Water Qual. Res. J. Canada, 2006
•
Volume 41, No. 4, 375–382
Copyright © 2006, CAWQ
Assessing the Disinfecting Power
of Chlorite in Drinking Water
Françoise Bichai* and Benoit Barbeau
CRSNG Industrial Chair on Drinking Water, CGM Department, École Polytechnique de Montréal,
P.O. Box 6079, Downtown Station, Montréal, Québec H3C 3A7
The goal of this study was to review and confirm experimentally chlorite effectiveness as a drinking water disinfectant. The
steps in the experiment were a series of laboratory assays performed on three types of microorganisms, Bacillus subtilis
spores, MS2 coliphages, as well as heterotrophic (HPC) bacteria, to verify chlorite action in water. The tests showed that
chlorites have no disinfectant effect on B. subtilis spores (CT > 106 mg min/L), that they slightly decrease HPC bacterial
regrowth and, finally, that a dose of 1 and 10 mg ClO2–/L can inactivate almost 2 log and 4.5 log of MS2 phages, respectively, after 9 days of contact time. It would therefore appear that chlorites are not a good primary disinfectant, but do
exhibit a bacteriostatic effect.
Key words: chlorite, disinfection, drinking water, B. subtilis spores, MS2 phages, HPC
Introduction
doubt, the late Belgian researcher, W.J. Masschelein,
who published the results of a test to confirm this
hypothesis (1979). However, even though other articles
have touched on this issue (Masschelein et al. 1981;
Liyanage et al. 1997; McGuire et al. 1999; Radziminski
et al. 2002), no project in due form has seemed to be
entirely devoted specifically to demonstrating the disinfectant effect of chlorites in water. In studying various
disinfection methods, where each aspect and each consequence of disinfectant use are worthy of interest, validating given disinfecting properties of chlorites could provide a more complete picture of chlorine dioxide use as a
disinfectant. In fact, even though it is not recommended
to use chlorites as a primary disinfectant or to try to
maintain higher than necessary residual chlorite concentrations in the system, it would be of interest to demonstrate that, for example, the residual dose of chlorites
present in the system in the form of a disinfection byproduct of chlorine dioxide is able to limit the bacterial
regrowth in the system.
The objectives of this paper were to: (i) review the
available literature on the efficacy of chlorite as a disinfectant, (ii) explain the apparent divergent results in
terms of primary and secondary disinfection, and (iii)
perform inactivation experiments to confirm that the
role of chlorite as a disinfectant is essentially limited to
secondary disinfection.
In scientific literature, one can find divergent
hypotheses regarding the disinfectant properties of chlorites. To date, little information is available on this
topic. Researchers will agree with one another that chlorites are an effective oxidizing agent (Aieta and Berg
1986). Regarding disinfection, conclusions are much
more divergent. The studies available to date can be
Since its first application in drinking water in Niagara
Falls (U.S.A.) in 1949, chlorine dioxide has been the
object of growing interest for its use in disinfecting drinking water, due to its many advantages in terms of effectiveness relative to chlorination. In fact, it has been
shown that chlorine dioxide does not produce any THM
and very little TOX (total organo halides) (Werdehoff
and Singer 1987), in addition to effectively inactivating
spores (Liyanage et al. 1997). Nevertheless, its limitation
resides in the generation of two potentially toxic by-products, chlorites and chlorates, which are substances that
can induce methaemoglobin, among others (Prokopov et
al. 1997). Research has reported that chlorine dioxide,
chlorites and chlorates all cause haemolytic anaemia
when administered to mice and rats in drinking water,
the most serious effect being observed with chlorites
(Werdehoff and Singer 1987). This is the reason for strict
standards for maximum residual concentrations allowed
in treated water. The U.S. Environmental Protection
Agency (U.S. EPA) used to recommend that the residual
concentration of the three products combined (ClO2,
ClO2- and ClO3-) not exceed 1.0 mg/L as Cl2 in distribution system water (U.S. EPA 1999). The U.S. EPA regulations now in effect call for a maximum chlorine dioxide
residual of 0.8 mg/L, whereas chlorite concentration in
the supply system must be lower than 1.0 mg/L.
