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Applied and Environmental Microbiology, November 1999, p. 4957-4966, Vol. 65, No. 11
0099-2240/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.
Impacts of the Reduction of Nutrient Levels on
Bacterial Water Quality in Distribution Systems
Christian J.
Volk1,* and
Mark W.
LeChevallier2
American Water Works Service Company Inc.,
Belleville, Illinois 62220,1 and
American Water Works Service Company Inc., Voorhees, New
Jersey 080432
Received 5 March 1999/Accepted 5 August 1999
 |
ABSTRACT |
This study evaluated the impacts of reducing nutrient levels on
bacterial water quality in drinking water. Two American Water System
facilities (sites NJ102a and IN610) with histories of coliform problems
were involved, and each water utility received two pilot distribution
systems (annular reactors). One reactor simulated the conventional
treatment conditions (control), while the other reactor was used to
assess the effect of biological filtration and subsequent reduced
biodegradable organic matter levels on suspended (water column) and
biofilm bacterial concentrations in the distribution systems.
Biodegradable organic matter levels were reduced approximately by half
after biological treatment. For site NJ102a, the geometric mean of the
assimilable organic carbon concentrations was 217 µg/liter in the
plant effluent and 91 µg/liter after biological filtration. For both
sites, plant effluent biodegradable dissolved organic carbon levels
averaged 0.45 mg/liter, versus 0.19 to 0.22 mg/liter following
biological treatment. Biological treatment improved the stability of
free chlorine residuals, while it had little effect on chloramine
consumption patterns. High bacterial levels from the biological filters
resulted in higher bacterial concentrations entering the test reactors than entering the control reactors. On average, biofilms in the model
systems were reduced by 1 log unit (from 1.4 × 105 to
1.4 × 104 CFU/cm2) and 0.5-log unit (from
2.7 × 105 to 7.8 × 104
CFU/cm2) by biological treatment at sites NJ102a and IN610,
respectively. Interestingly, it required several months of biological
treatment before there was an observable impact on bacterial water
quality in the system, suggesting that the effect of the treatment
change was influenced by other factors (i.e., pipe conditions or
disinfection, etc.).
| |
INTRODUCTION |
During the distribution of drinking
water, bacterial regrowth may lead to a deterioration of bacterial
water quality, amplification of corrosion, generation of bad tastes and
odors, and proliferation of macroinvertebrates (1, 7, 9, 13, 18,
26). Biofilm control is becoming recognized as an important part
of the operation of drinking water plants and distribution systems.
Bacterial regrowth and coliform occurrences are dependent on a complex
interaction of drinking water characteristics and engineering and
operational parameters (13, 21, 23, 25, 38, 40). The
concentrations of biodegradable organic matter (BOM) available for
microbial growth can be determined by several biological tests
(14). Bacterial regrowth can be controlled when the amount
of BOM entering the distribution system is limited. Van der Kooij
(34) showed that heterotrophic bacterial levels in
nonchlorinated systems did not increase when assimilable organic carbon
(AOC) levels were lower than 10 µg/liter. LeChevallier et al.
(23) suggested that the regrowth of coliform bacteria in
chlorinated water may be limited by AOC levels of less than 50 to 100 µg/liter. Block et al. (6) recommended an absence of BOM
after treatment to limit bacterial regrowth. Servais et al.
(32) associated biological stability with a biodegradable
dissolved organic carbon (BDOC) concentration of 0.16 mg/liter in the
finished water. Volk and Joret (36) indicated that BDOC
levels should be less than 0.15 to 0.30 mg/liter to limit coliforms.
