Water Safety Plan
Guide
Treatment Processes
– Ozone Disinfection
Version 1, Ref P7.3
January 2014
Citation: Ministry of Health. 2014. Water Safety Plan Guide:
Treatment Processes – Ozone Disinfection, Version 1, ref p7.3.
Wellington: Ministry of Health.
Published in January 2014
by the Ministry of Health
PO Box 5013, Wellington, New Zealand
ISBN: 978-0-478-42744-8 (print)
ISBN: 978-0-478-42745-5 (online)
Previously published in 2001 as Public Health Risk Management
Plan Guide: Treatment Processes – Ozone Disinfection, Version 1,
ref p7.3. This publication’s title and any reference within the text to
‘public health risk management plan’ were changed in January 2014
to reflect the December 2013 legislation change of the term ‘public
health risk management plan’ to ‘water safety plan’. No other
changes have been made to this document.
This document is available at: www.health.govt.nz
This work is licensed under the Creative Commons Attribution 4.0 International
licence. In essence, you are free to: share ie, copy and redistribute the material in any medium or
format; adapt ie, remix, transform and build upon the material. You must give appropriate credit,
provide a link to the licence and indicate if changes were made.
Contents
Introduction
1
Risk Summary
2
Risk Information Table
3
Contingency Plans
6
Water Safety Plan Performance Assessment
7
Appendix P7.3
8
Ref P7.3
Version 1, January 2014
Water Safety Plan Guide:
Treatment Processes – Ozone Disinfection
iii
Introduction
Ozone is used to disinfect, or oxidise contaminants in, drinking-water. This Guide is
concerned only with using ozone as a disinfectant.
If an event occurs during ozonation (ie, the process doesn’t work properly), the following
could happen:

If there is not enough ozone added to the water, germs can cause sickness

High concentrations of by-products from ozonation can cause sickness, even when
ozone concentrations are acceptable.
The on-site generation and use of ozone can present risks to the health of treatment plant
staff. These are acknowledged, but are not discussed further as such risks are the subject of
health and safety in employment legislation.
The ozonation process and the risks associated with it cannot be viewed in isolation. This
Guide only looks at the ozonation of the water. Ozone’s value as a disinfectant is also
affected by elements of the water supply system dealt with in other Guides.
Several factors influence how effective disinfection is:

ozone concentration

how long the ozone is in contact with the water (see Guide D1)

water temperature

turbidity of the water when the ozone is added to it; this can hinder the access of ozone
to germs (see the S1, P1, P5 and P6 series of Guides)

pH of the water; this may affect how much of the ozone is in a form that is good at
killing germs (see Guide P8.1).
If the whole ozonation process is going to work as well as possible, all these factors have to
be taken into consideration.
Ozone does not last long in water. Another disinfectant with a longer lifetime, such as
chlorine, chloramine or chlorine dioxide, should be added after ozone to kill any germs that
may get into the water after the ozone has decayed away.
Ref P7.3
Version 1, January 2014
Water Safety Plan Guide:
Treatment Processes – Ozone Disinfection
1
Risk Summary
The event creating the greatest risk involved in ozonising drinking-water is not having
enough ozone in the water during the contact period to kill germs (see P7.3.1).
The most important preventive measures are:

monitor the process to be sure there is enough ozone in the water, regardless of how
the quality of the incoming water might change (see P7.3.1.4)

put an alarm on the ozone generator to let you know if the generator fails (see P7.3.1.6)

monitor the pH of the treated water; use a properly calibrated pH probe (see P8.1.1.2
and P8.1.1.3).
(References in parentheses are to the Risk Information Table.)
2
Water Safety Plan Guide:
Treatment Processes – Ozone Disinfection
Ref P7.3
Version 1, January 2014
Risk Information Table
Reliable information about water quality is essential for the proper management of a water
supply. Knowledgeable and skilled staff are also essential for minimising the public health
risks associated with water supplies. Please read the staff training (Guide G1) and the
monitoring guides (Guide G2). While we haven’t pointed out every detail of how these
documents are linked with the present document, the links are many and are important.
Abbreviations: DWSNZ – Drinking-Water Standards for New Zealand
Causes
Preventive measures
Checking preventive measures
What to check
Corrective action
Signs that action is
needed
Event: OZONE CONCENTRATION TOO LOW
Possible hazards: Germs not killed.
Level of risk: High1
P7.3.1.1

Ozonator or
dosing
controller
malfunction.



P7.3.1.2
Dosing
controller’s
sensor
incorrectly
calibrated.

Routine
maintenance.

Microbiological
quality.

Ozone
concentration
after contact
period.

Replacement of
controller if suspect.

