Water Safety Plan
Guide
Pre-treatment Processes
– Destratification
Version 1, Ref P4.2
January 2014
Citation: Ministry of Health. 2014. Water Safety Plan Guide: Pretreatment Processes – Destratification, Version 1, ref p4.2.
Wellington: Ministry of Health.
Published in January 2014
by the Ministry of Health
PO Box 5013, Wellington, New Zealand
ISBN: 978-0-478-42718-9 (print)
ISBN: 978-0-478-42719-6 (online)
Previously published in 2001 as Public Health Risk Management
Plan Guide: Pre-treatment Processes – Destratification, Version 1,
ref p4.2. This publication’s title and any reference within the text to
‘public health risk management plan’ was 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
5
Water Safety Plan Performance Assessment
7
Ref P4.2
Version 1, January 2014
Water Safety Plan Guide:
Pre-Treatment Processes – Destratification
iii
Introduction
Destratification is used in lakes and reservoirs to avoid the formation of layers of water that
have little or no oxygen. This Guide is concerned with destratification by both aeration and
hydraulic mixing.
If an event occurs during the destratification process (ie, the water is not properly mixed), the
following could happen:

large growths of algae develop

the raw water becomes unsuitable for treatment

high concentrations of manganese and iron develop

variability in water quality makes treatment difficult to control.
These can lead sickness caused by germs, disinfection by-products, manganese or algal
toxins.
The operation of the equipment for destratification 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.
The destratification process, and the risks associated with it, cannot be viewed in isolation.
They influence elements of the water supply that are dealt with in other Guides. The
following elements can be affected:

chemical disinfection (see Guides P7.1, 7.2, 7.3)
–
–
water that is low in oxygen often contains substances that react quickly with
disinfectants so that not enough disinfectant is left to kill germs and disinfectant
by-products exceed the MAV
oxidation of iron and manganese can increase the turbidity of the water which
hinders the disinfectants in killing germs

ultraviolet disinfection (see Guide P7.4)
–
iron and manganese may foul the lamps or increase the turbidity of the water
which stops the ultraviolet light getting to the germs

processes removing natural organic matter (eg, coagulation/flocculation – see Guides
P5.1, 5.2, 5.3)

processes that are pH sensitive (eg, coagulation/flocculation).
Ref P4.2
Version 1, January 2014
Water Safety Plan Guide:
Pre-Treatment Processes – Destratification
1
Risk Summary
The event creating the greatest risk involved in destratification is poor mixing of the water
(see P4.2.1).
The most important preventive measures are:

good aerator design (see P4.2.1.1)

having intakes at different levels in the lake/reservoir or having a single intake that can
be used at a range of depths (P4.2.1.3)

starting the destratification process well before the different layers in the water are
likely to start forming (P4.2.1.4).
(References in parentheses are to the Risk Information Table.)
2
Water Safety Plan Guide:
Pre-Treatment Processes – Destratification
Ref P4.2
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: MAV – Maximum Acceptable Value; DWSNZ – Drinking-Water Standards for New Zealand
Causes
Preventive measures
Checking preventive measures
What to check
Corrective action
Signs that action is
needed
Event: POOR MIXING OF THE WATER BODY, LEADING TO:
– algal blooms (under some circumstances)
– raw water unsuitable for treatment
– difficulties with treatment control because of variability in raw water quality.
Possible hazards: Germs (disinfectant demand1 too high); manganese, trihalomethanes, haloacetic acids and
chloral hydrate (formed from elevated levels of organic matter), algal toxins.
Level of risk: Low–high2
P4.2.1.1
Poor design of
aerator or
hydraulic mixer
(including
insufficient
motor size).
P4.2.1.2
Inappropriate
location of
aerator or
hydraulic mixer.



Use a design that
has been
successfully used
previously.

Dissolved oxygen
(DO) with depth.

Temperature with
depth, to identify
the formation of a
thermocline.3
In a long narrow
reservoir (eg, a
dammed river) use

a longitudinal
aerator. It will be
more hydraulically
efficient than a point
source aerator.
Undertake studies
to determine the
location that will
achieve the best
mixing.



FAC (free
available chlorine)
leaving the
treatment plant

and through the
distribution
system.

Iron.

Manganese.

Ammonia.
As for P4.2.1.1.
Large differences
between DO and
temperature
levels at different
depths.

Redesign
destratification
unit.

Redirect intake
to upper levels
of the water
body until
destratification
can be
resumed.

Use low-level
intake to aid
destratification
in the
springtime and
early summer.

