APPLIED AND ENVIRONMENTAL MICROBIOLOGY, Mar. 1981, p. 646-651
Effects of Activated Carbon and Bacteriostatic Filters on
Microbiological Quality of Drinking Water
R. S. TOBIN,`* D. K. SMITH,2 AND J. A. LINDSAY2
Health Protection Branch, Health and Welfare Canada, Ottawa, Ontario KIA OL2,' and Ontario Research
Foundation, Sheridan Park Research Community, Mississauga, Ontraio L5KIB3,2Canada
Three activated carbon filters for point-of-use water treatment were tested in
laboratory and field studies for chemical removal and microbiological effects on
water. All removed free available chlorine in municipally treated water to below
the limit of detection, but removed only about 50 to 70% of the total available
chlorine and 4 to 33% of the total organic carbon. Standard plate count bacteria
in the effluent increased steadily with time for 3 weeks and remained elevated
over the 8-week period ofthe study. Total coliform bacteria were found to persist
and proliferate on the filters for several days after transient contamination ofthe
influent water. Silver-containing activated carbon filters suppressed total coliform
but not total bacterial growth. Pseudomonas aeruginosa was recovered from the
effluents of all filters at some time during the tests.
Increased awareness among the public of the
health implications of drinking water quality
and the continuing demand for aesthetically
pleasing water have resulted in a rapid prolifer-
ation in the types and numbers of point-of-use
water treatment devices sold (24). A major pro-
portion of all such devices are activated carbon
filters, including those releasing silver to act as
a bacteriostat. Some of these are claimed to
remove taste and odor, sediment, chlorine, tri-
halamethanes, carcinogens, pesticides, and a
number of other perceived problem substances.
Wallis and co-workers (27) were among the first
to demonstrate the tremendous proliferation of
bacteria in such devices, which they considered
to be a potential health hazard. In a later study,
Fiore and Babineau (11) also showed high bac-
terial counts in the effluent after periods of
stagnation, butmaintained that the proliferation
observed was not significantly greater than that
observed in untreated tap water after the same
period. Taylor et al. (24) clearly demonstrated
plate counts from activated carbon filters to be
greater than those from corresponding influent
water, but levels were lower than some previ-
The present study investigates the bacterial
growth on four types of activated carbon filters,
including a silver-releasing bacteriostatic filter.
The growth of total coliform bacteria and of
bacteria of potential health significance on the
filters is shown.
(This paper was presented in part at the An-
nual Meeting of the American Society for Mi-
crobiology, May 1980, in Miami Beach, Fla.).
MATERLALS AND METHODS
Filters. Filter A is a compact in-line filter that is
sold principally for the recreational vehicle, boat, and
vacation home market. The cartridge consists of a
carbon core sandwiched between thin outer and inner
layers of cellulose. It is designed to remove taste, odor,
andsediment. FilterB is a slightly larger unit, intended
for in-line attachment to cold water lines in homes.
The cartridge consists ofa resin-bonded cellulose fiber
filter section at the outlet, an activated carbon core,
and another resin-bonded cellulose network at the
outlet. It is designed to remove objectionable taste and
odor, as well as dirt, rust, and sand. Unit C is a small
unit that attaches to a kitchen faucet. It was tested
with two types of cartridge that are currently availa-
ble. One contained activated carbon, and the other
contained activated carbon plus silver. The silver re-
leased from the latter did not exceed 50,ug/literin the
effluent. Unit C is claimed to reduce chlorine, algae,
suspended particles greater than 8 u, and organic
chemicals such as detergents, pesticides, chloroform,
and polychlorinated biphenyls.
Testing of devices. Two of each of the three
devices were tested in the laboratory on a manifold
system described in detail elsewhere (9). Timers were
adjusted to give an "on" time of 30 s every 0.5 h, and
the flow rates on filters A and B were adjusted by
throttle valves to 7.6 liters/min (high flow rate) and
4.25 liters/min (low flow rate). Control counts were
taken from identically valved lines which were set at
the same flow rates and timing as the filters. The
manifold was operated for 16 h per day and left inac-
tive for 8 h per day. Total volumes processed were
approximately 123 liters per day at the fast flow rate
and 68 liters per day at the slow flow rate. Filter C
units were attached to faucets on the manifold, and
both were adjusted to a flow rate of 475 ml/min to
give a total of 7.6 liters per day. The portion of the
ACTIVATED CARBON FILTERS
manifold to which filter C devices were attached had
a bleed line which was open during the "on" cycle to
ensure that this portion of the manifold did not be-
come stagnant under the low flow conditions. This
bleed line would also simulate the use of the water
bypass function of filter C for nonconsumptive pur-
poses in home use.
