Water Safety Plan
Guide
Pre-treatment Processes
– Waste-Liquor Reintroduction
Version 1, Ref P4.4
January 2014
Citation: Ministry of Health. 2014. Water Safety Plan Guide: Pretreatment Processes – Waste-Liquor Reintroduction, Version 1, ref
p4.4. Wellington: Ministry of Health.
Published in January 2014
by the Ministry of Health
PO Box 5013, Wellington, New Zealand
ISBN: 978-0-478-42722-6 (print)
ISBN: 978-0-478-42723-3 (online)
Previously published in 2001 as Public Health Risk Management
Plan Guide: Pre-treatment Processes – Waste-Liquor Reintroduction,
Version 1, ref p4.4. 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
6
Ref P4.4, Version 1
January 2014
Water Safety Plan Guide:
Pre-Treatment Processes – Waste-Liquor Reintroduction
iii
Introduction
This Guide is concerned with the process of recycling waste liquor from the treatment process
back into the raw water entering the plant. This is done to reduce the amount of waste water
requiring disposal and in some cases to improve the effectiveness of the coagulationflocculation process.
The waste liquor is almost certain to contain germs and health-significant chemicals. Your
treatment processes must be able to produce a safe water despite this additional demand on
its capabilities, otherwise the process has a very high risk.
If an event occurs during the reintroduction of the waste liquor (ie, some goes wrong with
the process), the following could happen:

If previously removed contaminants are added back into the water, and following
processes do not remove them, germs and chemicals may cause sickness

If control over the coagulation process is lost, and following processes do not remove
them, germs and chemicals may cause sickness.
The reintroduction of waste liquor, and the risks associated with it, cannot be viewed in
isolation. If the process is used, it can influence how well the coagulation (see P5 series
Guides) and filtration processes (see P6 series Guides) work. The amount of waste liquor
that can be added back into the raw water without creating a risk to public health will depend
on:

the quality of the raw water

the nature of the waste produced by the treatment processes

how effective the treatment processes are in removing the contaminants.
Ref P4.4, Version 1
January 2014
Water Safety Plan Guide:
Pre-Treatment Processes – Waste-Liquor Reintroduction
1
Risk Summary
The event creating the greatest risk in the reintroduction of waste liquor is reintroducing
contaminants that have already been removed (see P4.4.1).
The most important preventive measure is to make sure that as much of the solid material as
possible is removed from the waste before it is reintroduced (P4.4.1.2).
(References in parentheses are to the Risk Information Table.)
2
Water Safety Plan Guide:
Ref P4.4, Version 1
Pre-Treatment Processes – Waste-Liquor Reintroduction
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.
Abbreviation: DWSNZ – Drinking-Water Standards for New Zealand
Causes
Preventive measures
Checking preventive measures
Corrective action
Signs that action
is needed
What to check
Event: REINTRODUCTION OF PREVIOUSLY REMOVED CONTAMINANTS1
Possible hazards: Germs and health significant chemical determinands.
Level of risk: High
P4.4.1.1

Contaminants in
the aqueous
phase.
P4.4.1.2

Inadequate
separation of
solids from
aqueous phase.


1
Avoid waste treatment 
conditions that
redissolve

contaminants.
Determine the range
of contaminant
concentrations in the
waste, and the
relative amounts in
the solids and the
reintroduced waste
stream.
Determine the
turbidity of the waste
stream above which
reintroduction should
be stopped because
of excessive
contaminant levels.
Ensure that
reintroduction can be
stopped when the
turbidity of the waste
stream is too high.
Colour – natural
organic matter.

Polymers/
monomers.
Elevated

concentrations
of contaminants
in combined

raw water waste
stream.

pH.

Metals, including
aluminium if used
as the coagulant.

Elevated
coagulant
demand.

Bacteria and
protozoa.


Turbidity, particle
content.
Elevated
coagulant
demand.

Colour – natural
organic matter.

Polymers/monom
ers.

Metals, including
aluminium if used
as the coagulant.

