Sustainability Assessment of Decentralized Hybrid Rainwater–Graywater Systems for Water Management in Arid and Semi-Arid Regions: A Systematic Review

Abstract

Water management in Arid and Semi-Arid Regions (ASAR) relied on large-scale, centralized systems that expanded potable water access. However, high energy requirements, rising operational costs, and limited adaptability to climate variability now put their sustainability under question. According to this study, hybrid rainwater–graywater systems (HRGSs) are emerging as decentralized approaches that can reduce the stress on centralized water systems, increase water supply during dry season, and lower the risk of flooding during rainy seasons. Identifying and evaluating a comprehensive sustainability framework of HRGSs for ASARs remains underexplored. To address this gap, a systematic review of literature indexed in two databases, Scopus and Engineering Village, was performed. Forty studies met the inclusion criteria and were critically appraised to delineate their scope, recurring patterns, and frameworks. Moreover, this study developed a comprehensive sustainability framework specific to the ASAR context, proposing key indicators for HRGS evaluation across environmental, economic, and social aspects with their indicators. Proposing a new sustainability framework provides a basis for guiding future research, technology design, and policy development aimed at implementing HRGS in ASAR contexts.

1. Introduction

The rising demand for water, deterioration of water quality, and insufficient infrastructure are key drivers of urban water scarcity, specifically in Arid and Semi-Arid Regions (ASAR), where natural freshwater resources are limited. These challenges are further exacerbated by climate change and rapid urbanization, all of which intensify pressure on existing water supply systems [1,2]. By 2050, global water consumption is projected to increase by 50%, while nearly half of the world’s population is anticipated to experience severe water scarcity by 2030 [3,4]. Water conflicts in ASAR are increasing due to rising water demand and geopolitical tensions over shared water resources. Increasing competition among agricultural, industrial, and domestic sectors has placed additional pressure on already limited water resources. For instance, Saudi Arabia’s pursuit of wheat self-sufficiency in the 1970s led to depletion of the groundwater [5]. On an international scale, disputes over transboundary water resources have increased, as seen in the case of the Grand Ethiopian Renaissance Dam (GERD), which has created challenges for downstream nations, particularly Egypt and Sudan, due to concerns over water availability and control [6].

ASARs, as drylands, are characterized by water scarcity, whereby mean annual evapotranspiration exceeds precipitation. Over the past century, these regions experienced rises in land temperature and spatial expansion over the previous century, which has exacerbated challenges to water availability [7]. Annual precipitation typically ranges from 25 to 200 mm yr⁻¹ in arid zones and 200–500 mm yr⁻¹ in semi-arid zones [8].

1.1. Assessment of the Water Resources Strategies for ASAR

In countries located in ASAR, such as in the Gulf Cooperation Council (GCC), which contains six countries: Bahrain, Kuwait, Oman, Qatar, Saudi Arabia, and the United Arab Emirates, water has long presented a significant challenge. To ensure limited freshwater supplies, societies have historically relied on conventional groundwater extraction methods, including well-digging. However, the discovery of oil in the mid-20th century caused a significant shift in water management applications. To meet their water needs in recent years, ASAR countries have combined non-conventional sources, such as desalinated seawater and treated wastewater, with traditional water sources, including surface water and groundwater [9]. Groundwater resources in these regions are already stressed, primarily due to excessive extraction rates and inadequate recharge rates [10]. This issue is particularly evident in ASARs such as the Beqaa Plain in Lebanon, where reliance on groundwater for irrigation has led to unsustainable withdrawal levels, emphasizing the urgent need for sustainable management practices [11]. Furthermore, studies indicate that groundwater depletion in the Iraq Shatt Al-Arab basin has intensified due to drought conditions and surface water shortages, emphasizing the requirement for effective resource management [12,13]. Despite the efficiency of the seawater desalination system, this approach consumes a significant amount of energy, contributing to environmental degradation and increased carbon dioxide emissions, and is also a less sustainable long-term water management solution due to the high operating costs [14,15]. Moreover, those countries have also adapted wastewater treatment plants to recover water for non-potable, industrial, and agricultural uses. However, issues with infrastructure costs and public acceptance will prevent this strategy from being widely adopted, especially in GCC. Large-scale implementation of current water centralized systems frequently necessitates high energy inputs and high expenses. Even though these strategies have made a significant contribution to ASAR’s water supply, their long-term sustainability and climate change implications are calling into question their viability [7].

