution system pipes. Seasonal differences and residence time were important factors in trihalomethane (THM) production, and surface biofilm had a more significant impact on chloroform formation in bulk water than the treatment process. Biofilm adsorption has been identified as one surface mechanism responsible for the removal of THM precursors from the bulk water phase. This study observed that compounds previously adsorbed and extracellular materials from biofilm bacteria (including protoplasm from lysed cells) could participate in the aqueous chloroform formation in the absence of available precursor. Under chlorination conditions typical to the Greater Vancouver Regional District (GVRD) utilities, additional precursors were masked by the chlorine-limited condition; however, experiments confirmed that the practice of rechlorination would increase THM production beyond its bulk water capacity. Heterotrophic plate counts (HPCs), measured from biofilm scrapings, revealed that higher chloroform production was accompanied by bacterial regrowth. A proposed model summarizes the major mechanisms interacting in a PVC pipe environment.
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293
Trihalomethane formation in drinking water
and production within a polyvinyl chloride pipe
environment
Karen C.W. Chan, Donald S. Mavinic, and John A. Brereton
Abstract: This project used pipe-incubated water samples collected from a pilot plant in North Vancouver, B.C., and
the material-specific simulated distribution system (MS-SDS) test (Brereton 1998) to investigate the interaction between
chlorine residuals in drinking water and the interior surface of polyvinyl chloride (PVC) distribution system pipes.
Seasonal differences and residence time were important factors in trihalomethane (THM) production, and surface biofilm
had a more significant impact on chloroform formation in bulk water than the treatment process. Biofilm adsorption has
been identified as one surface mechanism responsible for the removal of THM precursors from the bulk water phase.
This study observed that compounds previously adsorbed and extracellular materials from biofilm bacteria (including
protoplasm from lysed cells) could participate in the aqueous chloroform formation in the absence of available precursor.
Under chlorination conditions typical to the Greater Vancouver Regional District (GVRD) utilities, additional precursors
were masked by the chlorine-limited condition; however, experiments confirmed that the practice of rechlorination
would increase THM production beyond its bulk water capacity. Heterotrophic plate counts (HPCs), measured from
biofilm scrapings, revealed that higher chloroform production was accompanied by bacterial regrowth. A proposed model
summarizes the major mechanisms interacting in a PVC pipe environment.
Key words: biofilm, chloroform, distribution system, drinking water, material specific simulated distribution system
(MS-SDS), PVC pipe, regrowth, trihalomethane (THM) formation.
Résumé: Ce projet comporte l’utilisation d’échantillons d’eau incubés dans des tuyaux et collectés dans une usine pilote
à North Vancouver, Colombie-Britannique ainsi que le test de « système de distribution simulé spécifique au matériau »
(MS-SDS) (Brereton, 1998) pour étudier l’interaction entre les résidus de chlore dans l’eau potable et la surface interne
des tuyaux du système de distribution en chlorure de polyvinyle (PVC). Les différences saisonnières et le temps de
résidence sont des facteurs importants dans la production de THM, et le film biologique sur la surface a un impact
plus important sur la formation de chloroforme dans la masse d’eau que le procédé de traitement. L’adsorption du film
biologique est identifiée comme étant l’un des mécanismes de surface responsables du retrait des précurseurs de THM
dans la phase de la masse d’eau. La présente étude permet d’observer que les composés antérieurement adsorbés et les
matériaux extracellulaires des bactéries du film biologique (incluant le protoplasme des cellules lysées) pouvaient prendre
part à la formation aqueuse de chloroforme en l’absence d’un précurseur disponible. Sous des conditions de chloration
typiques des services publics du District régional de Vancouver, d’autres précurseurs sont masqués par cette condition
limitée en chlore. Cependant, des expériences confirment que la pratique de re-chloration pourrait accroître la formation
de THM au-del à de la capacité de la masse d’eau. La numération sur plaque des hétérotrophes (HPC), mesurée sur des
raclages du film biologique, montre que la production plus élevée de chloroforme est accompagnée d’une régénération
bactérienne. Un modèle proposé résume les principaux mécanismes en interaction dans un environnement de tuyaux en
PVC.
Mots clés: film biologique, chloroforme, système de distribution d’eau, eau potable, systèmes de distribution simulé
(MS-SDS), tuyau en PVC, régénération, formation de THM.
