Water Safety Plan
Guide
Treatment Processes
– Plant Construction and Operation
Version 1, Ref P11
January 2014
Citation: Ministry of Health. 2014. Water Safety Plan Guide:
Treatment Processes – Plant Construction and Operation, Version 1,
ref p11. Wellington: Ministry of Health.
Published in January 2014
by the Ministry of Health
PO Box 5013, Wellington, New Zealand
ISBN: 978-0-478-42760-8 (print)
ISBN: 978-0-478-42761-5 (online)
Previously published in 2001 as Public Health Risk Management
Plan Guide: Treatment Processes – Plant Construction and
Operation, Version 1, ref p11. 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
Water Safety Plan Performance Assessment
Ref P11, Version 1
January 2014
Water Safety Plan Guide:
Treatment Processes – Plant Construction and Operation
11
iii
Introduction
This Guide is concerned with the design, construction and operation of the treatment plant.
Few raw waters can be safely used without treatment. To keep the risks to public health as
low as possible the treatment plant must continuously provide effective, multiple barriers to
contaminants.
If an event occurs in the treatment plant (ie, it doesn’t work properly), the following could
happen:

If the plant is unable to produce water of good enough quality, sickness can come from
germs or chemicals that have not been removed from the water.

If the plant is unable to produce enough water, sickness may be caused by poor hygiene
and by low pressure allowing germs and chemicals to enter the distribution system.
Treatment plant operation 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.
Ref P11, Version 1
January 2014
Water Safety Plan Guide:
Treatment Processes – Plant Construction and Operation
1
Risk Summary
The two events creating the greatest risks in the design and operation of a treatment plant are
being unable to produce enough water (see P11.2) and being unable to produce water of good
enough quality (see P11.1).
The most important preventive measures are:

design the treatment plant so that the failure of any one part of it cannot stop the plant
from being able to produce enough water of acceptable quality (see P11.1.1, P11.2.1)

make sure the plant is properly maintained (see P11.1.3, P11.2.3)

make sure the plant is designed to be able to treat the expected range of raw-water
qualities (see P11.1.1)

develop and use quality assurance systems that will reduce the chances of something
going wrong with the operation of the plant (see P11.1.4).
(References in parentheses are to the Risk Information Table.)
2
Water Safety Plan Guide:
Ref P11, Version 1
Treatment Processes – Plant Construction and Operation
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: MAV – Maximum acceptable value – see Drinking-Water Standards for New Zealand:2000
Causes
Preventive measures
Checking preventive measures
Corrective action
Signs that action
is needed
What to check
Event: TREATMENT PLANT CANNOT PRODUCE WATER OF SATISFACTORY QUALITY
Possible hazards: Germs; health-significant chemical determinands.
Level of risk: High1
P11.1.1

Make sure, at the design 
stage, that sufficient
information is gathered
about the water quality
and its variability to allow
appropriate treatment
processes to be identified.

Make sure that, at the
design stage, the
proposed treatment
processes are capable of
producing water of the
required quality.

Periodically review the
performance of the
existing plant to assess
whether processes need
to be modified or added.

The plant should be
designed so that the
failure of no one part can
result in a whole
treatment process not
operating.

Where possible, the plant 
should be designed with
sufficient redundancy and
flexibility to allow
incorrectly functioning
units to be bypassed.

Undertake periodic
inspections of the
structural integrity of the
plant so that steps can be
taken to carry out repairs
before failure occurs.
Poor plant
design.
P11.1.2
Structural
failure of part
of the plant.
1
All

microbiological
and chemical
determinands
appropriate to
compliance with
the DWSNZ:
2000.
Visual
inspection of
plant.

Microbiological 
or chemical
determinands
monitored fail to
meet the
requirements of
DWSNZ:2000.
Signs of the
beginning of
structural
failure.
Undertake
treatment plant
modifications,
or consider
using another
source water.

Bypass nonoperational
units, if this can
be done without
the production
of water of
unsatisfactory
quality.