1.2. HRGS Strategy for ASARs

Responding to sustainable development and climate change challenges, there is a drive to transition from traditional centralized water systems toward decentralized water strategies [16]. Among these, hybrid rainwater–graywater systems (HRGSs) have become increasingly interesting for improving urban water security, as shown in Figure 1. HRGSs assist in providing continuous non-potable water by implementing graywater reuse (GWR) and Rainwater Harvesting (RWH) into an urban system, to lower reliance stress on municipal networks [17]. These systems are not only capable of enhancing potable supply but also reducing pressure on sewage infrastructure by intercepting household graywater, thereby minimizing wastewater generation [18,19]. RWH contributes to flood mitigation by managing runoff volumes, while GWR sustains water availability during periods of low rain precipitation. When strategically used in residential and commercial buildings, HRGS have been shown to reduce potable water demand by up to 42% [20,21], aligning directly with Sustainable Development Goal 6, which for access to clean water and sanitation, and Sustainable Development Goal 11, which promotes sustainable cities and communities [22,23].

Their decentralized water systems allow for site-specific customization, operational autonomy, and cost-effective scalability, making them a transformative strategy for climate-resilient urban development and aligning with sustainability goals [22,24]. Mostly non-potable applications, such as toilet flushing, garden irrigation, and car washing, have driven current research on RWH and GWR systems. Still, there is increasing interest in investigating their possibilities for using the laundry systems, which require water quality control. HRGS in residential and commercial buildings offers several advantages, including lower household water bills, less potable water consumption, and less pressure on urban drainage infrastructure [25,26,27].

Furthermore, financial assessments, including cost–benefit and payback analyses, are crucial for determining the economic feasibility of these systems, which in turn influences decision making processes at both individual and government policy levels [22,28]. To develop a practical strategic framework, ASAR, it is necessary to incorporate relevant indicators that monitor and assess various dimensions of water management sustainability. A strategic framework can be structured based on a range of sustainability indicators that include environmental, economic, and social aspects. Indicators are extensively acknowledged as vital tools for lowering uncertainty, assessing system performance, supporting evidence-based decision making, and early warnings of possible hazards. They also support monitoring development towards sustainability objectives and as tools for assessing the success of current policies and initiatives. Indicator-based assessment frameworks are significant tools for bridging scientific research and policy development by translating complex technical data into structured, accessible insights [7,29]. Through inclusive and transparent processes, these frameworks can reveal the core sustainability challenges in water systems into an interpretable score that resonates with both policymakers and the public. It is thus both necessary and beneficial to build water resources management indicators frameworks on a comprehensive and validated set of metrics that reflect sustainability priorities across various scales [30].