All of these objectives require very high levels of treatment. For
systems with high AOC or BDOC levels (e.g., an AOC level of >150
µg/liter and a BDOC level of >0.5 mg/liter), ways to reduce the
level of BOM entering the distribution system should be considered
(38). In addition to the amount of nutrients, the
composition of the BOM is also an important factor for controlling
microbial growth. Amino acids are only a small fraction of the natural
organic matter, but they represent a high regrowth potential (high
biomass production per unit of substrate) and are highly reactive with
chlorine (12, 15). A variety of processes can be used to
control BOM levels in drinking water. Coagulation can be efficient at
removing DOC and BDOC. However, AOC is only marginally affected by
coagulation, probably because AOC consists primarily of
low-molecular-weight, nonhumic substances that are not amenable to
coagulation (39). Other alternatives effective for
controlling BOM levels in water include the application of powered
activated carbon, biological filtration, or membrane processes. On the
other hand, oxidation with chlorine or ozone increases the amounts of
BOM in water (17, 20).
This research was conducted to assess the impact of treatment changes
on bacterial water quality monitored within model distribution systems
(i.e., annular reactors). The treatment change consisted of
implementing biological filtration. The study investigated (i) the
effects of biological filtration on nutrient levels entering the
distribution systems and (ii) the impacts of the decrease in nutrient
concentration on bacterial water quality (biofilm density and suspended bacteria).
| |
MATERIALS AND METHODS |
Study sites.
Two American Water Works Company utility
subsidiaries participated in this project. Sites were selected because
of historical coliform regrowth problems (23, 27).
Characteristics of plant effluent waters are presented in Table
1.
(i) Site NJ102a (New Jersey-American Water Company).
The
facility processes reservoir water which originates from the Swimming
River. The treatment train includes addition of powdered activated
carbon, a preoxidation step with chlorine, coagulation (with
poly-aluminum chloride and a cationic polymer), flocculation,
sedimentation, filtration with anthracite-sand-garnet, and
postdisinfection with free chlorine. The plant production is 36 million
gallons per day.
(ii) Site IN610 (Indiana-American Water Company).
The plant
is supplied from the White River and provides oxidation with chlorine
and potassium permanganate, settling, multimedium anthracite-sand-garnet filtration, and postdisinfection with
chloramines. Plant production averages 12 million gallons per day.
Annular reactor experimentation.
Annular reactors were used
as a model distribution system and consist of an outer cast iron
portion (pipe volume, 1.15 liters) and a rotating inner polyvinyl
chloride cylinder that simulates water shear stress (8).
Removable mild steel coupons located at the surface of the pipe allowed
the evaluation of bacterial biofilm density. The study used an original
experimental setup to specifically assess the benefit of a new
treatment. Each water utility received two annular reactors (Fig.
1). One reactor was used as a control,
modeling the distribution network; the other reactor was used as a test
device for assessing the benefit of reducing nutrient levels in
distribution systems. At the beginning of the study, both reactors were
fed with treatment plant effluent water to identically colonize both
pipes. Water retention times of 230 to 426 min were used, which
resulted in dissipation of the residual disinfectant within the reactor
(Table 2). When biofilm densities reached
a plateau, the water-detention time was gradually decreased to achieve
an effective disinfectant residual in the outlet of the reactor. At
this point, the treatment change was implemented in the test device
(Fig. 1; Table 2). A biological filter was installed to reduce nutrient
levels entering the test pipe. Prior to the biological filter, the
disinfectant residual was neutralized by filtration through a small
virgin-GAC filter. The dechlorinated water passed through a biological
GAC filter (originating from a full-scale biological active filter in
Montreal, Canada) and was subsequently postdisinfected by using an
intermediate clear well. The clear well was necessary to minimize the
influence of the bacterial levels emanating from the biological filters on the microbiology of the annual reactor. The goal was to supply the
test reactor with disinfectant residuals similar to those in the
full-scale plant effluent entering the control reactor. These steps of
flow rate augmentation, biofilter installation, and clear well
disinfection were successively implemented to verify the proper
functioning of each change before implementing the following steps.
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FIG. 1.
Experimental setup used to study the effects of
biological filtration on water quality. R., reactor; P, pump.
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Monitored parameters.