[Traduit par la Rédaction]
Introduction
In the United States and Canada many water utilities dis-
tribute treated water with chlorine residual to inactivate po-
tential pathogenic organisms and preserve water quality dur-
ing distribution. Chlorination has achieved much success in
the reduction of waterborne diseases. However, the reaction
of chlorine with natural organic materials also leads to the for-
Received 16 May 2001. Revision accepted 19 June 2002. Published
on the NRC Research Press Web site at http://jees.nrc.ca/ on
15 July 2002.
K.C.W. Chan,
1
D.S. Mavinic, and J.A. Brereton. Environmental
Engineering Group, Department of Civil Engineering, The Univer-
sity of British Columbia, Vancouver, BCV6T 1Z4, Canada
Written discussion of this article is welcomed and will be received
by the Editor until 30 November 2002.
1
Corresponding author (e-mail: kchan@stantec.com).
J. Environ. Eng. Sci. 1: 293–302 (2002) DOI: 10.1139/S02-019 © 2002 NRC Canada
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294 J. Environ. Eng. Sci. Vol. 1, 2002
mation of disinfection by-products (DBPs). One such group is
trihalomethanes (THMs), whose toxic and potential carcino-
genic properties have been undergoing intense scrutiny during
thelast20years.Controlling the levelsofTHMprecursorsprior
to chlorination should be deemed the most direct means of re-
solving THMs problems. Despite much research, the specific
precursors involved in the formation of THMs are still not clear
(Mallevialleet al. 1990; Hureiki et al. 1994; Pomes et al. 1999).
Moreover, recent studies have revealed that several aspects of
the pipe environment may contribute to a more complex reac-
tion system.
First, chemical decay as a result of reactions with pipe mate-
rial can compromise microbial reduction efficiency(LeCheval-
lier et al. 1987, 1990; Kerr et al. 1999; Hallam et al. 2001). The
water industry is currently replacing pipes with less reactive
synthetic materials such as polyvinyl chlorides (PVC); how-
ever, their biofilm-forming potentials are still under investiga-
tion. Second, biofilms are formed in distribution systems when
microbial cells attach to pipe surfaces and multiply, forming
a slime layer in the pipe. Attached bacteria produce extracel-
lular polysaccharides (EPS) to help anchor microorganisms to
the pipe surface, and the growth of microorganisms as biofilms
is supported by a substratum of accumulated nutrients in an
oligotrophic environment (Fletcher and Marshall 1982). Stud-
ies(Volketal.1997;Camper et al.1999)haveshownthathumic
substances, the principal precursors of THMs present in drink-
ing water, may become biologically utilizable to bacteria when
they are adsorbed to surfaces. Third, an inability to maintain a
chemical residual allows bacterial regrowth in drinking water
supplies. Microbial attachment has also been shownto be a ma-
jor factor in chlorination resistance and the persistence of bac-
terial regrowth (LeChevallier et al. 1988; Hallam et al. 2001).
It can then be hypothesized that surface biofilm may impose
certain impacts on the formation of THM during distribution.
Both bacterial inactivation (if any) and THM formation exert
a chlorine demand and rely heavily on natural organic matter
(as a precursor for THM formation and as a nutrient source for
biofilm).
Therefore, the commonly used simulated distributionsystem
(SDS) test, as specified in Standard Method 5710C (APHA et
al. 1995), may not be adequate to evaluate the formation of
THMs in a distribution system. The standard test assumes zero
contribution from the distribution system by incubating water
samples in standard glass bottles. Brereton (1998) developed a
modificationinwhichwatersamplesareincubatedincontainers
constructed from the same type of pipe material as the actual
distribution system under the same condition. This material-
specific (MS-) SDS test was proposed on the same premise as
the standard SDS test. If the two tests are run simultaneously,
it is then possible to distinguish THM production in drinking
waterunder the influence of twodistinct environments:the bulk
water phase and the pipe environment.
With these premises, this paper presents and discusses the
results from a research study on the production of disinfection
by-products in the presence of biofilm formed on PVC pipes.