One hypothesis often suggested is that, in spite of
chlorite toxicity, chlorite offers some disinfecting properties that have been put forth a few times by various
researchers. The greatest proponent of this idea is, no
* Corresponding author; [email protected]
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Bichai and Barbeau
divided into two groups: chlorite effectiveness as a primary disinfectant and as a secondary disinfectant.
Chlorite Effectiveness as a Primary Disinfectant
Effect on E. coli and Enterococci bacteria. It would
seem that Masschelein (1979) was the precursor to
assigning bactericidal properties to chlorites. In fact,
when he speaks of chlorine dioxide use as a bactericidal
agent in drinking water, he puts forth the hypothesis that
part of this effectiveness can be attributed to chlorites
resulting from the decomposition of chlorine dioxide in
water. Masschelein (1979) presents the results of a test
carried out on Escherichia coli and Enterococci. He concludes that applied dosages of 0.2 to 1 mg/L of sodium
chlorite significantly affect coliforms and fecal streptococci survival rates (Masschelein 1979). According to his
experiments, it can be observed that, for example, a 2 log
reduction of E. coli can be obtained with a 0.5-mg/L dose
of sodium chlorite after 4 to 5 d. However, Masschelein
states that the chlorite ion’s disinfectant capacity is
minor. He rationalizes that the post-disinfection of drinking water is caused by chlorine dioxide based on the
sequential action of chlorine dioxide, which is strongly
bactericidal, and of chlorites, which are bacteriostatic
and slightly bactericidal. He reports that, according to
Pichinoty et al. (1969), a ClO2– concentration of about
130 mg/L exerts a strong bactericidal power. Moreover,
it is interesting to note that Masschelein attributes the
toxic effect of chlorates to weak concentrations of chlorites produced from chlorates. He assumes, as well, that
chlorite ion can inactivate an enzyme that carries hydrogen and that is involved in nitrate reduction. Other
researchers have adopted this hypothesis, and this shall
be discussed further in the text.
Chlorite disinfectant effect on parasites and bacterial
spores. As greater interest in chlorine dioxide disinfection
arose, many researchers wanted to verify if the disinfectant effect was due to the action of certain chlorine dioxide by-products. In general, studies lead to the conclusion
that it was chlorine dioxide itself, and not its by-products,
such as chlorites and chlorates, that caused the disinfection observed. Such studies were carried out by various
researchers on f2 bacterial viruses, as well as on Bacillus
subtilis spores and Cryptosporidium parvum oocysts
(Radziminski et al. 2002). On this matter, Liyanage et al.
(1997) completed a thorough evaluation for Cryptosporidium, which suggested that chlorite ions, chlorate
and chloride have no disinfectant effect on C. parvum,
and neither does combining chlorites or chlorates with
sodium thiosulfate. Doses of 4 mg/L of various compounds had been applied for contact times up to 12 h and
no inactivation was observed, despite having obtained
more than 3 log of inactivation in less than 2 h with
3.3 mg/L of chlorine dioxide under the same conditions.
Liyanage’s conclusion does not necessarily contradict
Masschelein’s, since Cryptosporidium oocysts are
microorganisms that are much more resistant than E. coli
or Enterococci. Thus, the fact that chlorites have no disinfectant effect on oocysts does not necessarily eliminate the
possibility of a disinfectant effect on bacteria in general.
Oxidizing properties of chlorites in an acidic environment. Prokopov et al. (1997) presented a study on properties of chlorites. They underlined its solubility and stability in aqueous solutions, as well as its tendency not to
cause taste or odour problems. They further state that
chlorites have pronounced bactericidal properties, but
only in an acidic environment during chlorine dioxide
generation. They introduce the idea that this phenomenon occurs during digestion, because when chlorite
enters the stomach, it is entering an acidic environment.
