ISO 20468-7:2021

INTERNATIONAL ISO
STANDARD 20468-7
First edition
2021-06
Guidelines for performance evaluation
of treatment technologies for water
reuse systems —
Part 7:
Advanced oxidation processes
technology
Lignes directrices pour l’évaluation des performances des techniques
de traitement des systèmes de réutilisation de l’eau —
Partie 7: Technologie des processus d'oxydation avancés
Reference number
ISO 20468-7:2021(E)
©
ISO 2021

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ISO 20468-7:2021(E)

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© ISO 2021
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Published in Switzerland
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ISO 20468-7:2021(E)

Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms, definitions, and abbreviated terms . 1
3.1 Terms and definitions . 1
3.2 Abbreviated terms . 3
4 System components . 4
4.1 Chemical source feed unit . 4
4.2 UV unit . 4
4.3 ∙OH generation/contact unit . 4
4.4 ∙OH monitoring point. 4
5 Performance requirements and evaluation methods . 5
5.1 Functional requirements . 5
5.1.1 General. 5
5.1.2 Performance evaluation procedures . 5
5.1.3 UV transmittance . 8
5.1.4 Monitoring procedure . 8
5.1.5 Safety requirement . 8
5.2 Non-functional requirements . 8
5.2.1 General. 8
5.2.2 Environmental performance . 9
5.2.3 Economic performance . 9
5.2.4 Dependability performance .10
Annex A (informative) Main treatment technologies and target constituents for water reuse .11
Annex B (informative) Classification of AOPs.12
Annex C (informative) Reaction formulas and feature of indicator molecules capable of
measuring ∙OH .13
Annex D (informative) Representative ∙OH scavengers .14
Bibliography .15
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ISO 20468-7:2021(E)

Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out
through ISO technical committees. Each member body interested in a subject for which a technical
committee has been established has the right to be represented on that committee. International
organizations, governmental and non-governmental, in liaison with ISO, also take part in the work.
ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of
electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www .iso .org/ directives).
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of
any patent rights identified during the development of the document will be in the Introduction and/or
on the ISO list of patent declarations received (see www .iso .org/ patents).
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation on the voluntary nature of standards, the meaning of ISO specific terms and
expressions related to conformity assessment, as well as information about ISO's adherence to the
World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT) see the following
URL: www .iso .org/ iso/ foreword .html.
This document was prepared by Technical Committee ISO/TC 282, Water reuse, Subcommittee SC 3,
Risk and performance evaluation of water reuse systems.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www .iso .org/ members .html.
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ISO 20468-7:2021(E)

