Health and sanitation
<< Continued from page 23
Although the treatment wing had a smaller number of
positive cultures (3 of 12) than the control wing (8 of 12),
the researchers could not reach a conclusion on the role
of ozone in the inactivation of L. pneumophila. The study
indicated that when ozonation was stopped, L. pneumophila
regrew and reached levels close to the pre-test conditions
at the end of the stagnation phase. Moreover, the authors
pointed out one important factor for continual dosing of
ozone, namely that residual ozone at the faucet or shower
head led to the release of gaseous ozone into the air (an
issue discussed in Section 2.3.6.4).
Several laboratory studies have reported rapid and effective
inactivation of legionella with ozone (Jacangelo et al., 2002;
Domingue et al., 1988; Muraca et al., 1987).
• Loret et al. (2005) used a simulated distribution system
consisting of pipe loops to compare the effectiveness of
several disinfectants to control legionella in biofilms in
premise plumbing. The study concluded that ozone was
effective to control planktonic and biofilm-associated
populations within the pipe loops, but was ineffective
within dead end sections.
• Jacangelo et al. (2002) conducted laboratory studies
to evaluate multiple disinfectants, including ozone,
for inactivation of waterborne emerging pathogens
including legionella. The ozone dosage rate was
1.0mg/L. The model-predicted CT values for 2-log
(99-percent) inactivation of legionella at pH 7 were
2.5, 0.16 and 1.1 min-mg/L at 5, 15 and 25 degrees
C (41, 59 and 77 degrees F), respectively.
• Domingue et al. (1988) conducted laboratory
experiments to compare the bactericidal effects of
ozone, hydrogen peroxide and free chlorine on “free”
L. pneumophila cells. Ozone was the most potent
of the three disinfectants, with a greater than 2-log
(99-percent) inactivation in L. pneumophila occurring
during a 5-minute exposure to 0.10–0.3mg/L ozone.
The researchers reported little to no effect of pH and
temperature on ozone inactivation of L. pneumophila.
The pH ranged from 7.2 to 8.9. Experiments were
conducted at 25, 35 and 45 degrees C (77, 95 and
113 degrees F).
• Muraca et al. (1987) compared the efficacy of
chlorine, heat, ozone, and UV light for inactivating
L. pneumophila in a bench-scale model plumbing
system. L. pneumophila was added to the system and
allowed to circulate. Continuous ozonation for five hours
at a concentration of 1 to 2mg/L achieved a 5-log
inactivation of L. pneumophila at 25 and 43 degrees C
(77 and 109.4 degrees F, respectively). Neither turbidity
nor the higher temperature (43 degrees C, or 109.4
degrees F) was reported to affect the efficacy of ozone.
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25
Potential water quality issues
Ozone decomposes in water relatively rapidly. The
half-life of ozone in finished drinking water depends
on temperature, pH and alkalinity, and can vary from
minutes to hours. This time-scale is short relative to
chlorine-based disinfectants, and as such, ozone is
not generally considered to produce a disinfectant
residual. Therefore, water treated with ozone may,
in some cases, be susceptible to contamination
at downstream points. For this reason, more than
one type of treatment or control measure may be
necessary to protect the treated water.
Disinfection by-products formed from ozone
disinfection include bromoform, monobromoacetic
acid, dibromoacetic acid, dibromoacetone, cyanogen
bromide, chlorate, iodate, bromate,hydrogen peroxide,
hypobromous acid, epoxides, ozonates, aldehydes,
ketoacids, ketones and carboxylic acids (WHO, 2011b).
Ozonation of water containing inorganic bromide can
produce bromate, a regulated DBP with an MCL of
10µg/L. The disinfection process of a PWS will likely
have transformed any bromide in water to organically
bound bromine or inorganic bromamines. In either case,
these forms of bromine are less likely to contribute
to bromate formation via an ozonation process in a
premise plumbing system. As such, bromate formation
may not be as relevant as in the water treatment plant.
Other ozonation by-products such as aldehydes and
organic acids are more readily biodegradable and may
contribute to assimilable organic carbon (AOC) and
hence biological growth in the distribution system. In
addition, these ozonation by-products are more likely
to form some types of DBPs upon chlorination or
chloramination (Carlson and Amy, 2001; Shah and Mitch,
2012). However, these general concepts regarding
ozonation pertain to treatment of water at the plant.
Ozonation of water that has already undergone
treatment, including exposure to a chlorine or
chloramine residual in the distribution system en
route to the building (for example, hospital) has not
been studied to a great extent. Therefore, impacts
of ozonation on AOC or DBP formation in a premise
plumbing system are still unclear.
Loret et al. (2005) observed corrosion marks on mild
and galvanised steel coupons installed in pipe loops for
ozone treatment that were similar to corrosion effects
caused by other disinfectants (chlorine, chloramine,
chlorine dioxide and CSI), except that the coupons
exposed to CSI also had copper deposits. PA
As a primary
disinfectant,
ozone
is more
effective
than
chlorine.
August 2017 Volume 23 I Number 6