However, they refute the hypothesis that chlorites are
active in a neutral or alkaline pH.
Recently in the field of food industry, research has
been carried out regarding the effectiveness of numerous
methods for reducing the microbiological load on various fresh agricultural products. Many studies (Park and
Beuchat 1999; Kemp et al. 2000; Parish et al. 2003) carried out on pathogenic microorganisms (Salmonella,
E. coli) presented acidified sodium chlorite as one of the
potentially effective means for inactivating pathogenic
bacteria present on agricultural products. Its use in spray
form or as a soaking solution has also been approved for
certain meats, seafood, poultry, fruits and vegetables, at
doses varying from 500 to 1200 mg/L (Parish et al.
2003). However, these concentrations surpass, by far,
the value allowed for chlorite concentration in drinking
water (1.0 mg/L). The study highlights chlorite’s ability
to oxidize a great variety of germs. A test carried out by
Park and Beuchat (1999) showed that acidified sodium
chlorite had a substantial antimicrobial effect on E. coli
and on salmonella that can be found on cantaloupe,
honeydew melon and asparagus tips. A range of 3 log
reduction in these pathogenic microorganisms was
observed. Kemp et al. (2000) also demonstrated the
effectiveness of an antimicrobial treatment with acidified
sodium chlorite, tested on chicken carcasses, in considerably reducing natural microbial contamination. For various disinfectant concentrations, the effectiveness of
microbial reduction, after having immersed the carcasses
into an acidified sodium chlorite solution, was more
than 99% for E. coli and 86.1 to 98.5% for total coliform (Kemp et al. 2000).
In a study by McGuire et al. (1999), laboratory
experiments were carried out to verify whether or not
chlorites have a toxic effect on certain types of bacteria
such as E .coli and heterotrophic bacteria (HPC). Chlorite doses used varied between 0 and 1.0 mg/L ClO2– and
testing went on for a 72-h period. Results from these
tests lead to the conclusion that chlorites did not affect
Disinfecting Power of Chlorite
E. coli or HPC. These results contradict those of Masschelein’s tests (1979) in regard to E. coli. However,
Masschelein’s tests went on for a period of about one
week; one could therefore assume that the results from
McGuire et al. (1999) do not necessarily invalidate
Masschelein’s, since McGuire’s negative results may be
due to the differences in oxidant exposure (CT). Nevertheless, the study carried out by McGuire et al. (1999) is
still very conclusive regarding secondary disinfection, as
discussed in the next section.
Chlorite Effectiveness as a Secondary Disinfectant
Using chlorite to control nitrification. The nitrification
phenomenon is a common problem in drinking water
supply systems that use monochloramine as a disinfectant, especially those located in hot climates or that have
long residence times in their distribution system. Nitrification is the microbiological oxidation of ammonia into
nitrite, then into nitrate. There is always a weak concentration of free ammonia in water that contains monochloramine (McGuire et al. 1999). As monochloramine
dissipates in water, concentration of free ammonia
increases. In a favourable environment, certain bacteria
such as Nitrosomonas can oxidize ammonia into nitrites.
These nitrites consume monochloramine, which in turn
reduces monochloramine residual. Nevertheless, it was
noted that, in systems using chlorine dioxide (for example, to control taste and odour problems) simultaneously
with monochloramine (as a residual disinfectant), the
occurrence of nitrification events was reduced (McGuire
et al. 1999). It was therefore assumed that chlorite ion, a
by-product of chlorine dioxide, while remaining at a
steady concentration in the distribution system, could,
due to its bacteriostatic characteristic, diminish or stop
the nitrification phenomenon. McGuire et al. (1999)
tested this hypothesis with laboratory and field tests.
Laboratory assays showed that even a very low chlorite
concentration (for example 0.05 mg/L ClO2-) could inactivate AOB bacteria (ammonia-oxidizing bacteria) by 3 to
4 log in several hours. Furthermore, a higher concentration of chlorites (for example 1 mg/L ClO2-) can inactivate AOB by 2 to 3 log in 30 min (McGuire et al. 1999).