Background
Seymour Pilot Plant
This study utilized a water treatment pilot-plant facility lo-
cated in the Seymour Watershed (NorthVancouver) and owned
by the Greater Vancouver Regional District (GVRD). It em-
ployed treated water from the Seymour Reservoir with sec-
ondary chlorination to evaluate the formation of THMs. The
pilot-plant facility produced a maximumraw water flow capac-
ity of 14 m
3
/h and consisted of the following unit components
(Fig. 1):
rapid mix accomplished by ferric chloride (FeCl
3
) injec-
tion upstream of in-line static mixers
flocculation dissolved air flotation (DAF) unit that con-
sists of a single rectangular tank baffled to form an inte-
gral flocculator/DAF cell, a mechanical skimmer, an air
saturator, and associated controls
duel media (sand– anthracite) filter columns with auto-
matic control of outflow and backwashing, and
ZeeWeedultrafiltrationimmersedmembranesystem(Ze-
non Environmental Systems Inc., Edmonton, Alta) with
a cross-flow hollow fiber configuration that draws water
outside-in. The nominal pore size is 0.04 µm.
Source water
Seymour raw water is naturally soft and low in pH (pH
6.3), alkalinity (2.8 mg/L as CaCO
3
), and turbidity (0.4 to
0.6 nephelometric turbity units (NTU)). Chloroform is the only
THM formed upon chlorination, due to the naturally low bro-
midecontent of thiswater; forwaters thatdo contain significant
concentrationsof bromideion,brominated trihalomethanescan
be expected.
Materials and methods
Collection and preparation of water samples
Water samples were collected from the Seymour Pilot Plant
before and after the treatment process and were then trans-
ported to the Environmental Laboratory at The University of
British Columbia in an ice-filled insulated box for chlorina-
tion within 1 h. While there was little change in pH between
the water samples collected before and after the ultrafiltration
process (pH 6.2), water samples collected after the FeCl
3
coagulation–filtration process often displayed a lower pH (pH
5.1). All samples were spiked with chlorine at 2 to 3 mg/L
and then stored headspace-free in the test apparatus under both
short-term (3 to 5 h) and long-term (4 d) incubation periods.At
theendoftheincubationperiod,40-mLsamples werequenched
with sodium sulfite and subjected to gas chromatograph (GC)
analysis for chloroform. Chlorine residuals were reduced to
approximately 0.2 mg/L by the end of long-term incubation pe-
riod. The same residual concentration was targeted when water
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Chan et al. 295
Fig. 1. Schematic diagram of the modified pilot plant.
Raw
Water
Rapid
Mix
Flocculation/
DAF
Ultrafiltration
Membrane
(* recently
installed)
Sand / Anthracite
Filtration
reached residential homes in the GreaterVancouver area under
the municipal practice at the time of this publication.
Simulated distribution system tests
The standard SDS test was performed in accordance with
Standard Method 5710C (APHA et al. 1995). Spiked water
samples were stored in clean, 125-mL standard SDS glass bot-
tles, and sealed with PTFE-lined open-top screw type caps for
a preset incubation period at 25°C. The pH of the water sam-
ples ranged from 5.7 to 6.7 after incubation.All glassware was
washedwith detergent,rinsedwithtapwateranddistilled water,
and dried at 105°C prior to use.
TheMS-SDStestwasperformedinparallelwith thestandard
SDS test under the same incubation conditions for the same
period. Pipe incubators were used for storage in the MS-SDS
test. At the end of the incubation period, the pipe incubators
were uncapped and drained, and the waters were collected and
quenched for chloroform analyses.
Pipe incubators as storage apparatus
The storage apparatus used was pipe sections 360 mm in
length and 50 mm in diameter. Each pipe section was con-
structed from new PVC material and threaded at both ends to
connect to the raw water inlet at the Seymour water treatment
pilot plant.At the raw water inlet, continuous flow of untreated
water was passed through a gallery of connected PVC pipe seg-
ments and discharged to sewer. Initially, it took about 3 weeks
to grow a readily visible in situ biofilm on the internal sur-
face of the pipe walls. Once the biofilm was established, these
pipesegmentsweredisconnectedasstorageapparatusandfilled
headspace-free with chlorine-spiked waters.In an MS-SDS test
run,thepipeincubatorswerecompletelywrappedinTeflontape
at both ends andcapped off with PVC screw caps. Immediately
after each test, the pipe incubators were reconnected to the raw
water inlet for 1 week to reactivate the biofilm.