Repair or
replace failing
units, or
reconsider the
whole plant
design.
The consequences of the event, and therefore the level of risk, will be influenced by the type and concentration
of hazards present in the water, and length of time they have been present.
Ref P11, Version 1
January 2014
Water Safety Plan Guide:
Treatment Processes – Plant Construction and Operation
3
Causes
Preventive measures
Checking preventive measures
Corrective action
Signs that action
is needed
What to check
Event: TREATMENT PLANT CANNOT PRODUCE WATER OF SATISFACTORY QUALITY cont’d

P11.1.3
Inadequate
maintenance
resulting in
mechanical
failure.
P11.1.4

Where practicable,
backup equipment and
spares should be kept on
site to allow failed
equipment to be rapidly
replaced or repaired.

Make sure there is an
operator’s manual for the
treatment plant, so that
new staff have something
to reference, and so that
knowledge regarding the
plant’s operation is not
lost in the event of an
operator leaving.

Have in place a supply
water safety plan
(formerly known as a
Public Health Risk
Management Plan,
PHRMP) (such as
outlined in this series),
and supporting systems
(eg, documentation,
reporting).

Regularly review quality
assurance systems, and
make modifications where
it is evident that
deficiencies exist.

Develop a process
monitoring plan that
identifies:
Inadequate
quality
assurance
systems.
P11.1.6
Make sure all aspects of

the plant and its operation
that require maintenance
have a maintenance
schedule developed and
followed.
Inadequate
monitoring.
–
–
–
–
–

4
the reason for the
monitoring
where samples are to
be taken
Audit

maintenance
records. (Use
this information
to identify which
determinands
may need to be
monitored.)


All
microbiological
and chemical
determinands
appropriate for
the purposes of
the monitoring
programme.

No spare parts
available.
No manual
available for
new staff.

Instigate
maintenance
programmes.

Obtain spare
parts.

Prepare an
operator’s
manual for the
treatment plant.

Review quality
assurance
system and
modify where
deficient.
Water leaving

the plant fails to
comply with the

DWSNZ 2000
MAVs.
Instigate a
monitoring plan.
Train staff in
correct
monitoring
practices.
what is being sampled
the frequency of
sampling and
method of sampling.
Make sure that all
appropriate staff are
aware of, understand, and
follow the plan. Refer
also to Guide U4.
Water Safety Plan Guide:
Ref P11, Version 1
Treatment Processes – Plant Construction and Operation
January 2014
Causes
Preventive measures
Checking preventive measures
Corrective action
Signs that action
is needed
What to check
Event: TREATMENT PLANT CANNOT PRODUCE WATER OF SATISFACTORY QUALITY cont’d
P11.1.7

Inadequate
security
measures to
prevent
vandalism/
sabotage.
P11.1.8

Make sure all hatches are
lockable and are locked
when continuous access
is not required.

Make sure all ladders are
locked away.

Install an intruder alarm or
arrange for security
patrols.

Develop criteria for raw
water quality that define
when the water quality
has degraded to the point
at which the intake must
be closed and the plant
shut down.
Event
affecting the
source with
the result that
the plant
cannot treat
the raw water. 
P11.1.9
Unhygienic
practices
(including no
separation
between
water and
waste water
operations).
Provide a lockable

perimeter fence for the
treatment plant to
surround all components

of the plant. If the plant is
a single building, make
sure it is lockable and that
windows are protected
against forced entry.


Regular

inspection by
security patrols.
Evidence of
unauthorised
entry.

Review and
instigate
recommended,
or additional,
security
precautions.
Determinands

dictated by
evidence of
possible
intentional
contamination
(eg, chemical
packaging left in
the area).
Unexplained
deterioration in
water quality.
Appropriate raw 
water
determinands –
eg, turbidity,
odour and
colour.
Plant unable to
produce water
complying with
DWSNZ:2000.

See
Contingency
Plans.

Develop criteria
for plan
shut-down.

Extra preand/or posttreatment
storage
capacity.

Evidence of

contamination
being
transferred
from, or by staff.
Train staff in
hygiene
practices.

Staff with
infectious
diseases
working at the
plant.

