Water Safety Plan
Guide
Distribution System
– Post-treatment Storage
Version 1, Ref D1
January 2014
Citation: Ministry of Health. 2014. Water Safety Plan Guide:
Distribution System – Post-treatment Storage, Version 1, ref D1.
Wellington: Ministry of Health.
Published in January 2014
by the Ministry of Health
PO Box 5013, Wellington, New Zealand
ISBN: 978-0-478-42762-2 (print)
ISBN: 978-0-478-42763-9 (online)
Previously published in 2001 as Public Health Risk Management
Plan Guide: Distribution System – Post-treatment Storage, Version 1,
ref D1. 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
3
Risk Information Table
4
Contingency Plans
10
Water Safety Plan Performance Assessment
13
Ref D1
Version 1, January 2014
Water Safety Plan Guide:
Distribution System – Post-Treatment Storage
iii
Introduction
Post-treatment storage facilities (service reservoirs) are used in many water supplies to:

equalise demands on:
–
water sources
–
–
–
treatment plant
pumping stations
distribution mains

maintain uniform pressures in the distribution system

provide reserves for emergencies such as:
–
–
–
power failure
fires
equipment failures in the supply.
This Guide is concerned with tanks and reservoirs used for this purpose. These facilities
should not alter the quality of the water.
If an event occurs during the storage of treated water (ie, something happens to affect the
quality or quantity of water available from the storage facility), the following could happen:

If there is not enough water in storage to meet demand, sickness may be caused by
poor hygiene and by low pressure allowing germs and chemicals to enter the
distribution system.

If contamination gets into the tank or reservoir, germs or chemicals can cause sickness.

If sediment settles on the bottom of the tank or reservoir, or is stirred up, germs or
chemicals in the sediment can cause sickness.
Maintenance of reservoirs can present risks to the health of staff. These are acknowledged,
but are not discussed further as such risks are the subject of health and safety in employment
legislation. Shortage of water can lead to fire-fighting problems, and hence to the possibility
of injury. This is not dealt with in this Guide.
Post-treatment storage and the risks associated with it cannot be viewed in isolation. Having
enough storage in the reservoirs depends on:

being able to abstract enough water of adequate quality from the source (see P1 series
of Guides)

the operation of the treatment plant at a rate at which it can treat the water (see Guide
P11)

the system pumps working (see Guide P10)

being able to transfer water from the treatment plant to the storage facility (see Guide
P2)

the community water demand.
Having water stored of acceptable quality depends on:
Ref D1
Version 1, January 2014
Water Safety Plan Guide:
Distribution System – Post-Treatment Storage
1

the quality of the source water (see the S1 and P1 series of Guides)

the good operation of treatment processes before storage (see the P4, P5, P6, P7 and
P8 series of Guides)

events that affect the quality of the water once it is in the reservoir:
–
–
–
2
introduction of contaminated material into service reservoirs:

access by animals/birds/humans

build up of sediment or living organisms (including algal growth) (see
Guides S1.1 and P4.1)

construction materials dissolving or corroding

contaminated water running in from the surroundings or from the roof
short-circuiting; disinfection is incomplete because the water does not stay in
the reservoir long enough (see P7 series of Guides)
regrowth/resuspension of material within service reservoir:

biological regrowth or the disturbance of sediment or living organisms

post-treatment floc formation.
Water Safety Plan Guide:
Distribution System – Post-Treatment Storage
Ref D1
Version 1, January 2014
Risk Summary
The events creating the two greatest risks involved in treated water storage are having too
little water stored to meet demand (see D1.1), and contamination getting into the reservoir
(see D1.2).
The most important preventive measures are:

construct another reservoir or tank if having too little water seems like being a
continuing problem (see D1.1.3)

restrict access to the reservoir to reduce the chances of contamination (see D1.2.1–3)

carry out regular inspections for signs of structural deterioration or of contamination
having got into the storage (see D1.2.1)

make sure the facilities are designed to reduce the chances of contamination getting in,
and that the construction materials will not contaminate the water (see D1.2.3–6)

make sure there is enough disinfectant in the water to kill germs that get into the water
(see D1.2.1–3).
(References in parentheses are to the Risk Information Table.)
Ref D1
Version 1, January 2014
Water Safety Plan Guide:
Distribution System – Post-Treatment Storage
3
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; NTU –
nephelometric turbidity units
Causes
Preventive measures
Checking preventive measures
Corrective action
Signs that action
is needed
What to check
Event: NOT ENOUGH WATER IN POST-TREATMENT STORAGE TO MEET DEMAND
Possible hazards: Low pressure in the water supply system may allow entry of germs and chemical
determinands; hazards associated with poor hygiene.
Level of risk: High

