The Role of Scenario-Building in Risk Assessment and Decision-Making on Urban Water Reuse

Abstract

Urban resilience and water resilience are both increasingly relying on urban non-potable water reuse under the context of the Climate Emergency, but sound risk assessment is lacking. Compared to the state of art, the proposed framework for health risk assessment and management of urban non-potable water reuse includes (i) an additional step for establishing the context and (ii) the risk identification step being extended to introduce a description of the activities from which the hazard exposure scenarios may be built. This novel scenario-building process allows for a clear and comprehensive risk description, assessment, and treatment. The model of risk management is structured around three primary components: the decision-makers, i.e., the municipal services and the population at risk (users and workers); data elements relevant for the risk management process (reclaimed water quality, hazards, hazardous events, sites where exposure can happen, exposure routes, and activities developed by the population at risk and their vulnerabilities); and the links between the decision-makers and these elements and between the elements themselves. Its application in a representative case study shows that the framework comprehensively guides decision-making and communication to relevant stakeholders. From this practical exercise, the main recommendations were derived for risk mitigation by the municipal risk manager and the park users.

1. Introduction

Urbanization and climate change are two closely related phenomena that exacerbate water scarcity (i.e., a lack of freshwater to meet the water demand) in the cities, affecting the health and wellbeing of urban residents, urban environmental quality, and socioeconomic development [1]. In the context of urban planning, urban resilience has become an important strategy to tackle climate change [2]. According to Zeng et al. [3], urban vulnerabilities can be addressed by increasing cities’ (i) adaptive capacity (based on water, health, food, and education), (ii) absorptive capacity (via urban green space, protective infrastructure, access to transportation, and community support), and (iii) transformative capacity (using multi-stakeholder collaboration, communication technology, and government emergency services).

Wang et al. [4] assessed “clean-water scarcity” in terms of the availability of surface water with an acceptable quality, focusing on nitrogen pollution in rivers. These authors estimated a triple increase in the number of global river basins with water scarcity in 2050 when comparing quantity and quality-induced scarcity with quantity-induced scarcity. To decrease the pressure on freshwater supplies, it is important to support the transition to smart and more integrative water management. Water reuse is an important adaptation measure for tackling climate change because it is a rainfall-independent water source and reduces the discharge of contaminants into freshwater, including nutrients associated with eutrophication phenomena, i.e., phosphorus and the earlier-mentioned nitrogen.

Safe water reuse requires the existence of a comprehensive risk assessment and management process and benefits from articulation with existing sustainability policies or strategies. Figure 1 illustrates how the water reuse risk assessment framework contributes to urban resilience, which is crucial for improving urban sustainability.

While water reuse in agricultural irrigation has been recognized as a viable alternative for water supply [5,6,7], the integration of reclaimed water into the urban water supply has not reached its potential [8]. Water scarcity has been a catalyst for water reuse around the world, but, where there is no perceived severe water scarcity, there are few municipal reuse projects of meaningful scale [9]. This situation is becoming less and less sustainable in the context of the Climate Emergency. It is therefore necessary to prioritize water reuse at the municipal scale to promote a systemic change in water systems [10].

In contrast to a generally positive appreciation of water reuse as a possible solution for water supply, a usually negative public perception based on concerns related to risk, lack of trust, and cultural factors has hindered the implementation of various water reuse projects [9,11]. To solve this issue, scientists can contribute by simplifying the discussion around risk to enable informed decision-making on water reuse [12]. According to Durkin et al. [13], transparency, integrity, and stakeholder engagement should be the guiding principles for addressing concerns related to water reuse and recycling practices.

Understanding and managing risk, and communicating it in practice, is a complex process that should be carried out using clear and generally accepted procedures. The ISO, the International Organization for Standardization, has enabled consensus among global experts covering almost all aspects of technology, management and manufacturing. In the domain of risk management, two relevant international standards were published in 2018: the ISO 31000:2018 [14], which provides guidelines for risk management and was developed by the technical committee ISO/TC 262—Risk Management, and the ISO 20426:2018 [15], which provides guidelines for health risk assessment and management for non-potable water reuse and was developed by the technical committee ISO/TC 282—Water Reuse. The principles of the latter are embodied in the regulation (EU) 2020/741 of the European Parliament and of the Council of 25 May 2020 on the minimum requirements for water reuse [16].

ISO 31000:2018 (reviewed and confirmed in 2023) encompasses a generic approach for the management of risks. This standard provides a set of principles for guiding effective risk management; it also provides recommendations for establishing a risk management framework and a risk management process. According to ISO 31000:2018, risk is “the effect of uncertainty on an organization’s ability to meet its objectives”. By making the role of objectives explicit, risk management is presented as the activity of anticipating deviations from the plan and implementing corrective actions for achieving objectives despite uncertain futures events.

ISO 20426:2018 (reviewed and confirmed in 2023) provides guidelines for the assessment and management of the health risks related to non-potable water reuse. The risks are caused by potentially occurring pathogenic organisms when using reclaimed water and/or during the production, storage, and transportation of reclaimed water. Reclaimed water can be produced from different sources (raw sanitary sewage, treated municipal wastewater, industrial wastewater, and stormwater potentially influenced by sewage). Risks thus result from the combination of “the magnitude of consequences and the likelihood that those consequences can happen”, where “consequence” refers to potential adverse health impacts of hazard exposure scenarios, “likelihood” indicates the probability of occurrence of a hazardous event, in a certain period, with potential harmful effects, and risk scenarios are set in terms of the occurrence of hazardous events (ISO 20426:2018).