E. coli detected
in 100 ml of
water.
Where raw water can be
very variable, put in place
an ongoing monitoring
plan for raw water quality
to determine how
frequently such events
occur, and hence what
modifications to the
plant/storage may be
necessary. See also
P11.1.1.
Develop a code of

practice for the treatment

plant, which sets out

which unhygienic
practices must be
avoided, including (but not
limited to):
–
–
–
–
Ref P11, Version 1
January 2014
Staff health.
Staff activities.
Microbiological
quality of
treated water.
poor personal hygiene
staff with infectious
diseases working in
the plant
staff working at both
water and wastewater
plants
use of tools in both
water and wastewater
plants.
Water Safety Plan Guide:
Treatment Processes – Plant Construction and Operation
5
Causes
Preventive measures
Checking preventive measures
Corrective action
Signs that action
is needed
What to check
Event: TREATMENT PLANT CANNOT PRODUCE WATER OF SATISFACTORY QUALITY cont’d

P11.1.10
Inadequate
labelling of
chemical
storage/no
supervision of
chemical
delivery.


P11.1.11
Destruction of
part of plant
by chemical

fire or
explosion.
P11.1.12
Periodic
examination of
containers for
correct and
clear labelling.

Ensure that the
quality of all
chemicals used
in treatment is
certified by the
manufacturer.
Make sure that plant staff
are aware of delivery
dates and times of
chemicals, and that they
are present to direct the
delivery to the correct
container.
Make sure proper storage 
arrangements are made
for hazardous chemicals.
Make sure staff are
trained in the correct
procedures for handling
the chemicals used at the
treatment plant.
Include in the treatment
plant operator’s manual
instructions for evaluating
the need for the purchase
of emergency additional
chemical supplies in the
event of strike action
notification.

Make sure an alternative 
power source is available.
In most cases, this will be
a petrol or diesel-fuelled
generator.
Power failure.

6


Lack of
chemical
supplies due
to industrial
action.
P11.1.13
Make sure all chemical
storage containers are
clearly labelled with the
chemicals they should
contain, and their
concentration, if
appropriate.
Audit chemical
storing and
handling
practices.
Reserve fuel
level.


Use of incorrect 
treatment
chemicals
because of
delivery to the
wrong storage
container.

Fire or
explosion or
‘near miss’.
Instigate steps
to make sure
containers are
clearly labelled
and staff aware
of deliveries.
Suppliers to be
required to
ensure that the
operator is
present when
chemicals are
delivered.

Make proper
storage
arrangements

Identify staff
training needs
and provide
training.

Low stock
levels because
of strike action.

Instigate
emergency
stocking policy.

Inability to
operate the
plant because
of a power
failure.

Obtain a
generator.

Increase
reserve fuel
supply.
Make sure there is on-site
storage of sufficient fuel to
maintain plant operation
for several days.
Water Safety Plan Guide:
Ref P11, Version 1
Treatment Processes – Plant Construction and Operation
January 2014
Causes
Preventive measures
Checking preventive measures
Corrective action
Signs that action
is needed
What to check
Event: TREATMENT PLANT CANNOT PRODUCE ENOUGH WATER
Possible hazards: Germs; health-significant chemical determinands
Level of risk: High
P11.2.1

Structural
failure of part
of the plant.
P11.2.2

Visual
inspection of
plant.

Plant
construction
tenders.

Where possible, the plant
should be designed with
sufficient redundancy and
flexibility to allow nonoperational units to be
bypassed, so that the
failure of no one part of
the plant can put the
whole plant out of service.

Undertake periodic
inspections of the
structural integrity of the
plant so that steps can be
taken to carry out repairs
before failure.

In the design stages,

projections for future
water demand should be
undertaken for the
planned life of the plant to
assess what the design
capacity should be.

The plant should be
designed so that the
failure of no one part can
result in the whole plant
being put out of service.

Make sure all aspects of
the plant and its operation
that require maintenance
have a maintenance
schedule developed and
followed.
Inadequate
plant design.
P11.2.3
Make sure construction
contract requires
construction to
appropriate building
codes, and that these are
followed.
Inadequate
maintenance
resulting in
mechanical
failure.

Design the plant to make
as much use as possible
of gravity flow to allow
operation to continue in
the event of pump failure.

Where practicable,
backup equipment and
spares should be kept on
site to allow failed
equipment to be rapidly
replaced or repaired.
Ref P11, Version 1
January 2014
Water demand/
water use
restrictions.

