Water Safety Plan
Guide
Pre-oxidation
Version 1, Ref P4.3
January 2014
Citation: Ministry of Health. 2014. Water Safety Plan Guide: Preoxidation, Version 1, ref p4.3. Wellington: Ministry of Health.
Published in January 2014
by the Ministry of Health
PO Box 5013, Wellington, New Zealand
ISBN: 978-0-478-42720-2 (print)
ISBN: 978-0-478-42721-9 (online)
Previously published in 2002 as Public Health Risk Management
Plan Guide: Pre-oxidation, Version 1, ref p4.3. 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
4
Water Safety Plan Performance Assessment
5
Ref P4.3
Version 1, January 2014
Water Safety Plan Guide:
Pre-oxidation
iii
Introduction
Pre-oxidation is used, before the treatment plant, to improve the removal of some chemical
substances during treatment, or to reduce biological growths in the treatment plant. Oxidants
used include: chlorine, chlorine dioxide, ozone and potassium permanganate. This Guide is
concerned with the use of oxidants to:

control biological growths in the treatment plant

control tastes and odours

oxidise arsenic to allow its removal by coagulation

oxidise organic matter in the water, to improve the coagulation/flocculation process.
The removal of iron and manganese by oxidation/filtration is covered in Guide 8.2.
If an event related to pre-oxidation occurs (ie, the quality of water is affected by the preoxidation process), the following could happen:

if too little oxidant is added to the water, sickness may result because of the poor
performance of the coagulation process, and the poor removal of germs and chemicals,
such as arsenic, and toxins contained in algae or cyanobacteria

if too much oxidant is added to the water, the oxidants themselves, manganese,
disinfection by-products,1 or toxins released from algae, or cyanobacteria, when they
are killed may cause sickness

high concentrations of disinfection by-products1 or toxins released from algae, or
cyanobacteria, when they are killed, may cause sickness, even when the oxidant dose
is acceptable.
The oxidants used in pre-oxidation can present risks to the health and safety of the operators.
These are acknowledged, but are not discussed further as such risks are the subject of health
and safety in employment legislation.
Oxidants used in pre-oxidation are discussed in separate Guides. In this Guide reference is
made to:

chlorine (Guide P7.1)

chlorine dioxide (Guide P7.2)

ozone (Guide P7.3)

potassium permanganate (KMnO4).
Biological growths in the treatment plant arise because of poor quality source water. Guide
S1.1 (Event S1.1.21) discusses the control of algae, cyanobacteria, in source waters.
Pre-oxidation can help in improving the quality of the water, but even when used properly it
can produce disinfection by-products.1 This Guide assists in managing the risks associated
with pre-oxidation already in place. It does not encourage the use of pre-oxidation.
1
Many of the chemicals used as oxidants are also disinfectants. By-products that form from their reactions are
commonly referred to as disinfection by-products, even when they are used as oxidants.
Ref P4.3
Version 1, January 2014
Water Safety Plan Guide:
Pre-oxidation
1
Risk Summary
The event creating the greatest risk involved in the pre-oxidation process is the oxidant
concentration being too low (see P4.3.1).
The most important preventive measures are:

monitor the process to be sure there is enough oxidant in the water, regardless of how
the quality of the incoming water might change (see other Guides, P7.1.1.4, P7.2.1.4,
P7.3.1.4, P8.2.1.4)