Alarm system to
warn of inadequate
ozone
concentration.
Humidity of air
entering
generator.
Ensure air used as
the feed gas is
properly dried before
injection into the
generator.
Regular manual
checks on
calibration of sensor
(see DWSNZ:2000,
Section 3.3.4.7).

Microbiological
quality.

Ozone
concentration
after contact
period.

1
Maintenance
log.
Calibration
log.

Low ozone
residual (see
Appendix).

Identify cause
of fault and
rectify.

E. coli or
coliforms
detected in 100
mL of treated
water.

Manually dose
reservoir with
chlorine until
repaired.


Ozonator/
controller failure.
Replace
ozonator/
controller.

Frequent
maintenance
needed.

Maintenance log
not signed.


Water vapour
levels in feed
gas above those
recommended
by the
manufacturer.
Identify reason
for air drier not
working, and
rectify.

Low ozone
residual (see
Appendix).

Re-calibrate
sensor.


E. coli or
coliforms
detected in 100
mL of treated
water.
Increase
ozone dose
rate until
re-calibration
undertaken.

Calibration
schedule not
signed off.
The consequences of the event, and therefore the level of risk, will be influenced by the quality of the source
water, the effectiveness of treatment processes prior to the ozonation process, and whether a residual
disinfectant is added to the water following ozonation.
Ref P7.3
Version 1, January 2014
Water Safety Plan Guide:
Treatment Processes – Ozone Disinfection
3
Causes
Preventive measures
Checking preventive measures
What to check
Corrective action
Signs that action is
needed
Event: OZONE CONCENTRATION TOO LOW cont’d

P7.3.1.3
Dosing
controller set
to deliver the
wrong ozone
concentration.

P7.3.1.4
Incorrect dose
calculation.

P7.3.1.5
High ozone
demand
coupled with
poor dose
control.
P7.3.1.6
Adequate checks on
ozone concentration
at the sampling point
designated in
DWSNZ: 2000
(sections 3.3.1.2.3
and 3.3.4.7)
especially during
periods of water
quality variability.

Microbiological
quality.

Ozone
concentration
after contact
period.
Independent check
on calculations
(especially after a
system change and
expected dose rates
are uncertain).

Use a dose control
method that links
dose control to
ozone residual at
the sampling point
designated in
DWSNZ: 2000
(Sections 3.3.1.2.3
and 3.3.4.7).


Adjust ozone
dose controller
setting.

E. coli or
coliforms
detected in 100
mL of treated
water.

Frequent
calculation
errors found by
checks.

Re-calculate
dose rates and
change
settings.

Calculation
checks not
signed off.

Train staff in
making dose
calculations.

E. coli or
coliforms
detected in 100
mL of treated
water.

Replace dose
controller with
more suitable
unit.


Low ozone
residual (see
Appendix)
during periods of
poor water
quality.
Manual
monitoring and
manual ozone
control during
poor water
quality
episode.
Upstream processes
removing
substances
contributing to the
ozone demand from
the water.

TOC/colour
levels highly
variable.

Optimise
upstream
processes to
reduce ozone
demand.

Ensure ozonator has
sufficient capacity.

Inadequate
ozone dose
even when
ozonator running
at maximum.

Replace
ozonator with
one with
sufficient
capacity.

Stand-by generator.

Electricity
supply.

Poor continuity
of power supply.

Refuel
generator (if
appropriate).

Ensure that
contactor design has
been proven
effective elsewhere
before construction/
installation.

Ozone
concentration
after contact
period.

Low ozone
residual (see
Appendix).



Calculated
contact time.
E. coli or
coliforms
detected in 100
mL of treated
water r.
Obtain
engineering
advice to
assess what
modifications
can be made
to improve
contactor
design.
Poor
contractor
design

4
Ozone
concentration
after contact
period.
Low ozone
residual (see
Appendix).

Power failure.
P7.3.1.7
Dose
calculation.

Undertake pilotscale testing of
contactor before
construction.
Water Safety Plan Guide:
Treatment Processes – Ozone Disinfection
Ref P7.3
Version 1, January 2014
Causes
Preventive measures
Checking preventive measures
What to check
P7.3.1.8

Determine whether
there is a need to
pH control prior to
the contactor, and
install if necessary.


Provide staff training
in sample analysis
and record keeping.

Develop monitoring
schedule and roster.

Provide staff training
in sample analysis
and record keeping.
Water pH too
high (favours
rapid
decomposition
of ozone).
P7.3.1.9
Ozone
monitoring
samples taken
incorrectly, or
incorrectly
recorded (see
Guide D4).
P7.3.1.10
Method of
ozone
measurement
incorrect,
incorrectly
calibrated, or
analysis
reagents have
deteriorated.
Corrective action
Signs that action is
needed
Ozone
concentration
after contact
period.