Introduction
Global warming and climate change have become worldwide concerns as many countries suffer from
water shortages. There has been global investment to develop alternative water resources and secure
water supplies. One of such efforts is water reclamation/reuse since it is readily available. At the
same time, the implementation of the water reuse technology raises public and regulatory concerns
on potential human health, environmental, and its social impacts. The water reclamation/reuse
technology vendors and technology users have increased needs for defining various aspects of water
reuse projects, for regulation and for international standardization. Without ISO water reuse standards,
many opportunities for sustainable development based on water reuse could be lost.
Standardization of water reuse should include objective specifications, assessments of service level and
water reuse system performance dependencies such as safety, environmental protection, resilience,
and cost-effectiveness. Therefore, appropriate methods are needed to evaluate the performance of the
reuse system.
Varying amounts of persistent organic pollutants (POPs) can be found dependent on the biological
activity of the surrounding watershed and the geochemical circulation. POPs are organic compounds
that are resistant to degradation. POPs typically are halogenated organic compounds which exhibit
high lipid solubility, thus bioaccumulate in fatty tissues. Polyhalogenated organic compounds are of
particular concern because of the stability and lipophilicity which are often correlated to their halogen
content. Since POPs accumulate and are persistent, they can adversely affect human health and the
environment as a result.
The performance of the treatment technology for water reuse should be properly evaluated in order
to select the most appropriate technology to achieve the objectives of the water reuse project. Despite
considerable research and development on therapeutic techniques, such scientific knowledge is largely
depending on the scope of commercial interests. This document establishes a specific performance
evaluation method for advanced oxidation processes (AOPs) for water reuse systems based on
ISO 20468-1 as a generic standard. To address these issues, this document provides the evaluation
of the performance of water reuse systems in many applications by providing methods that most
stakeholders can accommodate.
At the ISO TC282/SC3 meeting, a general standard for performance evaluation based on the discussion
entitled "Guidelines for Performance Evaluation of Processing Technologies for Water Reuse Systems -
Part 1: General" in ISO 20468-1 was discussed. Technology, and combinations, thereof, and descriptions
of representative technologies should be included in the individual standards submitted in accordance
with ISO 20468-1. In this context, this document establishes a specific performance evaluation method
for advanced oxidation processes (AOPs) for water reuse systems based on ISO 20468-1 as a generic
standard.
AOP technologies represent a group of treatment processes (e.g., hydrogen peroxide/ozone, hydrogen
peroxide/UV, ozone/UV, pH elevated ozonation, etc.) that rely on the production of hydroxyl radicals as
a strong oxidant capable of the complete oxidation of most organic compounds.
In water reuse systems, AOP technologies are mainly applied for disinfection and for removing total
organic carbon (TOC) including persistent organic pollutants (POPs) that are barely decomposed by
conventional oxidation processes, as indicated in Table A.1 (Annex A). For instance, direct oxidation
of chlorobenzene by ozone is known to occur very slowly; this reaction’s second-order kinetic rate
-1 -1
constant is less than 1 M s . On the other hand, the oxidation of chlorobenzene by ·OH is extremely
9 -1 -1
rapid (up to 4 X 10 M s ).
AOPs as an advanced level treatment are generally applied to tertiary treated water, as shown in
Figure 1 of ISO 20468-1.
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INTERNATIONAL STANDARD ISO 20468-7:2021(E)
Guidelines for performance evaluation of treatment
technologies for water reuse systems —
Part 7:
Advanced oxidation processes technology
1 Scope
This document provides a performance evaluation method of treatment technology using advanced
oxidation processes (AOPs) for water reuse treatment. It introduces a system of evaluating water
quality to validate AOP performance through typical parameters such as the concentration of hydroxyl
radicals.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any amendments) applies.
ISO 20670:2018, Water reuse — Vocabulary
3 Terms, definitions, and abbreviated terms
For the purposes of this document, the terms and definitions given in ISO 20670 and the following apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at http:// www .electropedia .org/
3.1 Terms and definitions
3.1.1
persistent organic pollutant (POP)
chemical substances that persist in the environment, bio-accumulate through the food web, poses a
risk of causing adverse effects to human health and the environment, and can be subject to long range
transport away from its original source
Note 1 to entry: Substances are classified as POPs according to either The Protocol to the regional UNECE
Convention on Long-Range Transboundary Air Pollution (CLRTAP) on POPs, opened for signatures in June 1998
and entered into force on 23 October 2003 or the global Stockholm Convention on POP, opened for signatures in
May 2001 and entered into force on 17 May 2004.
[1]
[SOURCE: ISO 26367-2:2017, 3.8 ]
3.1.2
advanced oxidation process (AOP)
process that generates hydroxyl radicals in sufficient quantity to remove organics by oxidation
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ISO 20468-7:2021(E)

