ISO 18998:2026

International
Standard
ISO 18998
First edition
Water reuse in urban areas —
2026-01
Guidelines for decentralized water
reuse system — Management of a
decentralized water reuse system
Réutilisation de l'eau en milieu urbain — Lignes directrices
concernant les systèmes décentralisés de réutilisation de l'eau —
Gestion d'un système décentralisé de réutilisation de l'eau
Reference number
© ISO 2026
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ii
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Management models of decentralized water reuse systems . 1
4.1 General .1
4.2 Management models .3
4.2.1 Onsite systems management model.3
4.2.2 Cluster systems management model .3
4.2.3 Community systems management model .4
4.3 Management process .5
5 Management of source water . 6
6 Management of treatment processes for water reuse . 7
6.1 General .7
6.2 Selection of monitoring indicators .7
6.2.1 Principles of indicator selection .7
6.2.2 Indicator monitoring .8
6.3 Process adjustment .8
6.4 Operation and maintenance of equipment .8
7 Management of storage system . 9
8 Management of distribution system . 9
9 Management of end uses . . 9
9.1 Principles .9
9.2 Main items of end uses .10
10 Management of sludge treatment .11
10.1 General .11
10.2 Generation and collection .11
10.3 Selection and design .11
10.4 Monitoring and control .11
10.5 Safety and environmental measures .11
10.6 Quality control and evaluation .11
11 Management of monitoring .11
11.1 General .11
11.2 Baseline monitoring . . 12
11.3 Validation monitoring . 12
11.4 Operational monitoring . 12
11.5 Verification monitoring . 13
12 Management of incidents and emergencies . 14
13 Management of operations and maintenance staff .15
14 Review .15
Bibliography .16

iii
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
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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).
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This document was prepared by Technical Committee ISO/TC 282, Water reuse, Subcommittee SC 2, Water
reuse in urban areas.
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
With economic development, climate change, population growth, and rapid urbanization continuing to shape
the world, water has become an increasingly important strategic resource, particularly in arid and semi-
arid regions. However, even in areas with abundant water resources, challenges such as water shortages
and seasonal water scarcity remain. Water scarcity is recognized as one of the most serious threats to
sustainable development. To address these challenges, an increasing number of communities are turning to
use reclaimed water to meet water needs. This approach has been shown to be effective in improving the
reliability of long-term water supply, particularly in areas where water is scarce.
While centralized water reuse facilities have been widely implemented under different ownership and
management structures, there is also a need to develop decentralized/onsite water reuse systems in cost-
effective and resource-efficient ways, which can improve flexibility and convenience. Decentralized water
reuse systems have emerged as an essential component of water management in many cities and countries.
These systems typically consist of source water collection, wastewater treatment facilities, storage and
distribution systems, and monitoring systems. Management concepts and principles should be implemented
from source water to end users throughout the system, and appropriate strategies should be adopted for
each component.
This document provides management concepts and principles for decentralized water reuse systems in urban
areas, and it can be used by practitioners and regulatory authorities, who intend to implement management
concepts, principles, and supports on decentralized water reuse in a safe, reliable, and sustainable manner.
It considers and addresses key issues in the management process, which can help relevant practitioners and
users to adopt cost-effective methods to achieve safe, reliable, and appropriate reuse of reclaimed water. For
detailed information on the design of decentralized water reuse systems, refer to ISO 23056.