Chlorine residual and trihalomethanes
Free chlorine residual was measured by the N,N-diethyl-p-
phenylenediamine (DPD) colorimetric method using a Hach
fieldkit(ModelCN-70).Themethoddetectionlimitis0.02mg/L
for chlorine concentrations from 0 to 0.7 mg/L and 0.1 mg/L
for concentrations from 0.7 to 3.5 mg/L. Total trihalomethanes
were measured by liquid–liquid extraction gas chromatogra-
phy, using pentane (Fisher Chemical, HPLCgrade) exclusively
as the extraction solvent in accordance with Standard Method
6232B (APHA et al. 1995). Background interference from the
extraction solvent was eliminated by basic alumina column
chromatography (BrockmanActivity I; 60 to 325 mesh; Fisher
Chemical). The method detection limit ranges from 0.1 to
200 µg/L.
Organic-free water
Organic-free water was prepared from Vancouver tap water
byMilliporeAlpha-QUltra-purewatersystem,withitspurifica-
tion pack containing activated carbon, ion exchange resin, and
an organic scavenger as media. The system produces 18.2M-
cm reagent water (Type I water, ASTM) directly from potable
tap water.
Bacterial enumeration
A small interior surface area (82 mm
2
) of the incubator was
scraped with a sterile cotton-tipped swab to estimate active cell
counts prior to reconnecting to the raw water inlet. The swab
wasimmediatelyinsertedintoasteriletesttubecontaining5mL
dilution water (0.1692 g KH
2
PO
4
, 0.0020 g MgCl
2
6H
2
O) and
sonicated for 12 min (AQUASONIC ultrasonic cleaner 50HT,
VWR Scientific Products, West Chester, Pa.). The suspension
wasthenenumeratedforheterotrophsusingTGEagaronspread
plates(35°C,48h),accordingtoStandardMethod9215(APHA
et al. 1995).
Statistical analysis
Statistical analyses were completed using Microsoft Excel
97 (Microsoft Corporation, Redmond, Wash.).
Results
Chloroform reduction by treatment processes
Tables 1 and 2 show the percent reduction of chloroform lev-
els in water samples before and after the two process treatment
trains, namely, the sand/anthracite filtration and the ultrafiltra-
tion membrane system, based on the SDS test. Only five sets of
water samples were collected after the sand/anthracite filtration
treatment. Owing to the small sample size, it was felt that the
calculation of mean values and standard deviations could not
be justified.
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296 J. Environ. Eng. Sci. Vol. 1, 2002
Table 1. Chloroform reduction by sand/anthracite filtration (negative
reduction means an increase in chloroform production).
Sample
set
Raw water
chloroform
(µg/L)
Filtered effluent
chloroform
(µg/L)
Percent
reduction
(%)
SDS
incubation
time
1 6.2 6.8 8 3 h
2 18.9 22.4 16 15 h
3 6.4 10.4 39 4 d
4 3.8 17.0 78 4 d
5 18.1 17.9 –1 5 d
Note: SDS, simulated distribution system.
Table 2. Chloroform reduction by ultrafiltration membrane system
(negative reduction means an increase in chloroform production).
SDS
incubation Percent reduction (%) Standard Number
time Average Range deviation of runs
5 h 10.9 30.8 to –1.9 0.12 9
4 d 8.7 39.6 to – 43.8 0.35 6
Note: SDS, simulated distribution system.
These SDS test results indicated a wide range of chloroform
formation, even in effluent water samples collected after treat-
ment. Effluent samples from the sand/anthracite filtration treat-
ment produced chloroform measurements as high as 18.9 µg/L
within 15 h and as low as 3.8 µg/L within 5 days. Although
the ultrafiltration membrane system consistently produced ex-
cellent finished water quality with turbidity < 0.25 NTU, SDS
tests demonstrated a range of chloroform production in perme-
ate waters from 5.6 to 29.3 µg/L in 5-h incubation periods
and from 7.9 to 63.1 µg/L in 4-d incubation periods.
Table 2 indicates that the average chloroform formation lev-
els were reduced by about 11 and 9% for the 5-h and the 4-
d incubation periods, respectively. These reductions were not
high, and statistical analyses performed on these chloroform
measurements reveal a highly significant reduction for only the
5-h incubation (P = 0.001). This suggests that the ultrafiltra-
tionmembranesysteminstalledwasmoreeffectiveinremoving
fast-reacting precursors.