These should be context-sensitive and adaptable, especially in regions such as ASAR, where tailored, bespoke frameworks are critical. A diverse range of assessment frameworks has been developed in recent years to support sustainable water resources management, each incorporating indicators and evaluation methodologies. For instance, Alsaeed et al. [7] reviewed sustainability frameworks for water resources management in ASARs and concluded that existing indicators lack contextual information about water decentralization system conditions. Similarly, Ahmed et al. [31] evaluated 68 frameworks for RWH site selection but focused mainly on spatial suitability rather than the technical, social, and economic implications of RWH in the urban context. These studies demonstrate that although sustainability assessment is a well-established concept, its integration within decentralized hybrid systems such as HRGS remains underdeveloped, leading to methodological inconsistencies and a lack of unified indicators suitable for ASAR settings. Given the increasing water scarcity and the growing interest in decentralized water-reuse solutions, the current literature remains fragmented and limited in several respects. For example, previous studies focus on the technical and environmental indicators while neglecting social and institutional dimensions [23,27,28]. Moreover, there is no integrated sustainability assessment framework specifically developed for HRGS, nor a clear methodology for developing multi-criteria indicators that align with the nature of ASAR. This gap also reflects a broader issue in the literature, where sustainability assessment and decentralized water systems are discussed separately rather than being clearly connected. This research direction has clear scientific value and practical significance for improving water security and guiding system design and policy development. In this context, a systematic review is essential to understand how HRGS have been developed and implemented across different regions and over time. Existing studies vary widely in scope, methodology, technical and environmental context, resulting in fragmented, inconsistent, and context-dependent findings regarding HRGS performance and sustainability outcomes. These differences also extend to the selection and application of sustainability indicators, which limit comparability across studies. Therefore, conducting a systematic review provides a rigorous and transparent approach to synthesizing dispersed evidence, identifying common patterns and gaps, and evaluating the indicators used to assess HRGS sustainability. Such an approach is necessary to build a coherent understanding of HRGS development and to inform the creation of a context-appropriate sustainability assessment framework for ASAR.

Therefore, this study (“Sustainability Assessment of Decentralized Hybrid Rainwater–Graywater Systems for Water Management in Arid and Semi-Arid Regions: A Systematic Review”) proposes a novel, integrated framework that contains RWH and GWR systems. The framework presented incorporates the three pillars of sustainability, including social, economic, and environmental, alongside the technical performance tailored specifically to the conditions of ASAR. This study aims to systematically review research on the sustainability assessment framework for HRGS, with a particular focus on evaluating its applicability in the context of ASAR. To achieve this, the study formulates the following key research questions (RQ):

RQ1: To what extent are HRGS being applied in ASAR, and what are the trends in their implementation?

RQ2: In the context of ASAR countries implementing HRGS, how many studies provide comprehensive coverage of the environmental, economic, and social dimensions of sustainability?

RQ3: Which sustainability indicators are most relevant and context-appropriate for evaluating HRGS in ASAR, particularly in relation to climate resilience and long-term water security?

2. Methodology

To address the research questions outlined in Section 1.2, a systematic literature search was conducted using two databases: Scopus and Engineering Village, to identify relevant studies. Both databases were selected due to their extensive multidisciplinary and engineering-focused coverage, which has been shown to capture the vast majority of relevant studies. Engineering Village was included as a comprehensive platform that integrates three major engineering databases, including Compendex, GEOBASE, and Inspec, offering broad access. Scopus complements this by providing broader journal coverage and more robust citation tracking than Web of Science and PubMed [32,33]. Moreover, systematic review employs NVivo 12, a qualitative text mining software, to analyze, organize, and visualize data [34]. In the initial stage, a set of pertinent keywords was carefully selected and applied to search these databases using the title, abstract, and keywords fields. Given the large number of related studies, a filtering process was necessary to refine the results. The scope of this search was limited to peer-reviewed journal articles to ensure the credibility and reliability of the selected studies. Additionally, two key conditions were applied to all included documents:

• The publications needed to fall within the timeframe of 2000 to 2025. This period was chosen as several frameworks for assessing the HRGS emerged after 2000.
• Only documents written in English were considered to maintain consistency and accessibility in the review process.

The selected criteria align with methodologies used in previous studies that applied the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) framework [35] (Supplementary Materials), such as those that conducted similar systematic reviews in water management, Ahmed et al. [31], and Alsaeed et al. [7]. To further refine the research scope, a structured four-step clustering algorithm was employed, consisting of Scope, Target Group, Subject Domain, and Methods. This systematic approach ensured a more focused and relevant selection of studies within the broader research area. Additionally, a filtering process was applied to exclude studies that were not directly relevant. This was accomplished by using the OR operator within each category’s keywords and the AND operator across different groups, ensuring that only studies within the intersection of all four clusters were included in the review.

