Water Safety Plan
Guide
Treatment Processes
– Chlorination Disinfection
Version 1, Ref P7.1
January 2014
Citation: Ministry of Health. 2014. Water Safety Plan Guide:
Treatment Processes – Chlorination Disinfection, Version 1, ref p7.1.
Wellington: Ministry of Health.
Published in June 2001 by
Ministry of Health
PO Box 5013, Wellington, New Zealand
ISBN: 978-0-478-42740-0 (print)
ISBN: 978-0-478-42741-7 (online)
Previously published in 2001 as Public Health Risk Management
Plan Guide: Treatment Processes – Chlorination Disinfection,
Version 1, ref p7.1. 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
10
Water Safety Plan Performance Assessment
12
Appendix P7.1
13
Ref P7.1
Version 1, January 2014
Water Safety Plan Guide:
Treatment Processes – Chlorination Disinfection
iii
Introduction
Chlorination is used to disinfect, or oxidise contaminants in, drinking-water. This Guide is
concerned only with using chlorine as a disinfectant.
Chlorination is usually done at the treatment plant. If there is a problem in keeping the
chlorine concentration high enough, possibly because the distribution system is large, the
chlorine concentration can be boosted within the system. This process is also considered
here.
If an event occurs during chlorination (ie, the chlorination process doesn’t work properly),
the following could happen:

If there is not enough free available chlorine (FAC), germs may cause sickness

If there is too much FAC, sickness can come from either the high chlorine
concentration or from by-products of the chlorination process

High concentrations of chlorination by-products can cause sickness, even when FAC
levels are acceptable.
Chlorine (either gas, liquid or solid) 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 chlorination process and the risks associated with it cannot be viewed in isolation. This
Guide only looks at the chlorination of the water. Chlorine’s value as a disinfectant, and
what happens when chlorine is added to water, are also affected by elements of the water
supply system dealt with in other Guides.
Several factors influence how effective disinfection is:

whether the chlorine dose is large enough for other substances in the water to react
with the chlorine and still leave enough FAC to disinfect the water effectively
(discussed in this document)

how long the FAC is in contact with the water (see Guide D1)

pH of the water; this affects how much of the chlorine is in a form that is good at killing
germs (see Guide P8.1 and the figure in the Appendix of this Guide)

water temperature

turbidity of the water when the chlorine is added to it; this can hinder the access of
chlorine to target germs (see the S1, P1, P5 and P6 series of Guides).
If the whole chlorination process is going to work as well as possible, all these factors have
to be taken into consideration.
Ref P7.1
Version 1, January 2014
Water Safety Plan Guide:
Treatment Processes – Chlorination Disinfection
1
Risk Summary
The event creating the greatest risk involved in chlorinating drinking-water is not having
enough FAC to kill germs in the water, not only at the beginning of the process but all the
way through it (see P7.1.1).
The most important preventive measures are:

monitor the process to be sure there is enough FAC in the water, regardless of how the
quality of the incoming water might change (see P7.1.1.4)

put an alarm on the chlorine supply to let you know when the supply is running low.
Maintain records so you are aware of when this might happen; always have a spare
supply on hand (see P7.1.1.6)

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 – Chlorination Disinfection
Ref P7.1
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; MAV – Maximum acceptable value – see
DWSNZ:2000
Causes
Preventive measures
Corrective
action
Checking preventive measures
What to check
Signs that action is
needed
Event: NOT ENOUGH FREE AVAILABLE CHLORINE
Possible hazards: Germs not killed.
Level of risk: High1
P7.1.1.1

Dosing
malfunction
(see Guide
P10).
P7.1.1.2
Dose
controller’s
sensor
incorrectly
calibrated.
1
Routine maintenance of 
dose-controller and

dosing pump.

FAC.

FAC
concentration is
less than
0.2 mg/L.

E. coli or
coliforms detected
in 100 mL sample
of water leaving
the treatment
plant.
Microbiological
quality.
Maintenance
log.

Replacement of
controller if suspect.

Frequent repairs
recorded.

Alarm system to warn if
FAC concentration
incorrect.

Maintenance log
not signed off.

Regular manual checks
on calibration of sensor
(see DWSNZ:2000).

FAC
concentration is
less than
0.2 mg/L.