Sampling was performed on a weekly
(site IN610) or semimonthly (site NJ102a) basis. Water temperature,
chlorine residuals (amperometric titration or
N,N-diethyl-p-phenylenediamine [DPD] method), and heterotrophic plate count (HPC) levels were measured at
the inlets and outlets of the reactors (2). HPC levels were determined by using R2A agar (Difco, Detroit, Mich.) incubated for 7 days at 20°C. Removable pipe coupons were sampled to evaluate biofilm
density. Biofilm samples were homogenized (PT 1200C homogenizer; Brinkmann, Westbury, N.Y.) and plated on R2A agar. AOC concentrations were monitored at the inlets of the annular reactors by using the rapid
ATP method (24). BDOC levels were measured by using bacteria
attached to sand (37).
Because most of the data were not normally distributed, geometric means
were calculated. The Wilcoxon test (signed and ranks
test) was
performed to determine whether two data sets were statistically
different (comparison of
medians).
| |
RESULTS |
The effects of biological filtration were successively evaluated
by examining nutrient concentrations, disinfectant stability, and
bacterial concentrations in the bulk water and biofilm. The effects of
the reduction in nutrient levels on bacterial water quality were
evaluated by measurement of suspended and fixed bacteria, the amount of
time necessary to observe an impact, and the magnitude of the impact.
Reactor colonization.
Both reactors were identically colonized
with plant effluent water during winter months at both locations, using
cold waters (temperatures of <10°C). Average disinfectant residuals
at the inlets of the reactors were 1.3 mg of chlorine per liter for
site NJ102a and 2.4 mg of chloramines per liter for site IN610.
Disinfectant residuals were not detectable at the outlets of the
reactors. Consequently, suspended HPC levels in the reactor outlets
were very high (7.7 × 104 CFU/ml at site NJ102a and
9.8 × 104 CFU/ml at site IN610) (data not shown).
Nutrient levels.
During the course of the study, the average
DOC levels in the finished waters after conventional treatment were
2.00 and 2.34 mg/liter at NJ102a and IN610, respectively. BOM levels
were reduced approximately by half after biological treatment (Fig.
2 and Table 3). For site NJ102a, plant effluent AOC
concentrations varied from 153 to 319 µg/liter (geometric mean of 217 µg/liter), while after biological filtration, AOC concentrations
entering the test reactor ranged between 58 and 137 µg/liter
(geometric mean of 91 µg/liter). AOC reduction after biological
filtration was statistically significant (P < 0.05)
compared to plant effluent levels. BDOC levels ranged from 0.26 to 0.69 mg/liter (mean of 0.45 mg/liter) in the plant effluent water entering
the control reactor, versus <0.05 to 0.44 mg/liter (mean of 0.19 mg/liter) at the test reactor inlet. The removal of BDOC through the
biological filters averaged 60% and was statistically significant
(P < 0.01). A similar trend was observed at IN610,
where plant effluent BDOC levels averaged 0.45 mg/liter (range, 0.07 to
0.91 mg/liter) (Fig. 2; Table 3). After biological filtration, BDOC
concentrations entering the test reactor were reduced by 50% on
average (geometric mean of 0.22 mg/liter; range, <0.05 to 0.34 mg/liter [a significant difference, P < 0.05]).
Three AOC sampling campaigns were performed after implementation of
biological treatment, and average AOC concentrations were 160 µg/liter for the plant effluent and 76 µg/liter after biological
filtration.
Disinfectant residuals.
Total chlorine residuals in the plant
effluent of site NJ102a were slightly higher than residuals in the
inlet of the test reactor (average of 1.4 mg/liter for the control
reactor versus 1.2 mg/liter for the test pipe after implementation of
biological filtration [significant difference, P < 0.05) (Table 3; Fig. 3).
Disinfectant residuals in the biologically treated test reactor were
twice as high as the control reactor effluent chlorine residuals (P < 0.001). Disinfectant residuals leaving the
control reactor averaged 0.5 mg/liter (range of 0.2 to 0.7 mg/liter),
versus 1.0 mg/liter (0.6 to 1.2 mg/liter) for the test reactor (Fig.