Field experiments demonstrated that the presence of chlorites in the distribution system limits the decomposition
of chloramines and nitrification (compared to systems
that do not use chlorine dioxide but use monochloramine). These conclusions suggest that it may be possible to use monochloramination as a residual disinfectant
in distribution systems with high water temperatures, in
combination with chlorine dioxide use, which would
guarantee a certain concentration of chlorites in the system to control nitrification. Recently, McGuire et al.
(2006) also published results from assays that were carried out simultaneously for 6 months on a dozen pilots in
Tucson, Arizona, each of the 12 pilot units reproducing
377
different operational conditions (Cl2 and ammonia concentrations, Cl2:ammonia ratio, influent chlorite). The
pilot tests carried out by McGuire et al. (2006) under the
framework of this study showed that chlorite ion has the
ability to stop nitrification and stabilize ammonia concentrations, which confirms the results of their previous
laboratory studies as well as field studies. The assays used
various doses of monochloramine and tested two types of
treatment: either continuous or intermittent injections of
a dose of chlorite ion, which was applied to various doses
lower than the acceptable maximum concentration of
1.0 mg/L of chlorite ion. The results showed that concentrations of chlorite ion as low as 0.1 mg/L injected continuously were able to stop nitrification. Intermittent
doses temporarily stopped nitrification in systems that
were severely nitrified. The study also presents the estimated cost of applying sodium chlorite treatments such
as continuous addition of 0.1 mg ClO2-/L, intermittent
dosage of 0.2 mg ClO2-/L or even a hybrid option on a
large scale to control nitrification in drinking water systems and suggests that the cost of such treatments could
prove to be reasonable compared to the cost and the
usual methods used to deal with nitrification problems.
Following the results obtained by McGuire et al.
(1999), a second group of researchers (O’Connor et al.
2001) carried out a study on controlling nitrification by
adding weak concentrations of sodium chlorite to
treated water in a drinking water supply system. In
agreement with the work of McGuire et al. (1999), the
results of this study confirmed the effectiveness of
sodium chlorite use to reduce the nitrification phenomenon in a distribution system. An important benefit of
chlorites was observed 3 weeks following the initial
sodium chlorite addition to the system water. The
marked increase in residual monochloramine in the system water shows that nitrifying bacteria (AOB) activity
decreased due to the action of chlorite.
Chlorite effect on biofilm. Chlorine dioxide is known to
be effective in controlling biofilms (Aieta and Berg 1986;
Schwartz et al. 2003; Gagnon et al. 2005). The hypothesis was put forth that chlorites, resulting from chlorine
dioxide reduction, could also contribute to disinfecting
biofilms, because under acidic conditions inside the
biofilm, chlorites could be converted back into chlorine
dioxide and act against the biofilm, according to Turvey
(2006). This remains to be proven. On the other hand,
Gagnon et al. (2005) published the results of a study
assessing chlorite (and chlorine dioxide) efficacy in disinfecting distribution system biofilms. They concluded that
heterotrophic bacteria, either suspended or attached in
biofilm, were not effectively disinfected by low chlorite
concentrations. Using a pipe-loop simulating distribution
system conditions, they observed a 0.20 log reduction of
attached heterotrophic bacteria for a 0.1-mg/L dose of
chlorite ion and a 0.34 log reduction for a 0.25-mg/L
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Bichai and Barbeau
dose. They concluded that it is necessary to maintain a
chlorine dioxide residual in drinking water systems since
one cannot rely on chlorite disinfectant properties to
adequately control heterotrophic bacteria.
Effect on Pseudomonas bacteria. Masschelein et al.
(1981) lead a study on ways to limit microbial regrowth
in drinking water distribution systems. They consider
chlorine dioxide with partial and controlled chlorite generation a suitable technique for this application (Masschelein et al. 1981). In this study, Pseudomonas germs
were used as a microbiological indicator. The authors
concluded that chlorine dioxide is more persistent and
perhaps more bactericidal than free chlorine. They consider this effectiveness to be due, in part, to chlorine
dioxide reduction into chlorites, to which he attributes a
slow bactericidal effect that impedes microbial growth.