2.1. Keyword Selection

The search queries were structured to target specific fields, including “Article Title, Abstract, and Keywords” in Scopus and the equivalent “Subject, Title, and Abstract” fields in Engineering Village. The scope cluster included key terms related to the HRGS, capturing various terminologies and their derivatives commonly used in the literature.

These terms included “Hybrid rainwater-greywater system”, “Rainwater harvesting”, “Rainwater”, “Greywater”, and “Greywater recycling”. The target group cluster focused on keywords was selected to ensure comprehensive coverage of groups involved in or affected by the implementation and decision making processes of HRGS, including the following: “Sustainable”, “Stakeholders”, “Buildings”, “Household”, “Arid”, “Semi-arid”, and “Urban”. The subject domain cluster emphasized methodologies used in data collection, evaluation, and sustainability assessment. Key terms in this cluster were as follows: “Indicators”, “Framework”, “Criteria”, “Systems”, “Component”, and “Index”. Lastly, the methods cluster included terms related to data collection techniques aligned with participatory approaches. Terms such as “Survey”, “Interview”, “Questionnaire”, “Scenarios”, “Model”, and “Participatory”. A word cloud was generated using NVivo 12, illustrating the most frequently occurring keywords, as presented in Figure 2. This visualization serves to identify prevailing themes and recurrent terms within the retrieved literature, offering insight into the focal areas of existing research.

2.2. Database Search

A comprehensive search was conducted on 21 March 2025, using the Scopus and Engineering Village databases, yielding 786 and 843 results, respectively, for a combined total of 1629 articles. These results were imported into EndNote for data management and de-duplication, resulting in a refined dataset of 1340 articles. The establishment of clear inclusion and exclusion criteria is fundamental to ensuring the rigor, transparency, and relevance of a systematic review. In accordance with these predefined criteria, studies that did not align with the core scope of this review were excluded. This included the removal of numerous articles from unrelated fields such as business, medicine, and architecture. During the initial screening phase, the titles and keywords of all retrieved articles were assessed to ensure their relevance to the scope of this review. A total of 1019 studies were excluded at this stage, primarily because their titles and keywords did not align with the thematic boundaries of the analysis or they failed to demonstrate explicit linkage to HRGS frameworks. This initial exclusion was reduced to 321 studies.

Following this, the abstract screening was conducted to assess methodological appropriateness, research context, and the comprehensiveness of HRGS coverage. As a result of this stage of more refined filtering, the corpus was further reduced to 62 articles.

In the final screening stage, full-text articles were reviewed to assess their alignment with the research focus. As a result, 22 articles were excluded, yielding a final selection of 40 studies for inclusion in the analysis. The final set of 40 studies was characterized by publication in high-ranking journals, ensuring methodological rigor and relevance. The complete selection process is detailed in Figure 3.

3. Overview of Studies

The results from the reviewed articles are presented in

Table 1. Overview of results from the reviewed articles.