In the context of urban non-potable water reuse, other risk assessment approaches can be considered for enhancing the scenario-building process. The Framework for Cumulative Risk Assessment [17], for instance, integrates a clear framework for organizing and analyzing information about the combined effects of exposure to multiple environmental stressors. In the first of three sequential phases, the development of a conceptual model (with the identification of stressors, health or environmental effects, and relationships among various exposures and effects) and an analysis plan (including data needs, selected approaches, and expected result types) facilitates the understanding of the risks by the decision-makers and stakeholders [18].

In essence, the uncertainty or variability in risk estimates is assessed by two means: a qualitative approach and a quantitative approach [19]. The first is based on data judged by the risk assessors, and the second is based on deterministic or probabilistic data [20].

Qualitative risk assessment involves estimating the potential risk using increasing relative scales to represent the magnitude of consequences, the likelihood of occurrence, and the combination of impact and likelihood [15]. Quantitative microbial risk assessment is a mathematical approach for estimating risk posed by pathogens to human health; it is commonly performed to model possible risks in case of potable water reuse schemes [21]. It is important to note that qualitative risk assessment, like the one presented in ISO 20426:2018 [15], is often considered a semi-quantitative risk assessment (e.g., [22]). Even if the categorical labelling is based on a numerical definition (e.g., 10⁻³ and 10⁻⁴ probability of occurring in a year [20]), the selection of a category still depends on the judgment of the risk assessor. This view is aligned with Revitt et al. [23] and Vidottti et al. [24], who developed qualitative approaches for assessing the risk of non-potable water reuse in agricultural irrigation based on numerically described ordinal scales.

According to ISO 20426:2018, qualitative risk assessment is the most appropriate and economically feasible methodology for most non-potable reuse projects, whereas quantitative risk assessment should be limited to water reuse schemes with a “very high” health risk. This in line with Callahan and Sexton [25], who consider the qualitative approach for cumulative risk assessment as a practical solution to overcome the problems of complexity and data deficiencies and to improve the understanding about the nature and magnitude of risks.

Though it is a function of magnitude and likelihood [14,15], risk is often perceived only in terms of the magnitude of the consequences, even if these are highly unlikely [26]. It is essential for risk managers to understand the reasons for this type of risk amplification by the decision-makers and other stakeholders [27]. The qualitative risk assessment process, by describing how certain action developments may affect compliance with safe water reuse, can be instrumental to improving risk perception. This is particularly relevant in cases of municipal water reuse projects, and our aim is to accomplish this through a comprehensive scenario-building process.

The aim of this paper is to highlight the potential of the risk assessment process for facilitating the implementation of a sound risk management and increasing the transparency of the decision-making process in the context of urban non-potable water reuse. A framework for the health risk assessment of urban non-potable water reuse is proposed and applied in a case study. The scenario-building results and their contribution to decision-making are discussed.

2. Materials and Methods

2.1. Risk Assessment Framework

As previously mentioned, ISO 31000:2018 and ISO 20426:2018 provide similar structured approaches for understanding and managing risk, as presented in

Table 1. Main aspects of the risk assessment and management approaches in ISO 31000:2018 and ISO 20426:2018.

Risk Management Process (Based on ISO 31000:2018)	Health Risk Assessment and Management for Non-Potable Water Reuse (Based on ISO 20426:2018)
Scope, context, and criteria (clause (c.) 6.3) Definition of objective(s) Identification of relevant stakeholders Definition of risk tolerability criteria	[Although not included in ISO 20426:2018 shown in Figure 1, c. 4.2 presents the scope and c. 4.3 presents the context and the objective, i.e., safe water reuse]
Risk identification (c. 6.4.2) Identification of uncertain future events that may have an impact on the achievement of the objectives	Identification of hazard and hazardous events (c. 5.1) Identification of hazards (pathogenic organisms) Identification of hazardous events (detected at the points of use of reclaimed water)
Risk analysis (c. 6.4.3) Rating the potential impact of each risk and its likelihood of occurrence Determination of the risk severity via the combination of these factors	Assessment of risk levels (c. 5.2) Qualitative risk assessment: combined evaluation of the magnitude of consequences (potential adverse health impacts) and the likelihood that those consequences can happen
Risk evaluation (c. 6.4.4) Determination of the tolerability of each risk for risk treatment definition
Risk treatment (c. 6.5) Decision on risk mitigation for reducing the level of residual risk to an acceptable level	Risk management with risk-control measures (c. 6.1, 6.6, 6.3.1, and 6.4) Decision on risk-control measures: source control (preventing the entrance of hazards in source water); treatment control (removing hazards from source water); and end-use control (reducing exposure) Definition of a multiple-barrier approach for a reliable risk management
Monitoring and review (c. 6.6) Measuring risk management performance against indicators	Monitoring (c. 6.3.2, 6.3.3 and 7) Ensuring the continuous provision of safe water reuse
Communication and consultation (c. 6.2) Engaging stakeholders Obtaining information for reducing uncertainty	Training and documentation (c. 6.1) For improving the fulfilment of the risk-control measures determined in the risk management plan
Recording and reporting (c. 6.7) Documenting risk management	Review (c. 6.1) For recording the evidence of compliance and for planning continuous improvement

.