Determine best
location and
move
destratification
unit.
Elevated levels of
Fe and Mn
entering the
intake.
Complaints of
tastes and
odours.
FAC
concentration
below 0.2 mg/L.
As for P4.2.1.1.
1
Disinfectant demand is the difference between the amount of disinfectant added to the water and the disinfectant
residual concentration remaining after the disinfectant has reacted with other substances in the water.
2
The consequences of the event will depend on the chemical determinands that appear in the water, and the
efficacy of subsequent treatment processes in removing them from the water. The risk will be high if algal toxins
are in the water.
3
Thermocline is a region of rapid change in water temperature with the depth.
Ref P4.2
Version 1, January 2014
Water Safety Plan Guide:
Pre-Treatment Processes – Destratification
3
Causes
Preventive measures
Checking preventive measures
Corrective action
Signs that action is
needed
What to check
Event: POOR MIXING OF THE WATER BODY cont’d

P4.2.1.3
Poor selection
of Intake level.

P4.2.1.4
Destratification
process started
too late.
As for P4.2.1.1.
Design the intake
system with multiple
abstraction levels or
with a variable level
of abstraction.
As for P4.2.1.1.
Monitor water
quality and physical
parameters on an
annual basis to
assess when
stratification is likely
to become a
problem. Start
destratification
before then.
P4.2.1.5

Malfunction of
destratification
unit.
Routine preventive
maintenance
schedule.

Replace suspect
units.
P4.2.1.6

Stand-by electricity
generator.

Reserve fuel for
generators.
Power failure.
4

Intake structure
has single fixed
level of
abstraction.

Large differences
between DO and
temperature
levels at different
depths.

Elevated levels of
iron and
manganese
entering the
intake.

Complaints of
tastes and
odours.

FAC
concentration
below 0.2 mg/L.
As for P4.2.1.1.

Redesign intake
structure.

Redirect intake
to avoid
anaerobic
water.

Start data
collection so
that the problem
can be avoided
next year.

Redirect intake
to avoid
anaerobic
water.

Start preventive
maintenance
programme.

Maintenance
schedule (see
also P4.2.1.1).

Maintenance log
shows frequent
maintenance
needed (see also
P4.2.1.1).

Electricity supply
(see also
P4.2.1.1).

Poor continuity of 
power supply (see
also P4.2.1.1).
Refuel
generator (if
appropriate).

Obtain more
fuel.
Water Safety Plan Guide:
Pre-Treatment Processes – Destratification
Ref P4.2
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 – Algal bloom formation
Indicators:
Required actions:
Responsibility:

Dead fish in the water body.

Human sickness consistent with algal toxin poisoning.

Visual evidence of algal mats developing in the lake or
reservoir.

Notify the MOH, close down the supply. Provide another
source of potable water until water of acceptable quality can
again be supplied.

In conjunction with the MOH, evaluate the risk to health
posed by algal toxins that have been formed, or are likely to
form. This will require knowledge of the algae present, the
nature of toxins released, and their susceptibility to oxidation.

Consider (see Guide S1.1):
–
increased oxidant/disinfectant doses to destroy the
toxins passing through the treatment plant
–
–
an alternate water source
whether water in post-treatment reservoirs can be used
if adequately dosed with oxidant.

Once corrective measures are in place, monitor algal levels in
the raw water and assess whether toxin concentrations in the
treated water are acceptable before again reticulating the
water.

Flush the distribution system.

Warn consumers to flush their taps before resuming the
supply of water.

Record cause of system failure and steps taken to correct.

Modify water safety plan if necessary.
Manager designated responsible for the water supply.
Ref P4.2
Version 1, January 2014
Water Safety Plan Guide:
Pre-Treatment Processes – Destratification
5
Event – FAC concentration is lower than minimum acceptable level
Indicators:
Required actions:
Responsibility:
6

A detectable chlorine residual cannot be obtained in the water
leaving the treatment plant (high chlorine demand due to
poor destratification).

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 levels of 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 water safety plan if necessary.
Manager designated responsible for the water supply.
Water Safety Plan Guide:
Pre-Treatment Processes – Destratification
Ref P4.2
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:
How often:
What to do with the
results:
Responsibility:
Ref P4.2
Version 1, January 2014

Dissolved oxygen (DO) and temperature levels (at different
depths) in the water body

Iron, manganese, ammonia and total organic carbon (TOC)
levels entering the plant intake

FAC levels leaving the treatment plant

The operation of the motors driving the aerators or pumps.

FAC should be monitored as required by the DWSNZ:2000,
as should manganese if a P2 determinand

The determinands noted above could be monitored at a
frequency that the water supplier considers necessary for
process control. The easily-made measures of temperature
and DO could be made daily, although once results show
that the destratification unit is operation successfully it may
be sufficient to ensure that the motors are operating.

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 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 destratification process are
recognised in the plan.
Manager designated responsible for the water supply.
Water Safety Plan Guide:
Pre-Treatment Processes – Destratification
7
8
Water Safety Plan Guide:
Pre-Treatment Processes – Destratification
Ref P4.2
Version 1, January 2014