3.1.2.1
hydrogen Peroxide/Ozone (H O /O )
2 2 3
combination of H O and ozone which is able to generate ·OH
2 2
Note 1 to entry: When ozone is dissolved in water, it causes a complex chain of reactions that result in the
formation of radicals including ∙OH and superoxide radicals. The addition of hydrogen peroxide to ozone also
generates ∙OH radicals. The typical stoichiometry of hydrogen peroxide and ozone based on the mass ratio is
from 0,35 to 0,45 because 0,5 moles of H O are required to every mole of O for the complete reaction of ∙OH
2 2 3
production.
3.1.2.2
hydrogen Peroxide/UV (H O /UV)
2 2
combination of H O and UV light which is able to generate ∙OH through UV photolysis of H O
2 2 2 2
Note 1 to entry: The oxidation of organics can occur by either direct photolysis or reactions with ∙OH in H O /UV
2 2
system.
3.1.2.3
ozone/UV (O /UV)
3
combination of ozone and UV light which is able to generate ∙OH through UV photolysis of ozone
Note 1 to entry: UV photolysis of ozone where H O is generated as an intermediate, which then decomposes
2 2
–1 –1
to ∙OH. Due to the relatively high molar extinction coefficient of ozone (ε = 3 300 M cm ), ozone/UV
254 nm
radiation can generally produce more ∙OH than H O /UV radiation.
2 2
3.1.2.4
Fenton reaction
reaction between iron(II) and hydrogen peroxide to yield ∙OH
Note 1 to entry: Fenton reaction can occur either in homogeneous systems with dissolved ferrous iron or
in heterogeneous systems in the presence of complexed iron. The by-product, ferric iron, in turn reacts with
peroxide or superoxide radical to reproduce ferrous iron. The reaction cycle of iron between the ferrous and
ferric oxidation states continuous until the H O is fully consumed, producing ∙OH in the process. As in other
2 2
AOPs, the destruction of organics is primarily due to oxidation reactions initiated by the ∙OH. Similar reactions
can occur with copper (II) in place of iron (II).
3.1.2.5
hydroxyl radical scavengers
non-target substances that react to high degree of reactivity of hydroxyl radical
Note 1 to entry: Hydroxyl radical can oxidize a broad range of organic pollutants quickly and non-selectively.
A drawback resulting from such a high degree of reactivity is that the hydroxyl radical also reacts with “non-
target” materials in solution such as chloride, nitrite, bromide, carbonate, bicarbonate, and NOM, all of which are
referred to as radical “scavengers”.
Note 2 to entry: see Annex D
3.1.2.6
UV/TiO
2
-
semiconductor photocatalysts that absorb light and involve the generation of oxidants (e.g., ∙OH and O ∙
2
) for the destruction of organic pollutants
Note 1 to entry: When TiO , a semiconductor photocatalyst, is illuminated by UV light (≤ 400 nm), valence band
2
electrons are excited to the conduction band, resulting in the production of electron and hole pairs. These
generated electron and hole pairs are capable of initiating a wide range of chemical reactions (e.g., direct
oxidation/reduction, oxidants generation). Among them, ∙OH oxidation is the primary mechanism for the
destruction of POPs. The production of ∙OH can occur via several pathways but, as with many of the other AOPs
analysed, is readily formed from hydrogen peroxide.
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ISO 20468-7:2021(E)

3.1.3
hydroxyl radical (∙OH)
neutral form of hydroxide ion (OH-) which possesses an unpaired electron
Note 1 to entry: The unpaired electron makes it a powerful and non-selective chemical oxidant, which acts very
rapidly with most organic compounds.
3.1.4
redox potential
potential of a reversible oxidation-reduction reaction in a given electrolyte recorded on a standard
hydrogen electrode scale
[2]
[SOURCE: ISO 8044:2020, 6.1.37 ]
3.1.5
∙OH concentration
molar concentration of ∙OH in a unit volume of liquid
3.1.6
∙OH monitor
instrument capable of measuring ∙OH concentration (3.1.5) in samples
3.1.7
electrical energy per order
3
electrical energy in kWh which required to degrade a contaminant by one order of magnitude in 1m of
contaminated water
Note 1 to entry: Electrical energy per order as a Figure-of-merit for AOPs has been accepted by the International
[3]
Union of Pure and Applied Chemistry (IUPAC) in 2001 .
3.1.8
UV transmittance
the fraction of photons in the UV spectrum transmitted through a material such as water or quartz. It is
preferable that an online UVT sensor be installed and used to verify UVT
Note 1 to entry: The wavelength of the UVT (unit %) should be specified, often using a path-length of 1 cm. The
measurement is calibrated compared to ultra-pure water (ISO 3696 grade 1 or equivalent).
Note 2 to entry: UVT is related to the UV absorbance (A) by the following equation (for a 1- cm path length): %
UVT = 100 × 10-A.
3.2 Abbreviated terms
AOP advanced oxidation process
E electrical energy per order
EO
LCC life cycle cost
NOM natural organic matter
-
O ∙ superoxide anion radical
2
∙OH hydroxyl radical
-
OH hydroxide ion
O&M operation & maintenance
POP persistent organic pollutant
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ISO 20468-7:2021(E)

