Water Safety Plan
Guide
Treatment Processes
– Conventional Coagulation/
Flocculation/Sedimentation
Version 1, Ref P5.1
January 2014
Citation: Ministry of Health. 2014. Water Safety Plan Guide:
Treatment Processes – Conventional
Coagulation/Flocculation/Sedimentation, Version 1, ref p5.1.
Wellington: Ministry of Health.
Published in January 2014
by the Ministry of Health
PO Box 5013, Wellington, New Zealand
ISBN: 978-0-478-42724-0 (print)
ISBN: 978-0-478-42725-7 (online)
Previously published in 2001 as Public Health Risk Management
Plan Guide: Treatment Processes – Conventional
Coagulation/Flocculation/Sedimentation, Version 1, ref p5.1. 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
8
Water Safety Plan Performance Assessment
9
Ref P5.1
Water Safety Plan Guide: Treatment Processes –
Version 1, January 2014 Conventional Coagulation/Flocculation/Sedimentation
iii
Introduction
This Guide deals with the processes of coagulation, flocculation and sedimentation which
together are used to remove particles (including some large germs, eg, Giardia and
Cryptosporidium) and natural organic matter from drinking-water.
If an event occurs during the coagulation/flocculation/sedimentation process (ie, the process
doesn’t work properly), the following could happen:

If particles are not removed, sickness can come from larger germs not being removed

If natural organic matter is not removed, it reacts with the disinfectant and can cause
sickness from germs not being killed, and from disinfection by-products

If chemicals used in the coagulation/flocculation process are carried into the
distribution system, they can cause sickness.
The use of coagulant, polymer and pH-adjusting chemicals can present risks to the health of
treatment plant operators. These are acknowledged, but are not discussed further as such
risks are the subject of health and safety in employment legislation.
The coagulation/flocculation/sedimentation process and the risks associated with it cannot be
viewed in isolation. How well the process works affects:

filtration which comes after it (see P6 series of Guides); flocs not removed will more
rapidly clog the filters, or pass through them if the flocs are very small

disinfection (see P7 series of Guides); organic matter in the water will use up more
disinfectant leaving less for disinfection.
Several factors influence how effective the coagulation/flocculation/sedimentation process
is:

the quality of the source water; waters with little turbidity or of variable quality make
good coagulation difficult, and the composition of the organic matter also affects the
process (see Guide S1.1 and the P1 series of Guides)

coagulant and flocculant type and their dose control; poor dose control is likely to
cause poor floc formation

pH and alkalinity control; if the pH is too low or too high flocs will dissolve, and good
removal of organic matter requires a low pH during coagulation and a higher pH during
flocculation

contact times between chemicals and water; there must be enough mixing time to allow
flocs to form

hydraulics at each stage; the coagulant must be added with rapid mixing, but mixing
must be slowed during flocculation to avoid breaking up the flocs.
If the whole process is going to work as well as possible, care must be taken in the operation
of chemical dosing and control systems and the design and operation of the mixing chambers
and sedimentation tank.
Ref P5.1
Water Safety Plan Guide: Treatment Processes –
Version 1, January 2014 Conventional Coagulation/Flocculation/Sedimentation
1
Risk Summary
The event creating the greatest risk involved in the coagulation/flocculation/ sedimentation
of drinking-water is poor removal of particles (see P5.1.1).
The most important preventive measure is to make sure chemical dosing can be controlled to
cope with changing raw water quality and quantity (see P5.1.1.2).
(References in parentheses are to the Risk Information Table.)
2
Water Safety Plan Guide: Treatment Processes –
Conventional Coagulation/Flocculation/Sedimentation
Ref P5.1
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: 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: PARTICLES NOT REMOVED
Possible hazards: Germs not removed (particularly Giardia and Cryptosporidium).
Level of risk: High1
P5.1.1.1

Dosing
malfunction.


Routine maintenance
schedule for the
dosing system –
pumps and lines,
probes and meters,
electronics (see
Guide P10).

Maintenance
log.

Maintenance log not 
signed off.

Prior to
coagulation:

Prior to coagulation,
the following are not 
optimised for the
conditions:
Alarm systems to
indicate when turbidity
after the filter is too

high, and when
coagulation pH is
outside the target
range.
Routine monitoring of
coagulant residuals in
filtered water.
–
–
–
pH
jar test
Post-filter:
–
–
–
–
–
–
zeta
potential.
turbidity
colour
aluminium,
or iron, or
polymer.

Start monitoring
programme.
pH
jar test
zeta potential.
Post filter:
–
–
–
–
1
Identify cause
of fault and
rectify.
turbidity is more
than 0.2 NTU
colour is more
than 10 TCU
aluminium
residual is more
than 0.15 mg/L
elevated residual
levels of other
coagulants.
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 after coagulation, flocculation and sedimentation, ie, filtration
and disinfection.
Ref P5.1
Water Safety Plan Guide: Treatment Processes –
Version 1, January 2014 Conventional Coagulation/Flocculation/Sedimentation
3
Causes
Preventive measures
Checking preventive measures
Corrective action
Signs that action is
needed
What to check
Event: PARTICLES NOT REMOVED cont’d
P5.1.1.2

Dosing rates set
incorrectly or
inappropriately.