No	Title	Author(s), Year	Case Study	Journal
1	“The potential of rainwater harvesting and greywater recycling as an alternative domestic water resource in Bahnstadt-Heidelberg, Germany”	(Kasipiyawong et al., 2024) [36]	Germany	Journal of Water, Sanitation and Hygiene for Development
2	“Economic Feasibility of Rainwater Harvesting and Greywater Reuse in a Multifamily Building”	(Ghisi and Freitas, 2024) [20]	Brazil	Journal Water
3	“Application of the analytic hierarchy process for the selection of recycling rainwater/household grey water to improve SIDS sustainability targets”	(Chadee et al., 2024) [37]	Trinidad and Tobago	Journal Modeling Earth Systems and Environment
4	“Rainwater and Greywater as Alternative Water Resources: Public Perception and Acceptability. Case Study in Twelve Countries in the World”	(Stec, 2023) [38]	Poland	Journal Water Resources Management
5	“Integrated systems for rainwater harvesting and greywater reuse: a systematic review of urban water management strategies”	(Rodrigues et al., 2023) [17]	Brazil	Journal Water Supply
6	“Hybrid Decentralized Systems of Non-potable Water Supply: Performance and Effectiveness Analysis”	(Ferreira et al., 2023) [23]	Portugal	Journal Water Resources Management
7	“Economic analysis of hybrid rainwater–greywater systems between demand and supply sides based on cooperative theory”	(Chen et al., 2023) [39]	Japan	Journal of Cleaner Production
8	“Environmental performance of a hybrid rainwater harvesting and greywater reuse system: A case study on a high water consumption household in Colombia”	(Gómez-Monsalve et al., 2022) [40]	Colombia	Journal of Cleaner Production
9	“Water quality and financial analysis of a system combining rainwater and greywater in a house”	(Rosa and Ghisi, 2021) [27]	Brazil	Journal Water
10	“Spatially optimized distribution of household rainwater harvesting and greywater recycling systems”	(Stang et al., 2021) [41]	USA	Journal of Cleaner Production
11	“Minimum cost solution to residential energy-water nexus through rainwater harvesting and greywater recycling”	(Zhang et al., 2021) [42]	South Africa	Journal of Cleaner Production
12	“Feasibility analysis of decentralized hybrid rainwater-graywater systems in a public building in Japan”	(Chen et al., 2021) [18]	Japan	Journal Sustainable Cities and Society
13	“A Spatial Life Cycle Cost Comparison of Residential Greywater and Rainwater Harvesting Systems”	(Maskwa et al., 2021) [43]	USA	Journal Environmental Engineering Science
14	“A modeling evaluation of a system combining rainwater and greywater for potable water savings”	(Coutinho Rosa and Ghisi, 2020) [44]	Brazil	Urban Water Journal
15	“A hydroponic green roof system for rainwater collection and greywater treatment”	(Xu et al., 2020) [45]	China	Journal of Cleaner Production
16	“Modeling for sustainability: Life cycle assessment application to evaluate environmental performance of water recycling solutions at the dwelling level”	(Zanni et al., 2019) [22]	Italy	Journal Sustainable Production and Consumption
17	“Life cycle assessment and life cycle cost analysis of decentralised rainwater harvesting, greywater recycling and hybrid rainwater-greywater systems”	(Leong et al., 2019) [46]	Malaysia	Journal of Cleaner Production
18	“Environmental performance of hybrid rainwater-greywater systems in residential buildings”	(Marinoski and Ghisi, 2019) [47]	Brazil	Journal Resources, Conservation and Recycling
19	“Sustainable energy-water management for residential houses with optimal integrated grey and rainwater Recycling”	(Wanjiru and Xia, 2018) [48]	South Africa	Journal of Cleaner Production
20	“Seasonal patterns and socio-economic predictors of household rainwater and greywater use”	(Mason et al., 2018) [26]	Philippines	Urban Water Journal