In fact, among the assays carried out in this study, they
were able to observe that P. putida is very sensitive to
chlorite ion’s disinfecting action, and so is P. fluorescens
(wild strain). These results show that, for example, with
a 1.0-mg/L dose of ClO2–, approximately 2 log of inactivation of P. putida can be obtained after 40 h and
almost 3 log after 90 h. It is interesting to note that,
according to this study, chlorite ion had almost no
effect, or very little, on certain other types of germs such
as P. deva and Rhodotorula yeast. These results suggest
that the action of chlorite may vary among species.
Material and Methods
To be able to confirm chlorite action as a disinfectant,
assays were run in the laboratory by dividing the experiment into two parts: primary and secondary disinfection.
The first part of the experiment involved running disinfection assays using Bacillus subtilis spores and MS2 coliphages, and the second part was to run similar assays
using HPC bacteria. B. subtilis spores can be used as a
surrogate of organisms resistant to disinfection, such as
Cryptosporidium or Giardia (Ballantyne et al. 1999).
MS2 coliphages are commonly used as a surrogate for
enterovirus due to their similarity in size, isoelectric
point and configuration. Regarding HPC, they are an
indicator representing the general bacterial flora. The
elimination of spores and phages shall therefore be interpreted as a primary disinfection effect of chlorites, while
limiting the growth of HPC during the second round of
assays shall be considered as representing the effectiveness of chlorites as a secondary disinfectant.
Primary Disinfection: Testing B. subtilis
Spores and MS2 Coliphages
B. subtilis spore and MS2 phage assays were carried out
over a 9-day period. For both types of microorganism,
four 1-L reactors were filled and seeded with an initial
concentration of either B. subtilis spores (ATCC6633) or
MS2 (N0 = 104.5 CFU/mL). These amber glass reactors
had been previously soaked in the dark in a concentrated
solution of chlorites (100 ppm) for more than 24 h so as
to fulfill the chlorite demand of the glass. Each reactor
contained a magnetic bar and was placed upon an agitator, which created a weak vortex in the solution for the
duration of the test. For spores as well as phages, the four
reactors contained chlorite concentrations of ClO2– of 0
(control reactor), 1, 10 and 100 mg/L (as ClO2–), respectively. These chlorite doses, injected into the reactors,
originated from a chlorite stock solution of 1800 mg/L
ClO2– made from 80% NaClO2 sodium chlorite granules
(O’Connor 2001) dissolved in ultra-pure water (Milli-Q).
Water preparation. The microorganisms were injected
into buffered water with a pH of 7.0, ionic force, µ, of
0.05 M, and at room temperature (22 ± 0.5C). The
water used was ultra-pure (Milli-Q) buffered with phosphate salts (KH2PO4 and Na2HPO4).
Measurement of chlorite. Since it is assumed that chlorites are stable in solution (Narkis et al. 1987), concentrations were only measured at the beginning and at the
end of disinfection assays by using ionic chromatography (U.S. EPA 1993). As presented later, no chlorite
decay was observed during the assays.
Analysis of B. subtilis spores and MS2 coliphages. Samples collected from the B. subtilis spore reactors were
immediately filtered, i.e., without adding any reducer to
stop chlorite action and without storing samples prior to
their analysis. Filtration was done in duplicate and in
several dilutions, according to the filtration method outlined by Barbeau et al. (1997). Petri dishes were placed
in an incubator at 35°C for a 24-h period before counting colonies. Bacteriophage counts were done using the
double-layer agar technique (U.S. EPA 2001). Host bacterium is cultured in a TSB mixture and added to the
stock virus that is diluted on agar. This mixture is then
spread onto another layer of agar and incubated at 37°C
for 24 h. Lysis plaques are then counted in a similar way
to the bacterial colony counts. The results are expressed
as PFU/mL. Negative and positive controls were carried
out by taking counts from 1.5 mL of E. coli diluted
stock solution in buffered sterile water or MS2 diluted
stock solution, respectively.
