Information Sheet 1.6
Disinfection with ozone
General description
Ozone is generated on site by passing an electric discharge through clean dry air or oxygen. The
resultant ozone is a very strong biocide and oxidising agent and is effective in reducing colour,
iron, manganese, taste and odour.
The mechanism by which ozone inactivates microbes is not well understood. Ozone in aqueous
solution may react with microbes either by direct reaction with molecular ozone or by indirect
reaction with the radical species formed when ozone decomposes (Le Chevallier & Au 2004).
Ozone is known to attack unsaturated bonds, forming aldehydes, ketones or carbonyl compounds
(Langlais, Reckhow & Brink, 1991).
Free radicals formed by the decomposition of ozone are generally less effective for microbial
inactivation than molecular ozone, because microbial cells contain a high concentration of
bicarbonate ions that quench the free radical reaction, and many microbial cells also contain
catalase, peroxidase, or superoxide dismutase to control free radicals produced by aerobic
respiration. In addition, some bacteria contain carotenoid and flavonoid pigments that protect
them from ozone. These factors can account for reports that heterotrophic bacteria may be less
susceptible to ozone inactivation than Giardia (Wolfe et al., 1989).
Performance validation
Ozone is effective against bacteria and viruses. Of the vegetative bacteria, Escherichia coli is
one of the most sensitive (Table IS 1.7.1), while Gram-positive cocci (Staphylococcus and
Streptococcus), Gram-positive bacilli (Bacillus) and mycobacteria are the most resistant
(Langlais, Reckhow & Brink, 1991).
Viruses are generally more resistant to ozone than vegetative bacteria, although phage appear to
be more sensitive than human viruses (Langlais, Reckhow & Brink, 1991).
Ozone inactivation of the protozoa Giardia lamblia (Table IS 1.7.1) does not follow linear
kinetics, due to an initial latent phase. However, a Ct value of 0.53 mg/L.min for 2 log
inactivation at 5°C has been estimated (Wickramamayake et al, 1984).
Ozone is also effective for the inactivation of Cryptosporidium.
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Table IS1.7.1 Examples of Ct values for 99% (2 log) inactivation of various
microorganisms by ozone 1,2.3
Microorganism
Ozone
(mg/L.min)
Reference
Escherichia coli
0.02
USEPA 1999
Enteric viruses
0.6
USEPA 1999
0.5-0.6
Wickramamayake et al, 1984
Giardia
Cryptosporidium
32
NZ MoH 2008
o
Notes: (1) Temperature is 5 C unless stated.
(2) pH is within range of 6-9 unless stated.
(3) The values in the table are based on published values and should be viewed as the
minimum values necessary to achieve effective disinfection
The important conclusion to draw from Table IS1.7.1 is that ozone is more effective than
chlorine, chloramines, and chlorine dioxide for the inactivation of viruses, Cryptosporidium, and
Giardia.
Given that where the ozonation process is used as a primary disinfectant it will be a critical
control point (CCP), other important issues that will need to be considered to ensure the
effectiveness of the process are:

establishing target criteria (section 3.4.2) and critical limits for the ozonation process

preparing and implementing operational procedures (section 3.4.1) and operational
monitoring (section 3.4.2) for the process

preparing corrective action procedures (section 3.4.3) in the event that there are excursions in
the operational parameters

undertaking employee training (section 3.7.2) to ensure that the ozonation process operates to
the established target criteria and critical limits
Water quality considerations
Ozone is highly sensitive to turbidity. Turbidity should be less than one NTU at the time of
ozonation. The pH should be less than 8 for effective disinfection because ozone is unstable
above pH 8 (at pH 8, half of the ozone is lost in less than 30 minutes).
Practical considerations
Even though ozone systems are complex, using highly technical instruments, the process is
highly automated and very reliable, requiring only a modest degree of operator skill and time to
operate an ozone system (USEPA 1999). Maintenance of ozone generators requires skilled
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technicians. If trained maintenance staff are not available at the plant, this work can be done by
the equipment manufacturer.
Ozone is a toxic gas and the ozone production and application facilities should be designed to
generate, apply, and control this gas, so as to protect plant personnel. Ambient ozone levels in
plant facilities should be monitored continuously.
Persistence
Due to its low solubility in water and instability above pH 8, an ozone residual cannot be
maintained in a distribution system, particularly as temperature increases.
Byproducts
Ozone is a powerful oxidant and can convert naturally-occurring bromide to bromine, and this
can lead to the formation of brominated trihalomethanes (THMs), brominated acetic acids,
bromopicrin, brominated acetonitriles, as well as the formation of bromate. (USEPA 1999).
However, the brominated THMs produced in ozonation usually occur in lower concentrations
than chlorinated THMs produced by chlorination.
Low molecular weight aldehydes, such as formaldehyde and acetaldehyde, have also been
detected as byproducts of ozonation.
Application
Ozone can be used in medium to large treatment plants, although it has not been used in
Australia to date for the primary disinfection1 of a sizeable drinking water supply. It reacts with
natural organics to produce lower molecular weight compounds, which are more biodegradable
and promote the growth of bacteria in distribution systems, which may have significant
consequences for many Australian distribution systems, where elevated water temperatures
create a predisposition for bacterial growth.
The production of lower molecular weight compounds has been used to advantage in biological
filtration processes. Ozonation can break up high molecular weight organics before filtration
through a bed of granular activated carbon. The resulting low molecular weight compounds can
be used by bacteria that grow on the carbon, thereby reducing organic concentrations in the
water. Ozone has a long history of use for disinfection, and for the control of taste, odour and
colour. Ozone is more expensive than chlorine and has low solubility in water.
Operational monitoring
In general, the most important operational monitoring parameter is oxidant disinfectant
concentration measured at a point representing the end of the contact period. Both the dose and
the residual oxidant concentration should be measured online. Residual oxidant can be measured
directly using a residual analyser (NZ MoH 2008)
1
Ozone has been used in NSW for removal of algal toxins and for taste and odour control by Orange
Council for over ten years. Similarly, ozone has been used for such control by MidCoast Water, Rous
Water and Tweed Council for over four years.
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References
Langlais B, Reckhow DA, Brink DR (1991). Ozone in Water Treatment, Applications and
Engineering. Chelsea, MI, Lewis Publishers, Inc.
LeChevallier MW and Au K-K (2004) Water treatment and pathogen control. World Health
Organization, Geneva.
New Zealand Ministry of Health (2008) Drinking-water Standards for New Zealand 2005
(Revised 2008)
USEPA (1999) Alternative disinfectants and oxidants guidance manual. USEPA Washington DC
Wickramamayake GB, Rubin AJ, Sproul OJ (1984). Inactivation of Naegleria and Giardia cysts
in water by ozonation. Journal of the Water Pollution Control Federation, 56:983–988.
Wolfe RL et al. (1989). Inactivation of Giardia muris and indicator organisms seeded in surface
water supplies by peroxone and ozone. Environmental Science and Technology, 23:744–745.
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