Water Safety Plan
Guide
Treatment Processes
– Ultraviolet Irradiation Disinfection
Version 1, Ref P7.4
January 2014
Citation: Ministry of Health. 2014. Water Safety Plan Guide:
Treatment Processes – Ultraviolet Irradiation Disinfection, Version 1,
ref p7.4. Wellington: Ministry of Health.
Published in January 2014
by the Ministry of Health
PO Box 5013, Wellington, New Zealand
ISBN: 978-0-478-42746-2 (print)
ISBN: 978-0-478-42747-9 (online)
Previously published in 2001 as Public Health Risk Management
Plan Guide: Treatment Processes – Ultraviolet Irradiation
Disinfection, Version 1, ref p7.4. 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
Ref P7.4, Version 1
Water Safety Plan Guide:
January 2014
Treatment Processes – Ultraviolet Irradiation Disinfection
iii
Introduction
Shining intense ultraviolet (UV) light through drinking water (UV irradiation) can be used to
disinfect it. This Guide is concerned with this process.
If an event occurs during UV irradiation (ie, the UV disinfection process doesn’t work
properly), the UV dose may be too little to kill germs and stop them from causing sickness.
UV treatment can only kill germs while they are being irradiated. The UV treatment unit
should therefore be installed as close to the point of use as possible. This reduces the
likelihood of germs getting into the water after treatment, or germs that have been only
damaged reviving.
The use of UV irradiation 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 UV disinfection process and the risks associated with it cannot be viewed in isolation.
This Guide looks only at the UV irradiation of the water. The value of UV light 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:

intensity and wavelength(s) of the UV delivered; germs are killed fastest by intense
light in a particular wavelength range

exposure period; the longer germs are exposed to UV light, the higher the percentage
that are killed

design of the contact chamber where the water is irradiated; germs can escape full
exposure to the light if design is poor

water quality (turbidity, colour, iron, manganese, hardness); constituents of the water
that absorb light or form deposits on lamp surfaces reduce the effectiveness of the light
in killing germs (see the S1, P1, P5 and P6 series of Guides and Guide P8.2).
Ref P7.4, Version 1
Water Safety Plan Guide:
January 2014
Treatment Processes – Ultraviolet Irradiation Disinfection
1
Risk Summary
The event creating the greatest risk involved in UV irradiation of drinking-water is the UV
dose not being too low to kill germs in the water (see P7.4.1).
The most important preventive measures are:

regularly maintain the unit and clean the surface of the lamp, or lamp sleeve, to make
sure enough UV light passes into the water (see P7.4.1.1)

use a sensor to measure the UV irradiation (see P7.4.1.1)

put an alarm on the irradiation sensor to let you know when the light intensity is too
low and to cut off the water flow if this happens (see P7.4.1.1).
(References in parentheses are to the Risk Information Table.)
2
Water Safety Plan Guide:
Ref P7.4, Version 1
Treatment Processes – Ultraviolet Irradiation Disinfection
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
Corrective action
Signs that action
is needed
What to check
Event: UV DOSE TOO LOW
Possible hazards: Germs not killed.
Level of risk: High1
P7.4.1.1
Inadequate
UV intensity
at the
required
wavelength.






1
Routine cleaning and
maintenance schedule
for lamp sleeves and
UV sensor.
Regular replacement of
UV lamp.
The sensor for
measuring UV radiant
exposure should be on
the wall of the
irradiation chamber
farthest from the UV
lamp.
Lubrication of seal each
time the tube is
removed (with waterbased lubricant).
Reduce water hardness
to minimise scaling on
lamp sleeves and UV
sensor lens.
Link UV sensor to alarm
to show inadequate UV
intensity, and a solenoid
to shut off water flow.

Microbiological
quality.

UV intensity.

Alarm.

Signs of scaling
on the lamp’s
quartz sleeve

and sensor
lens.

Maintenance
records.


E. coli or

aerobic
bacterial spores
is detected in

100 ml sample
of water leaving 
the UV unit.
Activation of the
UV intensity

alarm.
Clean quartz
sleeve and sensor
lens.
Replace UV lamp.
Relocate sensor,
or purchase a unit
with correctly
located sensor.
Lubricate sleeve
seal.
Water flow shut
off.


Water inside
quartz sleeve
(ineffective
seal).
Check water
hardness and
treat to reduce if
necessary.


Scale formation
on the quartz
sleeve.

Absence of
maintenance
records.
Manually disinfect
storage tank
(chlorine) until
satisfactory water
quality can be
restored, or close
the supply down
and arrange for
another supply of
water.
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 UV disinfection process.
Ref P7.4, Version 1
Water Safety Plan Guide:
January 2014
Treatment Processes – Ultraviolet Irradiation Disinfection
3
Causes
Preventive measures
Checking preventive measures
Corrective action
Signs that action
is needed
What to check
Event: UV DOSE TOO LOW cont’d

P7.4.1.2
Time of
exposure to
UV radiation
inadequate

due to poor
flow rate
control,
incorrect dose 
calculation, or
low water
temperature.
Independent check on
calculations (especially
after alterations to the
system).
Routine maintenance
and calibration checks
on flow rate controller.
Establish how UV lamp
output is affected by
temperature, and
reduce water flow
accordingly.

Microbiological
quality.

UV dose
(intensity x
time).

Maintenance
records of flow
rate controller.

Minimummaximum
thermometer
readings of
water
temperature.
P7.4.1.3

Install adequate
filtration system.

Excessive
colour or
turbidity.
Microbiological
quality.

Regular replacement
and maintenance of
filters.

Filter
maintenance
schedule.

P7.4.1.4
Power supply
failure.
4


Turbidity/colour
post-filter.