Test procedure. Following spore injection into the reactors, a preliminary series of withdrawals was done to
determine the initial spore concentration in each reactor
(N 0). Specific concentrations of chlorites were then
injected into all reactors except for the control reactors.
Next, samples were taken from the four reactors after
24 h, 48 h, 6 d, 8 d and 9 d for spores, then at 30 min,
3 h, 24 h, 28 h, 48 h, 3 d, 7 d, 8 d and 9 d for phages.
Disinfecting Power of Chlorite
Secondary Disinfection: Testing on HPC Bacteria
As with the spores and the phages, HPC bacteria assays
took place over a 9-d period. Three amber glass reactors
that had previously been soaked in a 100 ppm chlorite
solution were used for the test. Each reactor contained a
magnetic bar and was placed upon an agitator at room
temperature for the duration of the test. The three reactors contained chlorite concentrations of 0 (control),
1 and 2 mg/L of ClO2-. Chlorite concentrations were
measured using ionic chromatography.
Water preparation. The water used for the HPC was
bottled natural spring water (Labrador Laurentienne)
with a dissolved mineral salt content of 160 mg/L. This
water did not undergo any preliminary treatment.
HPC measurement. Daily samples were withdrawn
from each reactor and immediately filtered. Two incubation methods were tested at the same time: 35°C for
48 h and 7 d at 22°C. The detection medium used was
R2A (Difco R2A Agar, #218263).
Test procedure. Prior to the injection of chlorite, initial
spore concentrations on each of the reactors were determined. Then, chlorite was injected into each reactor
except for the control reactor. Next, samples were taken
from the three reactors after 24 h, 48 h, 7 d, 8 d and 9 d
for the 35°C–48 h incubation conditions, and samples
were taken at 24 h, 48 h, 72 h, 4 d, 7 d and 9 d for the
22°C–7 d incubation conditions.
Results
Primary Disinfection: Testing on B. subtilis
Spores and MS2 Coliphages
Chlorite analysis results. Chlorite concentrations, for
samples taken from each reactor at the beginning and
379
at the end of the tests, showed no decrease throughout
the duration of the assays and were nearly identical to
the projected concentrations, as we measured 1.1 mg
ClO2-/L at time, T0, and after 9 d for the targeted concentration of 1.0 mg ClO2-/L, when the other measurements indicated exactly the expected chlorite concentrations of 10.0 and 100.0 mg ClO2-/L. These results
indicate that chlorite concentration is very stable over
time, as opposed to conventional water treatment oxidants (chlorine, ozone, chlorine dioxide). Thus, concentrations are identical for each reactor at the beginning and at the end of the test, following several days
of incubation.
Inactivation of B. subtilis spores and MS2 phages.
Results obtained for spore and coliphage disinfection are
shown in Fig. 1. Glancing at the overall look of the
spore inactivation graph (Fig. 1A), it is seen that the four
curves run closely along the N/N0 = 1 equation line.
Spore concentration remained stable enough despite the
presence of chlorite in the water. In general, adding 1,
10 and 100 mg/L lowered the spore concentration by
0.15, 0.16 and 0.26 log, respectively, for the duration of
the test. A t-test indicated that the variations observed
due to the action of chlorites, as minimal as they were,
resulted in spore concentrations that were statistically
significantly lower than the control reactor (p < 0.05).
Thus, in spite of conclusions drawn from statistical
tests, it would be presumptuous to declare that chlorites
have a disinfection effect on B. subtilis spores, given the
very slight decrease noted during the assay, which was in
the 0.2 log range, despite very long contact times and
high chlorite concentrations. Furthermore, referring to
the conditions normally encountered in drinking water
disinfection, it can be admitted that chlorites have no
notable effect as a primary disinfectant, with respect to
B. subtilis spores, and assuming that these results are a
good indicator of the effect of chlorite on resistant
microorganisms such as Cryptosporidium.