When a coliform challenge was used, a settled sew-
age sample was collected and diluted in a tank to
provide a challenge of 70 total coliform bacteria per
100 ml. When a challenge of bacteria was required to
determine the bactericidal efficacy ofsilver-containing
units, pigmented bacteria derived from the effluents
of colonized activated carbon filters were grown in
nutrient broth for 24 h at 280C. The bacteria were
diluted to about 1,780 colonies per ml in tap water left
to stand for 24 h in open tanks (no detectable free
available chlorine residual) and used to dose the water
to the test filters.
Another filter C unit was placed in a home supplied
by the same raw water supply but a different treat-
ment plant from that used in the laboratory tests. It
was used about two to five times a day with volumes
of 0.3 to 3 liters withdrawn at a time, for an average
daily use of 6 liters.
Sampling. Volumes of 200 ml (filters A and B) or
100 ml (filter C) were collected in sterile flasks con-
taining 0.1 ml of 10% (wt/vol) sterile sodium thiosul-
fate to neutralize any residual chlorine. When the
silver-carbon flter was used, samples were collected
in flasks containing 1 ml ofneutralizer solution (6) (5%
sodium thioglycolate plus 7.3% sodium thiosulfate) per
100 ml of sample. At least three sets of samples were
taken: the initial flush after overnight stagnation, after
30s offlow through the units, and after4h ofoperation
of the daily cycle. Influent samples were taken at the
Plates for standard plate counts (1) were incubated at
350C for 96 h except where noted. Total coliform
counts were performed on 100-ml samples using the
membrane filtration technique with incubation on m-
Endo LES agar (BBL) at 370C for 48 h. Pseudomonas
TABLE 1. Removal ofavailable chlorine and total
organic carbon by activated carbonfilters
Total organic carbon
aFAC, Free available chlorine.
bTAC, Total available chlorine.
CND, Not detectable (<0.05 mg/liter).
dParentheses indicate percent total organic carbon
aeruginosa was isolated by the membrane filtration
technique with incubation at 350C on Pseudomonas
isolation agar (Difco Laboratories), and identification
was performed by means of the Minitek system (BBL
Microbiology Systems) and by the scheme described
by Shayegani et al. (23).
Chemical analyses. Free available chlorine and
total available chlorine were measured by the N,N-
diethyl-p-phenylenediamine method (1) and a Hellige
color comparator disk. Total organic carbon analyses
were performed with an automatic TOC analyzer
(Beckman Instrument Co.).
The ability ofthe carbon filters to remove the
residual chlorine and total organic carbon from
municipally treated tap water was measured
(Table 1). All filters reduced the free available
chlorine content from 0.20 mg/liter to below the
limit of detection (0.05 mg/liter), although 'k to
½ ofthe total available chlorine remained in the
effluent. In unitsA and B, the units run at slower
flow rates were more effective in removing total
available chlorine than those run at the faster
flow rates. Total organic carbon removal ranged
between 4 and 33% in a typical experiment (Ta-
ble 1) and was not consistently related to the
flow rate through the units. Filter B was clearly
superior to the others in total organic carbon
removal, although at least two-thirds ofthe ma-
terial remained in the effluent. Total organic
carbon removal did not vary appreciably during
the period of testing these devices.
The microbiological quality ofthe water from
the filters was closely monitored during the 55-
day test. Standard plate counts (3500) were read
at 96 h to give optimum colony size and numbers
(12). Up to day 10 of the test, filters A and B
removed bacteria from the water to almost un-
detectable levels (Fig. 1). Filter C (not shown)
began to show appreciable colonization by day
6. By day 25, the colonization of the filters
appeared to have stabilized in both the initial
flush (0 s) after overnight stagnation and after 4
h of the regular daily cycle. The geometric
means of days 25 to 55 (Table 2) indicate a high
degree of bacterial contamination of effluent
water, particularly in the first flush after over-
night stagnation. Whereas counts decreased
markedly by 30 s and were even lower after 4 h
of cycling, they were, in all cases, considerably
higher than the influent values.