D1.1.1
Insufficient
supply of raw
water.
D1.1.2
Insufficient
water
treatment
capacity, or
output limited
by one or
more
treatment
stages not
operating.
D1.1.3
Inadequate
posttreatment
storage
capacity.
4
Provide sufficient raw water 
storage to cover seasonal
variations in source water
availability, periods of
untreatable raw water
quality, maximum limits on
consent for abstraction from
the source.

Develop auxiliary sources
that can be used when
secondary sources cannot
meet demand.

Refer also to Guide P1
series.

Design treatment plant to
meet expected demand for
projected life of the plant.

Identify treatment stages
that are limiting capacity
and undertake modification
of these.

Ensure appropriate water
safety plans are operational
for all treatment processes
to avoid or limit down time.

Construct additional
storage.

Encourage water
conservation during periods
when demand is likely to be
high.
Pre-treatment
storage levels.



Water
consumption.
Raw water

storage levels
or source
flows

unacceptably
low.
Plan
development of
new sources.
Post
treatment
storage levels
unacceptably
low.
Calculate future
water use and
plan actions to
meet likely
demand.

Identify problem
treatment stage
and increase
throughput if
possible.

Water use
restrictions.

Need for
water use
restrictions.

Posttreatment
storage levels
unacceptably
low.
Water Safety Plan Guide:
Distribution System – Post-Treatment Storage
Restrict
development in
distribution area
until adequate
water available.
Ref D1
Version 1, January 2014
Causes
Preventive measures
Checking preventive measures
What to check
Corrective action
Signs that action
is needed
Event: NOT ENOUGH WATER IN POST-TREATMENT STORAGE TO MEET DEMAND cont’d
D1.1.4

Inability to
transmit water 
from source
to plant, or
plant to posttreatment
storage
(including
power failure).
D1.1.5

Leakage from
the storage
facility.
Ensure pumps have standby generators.

Inspection log.

Electricity
supply.
Routine inspection and
maintenance of pipelines.
Carry out routine testing of
storage for leaks.

Change in
storage level.

Water
transmission
ceases.


Maintenance 
and
inspection not
carried out.

Change in
storage level
does not
equal water
input minus
water output
(including
evaporation).
Re-fuel
generators (if
necessary).
Clear
blockages/
repair pump
breakdowns or
pipe failures.

Locate and fix
leak.
Event: INTRODUCTION OF CONTAMINATING MATERIAL INTO SERVICE RESERVOIR
Possible hazards: Germs, arsenic, copper, chromium, iron, chlorine, aluminium.
Level of risk: High1
D1.2.1

For tanks in general:
–
Access by
animals/birds.
–
–
–

–
–
1
install mesh, grills, vents,
etc
undertake regular tank
inspections
document condition of
structure, mesh, grills,
vents, tank liner (for
gaps) etc and take
action.
For wooden tanks:
–

ensure the storage is
covered

Inspection
reports.

Free available
chlorine (FAC).

Unexplained
change in
water quality.

Replace
damaged
screens, etc.

Taste and
odour
complaints.


Visual
evidence of
entry of
animal or bird.