The ability to communicate the potential risks to human health to the population at risk is crucial in the case of urban water reuse because the exposure to hazards may be perceived as involuntary. To facilitate the identification of the situations where this exposure can happen, the framework includes in the scenario-building a description of regular activities developed on the green area irrigated with reclaimed water and during the operation of the water reuse system (reclaimed water treatment and distribution). This approach is aligned with Månsson et al. [23], who observed in an empirical study that the inclusion of a supporting narrative with contextual information facilitated the risk assessment as well as the decision-making about risk-control measures.

Figure 2 illustrates the proposed framework for health risk assessment and management of urban non-potable water reuse. Compared to ISO 20426:2018, it includes an additional step for establishing the context, and the risk identification step is extended to introduce a description of the activities based on which the risk scenarios may comprehensively be built.

Besides ISO 20426:2018, there are other ISO standards relevant to risk management of urban non-potable water reuse.

Table 2. Other ISO standards relevant for risk management of urban non-potable water reuse.

Risk Management	ISO Standards	Main Aspects
Establishing the context	ISO 20469:2018 [28]	Water quality grade classification for reuse application
Risk identification	ISO 20761:2018 [29]	Considerations of water reuse safety and public acceptance
Risk analysis and evaluation	ISO 20761:2018	Water reuse safety evaluation
Risk treatment	ISO 16075-2:2020 [30] ISO 16075-5:2021 [31] ISO 20469:2018 ISO 24416:2022 [32]	Water quality levels, irrigation barriers, multi-barrier approach Reclaimed water disinfection and equivalent treatments Notice boards signposting the water quality grades Reclaimed water stability evaluation
Monitoring	ISO 16075-4:2021 [33]	Monitoring the quality of reclaimed water

indicates the most useful standards as of July 2024. Updated information is provided on the technical committee ISO/TC 282 website.

The link between the proposed framework for health risk assessment of urban non-potable water reuse (Figure 2) and risk-informed decision-making [18] is established in Figure 3.

Table 3. Strengths and weaknesses of different approaches applicable to non-potable water reuse risk assessment.

Approach	Strengths	Weaknesses
Risk management process (ISO 31000:2018)	It is linked to an organizational-level risk management framework It promotes a clear definition of objectives ♦ It promotes the participation of stakeholders ♦	It is designed for general application
Health risk assessment and management for non-potable water reuse (ISO 20426:2018)	It is specific for non-potable water reuse ♦ It includes a qualitative risk assessment methodology that is adequate for comparing risk levels and making decisions on risk treatment - It is designed for enabling risk-informed decision-making as part of the process of continuous improvement in the management of risks ♦ It promotes the participation of stakeholders ♦	It may lead to an amplified risk perception regarding water reuse due to the fact the exposure scenarios are centered on the hazardous events It lacks a clear identification of the situations that may result in exposure to risks
Cumulative risk assessment framework (USEPA 2003)	It includes a framework for organizing and analyzing information It includes a conceptual model for risk description ♦ It promotes the participation of stakeholders ♦	It presents the risk assessment essentially as a sequential process, thus not formalizing the continuous improvement to be used in risk management
Proposed framework for health risk assessment and management of urban non-potable water reuse	It includes the aspects marked with “♦” It includes a water reuse system model structured around three primary components: decision-makers, data elements and links between the decision-makers and these elements, and links between the elements themselves ✸ It includes a novel rationale for the scenario-building process focused on everyday activities for risk-informed decision-making ✸ It is designed for facilitating the communication of risks to stakeholders ✸ It is implemented in a digital tool, in beta version [34]	The methodology is still at an initial stage of utilization by risk managers in urban water reuse projects

Notes: ^♦ aspect integrated in the proposed framework; ^✸ innovation of the proposed framework.

summarizes the main strengths and weaknesses of the risk assessment approaches analyzed and proposed, highlighting the latter’s novel aspects.

2.2. Configuration of the Water Reuse System

The configuration starts by establishing the context, i.e., defining the objectives set for water reuse, the relevant stakeholders, and how decision-making is integrated in the risk management process, including the criteria for risk acceptability.

Next, the risk identification provides the characterization of the settings where exposure to hazards may happen and is the basis of the risk management. The development of a site-specific conceptual model is very useful for the formulation of risk scenarios (e.g., [17]). The conceptual model should include the hypothesized source of hazards, transport routes of hazards, contaminated media, exposure routes, and endpoint receptors.

Figure 4 illustrates a conceptual model in the form of a flow chart, structured around three primary components:

• The municipal services and the population at risk (users and workers);
• The data elements relevant for the risk management process—reclaimed water quality, hazards, hazardous events (events in which people are exposed to a hazard within the system [15]), sites where exposure can happen, exposure routes, activities developed by the population at risk, and vulnerability of the population at risk;
• Links between the municipal services and the population at risk and these elements, and links between the elements themselves; the links can be actions, results, or features.

Table 4. Configuration of the conceptual model of a water reuse system for municipal green area irrigation.