Two filter C devices were installed in the
kitchen in homes where they were used to pro-
vide water for household use. One unit, operat-
ing on the same raw water source but a different
treatment plant from that supplying the labo-
ratory, gave results over a 55-day period that
VOL. 41, 1981
TOBIN, SMITH, AND LINDSAY
FIG. 1. Bacterial counts in the effluents ofthe ac-
tivated carbon filters duringthe55-daytest.Symbols:
(x) influent water at 4 h; (0) filter A, first flush; (0)
filter A, 4 h; (A) filter B, first flush; (A) filter B, 4 h.
TABLE 2. Bacterial numbers in filter influents and
Standard plate counta (colo
mesper ml) at tuxne:
a Geometric means from days 25 to 55. Plates were
incubated for 48 h at 35°C.
were comparable to those obtained in the labo-
ratory study. Some activated carbon filters are
designed to release silver ions into the water to
prevent excessive bacterial growth. We were
able to purchase replacement cartridges for filter
C with and without silver. The plate counts from
new filters before a bacterial challenge were
relatively low (Table 3). Both devices were chal-
lenged with 2.4 liters ofwater containing, per ml,
1,780 colonies of bacteria derived from growth
on another cartridge; then they were washed
with normal tap water and allowed to stand
overnight. Both the carbon andthe carbon-silver
units had high bacterial counts that remained
much higher than those in the influent water
throughout the day. These cycles were repeated
with similar results.
On day 41 of the study, transient low-level
contamination by total coliform organisms was
APPL. ENVIRON. MICROBIOL.
found in the supply to the filters (Table 4). The
level ofcontamination in the first flush from the
filterwas about 23 times greater than that meas-
ured in the influent. Furthermore, low levels of
coliforms were measured in the effluents for up
to 13 days after this incident even though they
were undetectable in the influent water. During
the laboratory portion of this study, low levels
of total coliforms were detected in the water
from filter A (low flow) on four occasions and in
that from the high-flow unit on one occasion.
The filter B (high flow) also yielded 5 coliforms
per 100 ml on one occasion. None of these were
To verify the ability of coliform organisms to
grow on carbon filters, tap water contaminated
with settled sewage to a level of 70 colonies per
100 ml was passed through the filter C carbon
and carbon-silver cartridges (Table 5). When
samples were taken from the carbon-silver unit
and neutralized immediately, some coliforms
could still be enumerated, whereas contact times
up to 30 min resulted in undetectable levels of
coliforms. After 2.4 liters of contaminated water
was passed at 400 ml/min, a further 2.4 liters of
normal water was passed through the devices,
TABLE 3. Bacterial growth on activated carbon
and bacteriostatic units after a challenge with a
Standard plate count (colonies
per ml) on unit:
Time of sample
bSilver-releasing bacteriostatic unit.
After one or two cycles of a challenge of 1,780
colonies per ml.
TABLE 4. Total coliform counts on filter C installed
in aprivate home
Total coliform count per 100 ml on daya:
a Day of test.
cSample was taken after 2 h of regular use.
VOL. 41, 1981
TABLE 5. Total coliform counts on activated
carbon and bacteriostatic filters after a coliform
Total coliform count per
aSampled at a flow rate of 400 ml/min.
bFirst flush sample.
cSamples were neutralized immediately or after
contact times of 5, 10, or 30 min after collection.
and they were allowed to stagnate overnight.
The first samples taken the next morning
yielded high coliform counts in the activated
carbon cartridge effluent, but none in the car-
In our previous studies of activated carbon
filters, P. aeruginosa and Flavobacterium spe-
cies were routinely isolated in the effluent water
and were undetectable in the influent. In the
present study, identification of some of the pig-
mented colonies picked from the standard plate
count agar and identification of presumptive
positive colonies on selective media demon-
strated that P. aeruginosa could be isolated, at
various times, from each ofthe activated carbon
Results presented here and those published
by others (4, 11, 13, 24, 27) indicate that acti-
vated carbon filters are susceptible to varying
degrees of colonization. The severity of this col-
onization would be expected to differ in various
studies, depending on such factors as levels and
types of organic material and residual chlorine
in the water, water temperature and pH, and the
quantity and variety of bacteria in the influent
water. Although flow rate is apparently not a
major factor in determining levels of bacteria in
the effluent from filters, the length of time that
the filter has been in service can be important.
For a certain period of time the devices can
produce almost sterile water until buildup of
organic material and concentration of bacteria
combine to foster growth and shedding of bac-
teria into the water.