FAC
concentration
less than
0.2 mg/L.
Isolate tank,
chlorinate and
check the
microbiological
quality. If there
is gross
contamination,
dump the water,
clean and refill
before
chlorinating.
install metal band
around wooden tanks to
prevent access from
ground level
remove vegetation near
tanks
install internal liner for
tank roof.
Maintain an FAC residual.
The level of risk from microbiological contamination is less if a chlorine residual is maintained.
Ref D1
Version 1, January 2014
Water Safety Plan Guide:
Distribution System – Post-Treatment Storage
5
Causes
Preventive measures
Checking preventive measures
Corrective action
Signs that action
is needed
What to check
Event: INTRODUCTION OF CONTAMINATING MATERIAL INTO SERVICE RESERVOIR cont’d

D1.2.2
–
–


D1.2.3
Human
access for

sampling, etc.
D1.2.4
Leaching/
corrosion
from
construction
materials.
6

locked perimeter fence
locked access towers
and ladders
Regular site

inspections with
checks on
security

measures.
locked hatches on tanks

FAC.
intruder alarm.

Microbiological
quality.
Maintain chlorine residual.
Train personnel who need
access to tanks.

Design internal walkways

that do not become
submerged with a solid floor
and raised edges to avoid
shoe scrapings entering the
water.

Maintain chlorine residual.

Isolation of tank.

Procedures for staff
entering tanks to clean
footwear, then to cover their
shoes with clean plastic
bags, or to go through a
chlorinated foot-bath.

Test sampling staff to
ensure they are not carriers
of water-borne disease, and
relieve them of sampling
duties if necessary.

Specify materials certified

for potable water use2 in
tenders and check these
specifications are being met

during construction.

Use accredited contractors.

Low residence time of water 
in tanks (while maintaining
disinfection contact time).

2
Provide:
–
–
Unauthorised
access/
vandalism/
sabotage.
Install roof liners in tanks
with treated timber roofs.

Evidence of
break-ins or
damage.

See
Contingency
Plan.
Unexplained
change in
water quality.

Determine how
access was
obtained and
rectify
shortcoming in
security.

Isolate tank,
chlorinate and
check the
microbiological
quality. If there
is gross
contamination,
dump the water,
clean and refill
before
chlorinating.
FAC residual
less than
0.2 mg/L and
cannot be
maintained.

E. coli or
coliforms
detected in
100 mL of
water.

Deterioration
of water
quality
following
operation
involving tank
access, in
particular
E. coli or
coliforms
detected in
100 mL of
water.

Isolate tank,
chlorinate and
check the
microbiological
quality. If there
is gross
contamination,
dump the water,
clean and refill
before
chlorinating.
Conduct regular 
inspection of
tanks.
Deterioration
of water
quality
following new
installation.

Replace substandard
materials.
Microbiological
quality.
Reservoir
construction.
Check
residence
times.

If roof is unlined
– copper/
chromium/

arsenic
(especially after
heavy rain).
Change in
water quality
after rain.
Concentration
of copper,

chromium or
arsenic is
more than
50% of its
MAV.
Replace lid with
more suitable
material.
See Guide D2.1 regarding approved materials.
Water Safety Plan Guide:
Distribution System – Post-Treatment Storage
Ref D1
Version 1, January 2014
Causes
Preventive measures
Checking preventive measures
What to check
Corrective action
Signs that action
is needed
Event: INTRODUCTION OF CONTAMINATING MATERIAL INTO SERVICE RESERVOIR cont’d
D1.2.5

Entry of roof
drainage.
Specify and audit roof
design:
–
–
–
rainwater to drain from
(not into) tank
restrict access to roof
top
hatches designed to
exclude water ingress.
D1.2.6

Repair leaks promptly.
Entry of
contaminated
groundwater
(for in-ground
tanks).

Inspect tank every 1–5
years (depending on the
age of the tank and risks
caused by its location).

D1.2.7
Animal entry
from
upstream
source
(animals
trapped and
destroyed by
action of
control
valves) –
where the
tank is filled
directly from
the source.


FAC.

Microbiological
quality.

Installation
design.

Annual roof
inspection
report.
Ref D1
Version 1, January 2014
Change in

water quality
following rain.

E. coli or
coliforms
detected in
100 mL of
water.

Flaws in the
design of the
roof.

Unexplained
change in
water quality.

Microbiological
quality.

Tank inspection
report.

Line the tank with a suitable
waterproof material.
Ensure there are screens
on the source outlet.


Regular screen
inspection.

Microbiological
quality.

Leaks
identified by
the
inspection.

Complaints of
petrol smell (if
near
underground
fuel tanks).

Taste and
odour
complaints.

