Water Safety Plan
Guide
Treatment Processes
– Chlorine Dioxide Disinfection
Version 1, Ref P7.2
January 2014
Citation: Ministry of Health. 2014. Water Safety Plan Guide:
Treatment Processes – Chlorine Dioxide Disinfection, Version 1, ref
p7.2. Wellington: Ministry of Health.
Published in January 2014
by the Ministry of Health
PO Box 5013, Wellington, New Zealand
ISBN: 978-0-478-42742-4 (print)
ISBN: 978-0-478-42743-1 (online)
Previously published in 2001 as Public Health Risk Management
Plan Guide: Treatment Processes – Chlorine Dioxide Disinfection,
Version 1, ref p7.2. 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
9
PHRMP Performance Assessment
12
Ref P7.2
Water Safety Plan Guide:
Version 1, January 2014 Treatment Processes – Chlorine Dioxide Disinfection
iii
Introduction
Chlorine dioxide (ClO2) treatment is used to disinfect, or oxidise contaminants in, drinkingwater. This Guide is concerned only with using chlorine dioxide as a disinfectant.
If an event occurs during chlorine dioxide treatment (ie, the treatment process doesn’t work
properly), the following could happen:

If there is not enough chlorine dioxide, germs can cause sickness

If there is too much chlorine dioxide, sickness can come from either the high chlorine
dioxide concentration or from chlorine dioxide by-products

High concentrations of by-products from chlorine dioxide treatment can cause
sickness, even when the chlorine dioxide levels are acceptable.
The on-site generation and use of chlorine dioxide 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 chlorine dioxide treatment process and the risks associated with it cannot be viewed in
isolation. This Guide only looks at the introduction of chlorine dioxide into the water.
Chlorine dioxide’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:

whether the chlorine dioxide dose is large enough for other substances in the water to
react with the chlorine dioxide and still leave enough chlorine dioxide to disinfect the
water effectively (discussed in this document)

how long the chlorine dioxide is in contact with the water (see Guide D1)

water temperature

turbidity of the water with the chlorine dioxide is added to it; this can hinder the access
of chlorine dioxide to germs (see the S1, P1, P5 and P6 series of Guides).
If disinfection with chlorine dioxide is going to work as well as possible, all these factors
have to be taken into consideration.
Treating poor quality water with chlorine dioxide can lead to the formation of high
concentrations of chlorite. For this reason, chlorite is always a Priority 2 determinand (see
the DWSNZ:2000 for a definition of Priority 2 determinands) when chlorine dioxide
treatment is used.
Ref P7.2
Water Safety Plan Guide:
Version 1, January 2014 Treatment Processes – Chlorine Dioxide Disinfection
1
Risk Summary
The event creating the greatest risk involved in the chlorine dioxide treatment of drinkingwater is not having enough chlorine dioxide in the water to kill the germs (see P7.2.1).
The most important preventive measures are:

monitor the process to be sure there is enough chlorine dioxide in the water, even when
the quality of the incoming water changes (see P7.2.1.4)

put alarms on the supplies of chemicals used for chlorine dioxide generation to let you
know when supplies are running low. Maintain records so you are aware of when this
might happen; always have spare supplies on hand (see P7.2.1.6).
(References in parentheses are to the Risk Information Table.)
2
Water Safety Plan Guide:
Ref P7.2
Treatment Processes – Chlorine Dioxide Disinfection 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.
NB: This water safety plan assumes chlorine dioxide generation through the chlorine-chlorite
process.
Abbreviations: DWSNZ – Drinking-Water Standards for New Zealand; MAV – Maximum acceptable value – see
DWSNZ:2000
Causes
Preventive measures
Checking preventive measures
What to check
Corrective action
Signs that action is
needed
Event: CHLORINE DIOXIDE (ClO2) CONCENTRATION TOO LOW
Possible hazards: Germs not killed, chlorite (if generator conversion efficiency of chlorite is low).
Level of risk: High1
P7.2.1.1
Dosing
malfunction
(see Guide
P10).



P7.2.1.2
Dosing
controller’s
sensor
incorrectly
calibrated.

Routine controller and 
dosing pump
maintenance.

Replacement of
controller if suspect.