There are currently no mandatory limits for
ACTIVATED CARBON FILTERS
the standard plate counts in drinking water reg-
ulations and guidelines in the United States or
Canada, although both countries suggest that
the general bacterial population should be lim-
ited to 500 organisms per ml in distribution
water (14, 25). Elevated levels of bacteria have
been shown to reduce the sensitivity ofthe total
coliform test for drinking water analysis (12).
When the plate count exceeded about 1,000/ml,
the total coliforn
results became unreliable.
When silver ions were present, with silver-car-
bon filters, they exertedaselective bacteriostatic
effect on the coliform bacteria, but had little
effect on the total bacterial population. In this
instance, total coliforms, the primary indicators
of finished drinking water quality (14, 25), were
inactivated, whereas a major segment of the
water-bome bacteria was unaffected.
The growth of coliform organisms on acti-
vated carbon filters has been shown in the field
and laboratory studies. Although two otherstud-
ies (11, 19) were unable to establish total coli-
form bacteria growth on such devices, at least
one other report (20) documents continued col-
iform isolation after treatment of a temporarily
contaminated municipal water supply. There
may be many reasons for this apparent discrep-
ancy in results, including factors unique to the
water supplies studied. It should be noted that
coliform levels should be considered miniimum
levels in view of the concomitant high plate
The demonstration of growth of P. aerugi-
nosa and Flavobacterium is indicative of the
potential problem associated with the use of
these devices. Both organisms are secondary
pathogenic organisms, capable of growth in low
substrate conditions and in the presence of high
chlorine levels (7, 10, 22). Some outbreaks of
gastrointestinal illnesses have been attributed to
dinking water contaminated with P. aerugi-
nosa (17). The organism is also the cause of
persistent ear and urinary infections (16, 28).
Seyfried and Fraser (22) determined that P.
aeruginosa persisted in swimming pool water
containing 0.4 mg of free available chlorine per
liter, and they also demonstrated ear infection
in 4 of 24 people swimming in a pool containing
less than 80 organisms per 100 ml (21). In hos-
pitals, this organism is particularly dangerous
for those with a compromised immune system,
for burn cases, and for those undergoing surgery
(2, 29). It has caused epidemics of diarrhea in a
nursery in which several children died (18). Fla-
vobacterium is also a pathogen of some impor-
tance, having been identified as the causative
agent in an epidemic of meningitis in newborns
(5) and in infections in surgical patients as well
(15). One hospital study (8) showed that the
TOBIN, SMITH, AND LINDSAY
upper airways of 195 patients of an intensive
care unit became colonized by Flavobacterium
that was ultimately shown to originate in the
municipal water supply.
It has been established that there is a mini-
mum infection dose for each pathogen below
which very few persons will become ill. In most
cases, sporadic or transient contamination of
drinking water will result in concentrations be-
low this dose. Thus, it is the possibility of an
amplification effect, by growth of these orga-
nisms on filters, that is a cause for concern (20).
Some persons such as the aged and very young
infants may also have significantly lower resist-
ance to the pathogens than the average healthy
adult. The many other uses of drinking water
such as preparing baby foods and formula, wash-
ing wounds, bathing and showering, and use in
mist humidifiers present different opportunities
for infection to occur.
Evidence presented here indicates the adverse
effects of the use of activated carbon fiters on
the microbiological quality of water. In addition
to the microbiological aspects, however, studies
of chemical removal have indicated that many
filters give unsatisfactory removal of chlorine,
trihalomethanes, and total organic carbon (4, 24,
26). The limited chemical removal efficiency
deteriorates further with continued use of the
filter (24), with no readily available means for
the consumer to determine when the capacity of
the filters is exceeded. When activated carbon is
used in the municipal water treatment plant,
organic removal can be monitored regularly, and
the problem posed by bacterial growth on media
(3) can be eliminated by subsequent filtration
and disinfection steps. Furthermore, federal,
state, and provincial legislation outline the re-
quirements that must be met by treated munic-
ipal water to produce a healthy, aesthetically
pleasing water. Taylor et al. (24) stated that
water treatment in municipal supplies should
remain in the hands of professionals 'and it
should not be necessary for the consumer to be
actively involved in further water treatment. In
view of the results presented here, the use of
point-of-use activated carbon filters, without
posttreatment disinfection, could be considered
a potential health hazard.
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VOL. 41, 1981