E. coli or
coliforms
detected in
100 mL of
water.

Turbidity
more than
0.2 NTU.
Turbidity.

Replace badly
designed
hatches.

Repair leaks.

Isolate tank,
chlorinate and
check the
microbiological
quality. If there
is gross
contamination,
dump the water,
clean and refill
before
chlorinating.

Replace and
clean screens.

Isolate tank,
chlorinate and
check the
microbiological
quality. If there
is gross
contamination,
dump the water,
clean and refill
before
chlorinating.
E. coli or
coliforms
detected in
100 mL of
water.

Repair leaky
joints/cracks/
hatches
promptly.
Water Safety Plan Guide:
Distribution System – Post-Treatment Storage
7
Causes
Preventive measures
Checking preventive measures
Signs that action
is needed
What to check

D1.2.8
Chemical
contamination
from incorrect
chemical

dosing.
Train staff in calculation of

dose rates and in the use of
methods for optimising

treatment plant operation.
Treatment
chemicals.

Elevated
turbidity/
treatment
chemical
concentration
posttreatment.

Absence of
written
descriptions
of dosing
procedures.

Turbidity
more than
0.2 NTU.
Turbidity.
Document treatment plant
dosing procedures
(especially calculations of
type and quantity of
materials used), and those
for the reservoir if it is
dosed.
Corrective action

See
Contingency
Plan.
Event: INTRODUCTION OF CONTAMINATING MATERIAL INTO SERVICE RESERVOIR cont’d

D1.2.9
Backflow from
drains.
Ensure that there is an

airgap or other suitable
backflow prevention device

between tank overflow and
sewer or stormwater drains. 
Microbiological
quality.

Change in
water quality
after rain
events.

Turbidity
more than
0.2 NTU.

E. coli or
coliforms
detected in
100 mL of
water.
Turbidity.
Overflow
construction.

Disconnect any
direct
connections.

Isolate tank,
dump the water,
clean,
chlorinate and
check the
microbiological
quality before
refilling.
Event: DEVELOPMENT, OR RE-SUSPENSION, OF SEDIMENT WITHIN TANK OR RESERVOIR
Possible hazards: Germs; manganese, disinfection by-products (trihalomethanes, haloacetic acids, chloral
hydrate).
Level of risk: Low–moderate
D1.3.1

Avoidance of rapid drain/fill.

FAC.

Sediment/
slime
accumulation
and release.

Undertake regular cleaning
of reservoirs (every 1–5
years depending on the
quality of the source water,
flow through and degree of
stagnation in the tank etc).

Microbiological
quality.
Visible slime/
sediment.


Turbidity.

Manganese.
Customers
complain of
slime/
sediment.

Audit cleaning
procedures and
operation.

8
Cleaning procedures need
to consider disinfection of all
equipment, satisfactory
training procedures,
preferably isolation during
cleaning, minimisation of
sediment stir-up if online
during cleaning.

Maintain chlorine residual.

Minimise conditions leading
to post-treatment floc
formation.

FAC residual
concentration
less than
0.2 mg/L.

E. coli or
coliforms
detected in
100 ml of
water.

Turbidity
more than
0.2 NTU.
Water Safety Plan Guide:
Distribution System – Post-Treatment Storage

Review
cleaning
procedures and
plant operation.

Isolate tank,
chlorinate and
check the
microbiological
quality. If there
is gross
contamination,
dump the water,
clean and refill
before
chlorinating.
Ref D1
Version 1, January 2014
Causes
Preventive measures
Checking preventive measures
What to check
Corrective action
Signs that action
is needed
Event: CHLORINE CONTACT TIME TOO SHORT
Possible hazards: Germs not killed.
Level of risk: High
D1.4.1

Storage tank
too small.
Check calculations of tank

hydraulics and the hydraulic
profile of the network before
construction; make use of

hydraulic models if
necessary.


Flow rates
(measured
through tank).

E. coli or

coliforms
detected in
100 mL of
water despite
adequate
FAC residual
concentration.

Unexplained
fluctuations in
water quality.

E. coli or

coliforms
detected in
100 mL of
water despite
adequate
FAC residual
concentration.