Seasonal differences
Figures2and3presentthemeasuredchloroformlevelsinwa-
ter samples, before and after the ultrafiltration membrane treat-
ment,fromthe SDStest.Themeanconcentration ofchloroform
productioninthewarmerseason(AprilandMay)wasmorethan
three times that in the colder season (February and March) for
the 5-h incubation. For the 4-d incubation, the first 2 weeks in
Aprilproduced amean chloroformconcentration fivetimes that
inthe previousmonths.Watertemperatures inthe colder season
were recorded to be 4°C, and water temperatures in the warmer
season ranged from 6 to 11°C. Seasonal differences in total
THMs concentrations have been reported previously (Chen et
al. 1998; Brett and Calverley 1979). One explanation for the
seasonal differences is that chemical reaction rate constants in-
crease with increasing temperature; higher temperatures in the
warmer season accelerated the rate of chloroform production.
The observed pattern for chloroform production may be also
explained by an increase in dissolved organic content in the
source water due to higher temperatures in the warmer season.
Unfortunately, an evaluation of how changes in the source wa-
ter organic matter might affect the THM concentration could
not be confirmed in this study, since organic carbon was not
measured (because of precision/detection limitations of avail-
ableinstrumentation).However,it isprobablethat the dissolved
organic carbon (DOC) content was still relatively high in the
permeate waters, since the chloroform precusor reduction by
ultrafiltration process was relatively low.
Incubation time
Table 3 summarizes pipe effect (PE) values obtained from
26 pairs of MS-SDS/SDS runs, for both raw water and ultra-
filtration permeate. The main factor of interest here was the
incubation time (5 h and 4 d). Pipe effect is defined as total
chloroform formed during pipe incubation (MS-SDS) divided
by chloroform formed during glass incubation (SDS). Pipe ef-
fectvalues>1.00indicateanoverallincreaseinTHMformation
as a result of pipe wall surface biofilm interaction with residual
chlorine. Pipe effect values <1.00 reveal a net loss of chloro-
form yield due to non-THM forming chlorine demand imposed
by the pipe environment (Brereton 1998; Brereton and Mavinic
2001).
In the 5-h runs PE values >1.00 in 9 cases out of 14, while
4-d runs resulted in PE < 1.00 in 10 cases out of 12. Further
statistical analysis of variance (ANOVA) shows that the impact
of incubation time was highly significant on PE values (P =
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Chan et al. 297
Fig. 2. Chloroform production in raw and permeate waters (5-h incubation).
0
5
10
15
20
25
30
35
23-Feb 24-Feb 01-Mar 02-Mar 04-Apr 07-Apr 20-Apr 27-Apr 02-May
chloroform (µg/L)
permeate raw
Fig. 3. Chloroform production in raw and permeate waters (4-d incubation).
0
10
20
30
40
50
60
70
23-Feb 24-Feb 01-Mar 02-Mar 03-Apr 06-Apr
chloroform (µg/L)
permeate raw
0.012 at α = 0.05). Chlorine consumption in pipe containers
was greater than its counterpart in SDS bottles under both incu-
bation times. Specifically, chlorine was completely exhausted
in most pipe containers by theend of the 4-d incubation period,
while measurable residuals remained in all SDS bottles. The
larger data spread evident at the 5-h contact time (Table 3) is
indicative of the variation in concentrations and reaction rates
of a heterogeneous mix of precursors across theindividual runs
and the difficulty in achieving steady state in this short incuba-
tion period.
Significance of treatment processes on chloroform
reduction
The average PE values of permeate waters (Table 3) are
slightly greater than that of the raw waters for both short and
long incubation periods. However, the differences are not sta-
tistically significant at 95% confidence level when analyzed by
the paired-sample t-tests. Although paired PE values of raw
and permeate waters help to account for the variability among
different process treatment runs, the small difference between
the two indicates that PE values may still be “drowned out”
by inherent variability in the MS-SDS pipe sections. Thus, the
primary purpose of this test is not so much to investigate the
effectiveness of the treatment process, but to investigate the
significance of the treatment process on chloroform reduction
in a pipe environment. In view of the significant chloroform
reduction by the ultrafiltration membrane system under short-
term incubation (as observed previously in Table 2), the pipe
environment seems to have drowned out the impact that the
treatment had on chloroform formation in the bulk water.