3). For site IN610, chloramine residuals entering the control and test
reactors were not statistically different (P 
0.05).
Average chloramine residuals were 2.3 mg/liter for the control (range, 1.9 to 2.7 mg/liter) and the test (range, 1.8 to 2.7 mg/liter) reactors. Chloramine residuals averaged 1.8 mg/liter at the control pipe outlet and 1.9 mg/liter in the test pipe effluent. The difference in disinfectant residuals at the pipe outlet was not statistically significant (P = 0.1).
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FIG. 3.
Disinfectant residuals measured at the inlets and
outlets of the test (TR) and control (CR) reactors.
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HPC levels entering the system.
HPC levels in the water
entering the control reactor averaged 14 CFU/ml and varied from 0 to
360 CFU/ml at site NJ102a (Table 3; Fig.
4). Plant effluent bacterial
concentrations at site IN610 were low and ranged between 0 and 36 CFU/ml, with a geometric mean of 1 CFU/ml (Fig. 4). After
implementation of biological filtration, bacterial concentrations in
the water entering the test reactor were slightly higher than those
entering the control reactor at both sites (significant difference,
P < 0.001). On the average, HPC levels after
disinfection of biologically filtered water were 93 CFU/ml (range, 4 to
1800 CFU/ml) and 74 CFU/ml (1 to 1,500 CFU/ml) for sites NJ102a and
IN610, respectively. Some bacteriological counts were also performed at
the outlet of the biological filter, before disinfection in the clear
well. HPC levels were as high as 5.7 × 105 CFU/ml at
site NJ102a and averaged 9.2 × 104 CFU/ml at site
IN610 (maximum of 6.2 × 105 CFU/ml) (data not shown).
Effect of reduced nutrient concentrations on bacterial levels.
Figure 5 shows HPC levels at the outlets
of the control and test reactors and after biological filtration. For
site NJ102a, a reduction in nutrient levels had only a slight effect on
HPC levels in the effluents of the reactors (difference not
significant, P > 0.05). The HPC level of 3.6 × 103 CFU/ml (range, 1.1 × 102 to 1.4 × 105 CFU/ml) in the test reactor effluent was comparable
to the geometric mean HPC level of 5 × 103 CFU/ml
(range, 1.4 × 102 to 1.8 × 105
CFU/ml) in the control effluent water for the same period of time (Fig.
5 and Table 3). The trend was similar at site IN610. On average, HPC
levels were 1.4 × 102 CFU/ml (range, 10 to 800 CFU/ml) and 1.1 × 102 CFU/ml (range, 10 to 730 CFU/ml) in the outlets of the test and control reactors, respectively.
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FIG. 5.
Bulk water HPC levels and temperatures at the outlets of
the test (TR) and control (CR) reactors.
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No drastic changes in concentrations of biofilm bacteria were observed
following the installation of the biofilters (Fig.
6). During the first 6 months at site
NJ102a, control reactor
biofilms averaged 3.1 × 10
5
CFU/cm
2, versus 1.6 × 10
5
CFU/cm
2 for the test reactor. At site IN610, geometric
means of biofilm
bacteria for the 5 months following the installation
of the biological
filtration were 5.7 × 10
5 and
3 × 10
5 CFU/cm
2 for the control and test
pipes, respectively (a marginal statistical
difference,
P = 0.03). However, much higher differences in biofilm
densities were
observed after a period of several months following
implementation of
biological filtration. After a period of 6 months,
a 10-fold difference
in biofilm levels in the test reactor was
apparent at site NJ102 (from
1.4 × 10
5 to 1.4 × 10
4
CFU/cm
2;
P < 0.001) (Table
3). On average,
the difference was less drastic
for site IN610, where biofilm levels
dropped from 2.7 × 10
5 to 7.8 × 10
4
CFU/cm
2, a 0.5-log-unit difference (
P < 0.001).
| |
DISCUSSION |
Bacterial regrowth in distribution systems results from the
proliferation and detachment of heterotrophic bacteria from pipe surfaces. This project evaluated the effect of biological treatment on
water quality in model distribution systems. The data from this study
showed that conditions that cause the growth of bacteria in
distribution systems are complex and site specific. Bacterial water
quality could be related to the levels of nutrients entering the pipe
network but also to other factors and their interactions. Annular
reactors were found to be a good tool to evaluate and predict the
effects of a treatment change before full-scale implementation.