Fig. 1. Primary disinfection test results of chlorites for 9 days on: (left) B. subtilis spores and
(right) MS2 phages.
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Bichai and Barbeau
In Fig. 1B, the inactivation graph for MS2 phages
indicates a decrease in MS2 phage concentration in the
three reactors containing chlorites. A decrease in the
0.5 log range in 9 d was also observed in the control
reactor. Nevertheless, almost a 2 log inactivation in 9 d
is seen for a chlorite dose of 1 mg/L. A 4.5 log reduction
is reached in less than a week for a high chlorite dose of
100 mg/L and disinfection of nearly 1 log was observed
in this reactor following the first 3 h of the test.
Figure 2 shows MS2 phages inactivation in terms of
CnT, according to the Chick-Watson law. According to
this model, MS2 phage inactivation can be estimated
using the following empirical equation, where C is
expressed in mg/L and T in min:
Log I = 0.00013 × C0.3963 × T
(R2 = 0.958)
(1)
The required CT for a 2 log inactivation of MS2
phages is therefore 15,385 mg ⋅ min/L at 22°C, for a
chlorite concentration of 1.0 mg/L.
Secondary Disinfection: HPC Testing
Chlorite measurement results. Chlorite concentration
analysis using ionic chromatography for the HPC assays
provides confirmation that true chlorite concentrations
found in both reactors were equal to the targeted values
and remained stable during the 9 days of the test.
HPC inactivation. The evolution of HPC concentrations in the three reactors is shown in Fig. 3. HPC were
incubated at the same time at 35°C for 48 h as well as at
20°C for 7 d, taking into consideration that incubation
conditions influence the type of bacterial population
recovered (Reasoner et al. 1989).
The overall appearance of the graphs presented in
Fig. 3A and 3B clearly shows that the presence of chlorites inhibited bacterial regrowth. In fact, the curves of
the reactors that underwent chlorite action remain
Fig. 2. Results of 9-d chlorite disinfection assays on MS2
phages - inactivation (log) of MS2 phages compared to
control for each reactor based on CnT (mg min/L), with
n = 0.3963.
located above the control reactor’s curve. In the two
incubation conditions studied, bacterial growth in the
control reactor can be observed at the beginning of the
test. For the HPC-35°C, bacterial regrowth continues in
the control reactor for about 6 d, followed by a decrease,
probably due to lack of available organic carbon. With
respect to the two other reactors, bacterial regrowth is
clearly limited by chlorite action. Furthermore, there is a
statistically significant difference between the two concentrations (1 versus 2 mg/L). Nevertheless, for the
HPC-22°C, the two curves for reactors that contain chlorites are not statistically different (p < 0.05). It would
therefore seem that there is no dose-response effect for
the concentrations studied (1–2 mg/L). The general
appearance of the curves (increase followed by decrease)
is still similar for both incubation conditions.
Discussion
The greatest effects observed within the framework of
this project were those obtained during MS2 phage inactivation. In fact, it was possible to describe MS2 phage
inactivation by chlorites by using a Chick-Watson Law
and to observe up to a 4.5 log MS2 inactivation at the
highest dose (100 mg/L). The standard for maximum
chlorite concentration in drinking water does not allow
it to be relied upon as a disinfectant for viruses. According to these results, the necessary CT values for a 2 log
inactivation are about 20 times higher than values typically observed in water treatment plants, assuming a
12-h contact time and a concentration of 1.0 mg
ClO2–/L. Nevertheless, in contrast to chlorine, chlorine
dioxide and ozone, chlorite concentrations are very stable in solution. This stability would allow it to remain
present during distribution into the water distribution
system. Chlorites would then act as a bacterial regrowth
inhibitor, as suggested by Masschelein et al. (1981).