ROS reactive oxygen species
SCE saturated calomel electrode
TOC total organic carbon
UVT ultraviolet transmittance
4 System components
AOP technologies generally follow tertiary treatment for the purpose of attaining higher quality treated
wastewater for specific water reuse applications. AOPs involve the following two stages of oxidation: 1)
the formation of strong radical oxidants (e.g., ∙OH) and 2) the reaction of these radical oxidants with
the water contaminants. However, the term AOPs refer specifically to processes in which oxidation of
organic contaminants occurs primarily through reactions with ∙OH. The ability of an oxidant to initiate
chemical reactions can be measured in terms of its redox potential and ∙OH is one of the most reactive
oxidants in an aqueous phase with an oxidation potential of 2,8 V (pH 0) vs. NHE (normal hydrogen
electrode). In water treatment applications, AOPs usually refer to a specific subset of processes that
involve O , H O , and/or UV light. All of these processes can produce ∙OH as non-selective oxidant enable
3 2 2
to rapidly destroy a wide range of organic contaminants. Although a number of the processes noted
above may have other mechanisms for destroying organic contaminants, in general, the effectiveness of
AOPs is proportional to its ability to generate ∙OH.
Established AOP systems include two representative cases:
1) one system produces ∙OH based on the combination of chemical sources only (e.g., H O /O , etc.);
2 2 3
and
2) the other system produces ∙OH based on the combination of chemical sources and UV light (O /UV,
3
H O /UV, etc.)
2 2
4.1 Chemical source feed unit
The chemical source unit supplies the generation/contact unit (e.g., combinations of O , H O , UV, etc.).
3 2 2
4.2 UV unit
A UV unit has simple components including an irradiation vessel and a power control panel. UV units
may be categorized into closed and open systems, based on the configuration of UV units in the
irradiation vessel. The closed system has a UV unit, comprised of a UV lamp and its sleeve, placed in the
flow chamber. Meanwhile, the open system has a UV unit immersed in an open channel or tank.
4.3 ∙OH generation/contact unit
The ∙OH generation/contact unit typically is a unit to produce ∙OH using feeds, and directly applies the
generated ∙OH to the water, due to the high reactivity of ∙OH. The unit normally includes jet injector
for feeds or/and mechanical agitation for an even distribution of ∙OH. The ∙OH generation/contact unit
occasionally includes a UV lamp for systems employing photons (e.g., H O /UV, O /UV).
2 2 3
4.4 ∙OH monitoring point
The ∙OH monitoring unit monitors the concentration of ∙OH at the ∙OH generation/contact unit either by
ex-situ sampling and in-situ detecting.
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ISO 20468-7:2021(E)