v
International Standard ISO 18998:2026(en)
Water reuse in urban areas — Guidelines for decentralized
water reuse system — Management of a decentralized water
reuse system
1 Scope
This document provides guidelines for the management of decentralized water reuse systems and water
reuse applications in urban areas.
This document addresses decentralized water reuse systems in their entirety and is applicable to any water
reuse systems component (e.g. reclaimed water, source water, treatment, storage, distribution, operation
and maintenance, and monitoring).
This document covers:
— management of each system component of a decentralized water reuse system;
— specific aspects for consideration and emergency response.
This document does not cover monitoring parameters and regulatory values of a decentralized water reuse
system.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content constitute
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 and definitions
For the purposes of this document, the terms and definitions given in ISO 20670 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/
4 Management models of decentralized water reuse systems
4.1 General
Effective management practices are indispensable for the successful reuse of treated wastewater in
decentralized water reuse systems. The management of decentralized water reuse systems should consider
the following aspects:
— source water assessment;
— system design;
— location selection;
— operations and maintenance;
— regulatory compliance;
— cost-benefit analysis;
— community engagement.
Decentralized water reuse management should align with utilization systems through various management
models, including onsite, cluster, and community models. Risk management and control should be
highlighted in decentralized water reuse systems. Aspects and specific points that should be considered in
the management of decentralized water reuse systems are shown in Table 1.
Table 1 — Considerations for management of decentralized water reuse systems
Primary aspects Contributing factors
— Water quantity monitoring
Water quality and quantity — Water quality testing
— Monitoring equipment
— Equipment maintenance
— Troubleshooting
— Waste removal and disposal
Operations and maintenance
— Leak detection and contamination control
— System regular checks and maintenance
— Odour control
— Operational costs
— Capital cost
— Sustainability
Economic impact — Tax policies
— Revenue and risk
— Reclaimed water pricing
— System operation efficiency
— Regulations
— Occupational health and safety standards
— Permits and applications
Legal compliance
— Responsibilities
— Monitoring and compliance
— Effluent standards
— Public acceptance
Social impact — Water resource conservation
— Environmental protection
4.2 Management models
4.2.1 Onsite systems management model
Onsite systems are used to treat wastewater generated from family/household-based systems and onsite
building scale systems. The onsite systems are usually managed and maintained by household owners or
users (Figure 1). This model offers flexibility, autonomy, and cost savings. Wastewater collection, treatment,
and use should align with water demands, adapting processes in less developed areas while prioritizing
users’ health. The implementation of management initiatives should also consider users' economic ability
and appropriate technology. For instance, onsite water reuse systems can be improved with autonomous
controls that use Internet of Things (IoT) and sensors to manage water quality accurately, and user
involvement remains crucial to prevent pollution and maintain compliance with water quality standards.
Key
1 onsite systems
2 wastewater treatment
3 water reclamation
4 end uses
Figure 1 — Onsite systems management model
4.2.2 Cluster systems management model
Cluster systems are designed to treat the wastewater from a collection of dwellings or facilities located
adjacent to each other with typically a few owners. The cluster systems are normally used for managing
multiple end uses through a unified wastewater treatment and water reclamation process (Figure 2). This
enables information sharing, resource integration, and optimization to improve water reuse efficiency and

economic benefits. Real-time monitoring and control enhance system stability and security, while unified
management strategies optimize operations. However, this model requires more financial and technical
resources, as well as resilience to single-point of failure. Coordination among subsystems is also crucial.
Key
1 cluster systems
2 wastewater treatment
3 water reclamation
4 end uses
Figure 2 — Cluster systems management model
NOTE The treatment process shown in Figure 2 is only an example of processes listed in ISO 24521.
4.2.3 Community systems management model
The community systems management model includes wastewater collection and treatment, as well as
reclaimed water storage and distribution through planning, implementation, monitoring, and evaluation
(Figure 3). This management model is relatively complex and requires professional system management
(such as automatic control systems, remote control) to achieve sustainability. It improves water utilization
and increases economic benefits, while reducing dependence on natural resources and pollution load.
However, it faces problems such as high operations and maintenance costs, technical difficulties, and
communication problems.
Key
1 community systems
2 wastewater treatment
3 water reclamation
4 end uses
Figure 3 — Community systems management model
4.3 Management process
The management process of decentralized water reuse systems is crucial for ensuring stable operation,
water quality safety, and efficient utilization. A comprehensive and efficient management system covers
source water collection, treatment technology, storage and distribution, end uses, etc. (Table 2).