Colonization.
The reactors became quickly colonized despite
the influx of chlorinated water. Biofilm densities reached a plateau
within 1 month at plant NJ102a and within 2 months at plant IN610. The rapid colonization of pipe surfaces is consistent with the findings of
other investigators. Depending on the experimental conditions, an
accumulation of steady-state biofilm can be observed at between 2 weeks
and 4 months (4, 8, 12, 30). Biofilm counts observed inside
the annular reactors at steady state were on the same order of
magnitude as the ones found in other pilot or full-scale distribution
systems (around 105 to 107
bacteria/cm2) (4, 8, 19, 29, 35).
Nutrient removal.
Biological filtration enhanced nutrient
removal by providing biofilm activity to consume assimilable organic
materials. Nutrient levels entering the distribution system could be
reduced by 50% by biological treatment. Various studies showed that
biological filtration can be highly effective for removal of BOM
(3, 16, 17). The percentage of BOM removal is dependent on
many parameters, such as organic matter characteristics, water
temperature, medium type, empty bed contact time, and backwashing
strategies. For example, during treatment of the Oise River (France),
biological sand and GAC filters achieved BDOC removals of 32% (BDOC
consumption of 0.25 mg/liter) and 37% (BDOC consumption of 0.30 mg/liter), respectively (11). Servais (31)
observed average BDOC removals of 50% during biological GAC filtration
of ozonated waters (empty bed contact time range of 10 to 20 min).
Disinfectant residuals.
For the site using free chlorine, the
biologically treated system showed higher disinfectant residuals than
the system supplied after conventional treatment. It is presumed that
the higher residuals in the test reactor outlet resulted from lower
organic matter and biofilm levels. Lower DOC and BDOC levels entering
the test reactor reduced chlorine demand and increased chlorine
stability within the system. Randon et al. (28) found
identical results in a distribution system fed with surface water
treated successively by ozone-GAC filtration (finished water BDOC
levels of 0.75 mg/liter) and nanofiltration (BDOC levels of 0.20 mg/liter). Chlorine residuals at the end of the distribution system
were more stable when the system was supplied with nanofiltered water,
despite the fact that plant effluent chlorine residuals were 0.85 mg/liter for the water treated with ozone-biological filtration versus
0.25 mg/liter for the nanofiltered effluent. Lower biofilm densities would also account for less chlorine consumption within the test reactor, since hypochlorite acid is highly reactive with
polysaccharides and cell material. Block et al. (5) reported
that chlorine disappearance in distribution systems could be attributed
to the reactions of chlorine with three components: organic matter in the water column, biofilm, and the pipe surface.
Disinfectant consumption within the reactors was lower with chloramine
than with chlorine, despite the fact that the retention
time of the
chloraminated water was longer. Sixty-five percent
of the chlorine was
consumed within the control reactor at site
NJ102a, while chloramine
residuals were reduced by only 20% at
site IN610. Both control and
test reactor effluents showed similar
disinfectant residuals,
suggesting that application of biological
filtration had little effect
on chloramine consumption patterns
(Fig.
3). It has been reported that
chloramines are a more stable
disinfectant because they are not
consumed by the polysaccharidic
matrix around biofilm cells and are
less reactive with corrosion
products (
13). They can
penetrate biofilm more effectively and
react specifically with DNA,
tryptophan, and sulfur-containing
amino acids (
13).