21	“Quantification of mains water savings from decentralised rainwater, greywater, and hybrid rainwater-greywater systems in tropical climatic conditions”	(Leong et al., 2018) [49]	Malaysia	Journal of Cleaner Production
22	“Financial feasibility of end-user designed rainwater harvesting and greywater reuse systems for high water use Households”	(Oviedo-Ocaña et al., 2018) [16]	Colombia	Journal Environmental Science and Pollution Research
23	“Environmental benefit analysis of strategies s for potable water savings in residential buildings”	(Marinoski et al., 2018) [50]	Brazil	Journal of environmental management
24	“Assessment of greywater quality and performance of a pilot-scale decentralised hybrid rainwater-greywater system”	(Leong et al., 2018) [51]	Malaysia	Journal of Cleaner Production
25	“End-User Cost-Benefit Prioritization for Selecting Rainwater Harvesting and Greywater Reuse in Social Housing”	(Domínguez et al., 2017) [52]	Colombia	Journal Water
26	“Analysing the financial efficiency of use of water and energy saving systems in single-family homes”	(Stec et al., 2017) [28]	Poland	Journal of Cleaner Production
27	“Prospects of hybrid rainwater-greywater decentralised system for water recycling and reuse: A review”	(Leong et al., 2017) [53]	Malaysia	Journal of Cleaner Production
28	“Water-energy nexus in houses in Brazil: Comparing rainwater and gray water use with a centralized system”	(Vieira and Ghisi, 2016) [54]	Brazil	Journal Water Supply
29	“Potential of rainwater harvesting and greywater reuse for water consumption reduction and wastewater minimization”	(López Zavala et al., 2016) [19]	Mexico	Journal Water
30	“Decentralized and user-led approaches to rainwater harvesting and greywater recycling: The case of Sant Cugat del Vallès, Barcelona, Spain”	(Vallès-Casas et al., 2016) [55]	Spain	Journal Built Environment
31	“Energy-Water Nexus: Potential Energy Savings and Implications for Sustainable Integrated Water Management in Urban Areas from Rainwater Harvesting and Gray-Water Reuse”	(Malinowski et al., 2015) [56]	USA	Journal of Water Resources Planning and Management
32	“Analysis of profitability of rainwater harvesting, gray water recycling and drain water heat recovery systems”	(Stec and Kordana, 2015) [24]	Poland	Journal Resources, conservation and recycling
33	“Rainwater and greywater harvesting for urban food security in La Soukra, Tunisia”	(Redwood et al., 2014) [57]	Tunisia	International Journal of Water Resources Development
34	“Comparing indicators to rank strategies to save potable water in buildings”	(Ghisi et al., 2014) [58]	Brazil	Journal Resources, conservation and recycling
35	“Rainwater harvesting and greywater treatment systems for domestic application in Ireland”	(Li et al., 2010) [59]	Ireland	Journal Desalination
36	“Alternative Water Resources for Rural Residential Development in Western Australia”	(Zhang et al., 2010) [60]	Australia	Journal Water resources management
37	“Socio-economic and psychological predictors of domestic greywater and rainwater collection: Evidence from Australia”	(Ryan et al., 2009) [61]	Australia	Journal of Hydrology
38	“Reuse of greywater and rainwater using fiber filter media and metal membrane”	(Kim et al., 2007) [62]	South Korea	Journal Desalination
39	“Potential for potable water savings by using rainwater and greywater in a multi-storey residential building in southern Brazil”	(Ghisi and Freitas, 2007) [63]	Brazil	Journal Building and Environment
40	“Potential for potable water savings by combining the use of rainwater and greywater in houses in southern Brazil”	(Ghisi and Mengotti de Oliveira, 2007) [64]	Brazil	Journal Building and Environment