FAC.

Microbiological
quality.

Calibration
schedule.

E. coli or
coliforms detected
in 100 mL sample
of water leaving
the treatment
plant.

Calibration
schedule not
signed off.

Identify cause
of fault and
rectify.

Manually
dose reservoir
with chlorine
until controller
repaired.

Replace
controller.

Recalibrate
controller.

Increase
chlorine dose
rate until
recalibration
undertaken.
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 the chlorination process.
Ref P7.1
Version 1, January 2014
Water Safety Plan Guide:
Treatment Processes – Chlorination Disinfection
3
Causes
Preventive measures
Corrective
action
Checking preventive measures
What to check
Signs that action is
needed
Event: NOT ENOUGH FREE AVAILABLE CHLORINE cont’d

P7.1.1.3
Dose
controller’s setpoint incorrect,
or incorrect
dose
calculation.

Independent check on
calculations (especially
after a system change
when expected dose
rates are uncertain.

Install visual flow
meters to allow chlorine
flow to be checked by
eye.

P7.1.1.4
High chlorine
demand2
coupled with
poor dose
control.

P7.1.1.5
Power failure.
2
4
Periodic manual checks 
on FAC concentration,

especially during
periods of water quality
variability
Ensure that monitoring
of the FAC residual is
adequate so that
chlorine dosing can be
adjusted to take
account of changes in
water quality, or use a
controller that
automatically adjusts
the chlorine dose to
maintain a satisfactory
residual.

FAC.
Dose flow rates
and dose
calculations.

FAC.

Microbiological
quality.

Total organic
carbon (TOC)
or colour level.
Use upstream
processes that remove
substances contributing
to the chlorine demand
of the water.

Ensure chlorinator is
designed to provide
enough chlorine to meet
the maximum required
dose.

Stand-by generator or
battery bank.


FAC
concentration is
less than
0.2 mg/L.

E. coli or

coliforms detected
in 100 mL sample
of water leaving
the treatment

plant.
Microbiological
quality.
Electricity
supply.


Frequent
calculation errors
found.

Flow rates lower
than expected.

FAC

concentration is
less than
0.2 mg/L or E. coli
or coliforms

detected in
100 mL sample of
water leaving the
treatment plant
during periods of
poor water
quality.

TOC/colour levels

highly variable.

Inadequate FAC
even when
chlorinator
running at
maximum setting.

Poor continuity of
power supply.
Adjust
controller setpoint.
Recalculate
dose rates
and change
settings.
Train staff in
making dose
calculations.
Replace dose
controller with
a more
suitable unit.
Manual
monitoring
and manual
chlorine
control during
poor water
quality
episodes.
Optimise
upstream
processes to
reduce
chlorine
demand.

Replace
chlorinator
with one with
sufficient
capacity.

Refuel
generator (if
one is used).
Chlorine demand is the difference between the amount of chlorine added to the water and the FAC residual
remaining after the chlorine has reacted with other substances in the water. If the chlorine 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:
Treatment Processes – Chlorination Disinfection
Ref P7.1
Version 1, January 2014
Causes
Preventive measures
Corrective
action
Checking preventive measures
What to check
Signs that action is
needed
Event: NOT ENOUGH FREE AVAILABLE CHLORINE cont’d
P7.1.1.6

Chlorine supply
exhausted.

P7.1.1.7
Chlorine
concentration in
the dosing
solution is low
because:
Ensure a spare chlorine
container is always kept
on site.

Ensure chlorine
solutions are kept dark
and cool (hypochlorite
chlorination).


FAC.

Chlorine level in
chlorine supply.

Chlorine usage.
Maintain records of
chlorine use to provide
a guide to the length of
time the supply is likely
to last.

 it has
decomposed
through the
solution being
old, or
exposed to

sunlight
 the chemical
used to
prepare the
solution is of
poor quality.
Place an alarm on the
chlorine supply to
indicate supply is close
to running out.

FAC.

Storage
conditions of
chlorine
solution.
Check that an adequate 
FAC level is produced
when a new chlorine
supply is first brought
into use.
Chlorine
concentration in
solution
generated from
brine.