Turbidity.
Cease
reintroduction of
waste stream.
Alter waste
treatment to
reduce
dissolved
contaminants.

Stop
reintroduction of
waste stream
manually.

Install
controller.
Contaminants will inevitably be reintroduced with the waste liquor; a small amount of particulate matter is needed
if the coagulation process is to be seeded. You need to be aware of the levels that your treatment system can
tolerate, and to keep contaminant levels within these limits. This event is concerned with reintroduced
contaminants beyond these limits.
Ref P4.4, Version 1
January 2014
Water Safety Plan Guide:
Pre-Treatment Processes – Waste-Liquor Reintroduction
3
Causes
Preventive measures
Checking preventive measures
Corrective action
Signs that action
is needed
What to check
Event: LOSS OF PROCESS CONTROL (primarily coagulation, due to variation in the combined raw
water/waste water composition).
Possible hazards: Germs and health significant chemical determinands.
Level of risk: Low–moderate

P4.4.2.1
Controller or
pump
malfunction
(see Guide
P10).
P4.4.2.2
Maintenance
log not signed
off.

Sudden change
in coagulant
demand.

Identify cause
of fault and
rectify.

Flow and rate of
waste stream
reintroduction.

Coagulant
demand not
proportional to
flow rate.

Install flow
proportional
controller.

Ensure coagulation
dose control (eg,
turbidity, colour,
streaming current
detector) sited so that
it responds to the
combined stream, not
just the raw water.

Turbidity.

Colour.

Re-site
coagulant dose
control.
Particle charge –
streaming current.
Elevated
turbidity, colour
and residual
coagulant in
filtered water.



Elevated

concentrations
of contaminants
in combined

raw water waste
stream.

Elevated
coagulant
demand.

Poor or no
control of waste
stream
composition.

4

Ensure reintroduction
rate is proportional to
the flow, not fixed
rate.
Control of
coagulation
processes does
not take
account of the
composition of
the combined
raw water and
waste stream.
P4.4.2.4
Maintenance log.

Rate of
reintroduction is
not proportional
to the raw water
flow.
P4.4.2.3
Routine controller and 
pump maintenance
schedule.
Avoid waste treatment 
conditions that redissolve

contaminants.
Ensure waste
treatment adequately
and consistently
separates solids from
the aqueous phase
using, for example,
polymers, centrifuge,
press, gravity
thickeners.
Colour – natural
organic matter.
Polymers/
monomers.

pH.

Metals, including
aluminium if used
as the coagulant.


Bacteria and
protozoa.


Turbidity, particle
content.

Colour – natural
organic matter.

Polymers and
monomers.

Metals, including
aluminium if used
as the coagulant.
Stop
reintroduction of
waste stream.
Alter waste
treatment.
Alter waste
treatment.
Elevated
coagulant
demand.
Water Safety Plan Guide:
Ref P4.4, Version 1
Pre-Treatment Processes – Waste-Liquor Reintroduction
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 – Previously-removed contaminants are reintroduced and not removed
Indicators
Required actions:
Responsibility:
Ref P4.4, Version 1
January 2014

Bacteria and protozoa in the treated water.

Elevated turbidity and colour.

Elevated coagulant residual in the treated water.

Follow Sections 3.4 and 4.4 of DWSNZ:2000.

Identify the reason for the failure of the process and rectify.

Monitor contaminant 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 quality.
Water Safety Plan Guide:
Pre-Treatment Processes – Waste-Liquor Reintroduction
5
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.

Bacteria and protozoa.

Turbidity and colour.

Coagulant residual.

Reasons for variability in coagulant dose rate.
How often:

Follow the protocols set out in DWSNZ:2000.
What to do with the
results:

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
waste-liquor reintroduction.

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 waste-liquor reintroduction
processes are recognised in the plan.
What to measure or
observe:
Responsibility:
6
Manager designated responsible for the water quality.
Water Safety Plan Guide:
Ref P4.4, Version 1
Pre-Treatment Processes – Waste-Liquor Reintroduction
January 2014