Unexplained
fluctuations in
water quality.
Microbiological
quality.
Installation
design.
Check
residence
times.
Increase size of
tank.
Event: CHLORINE CONTACT TIME TOO SHORT (cont’d)
D1.4.2

Shortcircuiting.
Specify and audit suitable
tank design to include:
– baffles
– fill at the top and empty

Flow rates
(measured
through tank).

Microbiological
quality.

Installation
design.

Check
residence
times.
from the bottom.

If necessary, redesign the
inlet/outlet or install baffles.
Ref D1
Version 1, January 2014
Operate the
tank to force
turnover
(deliberately run
up and down).
Water Safety Plan Guide:
Distribution System – Post-Treatment Storage
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 – Breach of the post-treatment storage tank/reservoir
Indicators:
Required
actions:
Responsibility:
10

Visual inspection shows evidence of structural deterioration or a
leak.

Water levels cannot be maintained despite water being pumped
into storage at an adequate rate.

Identify the location of the leak, and repair.

Bypass the tank/reservoir and take water directly from the
treatment plant.

If the normal water source does not meet the needs of the
community or treatment plant capacity (ie, the quantity is too
low at that time of year):
–
investigate and obtain alternative source(s) of water
–
implement water use restrictions.

Repair the storage tank/reservoir if a leak is the cause of the
shortage.

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:
Distribution System – Post-Treatment Storage
Ref D1
Version 1, January 2014
Event – Germs detected in the stored water
Indicators:
Required
actions:
Responsibility:

A detectable chlorine residual cannot be obtained in the water
leaving the reservoir.

In 100 ml samples of water from the reservoir, E. coli or
coliforms are continually detectable, or E. coli is present at
elevated levels (more than 10 per 100 mL).

Widespread levels of illness in the community.

Knowledge of sabotage or vandalism.

Follow the actions given in Figure 3.3 of the DWSNZ:2000. In
consultation with the MOH, consider dumping the stored water,
even though indicators are clear, if there have been high levels
of microbiological contamination.

Record cause of system failure and steps taken to correct.

Modify water safety plan if necessary.
Manager designated responsible for the water supply.
Ref D1
Version 1, January 2014
Water Safety Plan Guide:
Distribution System – Post-Treatment Storage
11
Event – High levels of chemical contaminants in the stored water
Indicators:
Required
actions:
Responsibility:
12

Knowledge of a major chemical spill, or chemical overdose (eg,
chlorine), into the reservoir; sabotage or vandalism.

Change in the appearance, smell or taste of the water.

Widespread levels of illness in the community.

Chlorine residual not restored.

Close down the reservoir, and if necessary the supply. Provide
another source of potable water until water of acceptable quality
can again be supplied.

Inform the MOH of the situation.

Identify the source of contamination and take steps to avoid
recontamination.

Dump the reservoir water. Consultation with the regional council
will probably be required with regard to disposal of the flushed
water.

Flush the reticulation system and monitor water quality until
determinand concentrations are again less than 50% of the
MAV.

Warn consumers to thoroughly 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.
Water Safety Plan Guide:
Distribution System – Post-Treatment Storage
Ref D1
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:
Inspection and sampling of service reservoirs.

E. coli and coliforms

Turbidity

Chemical Priority 2 determinands (if any exist)

FAC.
Follow the protocols set out in DWSNZ:2000.
How often:

For the monitoring frequencies for E. coli, turbidity and FAC
measurements see DWSNZ:2000 Section 3.3.2. For chemical
determinands see DWSNZ:2000 Table 4.1.

Other regular inspection and sampling should include, but not
be limited to, the following:

Ref D1
Version 1, January 2014
–
–
–
evidence of human or animal access
assessment of security
evidence of cracks or holes that may allow contaminants
to enter
–
appearance and odour of the water.
The frequency will be site-dependent and should be based on
previous findings.
Water Safety Plan Guide:
Distribution System – Post-Treatment Storage
13
What to do with the
results:
Responsibility:
14

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 service
reservoirs.

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 operation of post-treatment storage are
recognised in the plan.
Manager designated responsible for the water supply.
Water Safety Plan Guide:
Distribution System – Post-Treatment Storage
Ref D1
Version 1, January 2014