Signs of the
beginning of
structural
failure.


Insufficient

water produced
to meet demand
– limited by
plant capacity.

No spare parts
available.
Repair or
replace failing
units, or
reconsider the
whole plant
design.
Consider
modifications to
the existing
plant or the
construction of
a new one.

Instigate
maintenance
programmes.

Obtain spare
parts.
Water Safety Plan Guide:
Treatment Processes – Plant Construction and Operation
7
Causes
Preventive measures
Checking preventive measures
Signs that action
is needed
What to check
P11.2.4
Corrective action
See P11.1.7.
Inadequate
security
measures to
prevent
vandalism/
sabotage.
Event: TREATMENT PLANT CANNOT PRODUCE ENOUGH WATER cont’d
P11.2.5
See P11.1.11.
Destruction of
part of plant
by chemical
fire or
explosion.
P11.2.6
See P11.1.13.
Power failure.
P11.2.7

Natural
disasters.

8
When planning the
location of the plant, it
should be sited at an
elevation well above flood
levels of nearby bodies of
water.

Flooding of
plant.

Destruction of
the plant by
earthquake.
In regions prone to
earthquakes, structural
design criteria should be
selected to take account
of the possibility of
earthquake.
Water Safety Plan Guide:
Ref P11, Version 1
Treatment Processes – Plant Construction and Operation
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 – Treated water quality too poor
Indicators:
Required actions:
Responsibility:

Microbiological and/or chemical quality of the water leaving
the treatment plant does not meet the requirements of
DWSNZ:2000.

Outbreaks of sickness that are probably linked to water quality.

Close down the treatment plant, and supply water from posttreatment storage, or provide another source of potable water
until water of acceptable quality can again be supplied. (Make
sure that the history of any tanker used to cart water has been
investigated before it is used and that the tanker will not
contaminate the water.)

Notify the MOH and determine whether ‘boil-water’ notices
are needed.

Identify the reason for the failure and rectify.

If water of poor quality has passed into the distribution system,
flush and disinfect the network and notify consumers to flush
their taps before drawing water for use.

Record cause of system failure and steps taken to correct.

Modify water safety plan if necessary.
Manager designated responsible for the water supply.
Event – Not enough water produced by the treatment plant
Indicators:
Required actions:
Ref P11, Version 1
January 2014

Post-treatment storage levels dropping.

Drop in distribution system pressure.

Notify the public of the need to conserve water.

Identify the reason for the failure and rectify, if possible.

If the fault cannot be rectified within a few hours, provide
another source of potable water until water of acceptable
quality can again be supplied. (Make sure that the history of
any tanker used to cart water has been investigated before it is
used and that the tanker will not contaminate the water.)

Record cause of system failure and steps taken to correct.

Modify water safety plan if necessary.
Water Safety Plan Guide:
Treatment Processes – Plant Construction and Operation
9
Responsibility:
10
Manager designated responsible for the water supply.
Water Safety Plan Guide:
Ref P11, Version 1
Treatment Processes – Plant Construction and Operation
January 2014
Water Safety Plan Performance
Assessment
To make sure that your supply’s water safety plan 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:

E. coli (faecal indicator) and coliform bacteria.

Chemical Priority 2 determinands.

Turbidity.

Structural failure of any part of the plant.

Inoperative equipment.

Post-treatment storage level.

Reticulation network pressure.

Plant output.
Follow the protocols set out in DWSNZ:2000 (where
appropriate).
How often:
Ref P11, Version 1
January 2014

For the monitoring frequencies for E. coli and turbidity
measurements see DWSNZ:2000 Section 3.3.2. For
chemical determinands see DWSNZ:2000 Table 4.1.
Daily data for flows out the plant should provide an
indication of water quantity problems. Major failures will
become evident very quickly and should not need a
monitoring programme.
Water Safety Plan Guide:
Treatment Processes – Plant Construction and Operation
11
What to do with the
results:
Responsibility:
12

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 operation of the
treatment plant.

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.
Manager designated responsible for the water supply.
Water Safety Plan Guide:
Ref P11, Version 1
Treatment Processes – Plant Construction and Operation
January 2014