United States
Environmental Protection
Agency
* ' /,
Municipal Environmental Research v |% eS«f
Laboratory *
Cincinnati OH 45268 *
Research and Development
EPA-600/S2-84-011 Mar. 1984
Project Summary
Stability and Effectiveness of
Chlorine Disinfectants in Water
Distribution Systems
Vincent P. Olivieri, Michael C. Snead, Cornelius W. Kruse', and Kazuyoshi
Kawata
A test system for water distribution
was used to evaluate the stability and
effectiveness of three residual disinfec-
tants—free chlorine, combined chlorine.
and chlorine dioxide—when challenged
with a sewage contaminant. The test
distribution system consisted of the
street main and internal plumbing for
two barracks at Fort George G. Meade
in Fort Meade, Maryland. To the
existing pipe network, 152 m (500 ft) of
13-mm (0.5-in.) copper pipe was added
for sampling and 60 m (200 ft) of 2.54-
cm (1.0-in.) plastic pipe was added for
circulation. The levels of residual
disinfectants tested were 0.2 mg/Land
1.0 mg/L as available chlorine.
In the absence of a disinfectant
residual, microorganisms from the
sewage contaminant were consistently
recovered at high levels. The presence
of any disinfectant residual reduced the
microorganism level and frequency of
occurrence at the consumer's tap. Free
chlorine was the most effective residual
disinfectant and may serve as a marker
or flag in the distribution network. Free
chlorine and chlorine dioxide were the
least stable in the pipe network. The
loss of disinfectant in the pipe network
followed first-order kinetics. The half
life determined in static tests for free
chlorine, chlorine dioxide, and combined
chlorine was 140, 93, and 1,680 min,
respectively.
This Project Summary was developed
by EPA's Municipal Environmental
Research Laboratory, Cincinnati, OH,
to announce key findings of the research
project that is fully documented in a
separate report of the same title (see
Project Report ordering information at
back).
Introduction
The objectives of this study were to
evaluate the stability and effectiveness of
residual disinfectants in a test water
distribution system when challenged by a
sewage contaminant. The level of patho-
genic microoganisms that reach the
consumer's tap during cross connection
and back-siphonage episodes is a function
of dilution of the contaminating material,
natural die away, and inactivation by the
residual disinfectant. The dilution of the
contaminating material depends heavily
on the configuration and characteristics
of the pipe network and the flow of water
in the local area where the integrity of
the conduits was breeched. Because of
the infinite complexity and variety of
plumbing and flows found in water
distribution systems, the degree of
dilution cannot be predicted, even in
well-defined systems.
One aspect related to flow as a
mechanism for removing contamination
from a water distribution system is
washout. With continued use and consump-
ton of water, the contaminant slug will be
purged from the system in a relatively
short time. But dilution and washout
cannot be depended on to provide a safe
water or a water of good q ual ity at the tap.
The enteric microorganisms responsible
for transmitting diseases by water tend to
die away when introduced into the
aquatic environment, and proliferation of
these microorganisms has generally not
been observed. But the rates of die away
are relatively slow, and sufficient levels of
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microorganisms can be expected to
survive for the time periods found in most
water distribution systems.
Experimental Protocol
Test Distribution System
The test distribution system consisted
of several hundred feet of 10-cm (4-in.)
pipe and the internal plumbing of two
army barracks (Figure 1). Each building
contained four apartments with the
appropriate plumbing for bath and
kitchen facilities. The existing pipe
network in each building consisted of
galvanized pipe ranging in diameter from
5 to 1.3 cm (2 to 0.5 in.) for fixtures. The
test system consisted of eight loops
derived from the bathroom supplies to
each apartment and was plumbed to the
sample sink in the laboratory in building
T-152 with 1.3-cm (0.5-in.) copper pipe.
The total length of new plumbing for the
sampling lines added approximately 152
m (500 ft) to the distribution system. The
end of the pipe network in each building
was connected by 2.5-cm (1-in.) plastic
pipe to complete a loop and favor
circulation in the test system. The use of
cast iron, galvanized copper, and plastic
pipe simulated the mixed nature of the
materials used in real-world distribution
systems. The test system was isolated
from the Fort Meade water distribution
system by a back-flow preventer and an
air gap at the reservoir before the test
distribution system and the simulated
cross-connection. Pressure was main-
tained in the test distribution system with
a pump and hydropneumatic tank.
