Conventional Coagulation/Flocculation/Sedimentation

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Feb 22, 2014 (2 years and 9 months ago)

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Water Safety

Plan
Guide

Treatment Processes



Conventional Coagulation/
Flocculation/Sedimentation

Version 1, Ref P5.1

J
anuary

20
14



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
)

Previousl
y 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 pl
an’
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


Ref P5.1

Water Safety

Plan Guide: Treatment Processes



iii

Version 1,
January 2014

Conventional Coagulation/Flocculation/Sedimentation

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



1

Version 1,
January 2014

Conventional Coagulation/Flocculation/Sedimentation

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 no
t 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.


T
he 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 filt
ers, 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/sed
imentation 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
)



coa
gulant 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 coagula
tion 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 flocculati
on 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.

2

Water Safety

Plan Guide: Treatment Processes



Ref P5.1


Conventional Coagulation/Flocculation/Sedimentation

Version 1,
January 2014

Risk Summ
ary

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.)


Ref P5.1

Water Safety

Plan Guide: Treatment Processes



3

Version 1,
January 2014

Conventional Coagulation/Flocculation/Sedimentation

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 (
G
uide 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
-
Wate
r Standards for New Zealand:2000;

NTU


nephelometric turbidity units

Causes

Preventive measures

Checking preventive measures

Corrective action

What to check

Signs that action is
needed

Event:
PARTICLES NOT REMOVED

Possible hazards:
Germs not removed (particularly
Giardia

and
Cryptosporidium
)
.

Level of risk:
High
1

P5.1.1.1

Dosing
malfunction.



Rout楮i ma楮tenance
schedu汥 for the
dos楮i syste洠


pumps and 汩nes,
probes and meters,
e汥ltronics (
see
Guide P10
).



A污lm syste浳 to
楮i楣ate when turb楤楴y
after the f楬ter 楳 too
h楧i, and when
coagu污l楯i pH is
outs楤i the target
牡湧r.



Rout楮i monitor楮i of
coagu污lt res楤ials 楮
f楬tered water.



Ma楮ienance
汯l.



Pr楯i to
coagu污l楯i:



pH



jar test



zeta
potential.



Post
-
f楬ter:



turbidit
y



colour



aluminium,
or iron, or
polymer.



Ma楮ienance 汯l not
s楧ied off.



Pr楯i to coagu污t楯i,
the fol汯l楮i are not
optimised for the
cond楴楯is:



pH



jar test



zeta potential.



Post f楬ter:



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.



Ident楦y cause
of fault and
rectify.



Start monitor楮i
progra浭e.

1

周T consequences of the event, and therefore the 汥le氠of r楳k, w楬l be 楮i汵lnced by the qua汩ty of the source
water and the effect楶eness of treatment processes after coagulat楯i, f汯ccu污l楯i and sedi浥mtat楯i, 楥,
f楬trat楯i and dis楮fect楯i.


4

Water Safety

Plan Guide: Treatment Processes



Ref P5.1


Conventional Coagulation/Flocculation/Sedimentation

Version 1,
January 2014

Causes

Preventive measures

Checking preventive measures

Corrective action

What to check

Signs that action is
needed

Event:
PARTICLES NOT REMOVED

cont’d

P5.1.1.2

Dosing rates set
incorrectly or
inappropriately.



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.



Chemical
dosing flows.



Visual
inspection for
floc stability.



Source and
post
-
filter:



turbidity



colour



pH



alkalinity



coagula
nt
and/or
flocculant.



No jar test records.



No dosing control
based on source
water quality.



Elevated turbidity,
colour and residual
coagulant in filtered
water.



Determine
relationships
and document.









Install
appropriate
dosing control
equipment.



Adjus
t dose
rates.

P5.1.1.3

Power failure.



Stand
-
by generator.



Electricity
supply.



Poor continuity of
power supply.



Obtain a
generator or
alternative
power source.



Refuel
generator if
necessary.

P5.1.1.4

Chemical
supply
exhausted.



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.



Supply levels
of coagulant,
flocculant and
pH
-
adjusting
chemical.



Usage rates
of coagulant,
flocculant and
pH
-
adjusting
chemical.



Chemical supply
exhausted.



Elevated turbidity
and colour in filtered
water.



Install or repair
alarm system.



Urgent order
chemi
cals.



Start keeping
record of
chemical use.