Supplier’s
certificate of
analysis of
chemicals used
to prepare
chlorine dosing
solution.
Check quality of salt
(brine electrolysis), or
chlorinated chemicals
used (hypochlorite
chlorination).
Ref P7.1
Version 1, January 2014

FAC
concentration is
less than
0.2 mg/L.

E. coli or
coliforms detected
in 100 mL sample
of water leaving
the treatment
plant.

Level of chlorine
supply drops too
low to allow
replacement
before the
container in use
runs out.

FAC
concentration is
less than
0.2 mg/L.



Install alarm
system.

Hand-dose
chlorine until
system can
be brought
back on line.

Increase the
chlorine dose.

Obtain a fresh
container of
chlorine.
E. coli or
coliforms

detected in
100 mL sample of
water leaving the
treatment plant.
Chlorine solution
exposed to
sunlight and/or
allowed to warm.

No, or low,
chlorine level
produced in
dosing solution.

No chlorine
detectable when
solution first
prepared.

Quality of salt
inadequate.
Replace
chlorine
dosing
solution.

Increase the
chlorine dose.

Prepare a
new brine
solution with
salt of
satisfactory
quality.
Water Safety Plan Guide:
Treatment Processes – Chlorination Disinfection
5
Causes
Preventive measures
Corrective
action
Checking preventive measures
What to check
Signs that action is
needed
Event: NOT ENOUGH FREE AVAILABLE CHLORINE cont’d
P7.1.1.8

Chlorine supply
adequate, but
insufficient
chlorine

reaching dosing
point.


Install filters to avoid
blockage of valves or
venturi by particles in
the water.
FAC.

Pump
maintenance
log.
Routine maintenance of

pumps (chlorine

solution and venturi
booster) (see Guide
P10).

Check quality of salt;
insoluble solids may
cause blockages (brine

electrolysis).
Ensure that all lines
carrying chlorine are
properly dried after
opening for
maintenance to avoid
the development of
corrosion products and
their blocking the gas
lines (gas chlorination).
Chlorine lines.

FAC
concentration is
less than
0.2 mg/L.

Check filter
and unclog or
replace.

E. coli or

coliforms
detected in
100 mL sample of
water leaving the
treatment plant.

Identify cause
of pump fault
and rectify.
Gas flow meter
readings.
Chlorine
solution flow
meter readings.
Supplier’s
certificate of
analysis of salt.


Min-max
thermometer
readings.
Injector and
chlorine line
blockages.

Pump failure.

Chlorinator
dose rate
specifications.

Gas flow readings
insufficient to

provide adequate
dose.

Prepare fresh
brine solution.
Replace
chlorine lines
and dry
before
resealing
system.
Train staff in
correct
maintenance
procedures.

Install visual flow
meters to allow chlorine
flow to be checked by
eye.

Salt contains
compounds likely
to precipitate and
cause blockages.

Provide indirect heating
for drum storage and
chlorinator rooms to
ensure chlorine is kept
gaseous in supply lines
(gas chlorination).

Minimum
temperature less
than 10C.

Ensure chlorinator is
designed to provide
enough chlorine to meet
the maximum required
dose.

Maximum chlorine 
dose rate that can
be delivered too
low.
Replace
chlorinator
with one with
sufficient
capacity.

Ensure adequate
pressure for injector
venturi by:

Establish
reason for
poor injector
pressure and
rectify.
–
–
6

good hydraulics
design

Establish
reason for low
temperature
and rectify.
use of a booster
pump.
Water Safety Plan Guide:
Treatment Processes – Chlorination Disinfection
Ref P7.1
Version 1, January 2014
Causes
Preventive measures
Corrective
action
Checking preventive measures
What to check
Signs that action is
needed
Event: NOT ENOUGH FREE AVAILABLE CHLORINE cont’d
P7.1.1.9

Inadequate
output from
chlorine booster
stations (if
used).
P7.1.1.10
FAC monitoring
samples taken
incorrectly or
incorrectly
recorded (see
Guide D4).
P7.1.1.11
Method of FAC
measurement
incorrect,
incorrectly
calibrated, or
analysis
reagents have
deteriorated.
P7.1.1.12
pH too high
(resulting in a
lower
percentage of
the FAC
existing in its
more powerful
disinfecting
form).
Many of the causes and 
preventative measures

noted in P7.1.1.1–
P7.1.1.8 for chlorination
at the treatment plant
are also applicable to
chlorine boosting in the
distribution system.
Refer to these.