, which includes the author(s), year of publication, country, and journal name. The papers have been limited to those published in the last 20 years.

3.1. Analysis of Geographical Coverage

Figure 4 shows the distribution of the forty published articles included in the geographical map. Brazil emerged as the leading contributor to the field of HRGS, with ten publications, representing the largest share of studies identified. Malaysia followed with four publications, while Colombia, USA, and Poland accounted for three. Moderate research studies were observed in Australia, Japan, and South Africa, each with two publications. In contrast, a range of countries, including Germany, Mexico, Ireland, Tunisia, Spain, China, South Korea, Italy, the Philippines, Portugal, and Trinidad and Tobago, each conducted a single publication. This distribution highlights Brazil’s prominent role in advancing HRGS research and underscores the growing global interest in this domain, spanning both development and developing countries.

3.2. Analysis of Year of Publication

An analysis of the publication years shows that 2018 is the highest number of studies, with six publications, as illustrated in Figure 5. This was followed by 2021 with five publications and 2023 with four. The years 2024, 2019, 2017, 2016, and 2007 each conducted three publications, indicating equal interest during these periods. Meanwhile, 2020, 2015, 2014, and 2010 each contributed two publications, and the lowest counts were observed in 2009 and 2022, with one publication each. Overall, the data suggests a growing research trend in recent years, with a notable peak in 2018.

3.3. Thematic Analysis of Systematic Literature Studies

NVivo software was employed to code and analyze the selected forty articles [34]. While the inclusion and exclusion criteria guided the initial selection of studies, the coding strategy formed the basis of the subsequent analytical process, together establishing a rigorous and structured framework for the systematic review. The thematic structure was intentionally developed to incorporate technical aspects alongside sustainability-related themes, ensuring comprehensive coverage of the key dimensions of HRGS. Codes were subsequently organized into themes, which informed the development of a conceptual framework for decentralized water systems. In total, 40 articles met the review criteria. From these, 4 themes and 13 sub-themes were derived, providing an integrated assessment of current knowledge, including new insights and contributions grounded in established theoretical foundations. These themes were selected because they reflected the multidimensional nature of decentralized water systems solutions, which require simultaneous consideration of system design, environmental performance, economic feasibility, and social acceptance. Technical sub-themes are essential, as they determine the operational reliability, treatment effectiveness, and overall functionality of RWH and GWR systems. An environmental theme was included to assess the contribution of such systems to water conservation, climate mitigation, and regulatory compliance, which are central to sustainability evaluations. The economic theme is significant, given that financial viability and maintenance costs strongly influence adoption rates and long-term functionality. Social theme is incorporated because user behavior, health perceptions, and public acceptance ultimately shape the real-world applicability and successful implementation of HRGS. Together, these themes provide a balanced conceptual framework that aligns with sustainability assessment.

Figure 6 presents the NVivo hierarchy chart and sunburst diagram, illustrating the relative prominence of the four main themes and their sub-themes across the coded literature. Distinct colors are used to differentiate the four thematic categories, with each color corresponding to a specific theme and its associated sub-themes. The yellow color represents the technical theme, encompassing system components, building characteristics, and number of occupants, water end-uses, and water quality or treatment. The orange color reflects the environmental theme, which includes potential water savings, climate mitigation, green infrastructure integration, and alignment with environmental regulations. The gray color corresponds to the social theme, capturing user behavior and water use patterns, health and hygiene perceptions, and public acceptance. Finally, the blue color represents the economic theme, covering financial viability and operation, and maintenance costs. While the size of the boxes reflects the volume of studies addressing each sub-theme, larger boxes indicate sub-themes covered more extensively in the literature, whereas smaller boxes represent those examined in fewer studies. Moreover, prominence distribution across the 40 articles shows technical themes as the most dominant (49%), followed by environmental (24%), social (15%), and economic (12%) themes.

Table 2. Classification of 40 studies based on the sustainability indicators addressed under the environmental, economic, and social dimensions.

