ISO 20468-10:2025

International
Standard
ISO 20468-10
First edition
Performance evaluation of
2025-12
treatment technologies for water
reuse systems —
Part 10:
Guidelines for evaluation of
dependability of treatment systems
Évaluation des performances des techniques de traitement des
systèmes de réutilisation de l’eau —
Partie 10: Lignes directrices pour l'évaluation de la sûreté de
fonctionnement des systèmes de traitement
Reference number
© ISO 2025
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ii
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms, definitions, and list of abbreviated terms . 1
3.1 Terms and definitions .1
3.2 Abbreviated terms .3
4 Concept of dependability evaluation of treatment systems . 3
4.1 Dependability evaluation by availability .3
4.2 Treatment system and critical unit processes as performance control points (PCPs) .3
5 Evaluation procedure for dependability of treatment systems . 4
5.1 General .4
5.1.1 Setting boundary for dependability evaluation .4
5.1.2 Setting PCPs .4
5.1.3 Availability assessment of PCPs .4
5.1.4 Availability assessment of treatment system .4
5.2 Setting PCPs, confirming requirements and defining failure state .5
5.3 Qualitative assessment of availability .5
5.4 Quantitative assessment of availability .6
Annex A (informative) Examples of maintenance strategy as benchmark for qualitatively
assessing availability applied to key technologies . 7
Annex B (informative) Example of quantitative assessment of availability .11
Bibliography .12

iii
Foreword
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This document was prepared by Technical Committee ISO/TC 282, Water reuse, Subcommittee SC 3, Risk and
performance evaluation of water reuse systems.
A list of all parts in the ISO 20468 series can be found on the ISO website.
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.

iv
Introduction
The purpose of this document is to define a methodology to evaluate the dependability of treatment systems,
which is listed as one of “treatment technology non-functional requirements” in ISO 20468-1. Although the
targeted performance requirement can be selected depending on the purpose, functional requirements
related to water quality should be targeted with high priority in conventional water reuse projects.
As a consequence, the dependability of a treatment system can be represented and evaluated by availability
as the ability to perform long-term operation without a failure and the fulfilment of functional requirements
related to the water quality target to be conformed with. As well, dependability as a “non-functional
requirement” can be evaluated by benchmarking for making comparisons or improving treatment system
performance.
Performance indicators other than dependability for the non-functional requirements of treatment systems
have already been specified in ISO 20468-2 for environmental performance based on greenhouse gas
(GHG) emissions and in ISO 20468-8 for economic evaluations based on life cycle cost (LCC). If functional
requirements related to water quality performance are satisfied, dependability can provide appropriate
information on operation and maintenance used for calculating GHG emissions and LCC.
A comprehensive evaluation of dependability, environmental, and economic performance can be used to
select an appropriate and cost-effective treatment system in the planning phase and to identify improvement
for operations and optimization opportunities on a life cycle basis. A variety of stakeholders, including
managers of water reuse projects and owners of water infrastructure and facilities can find beneficial
opportunities and options through a comprehensive performance evaluation using the guidelines in the
ISO 20468 series.
v
International Standard ISO 20468-10:2025(en)
Performance evaluation of treatment technologies for water
reuse systems —
Part 10:
Guidelines for evaluation of dependability of treatment systems
1 Scope
This document provides guidelines for evaluating the dependability represented by the availability of
treatment systems for water reuse.
The document specifies methodologies for both qualitative and quantitative assessments of availability on a
life cycle basis.
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, Water reuse — Vocabulary
3 Terms, definitions, and list of abbreviated terms
For the purposes of this document, the terms and definitions given in ISO 20670 and the following apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
3.1 Terms and definitions
3.1.1
availability
ability of a treatment technology to be in a state to perform a required function under given conditions at a
given instant of time or over a given time interval
Note 1 to entry: This ability depends on the combined aspects of reliability (3.1.11) performance, maintainability (3.1.7)
performance, and maintenance support performance (3.1.8). Preventive maintenance (3.1.10) can extend reliability and
improve availability.
[SOURCE: ISO 20468-1:2018, 3.1.1, modified — “assuming that the required external resources are provided”
was removed from the definition; “Preventive maintenance can extend reliability and improve availability”
was added to Note 1 to entry; Note 2 to entry was removed.]
3.1.2
benchmarking
tool for performance improvement through systematic search and adaptation of leading practices
[SOURCE: ISO 20468-1:2018, 3.1.2]