P5.1.1.3

Determine and

document the
relationships between

source water quality
and coagulant type

and demand,
flocculant type and
demand, and
optimum pH.

Ensure that chemical
dosing is able to
respond quickly to
changing source
water quality and
quantity, ie.
appropriate choice of
manual, flow
proportional or
automatic dose
control (eg, streaming
current detector,
colour, turbidity).
Plant flow
rate.

Electricity
supply.
Stand-by generator.
Power failure.
P5.1.1.4

Chemical
supply
exhausted.
4
Place an alarm on the 
chemical supply tanks
to indicate when they
are close to running
out.

Maintain records of
chemical use to
provide a guide to the
length of time each
chemical supply is
likely to last.

Maintain a reserve
supply adequate to
cover the time before
resupply.


No jar test records.

No dosing control
based on source
water quality.

Elevated turbidity,
colour and residual
coagulant in filtered
water.
Chemical
dosing flows.
Visual
inspection for
floc stability.
Source and
post-filter:
–
–
–
–
–

Determine
relationships
and document.

Install
appropriate
dosing control
equipment.

Adjust dose
rates.

Obtain a
generator or
alternative
power source.

Refuel
generator if
necessary.

Install or repair
alarm system.
turbidity
colour
pH
alkalinity
coagulant
and/or
flocculant.

Supply levels 
of coagulant,
flocculant and

pH-adjusting
chemical.
Usage rates
of coagulant,
flocculant and
pH-adjusting
chemical.
Poor continuity of
power supply.
Chemical supply
exhausted.
Elevated turbidity

and colour in filtered
water.

Water Safety Plan Guide: Treatment Processes –
Conventional Coagulation/Flocculation/Sedimentation
Urgent order
chemicals.
Start keeping
record of
chemical use.
Ref P5.1
Version 1, January 2014
Causes
Preventive measures
Checking preventive measures
Corrective action
Signs that action is
needed
What to check
Event: PARTICLES NOT REMOVED cont’d
P5.1.1.5

Incorrect
chemical used.
Make sure chemicals
are supplied to the
correct storage tanks.
Clearly label supply
tanks; ensure
operator on site when
chemicals delivered.


–
–
–
–
–
–
P5.1.1.6

Chemicals of
poor quality.

No delivery records. 

Elevated turbidity
and colour in filtered
water.

Staff absent during
chemical delivery.
colour

No jar test records.
pH

No dosing control
based on source
water quality.
In source
water:
–


Chemical
delivery
records.
turbidity
alkalinity
coagulant
and/or
flocculant
Check strength of

chemicals supplied
and calculate dilutions
required for dosing.

Check chemicals
supplied for presence
of health-significant
chemical
contaminants.

Empty, clean
and refill
storage tank.

Introduce
and/or ensure
delivery
procedure.

Require
chemical
supplier to
ensure that
operator is
present when
chemicals are
delivered.

Trial several
coagulants and
flocculants.

Consider preoxidation
(beware of
disinfection
by-product
formation) (see
Guide P4.3).

Determine
relationships for
the most
appropriate
coagulant and
flocculant and
means of pH
control.

Check
calculations.
zeta
potential
jar test.
Carry out jar tests and
zeta potential
measurements to
determine the best
coagulants and
flocculants and their
optimum doses every
time raw water
characteristics
change.
Supplier’s
certificate of
analysis.


Coagulant,
flocculant and
pH-adjusting

chemicals.

Dilution
records.

Supplier’s
certificate of
analysis.

Chemicals of
possible
health
significance
following
dilution.
No supplier’s
certificate.
Elevated turbidity

and colour in filtered
water.
Calculation checks
not signed off.

No supplier’s
certificate for
chemical
composition.

Chemicals of
potential health
significance exceed
50% of their MAVs
in the treated water.
Ensure quality
of treated water
is safe. If not,
dump treated
water.

Adjust dilutions
and/or dose
rates as
necessary.
Change
supplier.
Ref P5.1
Water Safety Plan Guide: Treatment Processes –
Version 1, January 2014 Conventional Coagulation/Flocculation/Sedimentation
5
Causes
Preventive measures
Checking preventive measures
Corrective action
Signs that action is
needed
What to check
Event: PARTICLES NOT REMOVED cont’d
P5.1.1.7

Inadequate
mixing of
chemicals.
P5.1.1.8

Check mixing times
are adequate for
effective coagulation
and flocculation over
the range of flow rates
through the plant, and
that carry over to the
filter is not excessive.

Determine the range
of flow rates that will
allow flow to settle.
Insufficient
contact time for
floc formation.
P5.1.1.9
Ineffective
settling of floc.

P5.1.1.10
Floc removal
mechanism
malfunction.
6
Check that mixing of

coagulant, flocculant
and pH-adjusting
chemicals is adequate
over the range of flow

rates through the
plant.


Coagulant,

flocculant and
pH-adjusting
chemicals.

Plant flow
rate.
Turbidity.

Colour.

Visual
inspection of
floc stability.