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298 J. Environ. Eng. Sci. Vol. 1, 2002
Table 3. Effect of incubation period on PE values (actual chloroform
concentration measured in µg/L).
Incubation
time
Water
sample
Average
PE PE range
Standard
deviation
Number of
samples
5 h Raw 1.13 0.51–1.79 0.50 7
5 h Permeate 1.18 0.75–1.85 0.37 7
4 d Raw 0.75 0.61–1.10 0.19 6
4 d Permeate 0.83 0.59–1.24 0.23 6
Note: PE, pipe effect.
Fig. 4. The MS-SDS and parallel SDS tests on organic-free water (4-d incubation; applied chlorine = 2 mg/L) (chlorine residual
measurement in milligrams per litre is indicated above each bar).
0.05 0 000.02
1.8
1.8
2
2
2
0
5
10
15
20
25
30
35
PE = 1.0 PE = 4.3 PE = 15.0 PE = 7.8 PE = 4.0
chloroform (µg/L)
chloroform-MS-SDS
chloroform-SDS
Fig. 5. The MS-SDS and parallel SDS tests on organic-free water (5-h incubation; applied chlorine = 2 mg/L) (chlorine residual
measurement in milligrams per litre is indicated above each bar).
1.1 0.5 1.01.6
1.9
1.8
1.8 1.8
-1
0
1
2
3
4
5
6
7
8
9
PE = 2.3 PE = 1.3 PE = infinity PE = infinity
chloroform (µg/L)
chloroform-MS-SDS
chlofororm-SDS
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Chan et al. 299
Pipe environment
Figures 4 and 5 show chloroform concentrations and free
chlorine residuals measured at completion of a series of MS-
SDS and parallel SDS runs for organic-free waters under both
long (4 d) and short (5 h) incubation periods. Since the water
wasorganicfreein nature and precursor free, anyTHMproduc-
tion had to be considered as either background concentration
or end product resulting from the reaction of chlorine with pre-
cursors not originally present in the water itself. Chloroform
concentrations at or below 2 µg/L are generally accepted as
background.
Figure 4 shows chloroform levels measured after4dofin-
cubation. They are consistently about or below 2 µg/L in the
SDS test. Chlorine residual measurements at the end of each
run revealed almost zero chlorine consumption for SDS runs.
Similar results from the SDS test were observed under the 5-h
incubation period (Fig. 5), namely, almost zero chlorine con-
sumption and chloroform concentrations at background level.
IntheMS-SDStest,however,chloroformlevelsrangedfrom1.3
to 32.8 µg/L under the 4-d incubation period, while theapplied
chlorine was almost completely exhausted in each MS-SDS
pipe incubator. The MS-SDS tests under the short incubation
period indicated a lesser range of chloroform concentrations
from 2.2 to 7.9 µg/L and the presence of free chlorine residual
at the end of each run. Nevertheless, more chlorine was con-
sistently consumed in pipe containers than in SDS bottles by
the end of the incubation period. Regardless of incubation time,
chloroform production maintained PE 1.00 in every instance,
indicating that the pipe environment was definitely an active
contributor to aqueous chloroform formation.
Chloroform concentrations were observed to be 4, 5, or even
15 times background level in some MS-SDS runs. Previously,
from numerous laboratory and pilot scale experiments, Kiéné
et al. (1998) found that chlorine consumption by synthetic ma-
terials (such as PVC) was negligible.Therefore,the chloroform
concentrationsobserveddidnotappear tobea product ofthere-
actionbetween the pipewalland chlorine.This observationwas
further supported by having a section of the pipe not exposed to
the watersupply at the treatment facility and havingno measur-
able chloroform attributed to the pipe material itself (Kenway
2001).ThechloroformconcentrationsmeasuredintheMS-SDS
test provided evidence of transfer of precursor materials from
the pipe wall biofilm to the bulk water. In one case (Fig. 5),
chloroform concentration exceeding the background level was
observed in the MS-SDS run when the parallel SDS run pro-
duced no detectable chloroform formation. This suggests that
the source of these additional precursor compounds may be
organic substances previously adsorbed onto pipe biofilm sur-
faces when the pipe containers were exposed to flow-through
raw water for in situ biofilm establishment. Simultaneously,
extracellular materials and protoplasm from lysed cells ema-
nating from the biofilm bacteria could also be reacting with the
chlorine.