Test Protocol
The 1.5 m3 reservoir tank (4000-gal) was
filled with water drawn from the Fort
Meade water distribution system. Disin-
fectant residual in the tank was adjusted
on a batch basis by the addition of sodium
sulfate for dechlorination, chlorine,
chlorine plus ammonia, or chlorine
dioxide. The pH of the water was not
adjusted. Raw sewage was seeded with
f2 bacterial virus to a level of 106 plaque
forming units (PFU)/ml, and the tracer
dye (rhodamine, Tinopal RBS, * orTinopal
CBS) was added. An aliquot was removed
to determine the microbiological para-
meters and actual dye concentration. To
contaminate the distribution system, the
sewage slug was forced into a tee at the
head of the system by air pressure.
The studies of test distribution system
were divided into four sections:
1. Single tap, short-term - single home
2. Multi-tap, short-term - neighborhood
3. Multi-tap, long-term - small com-
munity, constant flow
4. Long-term, variable-flow - small
community, constant flow
In all cases, the reservoir water was
adjusted to greater than 30 mg/L free
chlorine between each run, and this
"Mention of trade names or commercial products does
not constitute endorsement or recommendation for
use.
VPi R
L
Existing Pipe
New Pipe
(Sample line)
Sample
\
rrh
, p j]
i i i
i i
. , |
1 i I '
~i— -i i u _,
ft i rl
1 1
1 L-T
—J
Figure 1. Schematic of water distribution system in Fort Meade buildings 152 and 162.
2
water was flushed throughout the system
for least 24 hr to clean and disinfect the
system thoroughly.
Results
The inactivation of microorganisms
contained in contaminating material
in the water distribution system depended
heavily on the disinfectant residual,
contact time, and temperature in the test
distribution system. Residual free chlorine,
combined chlorine, and chlorine dioxide
were compared as to their efficiencies in
inactivating natural populations of coli-
forms from sewage at various contact
times. Natural die away over the 240-min
test period was insignificant, but the
presence of any disinfectant residual
dramatically reduced the level and
frequency of coliforms at the tap. The
residual disinfectant inactivated natural
coliforms in the sewage challenge. Free
chlorine appeared to be the most effective
residual disinfectant for short contact
times and consistently yielded the lowest
level and frequency of coliform recovery.
The level of free chlorine in the pipe
network has a relatively short half life in
the absence of contamination and was
consumed by the added sewage.
The levels of free chlorine appeared to
flag the sewage slug. For short contact
times, combined chlorine residuals
decreased the density of the coliforms at
the tap, but the frequency of coliform
recovery (80%) was nearly as high as that
observed in the absence of residual
disinfectant. The levels of combined
chlorine residual were high throughout
the system. Coliforms were recovered at
combined chlorine residuals of 0.7 to 1.0
mg/L. Little suggestion of contamination
was indicated by the level of combined
chlorine. Chlorine dioxide residuals for
short contact times yielded low levels of
coliforms at a low frequency of recovery
and was more effective than combined
chlorine but not as active as free chlorine.
A similar comparison of residual
disinfectants was made for seeded f2
virus. Except for combined chlorine, the
disinfectant residuals showed activity
against viruses. Chlorine dioxide residuals
consistently yielded f2-free water at the
taps.
The levels of microorganisms observed
at the tap are presented as the log of the
coliform and f2 survival fraction in Table
1. The No was corrected for dilution by the
dye concentration. The mean log survival
of coliform and f2 virus observed was <
-3.8 and < -3.4, respectively, for free
chlorine. Chlorine dioxide yielded a mean
log survival of < -2.8 for coliforms and <
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Table 1. Log Coliform and f2 Virus Survival Fraction at the Tap in the Test Distribution System
During Multi-tap, Short Term Trials*
Disinfectant
Disinfectant Residual Coliform, log N/N0
Initial Tap
mg/L mg/L Mean a Range
12 Virus, log /V//V0
Mean a Range
Free chlorine
Chlorine dioxide
Combined chlorine
1.00-1.20 0.49-0.95 <-3.8 0.5 -2.6 to < 4.5 <-3.4 1.3 -1.0 to -5.1
0.85-0.95 trace-0.94 <-2.8 0.8 -1.4to<-4.7 <-4.4 0.5 -3.7 to<-5.3
0.81-1.08 0.49-1.08 <-3.2 1.1 -1.6 to <-5.3 -0.9-0.6-0.04 to -2.6
^Temperature, 14° to 77°C; pH, 7.3 to 7.7; flow, 2 gal/min.