Element	Group	Category (Examples)	Designation (Examples)
Components	Hazards	Pathogenic bacteria (indicator)	Legionella spp. Escherichia coli (enteric bacteria)
		Intestinal nematodes	Helminth eggs
	Exposure routes	Direct route	Inhalation Ingestion Dermal contact
		Indirect route	Ingestion Dermal contact
	Exposure sites	Vegetated area	Irrigated areas—lawns, flower beds Non-irrigated areas—meadows
		Non-vegetated area	Paths Water features—lakes
		Equipment	Drinking fountains Children playgrounds, fitness areas, etc. Terraces
	Activities	Stay	Lie/sit down on lawns, etc. Play sports—yoga, football, etc., on lawns Eat on terraces, lawns, etc. Drink from water fountains Use children’s playground, fitness equipment, etc.
		Pass by	Walk on paths, lawns, etc. Cycle on paths, etc.
		Work	Operation of the irrigation system Maintenance of vegetation Serving food on terraces
	Population at risk	Users	Immature immune system (young children) Competent immune system (older children, teenagers, and adults in general) Weakened immune system (elderly people and people with a poor immune system)
		Workers	Competent immune system (adults in general)
Hazardous events	Occurrence of hazards	Increase in hazards in the source water	Due to undue industrial discharge in the municipal drainage system
		Increase in hazards in the reclaimed water 1	Due to a failure in the reclaimed water treatment system Due to contamination of or regrowth in the reclaimed water distribution network
		Increase in hazards at the points of use	Due to the contamination of irrigation devices (sprinklers and drippers) by dog excrement
	Likelihood of exposure	Inadvertent misuse 1	Due to potential misuse: drinking from an unidentified water tap Due to inadequate education and information about permitted uses for the reclaimed water
		Accidental exposure 1	Due to design or operational deficiencies: pipe bursts or leaks, inadequate irrigation timing Due to end-use system failures resulting from sabotage, natural disasters, or extreme weather conditions
		Cross-connection 1	Due to an improper connection to a drinking water network, leading to its contamination by the reclaimed water Due to an improper connection to a sewage network, leading to the contamination of the reclaimed water network by wastewater
Barriers	Occurrence of hazards	Decrease in hazards in the reclaimed water 2	By additional, low-level disinfection [1 barrier ≈ 2 log reduction value (LRV)] By additional, high-level disinfection [2 barriers ≈ 4 LRV]
	Likelihood of exposure	Reduce the potential for accidental exposure 2	By access control [1 barrier ≈ 2 LRV] By irrigation technology to mitigate aerosol formation: use of drip irrigation [1 barrier ≈ 2 LRV] By requirements for sprinkler irrigation: maximum wind speed, distance to sensitive areas [1 barrier ≈ 1 LRV]
Preventive measures	Awareness	Reduce the potential for inadvertent misuse 3	By signage: “Reclaimed water is being used in irrigation” and “Water not suitable for drinking”
	Information	Reduce the potential for accidental exposure	By information about self-protection against contact with reclaimed water: on a website
		Reduce the potential for cross-connection	By clear identification of the reclaimed water network pipes and accessories, signaling the existence of non-potable water
	Training	Reduce the potential for accidental exposure	By implementation of good practices: use of protective equipment when working with vegetation and the irrigation system
		Reduce the potential for cross-connection	By adoption of procedures for detection of cross-connections between water distribution networks

Notes: ¹ ISO 20426:2018 (clause 5.1.2); ² ISO 16075-2:2020 (clause 4.3); ³ ISO 20469:2018 (clause 5).

illustrates how the different elements of the water reuse system can be described. The effect of the hazardous events and risk-control measures (i.e., barriers and prevention measures) on the risk level (by increasing or decreasing it) is clearly identified. This configuration scheme is the basis for building the risk scenarios and can easily be applied to other (i) types of non-potable water uses (e.g., street cleaning) or (ii) types of risk, namely, environmental risk. In the first case, the conceptual model is the same, and the exposure sites and the activities are site-specific. In the environmental risk assessment, the conceptual model involves different types of subjects at risk (surface water, ground water, or soil), hazards (e.g., nitrogen), exposure routes (e.g., infiltration), hazardous events (e.g., irrigation with reclaimed water exceeding the site’s demand for water and/or nutrients), and vulnerabilities (e.g., groundwater vulnerability). The main difference is that no activities are to be associated with the subjects at risk in terms of risk-control measures.

In Table 4, the hazardous events are described according to the convention suggested by the World Health Organization [35] for the development of the drinking water safety plans, specifically, “X happens because (of) Y”, where X is the effect on the water supply and Y is the cause.

2.3. Scenario-Building Process

The identification of the hazard exposure scenarios (the key result of the risk identification step) is critical for the success of the risk assessment; it should not be too narrow nor too broad. It is therefore important to establish a systematic approach to the scenario-building process, considering the following assumptions (built upon [36]):

• Each scenario should have an impact on the objective(s).
• Each scenario should be a plausible story, with information to support the potential action development.
• Each scenario should be psychologically effective, with a narrative which predisposes its acceptance.
• All scenarios should be structured in a consistent and logical manner.
• All scenarios are possible in principle but do not present the same likelihood.

Within the scope of the standard ISO 20426:2018, likelihood is mainly determined by the probability of human exposure to hazardous events. It is defined that human exposure to pathogens can occur through the combined likelihood of two circumstances: (a) potential for exposure to the media containing hazards (i.e., reclaimed water); and (b) the likelihood of the presence of hazards in the reclaimed water at a concentration higher than that defined for the reuse project (i.e., the reclaimed water presents lower quality than expected). In Table 4, the description of the hazardous events and the risk-control measures acknowledges these two conditions.