Ref P5.1

Water Safety

Plan Guide: Treatment Processes



5

Version 1,
January 2014

Conventional Coagulation/Flocculation/Sedimentation

Causes

Preventive measures

Checking preventive measures

Corrective action

What to check

Signs that action is
needed

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.



Chemical
delivery
records.



In source
water:



turbidity



colour



pH



alkalinity



coagulant
and/or
flocculant



zeta
potential



jar test.



No delivery records.



Elevated turbidity
and colour in filtered
water.



Staff absent during
chemical delivery.



No jar t
est records.



No dosing control
based on source
water quality.



Ensure quality
of treated water
is safe. If not,
dump treated
water.



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.



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.



Trial several
coagulants and
flo
cculants.



Consider pre
-
oxidation
(beware of
disinfection
by
-
product
formation) (
see
Guide P4.3
).



Determine
relationships for
the most
appropriate
coagulant and
flocculant and
means of pH
control.

P5.1.1.6

Chemicals of
poor quality.



Check strength of
chemicals supplied
and calculate dilutions
required for dosing.








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.



Check
calculations.



Adjust dilutions
and/or dose
rates as
necessary.



Check chemicals
supplied for presence
of health
-
significant
chemical
contaminants.



Change
supplier.

6

Water Safety

Plan Guide: Treatment Processes



Ref P5.1


Conventional Coagulation/Flocculation/Sedimentation

Version 1,
January 2014

Causes

Preventive measures

Checking preventive measures

Corrective action

What to check

Signs that action is
needed

Event:
PARTICLES NOT REMOVED

cont’d

P5.1.1.7

Inadequate
mixing of
chemicals.



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.



Turbidity of
clarified
water.



Frequency of
filter
backwash.



Elevated turbidity
and colour in filtered
water.



Turbidity of water
leaving the clarifier
in the range of
2
-
5

NTU.



Post filter:



turbidity is more
than 0.2 NTU



colour is more
tha
n 10 TCU



aluminium
residual is more
than 0.15 mg/L



elevated residual
levels of other
coagulants.



Review
treatment plant
hydraulics to
provide
improved
mixing of
chemicals.

P5.1.1.8

Insufficient
contact time for
floc formation.



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.

(See P5.1.1.7.)

(See P5.1.1.7.)



Review
treatment plant
hydraulics to
provide
improved
contact time
during
coagulation and
flocculation.

P5.1.1.9

Ineffective
settling of floc.



Determine the range
of flow rates that will
allow flow to settle.



Observe and record
conditions that bring
about buoyant flocs
and ‘boiling’. Where
controllable, avoid
these
conditions.
Where uncontrollable,
be prepared for
increased filter
loading.



Plant flow
rate.



Visual
inspection of
floc stability.



Turbidity of
clarified
water.



Frequency of
filter
backwash.

(See P5.1.1.7.)



Review
treatment plant
hydraulics to
allow adequa
te
settling of floc.

P5.1.1.10

Floc removal
mechanism
malfunction.



Routine maintenance
schedule.



Maintenance
log.



Frequency of
filter
backwash.



Frequent filter
backwash.



Elevated turbidity
and colour in filtered
water.



Identify cause
of fault and
repair.

Ref P5.1

Water Safety

Plan Guide: Treatment Processes



7

Version 1,
January 2014

Conventional Coagulation/Flocculation/Sedimentation

Causes

Preventive measures

Checking preventive measures

Corrective action

What to check

Signs that action is
needed

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:
Moderate
1

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:
Low
2

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, flo
cculation and sedimentation, ie,
filtration and disinfection.

2

Some low turbidity waters are difficult to treat with conventional coagulants.


8

Water Safety

Plan Guide: Treatment Processes



Ref P5.1


Conventional Coagulation/Flocculation/Sedimentation

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


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:



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.

Required
actions:



Follow Section 4.4 of
DWSNZ:2000
.



Identify the reason for the failure and rectify.



Monitor chemical until it re
aches acceptable limits.



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.


Ref P5.1

Water Safety

Plan Guide: Treatment Processes



9

Version 1,
January 2014

Conventional Coagulation/Flocculation/Sedimentation

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
deta
iled 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 faeca
l indicators may be influenced by
factors other than the adequacy of the coagulation/flocculation/
sedimentation process.

How often:



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.

What to do with the
results:



Results need to be recorded to m
eet 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 don
e 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 labo
ratory 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


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灲潣o獳e猠sr攠e散潧湩s敤⁩渠t桥⁰l慮a

Responsibility:

Manager designated responsible for the water supply.