-4.4 for f2 virus. For combined chlorine,
the mean log survival for coliforms was <
-3.2, but only -0.9 for f2 virus. Free
chlorine and chlorine dioxide were
effective against coliforms and test virus
during short-term trials (240 min).
Though combined chlorine residuals
were an effective bactericide, they were a
relatively poor viricide. The f2 virus was
seeded at high densities (106 PFU/ml) in
the added sewage to permit recovery and
evaluation of virus inactivation. Densities
of natural populations of human enteric
viruses in sewage are generally about 1
PFU/ml, and for the sewage in this study,
they were 0.01 to 0.1 PFU/ml. The data
for f2 exaggerate the virus survival. The
level of free chlorine was reduced in each
case when the virus were recovered at
relatively high densities. The difference
between seed virus titer and natural
human enteric virus also exaggerates the
decreased viricidal activity of combined
chlorine since f2 was more resistant to
combined chlorine than was human
enteric virus.
Clearly, the disinfectant residual
represents the primary barrier against
post-treatment contamination in a water
distribution system for short contact
times. The level of sewage used in these
trials was about 0.1 % of the test distribu-
tion system. At greater levels of contami-
nation, the residual disinfectants would
afford proportionately less protection.
The residual disinfectant will have little
effect on the levels of microorganisms
contained in a large intrusion of sewage
into the water distribution system. But
fortunately, high-level contamination
would offend the visual and olfactory
senses and flag the potable quality of the
water.
The extended contact time trials
emphasize several important factors in
the ability of the disinfectant to respond to
a challenge in the pipe network. Compari-
sons of coliform levels at the tap found
after long contact times (72 hr) were
similar to the short-term trials (240 min)
in that high coliform levels were consis-
tently recovered in the absence of a
disinfectant residual. The presence of any
free, combined, or chlorine dioxide
residual dramatically reduced the density
and frequency of coliform recovery at the
tap. Combined chlorine performed most
effectively against the coliforms, with
only 3 samples of 28 positive for coliforms
at low levels. Combined chlorine residuals
were effective bactericides given an
adequate contact time. Under the condi-
tions of this experiment, free chlorine
and chlorine dioxide were not as effective
as combined chlorine. Coliforms were
frequently recovered in the chlorine
dioxide trials and consistently recovered
with free chlorine. In the latter trial,
coliform levels were reduced markedly,
and most samples collected for the free
chlorine trial had no free chlorine
residual. The free chlorine was consumed
during extended contact in the distribution
system. Free chlorine was not as stable in
the pipes as combined chlorine. This fact
has been the bane of water utilities and
was responsible for the development of
the chlorammoniation process in the
1930's and its continued preference by a
segment of the water plant operators. A
similar situation existed with chlorine
dioxide.
The f2 virus levels at the tap were
compared for the multi-tap, long-term
trials. The bacterial virus f2 was recovered
at high densities at the tap in the test
system after 72 hr when no disinfectant
residual was present. As for coliforms in
the longer-term trials, combined chlorine
residuals were effective against f2. Free
chlorine and chlorine dioxide were less
effective. But the free chlorine and
chlorine dioxide were consumed in the
distribution system, and little or no
residual was observed. The coliform
level was compared with the f2 level for
long-term (72 hr) trials with an initial free
chlorine residual at 19°C and 10°C. At
10°C, the free chlorine was considerably
more stable. Though the level of free
chlorine decreased, only 7 of 28 samples
had no free chlorine residual. At 19°C,
free chlorine was absent in 21 of 28
samples and did not effectively represent
a free chlorine trial. When conditions
favored the stability of free chlorine in the
distribution system (decreased tempera-
ture), the residual functioned when
challenged.