Bioactivity
Heterotrophic plate count (HPC) measurements were car-
ried out for each pipe incubated with organic free water. The
HPCs revealed that bioactivity was reduced after chlorination
and that the applied chlorine residual of 2 mg/L was sufficient
for biofilm inactivation in most runs (Table 4). This is consis-
tent with previous observations by LeChevallier et al. (1990)
and Hallam et al. (2001), where bacteria grown on PVC pipe
surfaces were readily inactivated by a 1 mg/L (or less) of free
residual chlorine.
However,runs withhigher chloroform productionin the bulk
water were found to be accompanied by active bacterial growth
in the biofilm. It appears that chlorination acts selectively and
unpredictably on bacteria, which may result in some bacterial
maintenance and regrowth; this in turn seems to promote fur-
ther chloroform formation. Humic substances havebeen shown
to be utilizable as carbon and energy sources for biofilm bac-
teria when they were adsorbed to pipe surfaces (Camper et al.
1999).Sincethe bulkwaterisorganicfreeinnature, chlorinated
cells may, in fact, use the humic substances from those organic
compounds adsorbed to the pipe surface to repair cell damage
due to disinfection. There was also visual evidence that humic
substances were adsorbed, since scrapings of biofilm exhibit a
characteristic brown colour. Such a “survival strategy”, if true,
apparently occurs at chlorine concentrations sufficient to pene-
trate the biofilm yet insufficient to effect cell death. The HPCs
and high aqueous chloroform measurements strongly suggest
that these humic substances (as well as other organic material)
are subsequently released from the pipe wall biofilm, allowing
them to become available as precursors for subsequent THM
formation.Additional research is being pursued to conclusively
validate this hypothesis and identify specifically the nature of
these organics.
Rechlorination effect
Two sets of raw and effluent samples (treated by dual-media
direct filtration with FeCl
3
as a coagulant) were collected and
chlorinated. At time zero, a chlorine residual at 2 mg/L was
added to all water samples. After 24 h of incubation, the same
water samples were rechlorinated with 1 mg/L of free chlorine
(booster effect). By the end of the incubation period (4 d from
time zero), chlorine residual and chloroform levels were mea-
sured.Table 5 summarizes the results on two separate sampling
events.
As discussed earlier, most long incubation runs resulted in
PE values <1.00, regardless of water type. However, this trend
was observed only in raw waters in this experiment. The action
ofrechlorination offersadditional free chlorine andmore favor-
able reaction conditions for slow- reacting precursors, and the
filteredwatersshowsignsof takingadvantageofsuch favorable
conditions for chloroform formation. Therefore, the increased
chloroformproductionindicates that asubstantiallyhigher con-
centration of precursors is masked by the chlorine-limited state
of the bulk water. Moreover, PE values >1.00 implies the ad-
sorption of organic matter to pipe surfaces and subsequent re-
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300 J. Environ. Eng. Sci. Vol. 1, 2002
Table 4. Heterotrophic plate counts following MS-SDS test on organic-free waters
(Figs. 4 and 5).
Cl
2
remaining
(mg/L)
Chloroform
measured (µg/L)
HPC following
MS-SDS (cfu/mL)
HPC prior to
MS-SDS (cfu/mL)
Contact
time
0.05 1.33 None detected 750 4 d
0 8.93 None detected 900 4 d
0 11.32 200 1000 4 d
0.02 32.87 60 1000 4 d
0.04 21.32 4900 5500 4 d
1.1 2.82 None detected 750 5 h
0.5 2.2 None detected 900 5 h
1.6 7.88 10 340 5 h
1.0 2.76 None detected 450 5 h
Note: MS-SDS, material-specific simulated distribution system.
Table 5. MS-SDS and SDS test results.
Sampling
event
No. MS-SDS SDS PE
Water
type
Cl
2
residual
mg/L
chloroform
µg/L
Cl
2
residual
mg/L
chloroform
µg/L
1 0 8.61 0.65 10.45 0.82 Raw
1 0 7.41 1.3 6.41 1.16 Filtered
2 0 10.65 1.2 17.04 0.63 Raw
2 0 5.57 2.0 3.77 1.48 Filtered
Notes: MS-SDS, material-specific simulated distribution system; SDS, simulated distribution
system; PE, pipe effect.
lease, possibly translating into downstreamTHM control prob-
lemsina realdistributionsystem.The practiceofrechlorination
tends to provide a reaction condition similar to that of a THM
formation potential test, in which THM precursors react with
excessfree chlorineresidual, according to the StandardMethod
(APHA et al. 1995).