Alarm system to warn
if ClO2 residual
concentration is
incorrect.
Regular manual

checks on calibration
of controller (see

DWSNZ:2000 Section
3.3.4.8).

ClO2 residual
concentration.

Low ClO2 residual 
(see Appendix).
Microbiological
quality.

E. coli or
coliforms detected 
in 100 mL sample
of water leaving
the treatment
plant.

Frequent repair
needed.
Maintenance
log.


Maintenance log
not signed off.
ClO2 residual
concentration.

Low ClO2 residual 
(see Appendix).
Microbiological
quality.

E. coli or
coliforms detected 
in 100 mL sample
of water leaving
the treatment
plant.
Calibration
schedule.

1
Identify cause
of fault and
rectify.
Manually dose
reservoir with
chlorine until
repaired.
Replace
controller.
Recalibrate
controller
sensor.
Increase ClO2
dose rate until
recalibration
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 and the effectiveness of treatment processes prior to chlorine dioxide treatment.
Ref P7.2
Water Safety Plan Guide:
Version 1, January 2014 Treatment Processes – Chlorine Dioxide Disinfection
3
Causes
Preventive measures
Checking preventive measures
What to check
Corrective action
Signs that action is
needed
Event: CHLORINE DIOXIDE (ClO2) CONCENTRATION TOO LOW cont’d

P7.2.1.3
Dosing
controller’s setpoint incorrect
or incorrect
dose
calculation.



P7.2.1.4
High ClO2
demand2
coupled with
poor dose
control.

P7.2.1.5
Power failure.
2
4
Periodic manual
checks on ClO2
residual
concentration,
especially during
periods of water
quality variability.

ClO2 residual
concentration.

Low ClO2 residual 
(see Appendix).

Flow rates.


Dose flow rates.

Dose
calculations.
E. coli or
coliforms detected
in 100 mL sample
of water leaving
the treatment
plant.


Frequent
calculation errors
found.

Calculation
checks not signed
off.

Flow rates lower
than expected.

Low ClO2 residual 
(see Appendix) ,
or E. coli or
coliforms detected
in 100 mL sample

of water leaving
the treatment
plant, only during
periods of poor
water quality.
Independent check on
calculations
(especially after a
system change when
expected dose rates
are uncertain).
Install visual flow
indicators to allow
flow to be checked by
eye.
Ensure that

monitoring of the ClO2
residual is adequate

so that ClO2 dosing
can be adjusted to
take account of
changes in water
quality, or use a
controller that
automatically adjusts
the ClO2 dose to
maintain a
satisfactory residual.
ClO2 residual
concentration.
Total organic
carbon (TOC)
or colour.


Upstream processes
removing substances
contributing to the
ClO2 demand from the
water.

Ensure ClO2
generator can deliver
the maximum
required dose rate.

Stand-by generator or 
battery bank.
Electricity
supply.


TOC or colour
highly variable.

Inadequate ClO2
residual
concentration
even when
generator running

at maximum.
Poor continuity of
power supply.

Adjust controller
set-point.
Recalculate
dose rates and
change
settings.
Train staff in
making dose
calculations.
Replace dose
controller with
more suitable
unit.
Manual
monitoring and
manual ClO2
control during
poor water
quality
episodes.
Optimise
upstream
processes to
better reduce
ClO2 demand.
Replace
generator with
one with
sufficient
capacity.
Refuel
generator (if
appropriate).
Chlorine dioxide demand is the difference between the amount of chlorine dioxide added to the water and the
chlorine dioxide residual remaining after the chlorine dioxide has reacted with other substances in the water. If
the chlorine dioxide demand of the water increases without an increase in the dose, too little disinfectant will
remain to disinfect the water properly.
Water Safety Plan Guide:
Ref P7.2
Treatment Processes – Chlorine Dioxide Disinfection Version 1, January 2014
Causes
Preventive measures
Checking preventive measures
What to check
Corrective action
Signs that action is
needed
Event: CHLORINE DIOXIDE (ClO2) CONCENTRATION TOO LOW cont’d
P7.2.1.6

ClO2 supply
exhausted.

P7.2.1.7
Place alarm on supply 
of chemicals for ClO2
generation to indicate

when a supply is

close to running out.
Maintain records of
ClO2 use to provide a
guide to the length of
time the chlorine and
chlorite supplies are
likely to last.