Figure 5 illustrates the rationale developed for building the risk scenarios applicable to urban non-potable water reuse, which is structured around the following three elements:

• The cause, i.e., the hazard that may potentially affect human health.
• The events, i.e., the circumstances of the exposure. Everyday activities are the main reason why people can be exposed to reclaimed water. Thus, it is important to align the risk scenarios with the expected activities. In each, one or more hazardous events may apply, and it is necessary to consider the combined effect of these events in the assessment of the risk.
• The consequence of the exposure to that hazard, translated by the vulnerability of the population at risk.

Applying this rationale, a health risk scenario can be described as follows:

“The hazard A as present in reclaimed water of Class X may affect, via the exposure route B and on the exposure site C, the population at risk D when they carry out the activity E. In case the hazardous event(s) F occur, the baseline risk level corresponds to Y. If this risk level is acceptable, considering the risk criteria for this water reuse project, there is no need to reinforce the risk treatment. If the resulting risk level is not acceptable, the risk-control measures in place (barriers G and preventive measures H) need to be improved or the additional risk-control measures (barriers I and preventive measures J) need to be adopted to lower the level of risk to an acceptable value (final risk level).”

Scenario-building is a multi-factorial process that requires careful planning and execution. Figure 6 presents the process diagram for building risk scenarios for water reuse for municipal green area irrigation, where the proposed sequence for selecting the different data elements ensures that all relevant risks are analyzed.

3. Case Study Description

As with many cities, Lisbon, the capital city of Portugal, faces water challenges related to (i) an increased concentration of the population and a growing economy, (ii) climate change effects, (iii) a need to increase urban green areas to guarantee the quality of life of citizens and the sustainability of urban life, and (iv) a need to reduce the current use of drinking water for non-potable municipal uses. The green and blue infrastructure is a solution applied in Lisbon for tackling climate change impacts related to droughts, heat waves, heat islands, and floods (absorptive capacity/urban resilience). As reclaimed water is a sustainable source, largely independent of rainfall and climate uncertainty, it can contribute to reducing pressure on strategic freshwater resources to satisfy non-potable uses, such as irrigation of green areas, while allowing for resource recovery, such as phosphorus (adaptive capacity/urban resilience).

The case study herein presented is an urban park located in Lisbon. This public park is open 24 h/day (unrestricted access), it covers more than 29 hectares, and it is bordered on the south by the Tagus River estuary. The park is limited by residential areas to the west and to the north and by an urban wastewater treatment plant (WWTP) to the east. This park is very popular and used by people who live in the area and by visitors. Most of the activities carried out on site are walking, relaxing, picnicking, playing sports (yoga and others), and using the playgrounds, among others. As for sports, yoga was selected because it is a risk-prone activity since it is usually an early-morning activity involving close contact with the lawns irrigated overnight.

The case study’s park presents a mix of landscaped areas, serving multiple purposes—both recreational and ecological. It is made up of wide areas of lawns with trees and shrubs, intersected by a network of footpaths. It includes some areas specifically designed for sports (one fitness area, one skate park, one tennis center, and one padel center) and outdoor recreation (two children’s playgrounds). This park has a picnic area and a terrace. Other amenities include drinking fountains and benches, located on the border of the footpaths. The irrigation system is made up of sprinklers (lawn areas) and drippers (shrub areas).

Since March 2022, this urban park has been irrigated with reclaimed water produced by the nearby WWTP. Because access to the park is unrestricted and spray and sprinkler irrigation is used, this water reuse project was licensed for the use of reclaimed water with a Class-A quality (on a scale from A to D, A being the best quality water). The classification established in the Portuguese Decree-Law 119/2019 [37] is aligned with the classes proposed in the ISO standard 16075-2:2020 and defined in the Regulation (EU) 2020/741. The main barrier in place is the provision of additional (low-level) disinfection in the reclaimed water distribution network. Notice boards informing about the use of reclaimed water for irrigation (designed according to the Portuguese legislation Government Order 266/2019 [38]) are in place.

4. Scenario-Building Results and Contribution to Decision-Making

Risks may change over time, and hence, risk management must be dynamic and iterative. It is useful to review the risk management plan to improve the understanding and treatment of already-identified risks. Thus, scenario-building was applied to the selected case study to exemplify its role on the following aims:

• Support decision-making by the risk manager about risk-control measures already in place—evaluate if reinforcement is needed (scenarios (Sc) 1 and 5).
• Support decision-making by the users of the park—encourage yoga practitioners to adopt self-protection measures to reduce the likelihood of exposure (Sc2 and Sc6).
• Improve risk communication—present risks to the park’s neighbors for improving the quality of risk perception (Sc3 and Sc4).

Transformative capacity (urban resilience) includes, among other aspects, multi-stakeholder collaboration and safety promotion [3]. Risk scenario-building, if developed with this purpose in mind, can facilitate the implementation of changes to reduce the risk level associated with urban water reuse.

Next, the results of the application of the health risk assessment framework to the case study are presented as follows: establishing the context (

Table 5. Establishing the context.