5 Performance requirements and evaluation methods
The purpose of performance requirements and evaluation methods for AOP technology is to assess
whether the performance of AOP processes meets specific requirements for attaining reclaimed
water which satisfy the reclaimed water quality standards for the specific purposes of water reuse.
Performance requirements and evaluation methods include specific performance evaluation procedures
(e.g., water quality test, ∙OH quantification test, monitoring protocol), safety requirement, and
[4],[5]
environmental/energy performance as functional and non-functional requirements, respectively
.
5.1 Functional requirements
5.1.1 General
The design of AOPs is governed by the influent contaminant concentration, target effluent contaminant
concentration, desired flow rate, and background water quality parameters such as pH, bromide
concentration, alkalinity, etc. The key design parameters for AOPs include: chemical dosages and ratios
with other chemicals, reactor contact time, and reactor configuration. The optimum dosages, ratios, and
contact time are water-specific and treatment scenario-specific and are often determined through pilot
studies using the water matrix of interest. As can be expected, higher chemical dosages and contact
times are typically expected to result in higher removal rates; however, increasing dosages results in
higher O&M costs and possible by-product formation (e.g., bromate aldehydes, chlorate, etc.). However,
in some cases, the formation of by-products can be limited by higher chemical ratios. While AOPs have
been found to be effective for a wide variety of organic contaminants, this analysis will focus on the
practical implementation of AOPs in water reclamation, specifically for the treatment of tertiary treated
wastewaters. As previously mentioned, there are many water quality parameters that may impact the
effectiveness of any particular AOP. For example, nearly all dissolved organic compounds present in
the source water can negatively affect the removal efficiency of the target compound by consuming
∙OH. Below is a brief discussion of each of these water quality parameters and the mitigation measures
that can be taken to limit the adverse impact of these parameters on AOPs effectiveness. Regarding the
indirect parameters to judge whether AOP technology fulfills the requirements, ∙OH concentration in
the treatment system can be used.
AOPs can be divided into established and emerging technologies based on the existing literatures.
Emerging technologies are defined here as technologies that have very limited, if any, full-scale
applications for water reuse system.
Established Technologies includes:
1) Hydrogen Peroxide/Ozone (H O /O );
2 2 3
2) Ozone/UV (O /UV); and
3
3) Hydrogen Peroxide/UV (H O /UV)
2 2
Emerging Technologies includes:
1) Fenton Reaction; and
2) UV/TiO
2
5.1.2 Performance evaluation procedures
Established AOPs technologies can be divided into two cases: one is based on a coupling between
chemical oxidants and the other is based on a combination of chemical oxidants and UV light (Annex B).
CASE 1: AOPs based on the combination of chemical sources only (H O /O )
2 2 3
When O is added to water, it participates in a complex chain of reactions that result in the formation
3
-
of radicals such as the ∙OH and the O ∙ . Unlike ozone, these radical products could effectively destroy
2
POPs. For instance, direct oxidation of chlorobenzene by ozone is known to occur very slowly; this
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ISO 20468-7:2021(E)

-1 -1
reaction’s second-order kinetic rate constant is less than 1 M s . On the other hand, the oxidation
9 -1 -1
of chlorobenzene by ∙OH is extremely rapid (up to 4 X 10 M s ). The addition of hydrogen peroxide
enables the initiation of the decomposition of ozone, leading to the formation of ∙OH. . The typical
stoichiometry of hydrogen peroxide and ozone based on the mass ratio is from 0,35 to 0,45 because
0,5 moles of H O are required to every mole of O for the complete reaction of ∙OH production.
2 2 3
Two O and hydrogen peroxide contact configurations are continuously stirred basins and plug flow
3
reactors.
In an O unit, O is bubbled or injected through the base of the unit and allowed to diffuse through the
3 3
unit until it either escapes through the top or is completely reacted. These units are typically covered
so that excess O can be collected and directed to an off-gas decomposer. H O can be added either as a
3 2 2
single slug dose or at multiple points in the system. Automatic monitoring and control systems are used
to regulate chemical feed rates, pH, and other parameters. In addition, a variety of safety, monitoring,
and control systems are included to facilitate operation. A schematic of an H O /O system is shown in
2 2 3
Figure 1 (upper).
The major components of both a continuously stirred tank reactor and a plug flow reactor include a H O
2 2
storage tank, a H O injection system, an O generator, a liquid oxygen or oxygen from a concentrator,
2 2 3
in-line static mixers and mechanical agitation, O injector, an O contactor, an O decomposer and/or
3 3 3
off-gas recycling device, supply and discharge pumps and piping, monitoring and control systems.
CASE 2: AOPs based on the combination of chemical sources and UV light (O /UV and H O /UV)
3 2 2
A simplified schematic diagram of case 2 system is shown in Figure 1 (bottom).
For O /UV applications, O is introduced into the system at the bottom of each chamber by a stainless
3 3
steel sparger or injector. The O generator employed in the system can electrically generate O from
3 3
either air or liquid oxygen. Any O that is present in the off-gas is put through a decomposer and/or
3
recycling device such as activated carbon, catalyst media and heat, or combination of heat and catalyst
[6]
media .
The major components of an O /UV system include UV lamps, sleeves, and cleaning system, an O
3 3
generator and injectors, an O contactor, an O off-gas decomposer, an liquid oxygen or oxygen from a
3
...