And in the short-term trials, the disinfec-
tant residual was the primary barrier
against the sewage challenge. But during
long-term trials, the stability of the
disinfectant became an important factor.
When present, free chlorine was a
superior residual disinfectant. Similar
results were obtained for free chlorine at
10°C and for combined chlorine at 19°C.
As an alternative, combined chlorine (given
a sufficient contact time) was able to
provide water at the tap with low
coliforms and f2 virus levels.
Under variable flow conditions, levels
of bacteria and virus in the sewage
challenge were effectively reduced by
dilution, washout, and increased mass of
disinfectant. Few samples at the taps were
positive for coliforms and f2 virus. Data in
Figure 2 show standard plate count (SPC)
and disinfectant residual after contami-
nation with sewage for the test distribution
system operated under variable flow
conditions for extended periods of time.
Except for chlorine dioxide, residuals were
observed throughout the trials and
residuals at the tap were always greater
than 0.5 mg/L. The residual of the water
entering the distribution system was
approximately 1 mg/L in each case. Free
chlorine residuals consistently provided
water at the tap with an SPC of less than
10 colony-forming units (CFU)/ml.
The fate of naturally occuring viruses
was determined, and attempts were
made to distinguish between free and
solids-associated virus. In the presence
of disinfectant residuals, no enteric
viruses were recovered from either the
solids or the prefiltered fractions of the
distribution systems samples when the
contaminant contained detectable levels
of these viruses. Such was the case even
when combined chlorine served as the
disinfectant residual in the distribution
system and large numbers of enteric
virus were present in the sewage
contaminant. When compared with free
chlorine and chlorine dioxide, chloramines
are considered inferior viricides, but it is
possible that longer contact times allow
these relatively stable residuals to be
effective against viruses. Since no
surviving viruses were recovered from
the solids fraction in any of the test
distribution system samples, no conclu-
sive evidence on the protective effects of
virus association with particulate matter
could be obtained.
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§
i
O)
O
5. In the presence of 0.2 to 1.0 mg/L
residuals of free chlorine, chlora-
mines, and chlorine dioxide, no
surviving enteric viruses could be
concentrated from test distribution
system samples contaminated with
raw sewage containing detectable
levels of enteric viruses.
The full report was submitted in
fulfillment of Grant No. R806074 by the
Johns Hopkins University under the
sponsorship of the U.S. Environmental
Protection Agency.
4
40 80 120 160
Time. Hours
200
240
1.00
0.75
0.50
0.25
0
Free Cl.
\
k
4O 8O 12O 16O
Time. Hours
200
240
Figure 2. Mean SPC and halogen residuals for samples collected over the course of the
long-term trials.
Conclusions
1. The residual disinfectant in the water
distribution system represents the
primary barrier against transmission
of disease by post-treatment con-
tamination. The presence of any
disinfectant residual reduces the
level and frequency of occurence of
microorganisms at the consumer's
tap.
2. Free chlorine was the most effective
residual disinfectant and consis-
tently yielded the lowest level and
frequency of microorganisms.
3. Free chlorine can serve as a marker
for contamination. In a system
where free residual chlorine was
normally maintained, its absence is
evidence that chlorine-demanding
substances may have entered the
system. Few differences in combined
chlorine residuals were found, even
after the addition of sizable amounts
of contaminant. The detection of a
combined chlorine residual offers
little assurance of water potability.
4. Free chlorine and chlorine dioxide
were the least stable in the water
distribution system, and residuals
were difficult to maintain under low-
flow conditions.
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Vincent P. Ofivieri, MichaelC. Snead, Cornelius W. Kruse', and Kazuyoshi Kawata
are with The Johns Hopkins University. Baltimore, MD 21205.
Raymond Taylor was the EPA Project Officer (see below for present contact).
The complete report, entitled "Stability and Effectiveness of Chlorine Disinfectants
in Water Distribution Systems," (Order No. PB 84-140 201; Cost: $16.00,
subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
For further information contact Eugene Rice at:
Municipal Environmental Research Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Official Business
Penalty for Private Use $300
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