Context Elements	Description
Scope (water reuse project)	Use of reclaimed water: irrigation of an unrestricted-access urban park Irrigation system (end-use points): sprinklers and drippers Reclaimed water quality: E. coli ≤ 10 cfu/100 mL, BOD5 ≤ 10 mg/L O2), TSS ≤ 10 mg/L, turbidity ≤ 5 NTU (Decree-Law 119/2019 Class-A); Legionella spp.: ≤ 100 cfu/L (Act 52/2018 [39])
Objective	To use reclaimed water as fit-for-purpose water for irrigation without affecting the health of the park users and workers, aiming to provide a safe water reuse
Relevant stakeholders	Municipality services: reclaimed water end-user (responsible for the risk management), green area management, reclaimed water distribution system management Water utility: reclaimed water production Authorities (environment and public health): water reuse licensing Citizens: green area users, green area neighbors
Criteria for risk acceptability	Each risk scenario is evaluated qualitatively based on the levels of consequences and likelihood The severity of each risk is determined by a combination of the rating of its potential consequence on a five-level scale (1 to 5, as presented in Table A1) and the likelihood of occurrence on a five-level scale (A to E, as presented in Table A2) The tolerability of each risk is expressed on a three-level scale (low, moderate, and high) applied to different combinations of consequence and likelihood. A low level encompasses negligible risks that can be treated if the associated cost is insignificant. A moderate level refers to significant risks that require specific risk-control measures to lower the risk level. A high level represents outstanding risks that require a re-evaluation of the water reuse project’s suitability and, in case it is confirmed, specific risk-control measures to lower the risk level. Figure 7 presents the risk matrix Water reuse for irrigating the urban park can only happen when all the risk scenarios present a low level of risk

); risk identification (

Table 6. Configuration of the studied water reuse system.

Element	Group	Category	Designation
Components	Hazards	Pathogenic bacteria (indicator)	Legionella spp. Escherichia coli (enteric bacteria)
	Exposure routes	Direct route	Inhalation Ingestion
		Indirect route	Ingestion
	Exposure sites	Vegetated area	Irrigated area—lawns
		Non-vegetated area	Paths
		Equipment	Fitness area
	Activities	Stay	Carry out yoga on the lawns Work out on the fitness area
		Pass by	Walk on the paths around the park
	Population at risk	Users	Immature immune system (young children) Competent immune system (older children, teenagers, and adults in general) Weakened immune system (elderly people and people with poor immune system)
Hazardous events	Occurrence of hazards	Increase in hazards in the reclaimed water	Due to a failure in the reclaimed water (RW) treatment system Due to the contamination of the reclaimed water distribution network
Barriers	Occurrence of hazards	Decrease in hazards in the reclaimed water	By additional, low-level disinfection (1 barrier, 2 LRV). Additional information presented in [40]
	Likelihood of exposure	Reduce the potential for accidental exposure	By requirements for sprinkler irrigation: distance to sensitive areas (1 barrier, 1 LRV)
Preventive measures	Awareness	Reduce the potential for inadvertent misuse	By signage: “Reclaimed water is being used in irrigation” and “Water not suitable for drinking”
	Information	Reduce the potential for accidental exposure	By information on a website about self-protection against contact with reclaimed water

and

Table 7. Building the risk scenarios.

ID	Hazard	Exposure Route	Exposure Site	Activity	Population at Risk	Hazardous Events
Sc1	Legionella	Inhalation, direct route	Fitness area	Work out	Weakened immune system	Failure of the RW treatment Contamination of the RW distribution network Inadequate irrigation timing
Sc2	Legionella	Inhalation, direct route	Lawns	Carry out yoga	Competent immune system	Failure of the RW treatment Contamination of the RW distribution network Inadequate irrigation timing
Sc3	Legionella	Inhalation, direct route	Paths	Walk	Immature immune system	Failure of the RW treatment Contamination of the RW distribution network
Sc4	Legionella	Inhalation, direct route	Paths	Walk	Competent immune system	Failure of the RW treatment Contamination of the RW distribution network
Sc5	E. coli	Ingestion, indirect route	Fitness area	Work out	Weakened immune system	Failure of the RW treatment Contamination of the RW distribution network Inadequate irrigation timing
Sc6	E. coli	Ingestion, indirect route	Lawns	Carry out yoga	Competent immune system	Failure of the RW treatment Contamination of the RW distribution network Inadequate irrigation timing

); baseline risk analysis and risk evaluation (

Table 8. Baseline risk analysis and risk evaluation; l = likelihood; c = consequence.

ID	Hazardous Events	Risk Analysis		Baseline Risk Evaluation (Figure 7)	Comments
		l	c
Sc1	Failure of the RW treatment Contamination of the RW distribution network Inadequate irrigation timing	B	3	Moderate [not accepted, risk treatment needs to be reinforced]	If the use of the fitness area coincides with irrigation, inhalation of Legionella may occur when aerosols are produced in conjunction with water sprays (l = B) In case of a weakened immune system, a contact with Legionella at a low infectious dose could potentially result in minor illness (c = 3)
Sc2	Failure of the RW treatment Contamination of the RW distribution network Inadequate irrigation timing	B	2	Low [accepted, risk treatment will also be reinforced]	If the use of the lawns coincides with irrigation, inhalation of Legionella may occur when aerosols are produced in conjunction with water sprays (l = B) In case of a competent immune system, contact with Legionella at a low infectious dose could potentially result in minor health effects (c = 2)
Sc3	Failure of the RW treatment Contamination of the RW distribution network	B	3	Moderate [not accepted, risk treatment needs to be reinforced]	If the use of the paths coincides with irrigation, inhalation of Legionella may occur when aerosols are produced in conjunction with water sprays (l = B) In case of an immature immune system, contact with Legionella at a low infectious dose could potentially result in minor illness (c = 3)
Sc4	Failure of the RW treatment Contamination of the RW distribution network	B	2	Low [accepted, risk treatment will also be reinforced]	If the use of the paths coincides with irrigation, inhalation of Legionella may occur when aerosols are produced in conjunction with water sprays (l = B) In case of a competent immune system, contact with Legionella at a low infectious dose could potentially result in minor health effects (c = 2)
Sc5	Failure of the RW treatment Contamination of the RW distribution network Inadequate irrigation timing	B	3	Moderate [not accepted, risk treatment needs to be reinforced]	If RW reaches the fitness equipment and the latter is used wet, ingestion of E. coli may occur when a hand is brought to the mouth (l = B) In case of a weakened immune system, contact with E. coli at a low infectious dose could potentially result in minor illness (c = 3)
Sc6	Failure of the RW treatment Contamination of the RW distribution network Inadequate irrigation timing	B	2	Low [accepted, risk treatment will also be reinforced]	If the lawns are still wet with RW when used for practicing yoga, ingestion of E. coli may occur when a hand is brought to the mouth (l = B) In case of a competent immune system, contact with E. coli at a low infectious dose could potentially result in minor health effects (c = 2)