3.1.3
corrective maintenance
maintenance carried out after fault detection to effect restoration
[SOURCE: IEC 60050-192:2015, 192-06-06, modified — Note 1 to entry was removed.]
3.1.4
dependability
ability of a system represented by the availability (3.1.1) performance and its influencing factors, namely
reliability (3.1.11) performance, maintainability (3.1.7) performance and maintenance support performance (3.1.8)
[SOURCE: ISO 20468-1:2018, 3.1.5, modified — “collective term used to describe” was replaced with “ability
of a system represented by”; the list of influencing factors was added to the definition.]
3.1.5
downtime
amount of time that a system or a component is not able to operate or meet required functions
[SOURCE: ISO 20468-1:2018, 3.1.6]
3.1.6
failure
state in which a treatment technology does not meet a functional or a non-functional requirement (3.1.9)
[SOURCE: ISO 20468-1:2018, 3.1.7]
3.1.7
maintainability
ability of a treatment technology, under given conditions of use, to be retained in, or restored to, a state in
which it can perform a required function, when maintenance is performed under given conditions and using
stated procedures and resources
[SOURCE: ISO 20468-1:2018, 3.1.9]
3.1.8
maintenance support performance
ability of a maintenance organisation, under given conditions and maintenance policy, to provide, upon
demand, the resources required to maintain the treatment technology
[SOURCE: ISO 20468-1:2018, 3.1.10, modified — Notes to entry were removed.]
3.1.9
non-functional requirement
requirement that specifies criteria or constraints on the design or implementation of a treatment technology
[SOURCE: ISO 20468-1:2018, 3.1.12]
3.1.10
preventive maintenance
maintenance carried out to mitigate degradation and reduce the probability of failure (3.1.6)
[SOURCE: IEC 60050-192:2015, 192-06-05, modified — Notes to entry were removed.]
3.1.11
reliability
ability that a device, system or process will perform its prescribed function without failure (3.1.6) for a given
time when operated correctly in a specified environment
[SOURCE: ISO 20670:2023, 3.79, modified ― “probability” was replaced with “ability”.]
3.1.12
uptime
amount of time that a system or a component is able to operate or meet required functions

3.2 Abbreviated terms
NF nanofiltration
PCP performance control point
RO reverse osmosis
TMP transmembrane pressure
UV ultraviolet
4 Concept of dependability evaluation of treatment systems
4.1 Dependability evaluation by availability
The dependability of a treatment system is evaluated mainly from the availabilities of unit processes, which
make up the system facility, to achieve functional requirements related to water quality over the course of
operation. In planning or design stage, inherent availability is generally expressed by Formula (1) with two
[9]
influencing factors: reliability and maintainability. The introduction of preventive maintenance to prevent
failures of the treatment system facility can extend the reliability represented by mean time between
failures and improve the inherent availability. While preventive maintenance is usually more effective and
preferable, corrective maintenance for the rapid recovery from a failure can reduce mean time to recovery
and lead to improved maintainability, which in turn contribute to increased availability.
T
b
A = (1)
i
TT+
br
where
A is the inherent availability;
i
T is the mean time between failures (in d);
b
T is the mean time to recovery (in d).
r
The primary functional requirement of a treatment system is to produce reclaimed water that fulfils
quality and quantity requirements, and a failure can be in the context of water quality or quantity. As a
result, dependability represented by availability can be explained as an ability related to the duration of
system operation without intervention or failure and fulfilment of functional requirements related to a
water quality target. As well, dependability as a non-functional requirement can usually be evaluated by
benchmarking for making comparisons or improving treatment system performance.
4.2 Treatment system and critical unit processes as performance control points (PCPs)
For a treatment system, the use of treatment processes as multiple barriers is an effective and commonly
used strategy to reduce residual health risks in reclaimed water. The required level of treatment is
determined based on the degree of required water quality, which is ultimately associated with intended
end-use.
The availability of a treatment system depends on the availability of each of the individual unit treatment
processes that make up the system, and the way in which these processes are integrated. To fulfil the
requirements for the high reclaimed water quality of a treatment system, in addition to monitoring water
quality, ISO 20426 recommends a risk control approach by setting process PCPs. The availability of each
of the unit processes determined as PCPs should be assessed for practical and effective assessments of
treatment system availability.
The availability of unit processes can be assessed by selecting water quality criteria or surrogate parameters
and the measurable control numerical limits that are set for each PCP. For monitoring the fulfilment of
functional requirement of processes as possible PCPs, the ISO 20468 series guidelines for evaluating the
performance of treatment technologies can be used to select appropriate monitoring parameters.

5 Evaluation procedure for dependability of treatment systems
5.1 General
5.1.1 Setting boundary for dependability evaluation
The boundary conditions for a dependability evaluation of a treatment system should be established
according to the relevant project requirements. Normally, the treatment system consists of multiple
treatment processes using physical, biological or chemical means, or a combination of these. The boundary
conditions should be the treatment system or be limited to part of the treatment system and associated
facilities of the treatment processes.
5.1.2 Setting PCPs
Critical processes should be selected as PCPs among the treatment processes included in the boundary, and
prerequisite information on each PCP operation should be confirmed for the availability assessment. Setting
a benchmark
...