Elevated turbidity

and colour in filtered
water.
Turbidity of water
leaving the clarifier
in the range of
2-5 NTU.
Post filter:
–
–

Turbidity of
clarified
water.
–

Frequency of
filter
backwash.
–

turbidity is more
than 0.2 NTU
colour is more
than 10 TCU
aluminium
residual is more
than 0.15 mg/L
elevated residual
levels of other
coagulants.
(See P5.1.1.7.)
(See P5.1.1.7.)

Review
treatment plant
hydraulics to
provide
improved
contact time
during
coagulation and
flocculation.
Plant flow
rate.
(See P5.1.1.7.)

Review
treatment plant
hydraulics to
allow adequate
settling of floc.

Identify cause
of fault and
repair.

Observe and record
conditions that bring
about buoyant flocs

and ‘boiling’. Where
controllable, avoid
these conditions.
Where uncontrollable, 
be prepared for
increased filter
loading.
Visual
inspection of
floc stability.

Maintenance
log.

Frequent filter
backwash.

Frequency of
filter
backwash.

Elevated turbidity
and colour in filtered
water.
Routine maintenance
schedule.
Review
treatment plant
hydraulics to
provide
improved
mixing of
chemicals.
Turbidity of
clarified
water.
Frequency of
filter
backwash.
Water Safety Plan Guide: Treatment Processes –
Conventional Coagulation/Flocculation/Sedimentation
Ref P5.1
Version 1, January 2014
Causes
Preventive measures
Checking preventive measures
Corrective action
Signs that action is
needed
What to check
Event: PARTICLES NOT REMOVED cont’d
P5.1.1.11

Ineffective floc
removal.

Ensure design and

control of floc removal
system are
appropriate for the

capacity and design
of the clarifier and its
sludge loading.

Ensure there is
sufficient capacity in
the sludge disposal
system to handle
peak load.
Visual
inspection of
floc stability.
Turbidity of
clarified
water.
Frequency of
filter
backwash.

Frequent filter
backwash.

Elevated turbidity
and colour in filtered
water.

Odour and
increased chlorine
demand from
anaerobic
conditions.

Review design
and control
system.

Increase
capacity of
sludge disposal
system.
Event: NATURAL ORGANIC MATTER NOT REMOVED
Possible hazards: Germs; disinfection by-products (trihalomethanes, haloacetic acids, chloral hydrate (chlorine),
chlorite (chlorine dioxide), bromate, bromoform, formaldehyde (ozone)).
Level of risk: Moderate1
P5.1.2
The causes, preventive measures, etc., for this event are the same as those given for event P5.1.1. See
P5.1.1.1–P5.1.1.11.
Event: TREATMENT CHEMICALS CARRIED INTO DISTRIBUTION SYSTEM
Possible hazards: Health significant chemicals (acrylamide, epichlorohydrin)
Level of risk: Low2
P5.1.3
The causes, preventive measures, etc., for this event are the same as those given for event P5.1.1. See
P5.1.1.1–P5.1.1.11.
1
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 after coagulation, flocculation and sedimentation, ie, filtration
and disinfection.
2
Some low turbidity waters are difficult to treat with conventional coagulants.
Ref P5.1
Water Safety Plan Guide: Treatment Processes –
Version 1, January 2014 Conventional Coagulation/Flocculation/Sedimentation
7
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 – Turbidity higher than maximum acceptable level
Indicators:

A turbidity of less than 0.2 NTU cannot be maintained in the
water leaving the treatment plant.
Required
actions:

Follow the actions given in Figure 3.4 of DWSNZ:2000.

Identify the reason for the failure and rectify.

Record the reason for the failure and the steps taken to rectify.

Modify the water safety plan if necessary.
Responsibility:
Manager designated responsible for water supply.
Event – Excessive levels of treatment chemicals carried into the distribution system
Indicators:
Required
actions:
Responsibility:
8

Consumer complaints of discoloration, poor taste, sensation,
odour.

Excessive chemical concentrations measured in the water
leaving the treatment plant or in the distribution system.

Notification of error by operator or chemical delivery contractor.

Follow Section 4.4 of DWSNZ:2000.

Identify the reason for the failure and rectify.

Monitor chemical until it reaches acceptable limits.

Record the reason for the failure and the steps taken to rectify.

Modify the water safety plan if necessary.
Manager designated responsible for water supply.
Water Safety Plan Guide: Treatment Processes –
Conventional Coagulation/Flocculation/Sedimentation
Ref P5.1
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:

Turbidity, colour and aluminium in filtered water.

E. coli (faecal indicator), coliforms.
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 coagulation/flocculation/
sedimentation process.
How often:
What to do with the
results:
Responsibility:

For the monitoring frequencies for turbidity and E. coli
measurements see DWSNZ:2000 Section 3.3.2.

Monitoring for colour and aluminium will likely be part of
regular process control, but as a minimum should be
immediately following a change in source water quality
that requires a change to manual chemical dosing.

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 coagulation/flocculation/sedimentation.

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.
Ref P5.1
Water Safety Plan Guide: Treatment Processes –
Version 1, January 2014 Conventional Coagulation/Flocculation/Sedimentation
9