); and final risk evaluation after risk treatment (

Table 9. Final risk evaluation after risk treatment; l = likelihood; c = consequence.

ID	Risk-Control Measures	Risk Analysis		Final Risk Evaluation (Figure 7)	Comments
		l	c
Sc1	Additional, low-level disinfection [1 barrier, in place]. Park signage informing about water reuse [can be improved]	A	3	Low [accepted]	The barrier ensures the RW’s microbiological stability, maintaining Class-A quality throughout the RW distribution network If people are properly informed about potential risks, contact with RW is highly improbable (l = A) In case of a weakened immune system, contact with Legionella at a low infectious dose could potentially result in minor illness (c = 3)
Sc2	Additional, low-level disinfection [1 barrier, in place]. Park signage informing about water reuse [can be improved] Online information about good practices [new measure]	A	2	Low [accepted]	The barrier ensures the RW’s microbiological stability, maintaining Class-A quality throughout the RW distribution network If people are properly informed about potential risks and encouraged to take self-protection measures, contact with RW is highly improbable In case of a competent immune system, contact with Legionella at a low infectious dose could potentially result in minor health effects (c = 2)
Sc3	Additional, low-level disinfection [1 barrier, in place]. Park signage informing about water reuse [can be improved]	A	3	Low [accepted]	The barrier ensures the RW’s microbiological stability, maintaining Class-A quality throughout the RW distribution network If people are properly informed about potential risks, contact with RW is highly improbable (l = A) In case of a weakened immune system, contact with Legionella at a low infectious dose could potentially result in minor illness (c = 3)
Sc4	Additional, low-level disinfection [1 barrier, in place]. Park signage informing about water reuse [can be improved] Face-to-face communication [new measure]	A	2	Low [accepted]	The barrier ensures the RW’s microbiological stability, maintaining Class-A quality throughout the RW distribution network If people are properly informed about potential risks, contact with RW is highly improbable (l = A) In case of a competent immune system, contact with Legionella at a low infectious dose could potentially result in minor health effects (c = 2)
Sc5	Additional, low-level disinfection [1 barrier, in place] Park signage informing about water reuse [can be improved] Face-to-face communication [new measure]	A	3	Low [accepted]	The barrier ensures the RW’s microbiological stability, maintaining Class-A quality throughout the RW distribution network If people are properly informed about potential risks, contact with RW is highly improbable (l = A) In case of a weakened immune system, contact with Legionella at a low infectious dose could potentially result in minor illness (c = 3)
Sc6	Additional, low-level disinfection [1 barrier, in place] Park signage informing about water reuse [can be improved] Online information about good practices [new measure]	A	2	Low [accepted]	The barrier ensures the RW’s microbiological stability, maintaining Class-A quality throughout the RW distribution network If people are properly informed about potential risks and encouraged to take self-protection measures, contact with RW is highly improbable (l = A) In case of a competent immune system, contact with E. coli at a low infectious dose could potentially result in minor health effects (c = 2)

).

This methodology was primarily designed to be user-friendly for risk assessment teams. However, the resulting information is not easily readable by a non-expert. In Commedia dell’Arte, a scenario involves a play sketch to be used as a basis for the actors’ improvisation on stage. Applying this way of thinking to the information presented in Table 5, Table 6, Table 7, Table 8 and Table 9, the scenario Sc1 can be narrated as follows: “The bacteria Legionella as usually occurring in Class-A reclaimed water may affect, via inhalation and on the fitness area, the senior population when working out. In case there is a degradation of the reclaimed water quality (due to a failure in the treatment system or the contamination of the irrigation network) and if the use of this open-air area coincides with the irrigation time, the inhalation of Legionella may occur. Because senior people are expected to have a weakened immune system, contact with Legionella potentially results in minor illness. The combination of these factors leads to a baseline risk level equal to moderate, which is unacceptable. Thus, it is necessary to apply risk control measures to ensure safe water reuse. The already in place barrier “additional, low-level disinfection” provides the necessary microbiological stability for the reclaimed water. The prevention measure “signage” will be reinforced by using additional informative panels, more attention-catching and including suggestions of self-protection measures when using the fitness area.” Similar narratives can be applied to the remaining risk scenarios.