As for the effect observed on HPC, it is seen that the
effect is somewhat limited in comparison to the effect of
residual chlorine in a distribution system. Nevertheless,
the effect of chlorites in this case would seem sufficient
for reducing the regrowth of free bacteria by about
0.5 log, at least for a period of about a week, assuming a
1 mg/L concentration. However, these results are limited
to test conditions on HPC within the context of this
study. The type of water used, the room temperature
and the available organic matter concentration are all
factors that can vary the natural bacterial growth. Also,
the assay conditions do not account for the influence of
biofilm which was assessed in a previous study by
Gagnon et al. (2005).
In the end, the results obtained for B. subtilis spores
lead to the conclusion that this microorganism is very
resistant to chlorite action, which would lead to the supposition that parasites such as Giardia lamblia and Cryptosporidium parvum would probably remain unaltered
Disinfecting Power of Chlorite
381
Fig. 3. Results of 9-d chlorite disinfection assays on HPC for two incubation conditions: (left)
incubation at 35°C for 48 h and (right) incubation at 22°C for 7 d.
after being exposed to chlorites, even at very high doses
and for prolonged contact times beyond a week. All
together, the experimental tests carried out within the
context of this research project would lead to the conclusion that chlorites have weak disinfection power, probably due to their limited oxidation capacities. However,
chlorite action on HPC would lead to the assumption
that chlorites can interfere with bacterial metabolism.
Results observed during this study explain the disparity between conclusions that were drawn in the literature
regarding chlorite effect on disinfection. With regard to
primary disinfection, according to Liyanage et al. (1997),
chlorite ions have no disinfectant effect on Cryptosporidium parvum. This result presents some coherence with our
test results on Bacillus subtilis spores, since spores and
Cryptosporidium are microorganisms that both show
great resistance to oxidants. On the other hand, Masschelein (1979) observed almost a 3 log inactivation of E.
coli in eight days for a 0.5-mg/L dose of sodium chlorite,
which can be compared to the chlorite effect observed on
MS2 phages, while for a 1-mg ClO2–/L dose, our tests
resulted in almost a 2 log inactivation in 9 d.
With regard to secondary disinfection, several results
deserve mention here. First of all, laboratory test results
obtained by McGuire et al. (1999) indicated that a chlorite concentration of 1 mg/L ClO2- can inactivate 2 to
3 log of AOB in 30 min. Similarly, O’Connor et al.
(2001) observed during a field study that nitrifying bacterial activity (AOB) was significantly decreased by sodium
chlorite action. Finally, following a study on limiting
microbial regrowth in drinking water distribution systems, Masschelein et al. (1981) found that chlorite ions
have a slow bactericidal effect that can stop microbial
development. Their tests carried out on Pseudomonas
germs showed, however, that certain types of germs are
very sensitive to chlorite action, while others suffer no
effects. Results observed during this study on HPC bacteria in the bulk phase confirmed that chlorite induces a
reduction in bacterial regrowth and could therefore be
viewed as having a bacteriostatic action.
Conclusions
Results published in scientific literature regarding chlorite effectiveness are variable and suggest that the effect
is specific to the type of microorganism. Microorganisms
in a dormant state (protozoa cysts and bacterial spores),
or which have no metabolism (viruses), are minimally
impacted by chlorites. The mode of action of chlorites
appears related to the bacterial metabolism, which
would explain the significant effect observed for controlling biofilm and bacterial regrowth, including nitrification. These results are supported by the present study,
which observed no significant effect of chlorites on MS2
coliphages nor on B. subtilis spores, while HPC
regrowth (24 h–35°C and 7 d–22°C) was decreased after
having added 1 or 2 mg/L of chlorites. Chlorites can
therefore have a secondary disinfecting effect, but
remain a poor primary disinfectant.
Acknowledgements
We extend our thanks to Mélanie Rivard, Jacynthe
Mailly and Julie Philibert for their help and contributions
during the laboratory assays. Our thanks, as well, to Clément Cartier for his judicious advice regarding laboratory
techniques. We would also like to thank NSERC and the
industrial partners of the Chair: the municipalities of
Montreal and Laval and John-Meunier Inc.
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Received: July 18, 2006; accepted: October 3, 2006.