HPC levels entering the system.
Bacterial counts in the
biological filter effluent were very high (approximately
105 CFU/ml). Such high plate counts are not uncommon in the
effluents of biological filters (22). High levels of
bacteria at the inlet of the distribution system could be of concern.
Block et al. (6) reported a linear relationship between the
concentration of suspended bacteria in the finished water and the
concentration of attached biofilm cells. Those authors concluded that
limiting the input of the bacteria entering the distribution system
would improve the biological stability of the system. However, a more
recent study (22) showed that bulk water bacteria had a
minor impact on biofilms because bacteria in the water column were
different from those attached to the pipe surface. Biofilm tended to be dominated by gram-negative bacteria, while bacteria present in the
chlorinated water column were gram positive. This work suggested that
even if bacteria from the water column initially colonize the pipes,
the biofilm develops its own, unique ecosystem and represents a
constant source of inoculation of new pipes introduced into the
distribution system.
Bacterial water quality.
This study showed that biofilm
densities were related to the amount of biodegradable material entering
the system. Bacterial levels within model distribution systems could be
reduced by 0.5 to 1.0 log10 unit. Similarly, Servais et al.
(32) observed a relationship between biofilm bacteria and
the concentration of BDOC at the points of entry of several full-scale
distribution systems. Another study (30) compared the
bacterial water quality in two annular reactors supplied with water
containing different levels of organic matter. The first reactor was
fed with ozonated water, while the second one was supplied with
biologically filtered water. Lowering nutrient levels with biological
filtration led to lower biofilm densities. Biofilm counts were
106 to 107 CFU/cm2 for the ozonated
water reactor, compared to 105 to 106
CFU/cm2 for the reactor fed with biologically filtered
water (30).
However, the effect of nutrient reduction on biofilm concentrations was
not immediate. It required several months (approximately
6 months) of
biological filtration before there was an observable
impact on
bacterial water quality. In two other studies (
28,
33),
bacterial water quality changes were monitored after implementation
of
nanofiltration in a pilot or full-scale system that was initially
supplied with surface water treated with ozonation and biological
filtration. For the cast-iron pipe loop pilot study (
33),
BDOC
levels decreased from 0.25 mg/liter in the ozonated and filtered
water to <0.1 mg/liter after nanofiltration. The authors did not
observe drastic changes in the biofilm or suspended bacterial
levels
after the treatment change. At the beginning of the study,
total and
cultivatable counts were 4.9 × 10
6
cells/cm
2 and 4 × 10
5 CFU/cm
2
for the biofilm and 2.6 × 10
5 cells/ml and
10
3 CFU/ml in the bulk water. Bacterial concentrations
decreased
slightly after 6 weeks of supplying nanofiltered water
(suspended
bacteria, 1.4 × 10
5 cells/ml and 259 CFU/ml; biofilm, 2.3 × 10
6 cells/cm
2 and
1.2 × 10
5 CFU/cm
2). After 1 year of
supplying nanofiltered water, biofilm levels
were 1.9 × 10
5 CFU/cm
2 and suspended bacterial levels were
50 CFU/ml. Similarly, bacterial
water quality changes were not obvious
when the same treatment
conversion was performed in a full-scale
distribution system supplying
a Paris, France, suburban community
(
28). Biofilm densities
were low before the treatment change
due to the high chlorine
levels (0.85 mg/liter) used to combat
microbial growth caused
by the high BDOC levels (0.75 mg/liter).
Following application
of nanofiltration, biofilm levels remained low,
but a low disinfectant
residual could be used because of the improved
biostability (low
BDOC level) of the
water.