Discussion and summary
In spite of an excellent level of performance, water treat-
ment processes used in this study still produced effluents with
a wide range of chloroform yield under chlorination. This in-
dicated that the water treatment technology employed herein
did not selectively remove those organic precursors that were
responsible for the formation of chloroform in the bulk water
phase.Moreover,severalcharacteristic aspects of the pipeenvi-
ronment would drown out the impact that the treatment had on
chloroformformationinthebulkwater.Therefore,theMS-SDS
procedureprovedextremelyusefulintheassessmentofaqueous
THM formation in a pipe environment. Generally, extra chlo-
rine demand exertedby the pipe environment may contributeto
the decreased chloroform production in bulk water at chlorine
concentrations typical of mostutility practices. Declining yield
due to the loss of chlorine residual through intended micro-
bial reduction is then merely a problem of balancing the need
to maintain effective chlorination in the water and the effort
to control THM production. However, the results of this study
reveal the possibility of more complex reaction dynamics.
First, the impact of residence time on PE values is highly
significant. Chlorinated water incubated in pipe containers pro-
ducedmorechloroformthanthatinSDSglassbottlesduringthe
rst5hofcontact. This phenomenon may be attributed to the
transfer of fast-reacting precursor materials from the pipe wall
biofilm to the bulk water, thereby allowing the applied chlo-
rine to combine with the additional precursors to form aqueous
chloroform. This hypothesis is supported by the higher mea-
surements of free chlorine remaining in SDS glass bottles than
in pipe containers after5hofcontact (data not shown). Sec-
ond, in the chlorine booster effect experiments, rechlorination
significantly increased THM production beyond SDS capacity
in the treated water; this indicates a higher concentration of
additional precursors that were masked by a chlorine-limited
condition. Thus, at least in some cases, the potential of rechlo-
rination to prevent biofilm regrowth may be outweighed by its
potential to pose a threat downstream on THM control efforts.
This would depend on in situ factors such as the organic com-
position of the water and the dynamic state of the biofilm in a
distribution system.
Finally, and perhaps most interesting, is the substantial chlo-
roform occurrence in pipe-incubated organic-free water as re-
vealed by the MS-SDS test.A pattern of increasing chloroform
production and bioactivity emerges in response to the bacte-
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Chan et al. 301
Fig. 6. Proposed model for THM evolution within a PVC pipe environment.
PE = 1.00 PE > 1.00
Cl
2
re-Cl
2
FINISHED
WATER
Bulk Water Phase:
Cl
2
+ P THM + I
Cl
2
+ I THM
I
THM
Cl
2
+ OD nonTHM + Cl
-
Bulk Water Phase:
Cl
2
+ P THM + I
Cl
2
+ I THM
I
THM
Cl + OD
2
nonTHM + Cl
-
P P
adsorption
release
SINK / SLOUGHING
disinfection
biofilm cell repair
biofilm inactivation
PE > 1.00 PE < 1.00
Abbreviations 5 h 4 d
P organic precursors
I intermediates
THM trihalomethanes
OD nonTHM-yielding chlorine demand
rial inactivation exerted by chlorination. This is indicative of a
biolm resistance mechanism that results in the release of ad-
sorbed humic substances or extracellular material for aqueous
chloroformformation. Previously, Camperet al. (1999)showed
thepotential of humicsubstances as acarbon and energy source
for bacterial growth in an otherwise low nutrient environment
when they are adsorbed to surfaces. However, the present study
potentially expands this knowledge to chlorination practice of
drinking water and, for the rst time (based on a literature sur-
vey), observes and reports an associated chloroform formation.
A model is provided in Fig. 6 to illustrate possible reac-
tion dynamics for THM production in water chlorination (typ-
ical of practice in Vancouver) within PVC pipes, based on the
completed research to date. Follow-up research is now being
planned at the same test facility.
Acknowledgments
This study is nancially supported by an NSERC Grant
awarded to the second author. Industry support included the
B.C. Hydro Water and Wastewater Research Centre and the
GVRD, which generously permitted the use of the Seymour
Pilot Plant facility in this study.
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