Ensure spare supplies
of chemicals are
always kept on site.

Preventive
maintenance
programme.
ClO2 generator
malfunction.

P7.2.1.8
ClO2 residual
concentration.

Low ClO2 residual 
(see Appendix).
Install alarm
system.
ClO2 usage.

E. coli or

coliforms
detected in
100 mL sample of
water leaving the
treatment plant.
Hand-dose
chlorine until
system can be
brought back on
line.

Level of ClO2
generation
chemicals too low
to allow
replacement
before they run
out.
Levels of
generation
chemicals.

ClO2 residual
concentration.

Low ClO2 residual 
(see Appendix).

Maintenance
log.

E. coli or
coliforms is

detected in
100 mL sample of
water leaving the
treatment plant.

Soften the water used
to make up the
sodium chlorite
solution to avoid
clogging by scale (if
water is hard).
Frequent repairs
needed.

Identify cause
of fault and
rectify.
Dose with
chlorine until
system can be
brought back on
line.
Clear scale
from lines and
valves.
(See Guide P7.1 for preventive measures, checks and corrective actions.)
Chlorine flow
not reaching
generator.
P7.2.1.9

ClO2 generation
chemistry not
optimised
(wrong chlorine 
and chlorite
ratio,
concentrations,

or pH);
chemicals of
unsatisfactory
quality; or
wrong

chemicals used.
Carry out pilot tests to 
determine optimum
chemical feed rates

for ClO2 production.
Check chemical flows
are correctly set; pH
set point is correct.
Ensure chemicals are
of adequate quality.
Obtain manufacturer’s
certification.
Ensure chemical
storage containers
are properly labelled
and an operator is
present to supervise
delivery.
ClO2 residual
concentration.

Supplier’s

certificate of
analysis for
chemicals used.

Low ClO2 residual 
(see Appendix).
E. coli or
coliforms detected
in 100 mL sample
of water leaving
the treatment
plant.
Modify chemical
feed rates until
ClO2 production
maximised.

Change
chemical
supplier.

Require
chemical
supplier to only
deliver when
the operator is
present.
Unsatisfactory
chemical quality.
Ref P7.2
Water Safety Plan Guide:
Version 1, January 2014 Treatment Processes – Chlorine Dioxide Disinfection
5
Causes
Preventive measures
Checking preventive measures
Corrective action
Signs that action is
needed
What to check
Event: CHLORINE DIOXIDE (ClO2) CONCENTRATION TOO LOW cont’d

P7.2.1.10
ClO2 supply
adequate, but
insufficient ClO2 
reaching dosing
point.

P7.2.1.11
Exposure to
sunlight
resulting in
photodecompos
ition of ClO2.

P7.2.1.12
ClO2 monitoring
samples taken
incorrectly or

incorrectly
recorded (see
Guide D4).

P7.2.1.13
Method of ClO2
measurement
incorrect,
incorrectly
calibrated, or
reagents used
in analysis have
deteriorated.
Routine maintenance 
of pumps dosing ClO2
solution.
ClO2 residual
concentration.
Pump capacity too
low to meet maximum
ClO2 demand.

Low ClO2 residual 
(see Appendix).

Injector
blockages.

Pump failure.

Inadequate pump
specifications.

Low ClO2 residual 
(see Appendix).

E. coli or
coliforms
detected in
100 mL sample of
water leaving the
treatment plant.

Identify and
rectify causes of
pump failure.
Obtain a new
pump.
Ensure ClO2 dosing
solution is protected
from sunlight.

Provide staff training
for sample analysis
and record keeping.

Analysis
records.

Independent
checks show
monitoring
inaccuracies.

Identify staff
training needs
and provide
training.

Analysis
records.

Independent
checks show
monitoring
inaccuracies.

Identify staff
training needs
and provide
training.

Identify cause
of fault and
rectify.

Replace
controller with
new unit.

Recalibrate
controller
sensor.

Manually
monitor and
adjust ClO2
dose rate until
recalibration
undertaken.
ClO2 residual
concentration.
Develop monitoring
schedule and roster.
Provide staff training
for sample analysis
and record keeping.
Cover open
channels or
reticulate in
pipes.
Event: CHLORINE DIOXIDE (ClO2) CONCENTRATION TOO HIGH
Possible hazards: Chlorite, chlorate, chlorine dioxide.
Level of risk: Low–moderate

P7.2.2.1
Dosing
malfunction
(see Guide
P10).