Pressure drop
across the filter
(breakthrough
or rupture may
lead to low
pressure drop).
Regular maintenance of 
UV power supply.

An alarm to indicate
power failure.

Stand-by generator or
battery bank.

Solenoid shut-off valve
to stop flow.

E. coli or

aerobic
bacterial spores
is detected in

100 ml sample
of water leaving

the UV unit.

Calculated UV
dose at
wavelength of
240-290 nm is
less than
400 J/m2
[40 mW.s/cm2].

Calculations not
cross-checked
and signed off.

Maintenance
and calibration
checks not
signed off.

E. coli or

aerobic
bacterial spores
is detected in

100 ml sample
of water leaving
the UV unit.

Excessive
turbidity or
colour
(sufficient to
reduce the UV
intensity to a
level that is too
low).

Absence of
maintenance
records.
Alarm.

Power supply
maintenance
schedule.
No electrical
power.

Absence of
maintenance
records.
Recalculate dose
rates and change
flow settings.
Identify cause of
fault and rectify.
Undertake flow
controller
calibration.

Reduce water
flow until UV
irradiation dose
exceeds
40 mW.s/cm2 is
acceptable.

Manually disinfect
storage tank
(chlorine) until
satisfactory water
quality can be
restored, or close
the supply down
and arrange for
another supply of
water.
Obtain a more
suitable filter
system.
Replace filter
cartridge.

Make
arrangements for
a more reliable
power supply to
be provided.

Manually dose
chlorine into the
holding tank, or
close the supply
until power is
restored and
arrange for
another supply of
water.
Water Safety Plan Guide:
Ref P7.4, Version 1
Treatment Processes – Ultraviolet Irradiation Disinfection
January 2014
Causes
Preventive measures
Checking preventive measures
Corrective action
Signs that action
is needed
What to check
Event: UV DOSE TOO LOW cont’d
P7.4.1.5

Monitoring
samples not
taken or

incorrectly
recorded (see
Guide D4).
P7.4.1.6

Method of UV
dose/turbidity/
colour
measurement 
incorrect, or
instruments
incorrectly
calibrated
(see Guide
D4).
Staff training for sample 
analysis and record
keeping.
Develop monitoring
schedule and roster.
Audit staff
training and
sampling
records.


Monitoring
schedule not
followed.
Maintenance
log and
schedule for
equipment.

E. coli or

aerobic
bacterial spores

detected in
100 ml sample
of water leaving
the UV unit.
Develop a quality
assurance system for
monitoring, and
instrument calibration.

Staff training in quality
assurance methods.

Monitoring
schedule.

Audit staff

training and
methods used
to monitor water
quality.


Inadequate
records.
Equipment fails
quality control
checks.
Evidence that
monitoring
schedule has
not been
followed.

Train staff.

Put monitoring
schedule in place.
Replace faulty
equipment.
Start using
approved
methods for
monitoring (see
DWSNZ:2000
where
necessary).

Put in place a
maintenance
schedule for
instruments.

Train staff.
Evidence of use
of
unsatisfactory
methods and
instruments for
monitoring.
Event: AFTER TREATMENT, WATER IS RE-INFECTED, OR GERMS REVIVE
Possible hazards: Germs.
Level of risk: Low–High2
P7.4.2.1
A number of practices may lead to
re-infection. (See Guides D2.3 and D2.4 for
causes and preventive actions.)
Resuscitation of micro-organisms is a
natural process, therefore Corrective
Actions, rather than Preventive Measures,
are required.
2

Microbiological
water quality.

E. coli or

aerobic
bacterial spores
is detected in
100 ml sample
of water leaving
the UV unit.

Heterotrophic
plant count in
distribution
system is higher
than that
leaving the UV
unit.
Periodic shock
dose of the
distribution
system with a
strong
disinfectant, eg,
chlorine.
The level of risk will be high if UV disinfection takes place a long way from the point of use, or is stored after
treatment before use.
Ref P7.4, Version 1
Water Safety Plan Guide:
January 2014
Treatment Processes – Ultraviolet Irradiation 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 – Dose of UV light is lower than minimum acceptable level
Indicators
Required
actions:

Action required if UV radiant exposure is less than 400 J/m2 at
240-290 nm.

In 100 ml samples of water leaving the treatment plant, E. coli or
aerobic bacterial spores is continually detectable, or is present at
elevated levels (more than 10 per 100 mL).

Widespread illness in the community.
Immediately take the treated water off line. Provide another source of
potable water until water of acceptable quality can again be supplied.
Then:

if excessive turbidity detected in water after filtration:
–
identify and rectify cause of inadequate filtration
–
monitor turbidity until it reaches acceptable limits

if excessive colour detected in water after filtration:
–
identify and rectify cause of excess colour
–
monitor colour until it reaches acceptable limits

if low UV dose not due to poor influent water quality:
–
identify and rectify cause of reduced UV reading at the
sensor (ie, low tube output, dirty tube/quartz sleeve,
increased flow, etc).
Then, in all above situations:
Responsibility:
6

if indicator organisms or pathogens have entered the reticulation,
inform MOH of the transgression

restart plant operation

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:
Ref P7.4, Version 1
Treatment Processes – Ultraviolet Irradiation Disinfection
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:

UV intensity (calculate UV dose).

E. coli (faecal indicator). Aerobic bacterial spores.
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 UV treatment process (eg,
reinfection downstream).
How often:

For the monitoring frequency 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 UV disinfection.

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:
Manager designated responsible for water quality.
Ref P7.4, Version 1
Water Safety Plan Guide:
January 2014
Treatment Processes – Ultraviolet Irradiation Disinfection
7