Low ozone
residual (see
Appendix).

Introduce a
system for pH
adjustment.

E. coli or
coliforms
detected in 100
mL of treated
water.

Analysis
records.

Audits show
monitoring
inaccuracies.

Identify staff
training needs
and provide
training.

Analysis
records.

Audits show
monitoring
inaccuracies.

Identify staff
training needs
and provide
training.
Event: EXCESSIVE FORMATION OF OZONATION BY-PRODUCTS
Possible hazards: Bromate, bromoform (and other organic DBPs containing only bromine), formaldehyde.
Level of risk: Low–moderate2
P7.3.2.1
Natural
organic matter
and/or
bromide
present in the
water being
ozonated.
2


Provision of
treatment processes
upstream to reduce
levels of organic
matter in the water.
Adjustment of water
alkalinity to minimise
bromate formation.

TOC/colour.

Bromide.

Elevated TOC/
colour and/or
bromide.

Elevated
disinfection
by-product
formation.

Optimise
upstream
processes to
maximise
organic matter
removal.
The concentrations of disinfection by-products formed, and therefore the risk they present, will depend on the
amount of natural organic matter in the water.
Ref P7.3
Version 1, January 2014
Water Safety Plan Guide:
Treatment Processes – Ozone Disinfection
5
Contingency Plans
If an event happens despite preventive and corrective actions you have taken, you may need
to consult with the Medical Officer of Health to assess how serious a problem is.
Event – Ozone concentration is lower than minimum acceptable level
Indicators:
Required actions:
Responsibility:
6

A detectable ozone residual cannot be obtained in the
water at the outlet of the contact tank, and the disinfectant
used to provide a disinfecting residual cannot be detected
in the water leaving the treatment plant.

In 100 ml samples of water leaving the treatment plant,
E. coli or coliforms continually detectable, or is present at
elevated levels (more than 10 per 100 mL).

Both the ozoniser and residual disinfectant dosing system
are not operational.

Widespread illness in the community.

Follow the actions given in Figure 3.2 of the
DWSNZ:2000.

Manually dose chlorine into the post-treatment reservoir
until the disinfecting systems are operational again.

Identify the reason for the failure and rectify.

Record cause of system failure and steps taken to correct.

Modify water safety plan if necessary.
Manager designated responsible for the water supply.
Water Safety Plan Guide:
Treatment Processes – Ozone Disinfection
Ref P7.3
Version 1, January 2014
Water Safety Plan Performance
Assessment
To make sure that your supply’s water safety plan (formerly known as a Public Health Risk
Management Plan, PHRMP) is working properly, periodic checks are needed. The overview
document outlines what needs to be done. The following table provides the detailed
information for checking this particular supply element.
What to measure or
observe:

Ozone concentration at contact tank outlet.

E. coli (faecal indicator) or coliforms (bacteria not killed).
Follow the protocols set out in DWSNZ:2000.
Note that the presence of faecal indicators may be influenced by
factors other than the adequacy of the ozonation process (eg, the
pH control).
How often:

For the monitoring frequencies for FAC and E coli.
Measurements see DWSNZ:2000 Section 3.3.2.
What to do with the
results:

Results need to be recorded to meet legislative requirements
or to allow water safety plan performance assessment. The
WINZ database is good for this.

The collected data need to be periodically reviewed to see
whether problems with this supply element are developing.
This should be done as frequently as the manager
responsible considers necessary to minimise risk to public
health arising from this supply element.

Should this review show any unusual incidents, indicate
that proper procedures are not being carried out, highlight
poor laboratory results or indicate that poor water quality is
reaching customers, then review the procedures for
managing ozonation.

Evaluate the monitoring results, and any actions taken as
the result of having to implement a contingency plan, to see
if the water safety plan needs modification – eg, preventive
measures are up to date; the contingency plan steps are still
adequate; and changes to the treatment processes are
recognised in the plan.
Responsibility:
Ref P7.3
Version 1, January 2014
Manager designated responsible for the water supply.
Water Safety Plan Guide:
Treatment Processes – Ozone Disinfection
7
Appendix P7.3
Table P7.3:
C.t values (mg.min/L) for inactivation of Cryptosporidium by ozone*
Temp (°C)
99%
99.9%
0.5
57.9
83
5
32.5
46.6
10
17.5
25.1
15
9.6
13.8
20
5.39
7.73
25
3.08
4.42
30
1.79
2.96
*
From Table 13.1 of the Drinking-Water Standards for New Zealand 2000.
8
Water Safety Plan Guide:
Treatment Processes – Ozone Disinfection
Ref P7.3
Version 1, January 2014