Put in place either
manual monitoring or inline monitoring of the
FAC residual to provide
reliable control of the
booster dose rate.

Provide staff training for 
sample analysis and
record keeping.

Develop monitoring
schedule and roster.

Provide staff training for 
sample analysis and
record keeping.
FAC.

FAC
concentration is
less than
0.2 mg/L.

E. coli or
coliforms
detected in
100 mL sample of
water leaving the 
treatment plant.
Microbiological
quality.

Take steps
noted in
P7.1.1.1–
P7.1.1.8, as
appropriate.
Adjust dose
setting.
Records of FAC 
analysis results.
Monitoring
inaccuracies.

Identify
shortcomings
in staff
training and
rectify.
Records of FAC 
analysis results.
Monitoring
inaccuracies.

Identify
shortcomings
in staff
training and
rectify.
For information on causes of pH being too high and preventive measures, see Guide P8.1.
See the Appendix for a graph showing how the effectiveness of chlorine as a disinfectant
decreases as the pH increases.
Ref P7.1
Version 1, January 2014
Water Safety Plan Guide:
Treatment Processes – Chlorination Disinfection
7
Causes
Preventive measures
Corrective
action
Checking preventive measures
What to check
Signs that action is
needed
Event: TOO MUCH FREE AVAILABLE CHLORINE
Possible hazards: Chlorine; possibly high levels of trihalomethanes, haloacetic acids, chloral hydrate and
chlorate (hypochlorite chlorination); possible heavy metals (from corroded fittings).
Level of risk: Low–moderate3

P7.1.2.1
Dosing
malfunction
(see Guide
P10).

Replacement of
controller if suspect.

Install an alarm system
to indicate when FAC
concentration is outside
designated limits.

P7.1.2.2
Dose controller
incorrectly
calibrated.

P7.1.2.3
Dose controller
set-point
incorrect, or
incorrect dose
calculation.



P7.1.2.4
Low chlorine
demand2
coupled with
poor chlorine
dose control.
Routine maintenance of 
dose controller and

dosing pumps.
Regular manual checks
on calibration of FAC
controller.
FAC.
Examination of
maintenance
log.

FAC.

Calibration
schedule.
Periodic manual checks 
on FAC concentration,

especially during
periods of water quality
variability.
FAC.

Install visual flow
indicators to allow flow
to be checked by eye.
FAC.
FAC

concentration is
more than 50% of
its MAV.4

Maintenance log
shows frequent
maintenance
needed.

Maintenance log
not signed off.

FAC

concentration is
more than 50% of

its MAV.

Calibration
schedule not
signed off.

FAC

concentration is
more than 50% of
its MAV.

Frequent
calculation errors
found by checks.

Calculation
checks not signed 
off.
Dose flow rates
and dose
calculations.
Independent check on
calculations (especially
after a system change
when expected dose
rates are uncertain).
Use a control method

that links dose control to
FAC residual at
appropriate location.



Flow rates above
expected values.

FAC

concentration is
more than 50% of
its MAV.

Identify cause
of fault and
rectify.
Replace
controller with
new unit.
Recalibrate
controller.
Decrease
chlorine dose
rate until
recalibration.
Adjust
controller setpoint.
Recalculate
dose rates
and change
settings.
Train staff in
making dose
calculations.
Replace dose
controller with
more suitable
unit.
Manual
monitoring
and manual
chlorine
control when
water quality
is variable.
2
Chlorine demand is the difference between the amount of chlorine added to the water and the FAC residual
remaining after the chlorine has reacted with other substances in the water. If the chlorine demand of the water
increases without an increase in the dose, too little disinfectant will remain to disinfect the water properly.
3
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.
8
Water Safety Plan Guide:
Treatment Processes – Chlorination Disinfection
Ref P7.1
Version 1, January 2014
Causes
Preventive measures
Corrective
action
Checking preventive measures
What to check
Signs that action is
needed
Event: TOO MUCH FREE AVAILABLE CHLORINE cont’d
P7.1.2.5

Chlorine dose
solution
strength too
high.
P7.1.2.6

Spillage of
chlorine
compound or
solution into
dosing solution.
P7.1.2.7
Check that a

satisfactory FAC level is

produced when a new
chlorine container is
brought into use.
Train staff in handling
and labelling of
chemicals.
Chlorine level in
chlorine dosing
solution.