P7.2.2.2
Dosing
controller’s
sensor
incorrectly
calibrated.
6
Routine controller and 
dosing pumps
maintenance.

Replacement of
controller if suspect.
Fit an alarm to
indicated incorrect
CIO2 concentration.
Regular manual

checks on calibration
of controller (see
DWSNZ:2000 Section
3.3.4.8).
ClO2 residual 
concentration.
Maintenance
log.
ClO2 residual
concentration more
than 50% of its
MAV.

Frequent repair
needed.

Maintenance log not
signed off.
ClO2 residual 
concentration.

ClO2 residual
concentration more
than 50% of its
MAV.
Calibration
schedule not signed
off.
Water Safety Plan Guide:
Ref P7.2
Treatment Processes – Chlorine Dioxide Disinfection Version 1, January 2014
Causes
Preventive measures
Checking preventive measures
Corrective action
Signs that action is
needed
What to check
Event: CHLORINE DIOXIDE (ClO2) CONCENTRATION TOO HIGH cont’d
P7.2.2.3
Dosing
controller’s setpoint incorrect
or incorrect
dose
calculation.
P7.2.2.4



ClO2 residual 
concentration.

Flow rates.

Dose
calculations.
Independent check on
calculations
(especially after a
system change when
expected dose rates
are uncertain).

Install visual flow
indicators to allow
flow to be checked by
eye.

Use a control method 
that links dose control
to ClO2 residual at
appropriate location.
Low ClO2
demand
coupled with
poor ClO2 dose
control.
P7.2.2.5
Periodic manual
checks on ClO2
concentration,
especially during
periods of water
quality variability.

ClO2 solution
strength from
generator too
high.
Check that a
satisfactory ClO2
concentration is
produced when new
supply of chemicals
for generation is first
brought into use.
ClO2.

ClO2.

ClO2 level in
the ClO2
solution.
ClO2 residual
concentration more
than 50% of its
MAV.

Adjust controller
set-point.

Frequent calculation
errors found by
checks.


Calculation checks
not signed off.

Flow rates below
expected values.

Train staff in
making dose
calculations.

ClO2 residual
concentration more
than 50% of its
MAV.

Replace dose
controller with
more suitable
unit.

Manual
monitoring and
manual ClO2
control when
water quality is
variable.

Determine the
cause of the
high ClO2
concentration
and rectify.

Provide training
in the
preparation of
solutions
(including
calculations).

ClO2 residual
concentration more
than 50% of its
MAV.

ClO2 concentration
in dosing solution is
too high.
Recalculate
dose rates and
change
settings.
Event: EXCESSIVE FORMATION OF BY-PRODUCTS FROM CHLORINE DIOXIDE TREATMENT
Possible hazards: Chlorite, chlorate.
Level of risk: Moderate
P7.2.3.1

ClO2 generation
chemistry not
optimised
(wrong chlorine 
and chlorite
ratio,
concentrations,
or pH).

Carry out pilot tests to 
determine optimum

chemical feed rates
for ClO2 production.
Checks to ensure
chlorite solution
concentration is
correct.
Provide staff training
in chlorite solution
preparation.
Chlorite.
Chlorate.

Elevated levels of
chlorite and
chlorate.

Modify chemical
feed rates until
ClO2 production
maximised, and
by-products
minimised.

Identify staff
training needs
and provide
training.
Ref P7.2
Water Safety Plan Guide:
Version 1, January 2014 Treatment Processes – Chlorine Dioxide Disinfection
7
Causes
Preventive measures
Checking preventive measures
Corrective action
Signs that action is
needed
What to check
Event: EXCESSIVE FORMATION OF BY-PRODUCTS FROM CHLORINE DIOXIDE TREATMENT cont’d

P7.2.3.2
Natural organic
matter present
in the water
being
chlorinated
(reduction of
ClO2 to chlorite).
8
Provision of treatment 
processes upstream
to reduce levels of
organic matter in the
water.
TOC/colour.

Elevated
TOC/colour.

Elevated chlorite
formation.