put an alarm on the oxidant supply to let you know when the supply is running low.
Maintain records so you are aware of when this might happen; always have a spare
supply on hand (see other Guides, P7.1.1.6, P7.2.1.6, P7.3.1.6, P8.2.1.6).
(References in parentheses are to the Risk Information Table.)
2
Water Safety Plan Guide:
Pre-oxidation
Ref P4.3
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: DWSNZ – Drinking-Water Standards for New Zealand.
Causes
Preventive measures
Checking preventive measures
Corrective
action
Signs that action is
needed
What to check
Event: OXIDANT DOSE TOO LOW
Possible hazards: Germs and arsenic (poor coagulation performance); algal toxins (algal growth not stopped).
Level of risk: High1
P4.3.1.1 Where chlorine, chlorine dioxide or ozone is used as the oxidant, see the Guide for the individual
oxidant for information on possible causes of the oxidant dose being too low, preventive measures etc. See:
Chlorine (P7.1); Chlorine dioxide (P7.2); Ozone (P7.3).
Where potassium permanganate is the oxidant see Guide P8.2. Guide P8.2 discusses its use for iron and
manganese removal, but what is said also applies to use as a pre-oxidant.
Event: OXIDANT DOSE TOO HIGH
Possible hazards: Chlorine and manganese; trihalomethanes, haloacetic acids, chloral hydrate and chlorate
(hypochlorite and chlorine dioxide); bromate (ozone); chlorate and chlorite (chlorine dioxide); algal toxins (from
algal cell destruction).
Level of risk: Moderate2
P4.3.2.1 Where chlorine, chlorine dioxide or ozone is used as the oxidant, see the Guide for the individual
oxidant for information on possible causes of the oxidant dose being too high, preventive measures etc. See:
Chlorine (P7.1); Chlorine dioxide (P7.2); Ozone (P7.3).
Where potassium permanganate is the oxidant see Guide P8.2. Guide P8.2 discusses its use for iron and
manganese removal, but what is said also applies to use as a pre-oxidant.
Event: EXCESSIVE FORMATION OF OXIDANT BY-PRODUCTS (oxidant dose satisfactory)
Possible hazards: trihalomethanes, haloacetic acids, chloral hydrate and chlorate (hypochlorite and chlorine
dioxide); bromate (ozone); chlorate and chlorite (chlorine dioxide).
Level of risk: Moderate2
P4.3.3.1

Oxidant
reacting with
a high
concentration
of organic
matter in the
water.

Avoid pre-oxidation where

possible. When designing
the plant, test to find out the

levels of oxidant byproducts formed. If levels
are high, check to see if
there are other ways to
achieved the desired effects
of pre-oxidation. A different
oxidant may be preferable.
If pre-oxidation is to be
used, carry it out on an
intermittent basis.
Oxidant byproducts.

TOC concentrations 
greater than 2 mg/L
are likely to lead to
chlorination byproduct
concentrations
more than 50% of
their MAV.

Oxidant by-product
concentrations
more than 50% of
their respective
MAVs.
Total
organic
carbon
(TOC) (raw
water).
Reconsider
the
intermittent
use of preoxidation, or
avoid its use
altogether.
1
Where pre-oxidation is important for the good performance of the coagulation process, or the control of algae
capable of releasing toxins into the water, the risk associated with the event should be considered to be high.
Lower levels of risk are linked to the use of pre-oxidation for other purposes.
2
The level of risk depends on the concentration of natural organic matter in the raw water, and therefore the
possible extent of by-product formation. Algal toxins may be released by cell destruction, but later destroyed
by oxidation.
Ref P4.3
Version 1, January 2014
Water Safety Plan Guide:
Pre-oxidation
3
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 – Oxidant concentration very much higher than acceptable
Indicators:
Required actions:
Responsibility:
4

Knowledge of a major spillage or overdose of oxidant into
the water.

Strong pink coloration of the water (KMnO4 overdose).

Inability to obtain pink colour from DPD indicator despite
high oxidant dose rates. (NB: This indicates chlorine or
chlorine dioxide levels very much greater than the MAV –
very high concentrations of these oxidants bleach the pink
colour that normally develops in their presence).

Water develops a strongly chlorinous odour.

Widespread complaints of taste and odour, or black
particles in the water or staining (KMnO4 overdose), or
illness in the community.

Close down the plant. Provide another source of potable
water until water of acceptable quality can again be
supplied.

Inform the MOH of the situation.

Identify the reason for the oxidant overdose and rectify.

Dump the reservoir water, or add chemicals to neutralise
the oxidant if more appropriate (neutralisation may be
required before any water is dumped, anyway).

Flush the distribution system, if excessive levels of oxidant
are also present in the distribution system, and monitor
water quality until oxidant concentrations are again back to
normal operating levels.

Warn consumers to thoroughly flush their taps before
drawing water for use (if they are likely to have been
affected).

Record cause of system failure and steps taken to correct.

Modify water safety plan if necessary.
Manager designated responsible for water supplies.
Water Safety Plan Guide:
Pre-oxidation
Ref P4.3
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:

Chlorine, chlorine dioxide or KMnO4 residuals.

Manganese concentrations greater than 50% of the MAV in
the treated water.
Follow the protocols set out in DWSNZ:2000.
How often:

Chlorine and manganese should be monitored in accordance
with DWSNZ:2000.
What to do with the
results:

Record results 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 pre-oxidation.

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 pre-oxidation system are
recognised in the plan.
Responsibility:
Ref P4.3
Version 1, January 2014
Manager designated responsible for the water supply.
Water Safety Plan Guide:
Pre-oxidation
5