FAC.

Chlorine level in
chlorine dosing
solution.

Many of the causes and 
preventative measures
noted in P7.1.2.1–
P7.1.2.6 for chlorination
at the treatment plant
are also applicable to
chlorine boosting in the
distribution system.
Refer to these.

Put in place either
manual monitoring or inline monitoring of the
FAC residual to provide
reliable control of the
booster dose rate.
Chlorine
overdose at a
chlorine booster
station.
FAC.
FAC.

FAC

concentration is
more than 50% of
its MAV.

Chlorine levels in
dosing solution
too high.

Determine the
cause of the
high chlorine
concentration
and rectify.
Provide
training in the
preparation of
chlorine
solutions
(including
calculations).

FAC

concentration is
more than 50% of
its MAV.

Chlorine levels in
dosing solution
too high.

FAC

concentration is
more than 50% of
its MAV.
Take steps
noted in
P7.1.1.1–
P7.1.1.8, as
appropriate.

Adjust dose
setting.
Identify the
reasons for
the spillage
and rectify if
possible.
Event: EXCESSIVE FORMATION OF CHLORINATION BY-PRODUCTS
Possible hazards: Trihalomethanes, haloacetic acids, chloral hydrate and chlorate (hypochlorite chlorination).
Level of risk: Low–moderate3
P7.1.3.1
Natural organic
matter present
in the water
being
chlorinated.
3

Provision of treatment

processes upstream to
reduce levels of organic
matter in the water.
TOC/colour.

Elevated TOC or
colour.

Elevated
disinfection byproduct formation.

Optimise
treatment
parameters in
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 P7.1
Version 1, January 2014
Water Safety Plan Guide:
Treatment Processes – Chlorination Disinfection
9
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 – FAC concentration is lower than minimum acceptable level
Indicators:
Required
actions:
Responsibility:
10

A detectable chlorine 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 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.
Water Safety Plan Guide:
Treatment Processes – Chlorination Disinfection
Ref P7.1
Version 1, January 2014
Event – FAC concentration is very much higher than maximum acceptable value
Indicators:
Required
actions:
Responsibility:

A major spillage or overdose of chlorine into the water.

Inability to obtain pink colour from DPD chlorine indicator
despite high chlorine dose rates. (NB: This indicates chlorine
levels well in excess of the MAV – very high chlorine levels
bleach the pink colour that normally develops in the presence of
chlorine.)

Water develops a strongly chlorinous odour.

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 chlorine overdose and rectify.

Dump the reservoir water, or add chemicals to neutralise the
chlorine if more appropriate (neutralisation may be required
before any water is dumped, anyway).

Flush the distribution system, if excessive levels of chlorine are
also present in the distribution system, and monitor water quality
until chlorine 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.
Ref P7.1
Version 1, January 2014
Water Safety Plan Guide:
Treatment Processes – Chlorination Disinfection
11
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:

FAC.

E. coli (faecal indicator) or coliforms.
Follow the protocols set out in DWSNZ:2000.
The presence of faecal indicators may be influenced by factors
other than the adequacy of the chlorination process (eg, chlorine is
not in contact with the water long enough). The presence of
coliforms indicates that disinfection is not killing all bacteria.
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:

Record results 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 chlorination.

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:
Treatment Processes – Chlorination Disinfection
Ref P7.1
Version 1, January 2014
Appendix P7.1
Figure P7.1: Graph showing the increase in the FAC needed for satisfactory disinfection
as the pH of a water increases
FACmg/L
1.6
1.4
1.2
1
0.8
0.6
0.4
0.2
0
7.8
8
8.2
8.4
8.6
8.8
9
9.2
pH
This plot shows how the FAC concentration needed to disinfect as well as an FAC
concentration of 0.2 mg/L at pH 8 increases as the pH of the water increases.
Ref P7.1
Version 1, January 2014
Water Safety Plan Guide:
Treatment Processes – Chlorination Disinfection
13