The results of the case study (Table 5, Table 6, Table 7, Table 8 and Table 9) strongly support the use of a coherent system in the building of risk scenarios for effective risk management of urban non-potable water reuse. The herein-presented scenarios, which exemplify different combinations of hazards (Legionella spp. or E. coli), exposure routes (inhalation or ingestion), activities (workout, carrying out yoga, or walking), and population at risk (children, adults, or seniors), enable risk-informed decision-making by the park’s risk manager as well by the park’s users. Green area managers and users are key elements for the sustainable risk management of water reuse in irrigation, and the same applies in other non-potable water uses, e.g., street cleaning, car washing, fire protection, air conditioning, and toilet flushing.

The main recommendations derived from this scenario-building exercise are as follows:

• Support decision-making by the risk manager of the water reuse project, Sc1 and Sc5: (a) Maintain the additional, low-level disinfection for an effective multi-barrier risk management; (b) introduce additional park signage with information about water reuse, especially near the areas where the risk level is considered higher. The information to be presented in these additional panels should include clear messages on how to adopt self-protection measures, e.g., “Don’t use the fitness equipment when the nearby lawn is being irrigated” or “Please, wipe the fitness equipment before use if it is wet”. To improve the acceptance of water reuse, and taking advantage of these panels, information should also be given about related environmental benefits; for example, “Reusing water for irrigation in this park avoids abstracting from nature (value) m³ of water per year. Keep safe, help save natural water!”.
• Support decision-making by the people who use the park, Sc2 and Sc6: (a) Maintain the additional, low-level disinfection for effective multi-barrier risk management; (b) introduce additional park signage with information about water reuse. To promote the awareness and acceptance of the yoga practitioners, attention-catching messages, e.g., “Wait for the grass to dry so that your yoga may shine” can be used in the new panels. Again, it is important to use the latter to inform about water reuse benefits; for instance, “By reusing water to irrigate the park’s lawns, the use of (value) kg of chemical fertilizers is avoided every year”. It is recommended, as an additional risk-preventive measure, to use the municipal website to inform the yoga practitioners about good practices when practicing yoga in urban green areas. This communication may include information about yoga, in addition to recommendations of risk-control measures.
• Improve communication, Sc3 and Sc4: (a) Maintain the additional, low-level disinfection for effective multi-barrier risk management; (b) introduce additional park signage with information about water reuse. Similarly to the previous scenarios, the information about risk management and the benefits of water reuse should be appellative. In this case, it is recommended, as a new risk-preventive measure, to have face-to-face communication, such as an open-house meeting, for presenting the water reuse risks to the park’s neighbors in a clear and simple way that is understandable by non-scientists. This type of initiative provides an opportunity for risk managers to understand risk perceptions of park neighbors, which can be very useful for future water reuse projects.

The example provided is not exhaustive since many activities were not covered, e.g., eating, using the children’s playground, and cycling. However, the effort of completing the process is greatly facilitated by the modular way in which the scenarios are built and by the fact that it is easy to identify the specific circumstances that can enable human exposure to pathogens. Additionally, the identification of the objective(s) for the water reuse project at the beginning of the risk assessment process helps prioritize the circumstances to be evaluated in terms of risk and, thus, limits the number of risk scenarios.

The framework’s coherence with the reference ISO standards in the field of risk management and its flexibility for assessing the risks of non-potable water reuse is expected to catalyze its application to non-potable water reuse projects in different cities or regions.

5. Conclusions

In the case of urban non-potable water reuse, the contribution of the population at risk is often neglected because it is considered that “they” do not understand the risks and, for that reason, “they” are naturally excluded from the risk management process. This rationale presents two main setbacks: (i) it results in higher requirements for the reclaimed water quality since, in case of direct contact, the population at risk is not informed about the applicable self-protection measures; and (ii) it can induce an amplified risk perception by the population because, due to a lack of knowledge about the water reuse system, the exposure to hazards can be perceived as involuntary. Thus, it is important to invest in the development of risk assessment solutions that facilitate the risk communication to the population at risk via the scenario-building process.

This study presents a framework for the health risk assessment and management of urban non-potable water reuse, mainly based on the relevant ISO standards (especially ISO 31000:2018, ISO 20426:2018, ISO 16075-2:2020, and ISO 20469:2018) and on aspects of the USEPA Cumulative Risk Assessment Framework. It includes a novel scenario-building process that helps define how far to take the building of risk scenarios, a recurrent challenge for water reuse practitioners and regulators, and it facilitates the validation of the risk management framework. The framework makes available expert knowledge for risk managers and stakeholders responsible for non-potable water uses, guiding them through the often-complex process of licensing urban non-potable water reuse projects. Hopefully, this standardized solution may facilitate urban management policies for achieving a full integration of reclaimed water into the urban water supply systems, i.e., increasing urban water resilience.

The framework is implemented in the digital tool “Risk Assessment for Urban Water Reuse Module” (TRL 8, in beta version) developed within the B-WaterSmart project. The user-friendly tool for risk assessment and management is integrated in a set of four applications designed to provide water-smart allocation for urban non-potable uses, assessing combinations of water supply/demand alternatives, including reclaimed water, to enable prioritizing strategic and tactical options in urban planning [40].

Future studies could analyze the influence of the cultural and socio-economic context in water reuse risk evaluation, namely, if the description of the consequence and likelihood categories needs to acknowledge local factors. Another aspect of interest for future research is the role of how the risk associated with urban water reuse is assessed and managed as a starting point for the implementation of wider processes of multi-stakeholder collaboration on the path to increasing sustainability in cities.