Field experience shows that a variety of parameters influence biofilm
growth. Factors such as water temperature, disinfectant
type and
residual, selection of the pipe material, corrosion control,
and
hydraulic conditions may be more influential than the levels
of organic
matter for regulating the biological activity of the
biofilm. In this
study, these (and other) parameters probably
influenced the microbial
water quality data as much as the tested
variable (e.g., biological
filtration). Lower nutrient levels
in the system did not affect the
concentration of suspended bacteria
in the water column. This
observation may be related to the design
of the pilot system. Shear
forces exerted by the inner cylinder
would seem to be the factor most
influential on suspended cell
concentrations. However, concentrations
of suspended bacteria
decreased simultaneously in the control and test
pipes at site
IN610 when temperatures started to decrease in the fall
(Fig.
5). Although bacterial regrowth occurred all year long in the
model distribution system, it was amplified at elevated temperatures.
Many investigators have shown increased bacterial problems and
coliform
occurrences when temperatures were more than 15°C (
10,
25,
36).
Biofilm reduction was more pronounced at site NJ102a, which used free
chlorine, probably because lower BOM levels increased
the stability of
the disinfectant (greater reductions in biofilm
densities would be
expected with simultaneous lowering of nutrient
levels and maintaining
of higher disinfectant residuals within
the
system).
Pipe characteristics and conditions are also a determining factor in
bacterial regrowth. The use of a cast-iron pipe for the
outer portion
of the reactor may have significantly influenced
the density of fixed
biomass and changes in bacterial water quality.
Trends might have been
different for polyvinyl chloride pipes
in absence of corrosion
products. Iron pipe surfaces have been
shown to stimulate bacterial
growth (
8,
18,
21,
22).
Corrosion, pitting, and
tuberculation are fundamental to the presence
of biofilms, their
metabolic activity, and the release of bacteria
into the water column
(
22). Pipe conditions also indirectly
affect biofilm
bacteria by affecting disinfection efficiency,
especially with free
chlorine. There is a relationship between
the corrosion of the iron
surface and the protection of biofilm
bacteria from chlorine
disinfection (
21,
22). Rompre et al.
(
30)
reported that free chlorine produced a rapid decrease in
biofilm
density (>1 log CFU/cm
2) in polycarbonate annular
reactors, while the chlorine did not
affect biofilm in gray-iron
reactors. Moreover, organic matter
tends to adsorb to corrosion
products on iron pipe surfaces, and
the elevated concentration of
organic molecules can stimulate
bacterial
regrowth.
In conclusion, the decrease in nutrient concentrations following
implementation of biological filtration resulted in lower
biofilm
densities over a period of several months (0.5- to 1-log-unit
reduction). Steps to reduce bacterial nutrient levels should be
implemented for systems with high AOC and BDOC levels. In addition
to
nutrient levels, other parameters, including temperature, disinfection,
and the condition and composition of the pipe material, control
the
development of biofilm bacteria. Therefore, it is difficult
to predict
the effects of a treatment change at a specific site,
which depend on
the weights of the different factors regulating
bacterial growth.
Biofilm problems could be limited by simultaneously
addressing the
three following issues: nutrient levels, corrosion,
and disinfection.
High removal of organic matter during water
treatment and effective
corrosion control during distribution
would improve disinfection and
limit bacterial regrowth within
the distribution
system.
| |
ACKNOWLEDGMENTS |
We thank Jeff Robinson, Larry Wood, and Mary Anderson
(Indiana-American Water Company); Kevin Dixon, Carol Storms, and Judith Lorimer (New Jersey-American Water Company); Michele Prevost (Ecole Polytechnique de Montreal, Montreal, Canada); Calvin Abernathy (Montana
State University); and Melinda Friedman (Economic and Economic
Services, Bellevue, Wash.) for their assistance.
This project was funded by the National Water Research Institute and
the American Water Works Service Company, Inc. (Voorhees, N.J.).
| |
FOOTNOTES |
*
Corresponding author. Mailing address: Indiana-American
Water Company, Inc., 1420 S. Burlington Dr., P.O. Box 1152, Muncie, IN
47308-1152. Phone: (765) 741-1274. Fax: (765) 741-1258. E-mail: cvolk{at}amwater.com.
| |
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Abstract
Introduction