Optimise
treatment
parameters in
upstream
processes to
maximise
removal of
organic matter.
Water Safety Plan Guide:
Ref P7.2
Treatment Processes – Chlorine Dioxide Disinfection Version 1, January 2014
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 – ClO2 concentration is lower than minimum effective level
Indicators:
Required
actions:
Responsibility:

A detectable ClO2 residual cannot be obtained in the water
leaving the treatment plant.

In 100 ml samples of water leaving the treatment plant, E. coli or
coliforms are continually detectable, or E. coli is present at
elevated levels (more than 10 per 100 mL).

Widespread illness in the community.

Follow the actions given in Figure 3.2 of the DWSNZ:2000.

Identify the reason for the failure and rectify.

Record cause of system failure and steps taken to correct.

Modify your water safety plan if necessary.
Manager designated responsible for the water supply.
Ref P7.2
Water Safety Plan Guide:
Version 1, January 2014 Treatment Processes – Chlorine Dioxide Disinfection
9
Event – ClO2 concentration is higher than maximum acceptable value
Indicators:
Required
actions:
Responsibility:
10

Knowledge of a major overdose of ClO2 into the water.

Inability to obtain pink colour from DPD indicator despite high
ClO2 dose rates. (NB: This indicates ClO2 levels well in excess
of the MAV – very high ClO2 levels bleach the pink colour that
normally develops in the presence of ClO2.)

Change in the odour or taste of the water.

Widespread levels of taste and odour complaints, or illness, in
the community.

Close down the plant. Provide another source of potable water
until water of acceptable quality can again be supplied.

Inform the MOH of the situation.

Identify the reason for the ClO2 overdose and rectify.

Dump the reservoir water, or add chemicals to neutralise the
ClO2 if more appropriate (neutralisation may be required as a
condition of the Resource Consent, anyway).

Flush the distribution system, if excessive levels of ClO2 are also
present in the distribution system, and monitor water quality
until ClO2 concentrations are again back to normal operating
levels.

Warn consumers to thoroughly flush their taps before drawing
water for use (if they are likely to have been affected).

Record cause of system failure and steps taken to correct.

Modify your water safety plan if necessary.
Manager designated responsible for the water supply.
Water Safety Plan Guide:
Ref P7.2
Treatment Processes – Chlorine Dioxide Disinfection Version 1, January 2014
Event – Chlorite or chlorate concentrations higher than maximum acceptable value
(MAV)
Indicators:
Required
actions:
Responsibility:

Chlorite concentration more than 0.3 mg/L.

Chlorate concentration more than 0.3 mg/L.

Decrease the ClO2 dose, and monitor E. coli and coliforms to
ensure that reduction in the ClO2 has not led to inadequate
disinfection. The ClO2 dose should still meet the C.t
requirements for Cryptosporidium inactivation (see Appendix
P7.2).

Inform the MOH of the situation, and determine what options for
treatment changes could be considered. Supplementary use of
chlorine may provide satisfactory disinfection.

Record the steps taken to correct the problem.

Modify your water safety plan if necessary.
Manager designated responsible for the water supply.
Ref P7.2
Water Safety Plan Guide:
Version 1, January 2014 Treatment Processes – Chlorine Dioxide Disinfection
11
Water Safety Plan Performance
Assessment
To ensure 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:

ClO2 concentration.

E. coli (faecal indicator), coliforms.

Chlorite and chlorate.
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 chlorine dioxide process (eg,
chlorine dioxide is not in contact with water long enough).
How often:

For the monitoring frequencies for 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
chlorine dioxide treatment.

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:
12
Manager designated responsible for the water supply.
Water Safety Plan Guide:
Ref P7.2
Treatment Processes – Chlorine Dioxide Disinfection Version 1, January 2014
Appendix (P7.2)
Table (P7.2): C.t values (mg.min/L) for inactivation of Cryptosporidium by chlorine dioxide*
Temp (°C)
*
99%
99.9%
0.5
1530
2170
5
829
1180
10
429
609
15
227
322
20
123
174
25
67.8
96.3
30
38.2
54.2
From Table 13.1 of the Drinking-water Standards for New Zealand 2000.
Ref P7.2
Water Safety Plan Guide:
Version 1, January 2014 Treatment Processes – Chlorine Dioxide Disinfection
13