Table 2 — Management process of decentralized water reuse systems
Management process Considering elements
— Water quality
Source water collection — Water quantity
— Routine checking
— Evaluation indicator
— Potential treatment technologies
Treatment technology
— Regular operation checking
— Regular monitoring of the water quality
— System operation
— Objectives
Storage and distribution
— Methods for quality and quantity control
— Routine checking of operations and maintenance
— Operability
End uses
— Safety, quantity & quality
— Treatment
— Disposal
Sludge
— Possible reuse options
— Safety, quality and quantity checking
— Frequency of monitoring
Monitoring — Data recording, data keeping, corrective action
— Improvement actions related to monitoring report
— Identification
— Prevention
Incidents and emergencies
— Verification
— Recording, reporting, and taking action
— Awareness
Stakeholder — Training
— Public outreach
5 Management of source water
Management of source water in decentralized water reuse systems depends on different scenarios and types
of source water. For instance, the management of source water for onsite and cluster systems is primarily
the responsibility of end users, whereas the source water management for community systems are more
frequently carried out by authorized practitioners.
Management of source water focuses on source water quality and quantity. In terms of source water quality,
it is recommended that the management practices align with those of the facility to measure and monitor
the quality of source water. For example, an early warning system can be incorporated into the program to
facilitate the provision of timely information for the detection of sudden changes in source water quality.
At the same time, contaminant concentrations and the various alternatives for diverting the contaminated
waters should be documented in the source water control program. Accordingly, informed decisions or

responses can be made with regards to changing the treatment and operational methods or closing the
intakes for the source control programs.
Information on the quantity of source water should be monitored and recorded in a timely manner in order
to maintain the water balance between source water and reclaimed water. Furthermore, a response and
management plan for mitigating reclaimed water shortages may be developed and maintained based on the
importance of reclaimed water supply to end users. The plan should include strategies for the provision
of alternative water resources for the provision of essential services on a short-term basis. An additional
consideration that should be included in the plan is whether availability of the reclaimed water would only
be on a seasonal or interruptible basis. Alternatively, a clear time schedule for the period of delivery should
be included.
6 Management of treatment processes for water reuse
6.1 General
An indispensable component of managing treatment processes for water reuse is a comprehensive
performance monitoring plan, including monitoring indicators, process adjustment, and operation and
maintenance of equipment. This plan serves a vital role in validating both the operation normality and
effectiveness of the treatment processes, and the conformity of reclaimed water with purpose criteria.
6.2 Selection of monitoring indicators
6.2.1 Principles of indicator selection
Monitoring indicators are essential for ensuring the proper functioning of decentralized water reuse
systems, and also help to achieve the desired outcomes. The following are principles for selecting monitoring
indicators for decentralized water reuse systems.
a) The water quality and water quantity demands of end users should be considered, including the process
operational indicators specified in the relevant water quality standards.
b) The indicators should meet local environmental protection and water reuse standards to ensure the
safety and hygiene of the reclaimed water quality.
c) The indicators should be easy to measure, and appropriate responses can be made accordingly.
d) The treatment processes’ effectiveness and efficiency should be demonstrated through real-time
indications of process performance.
e) It should be ensured that the operating and construction costs of the system are reasonable.
f) System design and equipment selection should conform with local technical standards and building
codes.
The treatment processes for water reuse include natural treatment processes, aerobic, anaerobic, and
combined processes, disinfection, etc. A detailed description of wastewater treatment processes is provided
in ISO 24521. Furthermore, advanced treatment technologies are necessary when terminal reclaimed
water criteria become stricter. Different treatment processes have some commonalities in the selection
of operating indicators, such as common water quality indicators (e.g. chemical oxygen demand (COD),
biochemical oxygen demand (BOD), total nitrogen (TN), total phosphorus (TP), Escherichia coli) and water
quantity indicators. Technical assessment can refer to ISO 20761, ozone treatment technology can refer
to ISO 20468-3, UV disinfection technology can refer to ISO 20468-4, membrane filtration technology can
refer to ISO 20468-5, ion exchange and electrodialysi
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