Author(s), Year	Case Study	Environmental Theme				Economic Theme		Social Theme
		Potential Water Savings	Climate Mitigation	Green Infrastructure Integration	Environmental Regulations Alignment	Financial Viability	Operation and Maintenance Cost	User Behavior and Water Use Patterns	Health and Hygiene Perception	Public Acceptance
(Kasipiyawong et al., 2024) [36]	Germany	✓	✓	✓	✓		✓	✓	✓	✓
(Ghisi and Freitas, 2024) [20]	Brazil	✓	✓	✓		✓		✓
(Chadee et al., 2024) [37]	Trinidad and Tobago	✓	✓	✓	✓			✓	✓	✓
(Stec, 2023) [38]	Poland	✓						✓	✓	✓
(Rodrigues et al., 2023) [17]	Brazil	✓	✓	✓		✓		✓	✓
(Ferreira et al., 2023) [23]	Portugal	✓				✓	✓
(Chen et al., 2023) [39]	Japan	✓	✓	✓	✓	✓	✓	✓		✓
(Gómez-Monsalve et al., 2022) [40]	Colombia	✓	✓			✓		✓
(Rosa and Ghisi, 2021) [27]	Brazil	✓				✓		✓
(Stang et al., 2021) [41]	USA	✓				✓		✓
(Zhang et al., 2021) [42]	South Africa	✓	✓			✓
(Chen et al., 2021) [18]	Japan	✓	✓			✓	✓	✓
(Maskwa et al., 2021) [43]	USA	✓	✓			✓
(Coutinho Rosa and Ghisi, 2020) [44]	Brazil	✓				✓		✓
(Xu et al., 2020) [45]	China	✓	✓	✓			✓
(Zanni et al., 2019) [22]	Italy	✓	✓			✓
(Leong et al., 2019) [46]	Malaysia	✓	✓	✓		✓	✓	✓
(Marinoski and Ghisi, 2019) [47]	Brazil	✓	✓				✓	✓
(Wanjiru and Xia, 2018) [48]	South Africa	✓	✓			✓	✓
(Mason et al., 2018) [26]	The Philippines	✓				✓		✓	✓	✓
(Leong et al., 2018) [49]	Malaysia	✓
(Oviedo-Ocaña et al., 2018) [16]	Colombia	✓	✓		✓	✓	✓	✓	✓	✓
(Marinoski et al., 2018) [50]	Brazil	✓	✓			✓		✓
(Leong et al., 2018) [51]	Malaysia	✓						✓
(Domínguez et al., 2017) [52]	Colombia	✓				✓	✓	✓	✓	✓
(Stec et al., 2017) [28]	Poland	✓	✓			✓
(Leong et al., 2017) [53]	Malaysia	✓			✓			✓
(Vieira and Ghisi, 2016) [54]	Brazil	✓	✓					✓	✓
(López Zavala et al., 2016) [19]	Mexico	✓				✓	✓
(Vallès-Casas et al., 2016) [55]	Spain	✓			✓			✓	✓	✓
(Malinowski et al., 2015) [56]	USA	✓	✓	✓		✓	✓	✓
(Stec and Kordana, 2015) [24]	Poland	✓	✓			✓
(Redwood et al., 2014) [57]	Tunisia	✓						✓		✓
(Ghisi et al., 2014) [58]	Brazil	✓	✓			✓		✓
(Li et al., 2010) [59]	Ireland	✓				✓		✓	✓	✓
(Zhang et al., 2010) [60]	Australia	✓	✓	✓				✓
(Ryan et al., 2009) [61]	Australia	✓						✓	✓	✓
(Kim et al., 2007) [62]	South Korea	✓
(Ghisi and Freitas, 2007) [63]	Brazil	✓				✓		✓
(Ghisi and Mengotti de Oliveira, 2007) [64]	Brazil	✓				✓	✓	✓

further details the frequency of individual indicators within the reviewed publications. All studies included the four technical indicators, reflecting their essential role in evaluating HRGS performance. The proportional emphasis on each sustainability dimension was quantified based on the total number of verified indicators (✓) reported across the studies. Environmental indicators, particularly those relating to water-saving potential, were also widely represented, highlighting the significance of RWH and GWR. In contrast, economic and social indicators appeared less frequently.

4. Technical Characteristics of HRGS

When incorporated into buildings, RWH and GWR systems are categorized as decentralized water management solutions that provide on-site collection, treatment, and reuse, thereby eliminating the need for long-distance transport, as is the case with centralized systems [23]. These systems represent the most extensively researched decentralized techniques globally, with the ability to function either independently or in combination with centralized systems networks. However, their performance depends on climate conditions, water supply reliability, consumption patterns, and socioeconomic factors [18,43,59]. RWH and GWR systems are increasingly recognized as a cost-effective and efficient approach to non-potable water supply, particularly in urban areas.

Table 3. Global overview of rainwater harvesting and graywater reuse.

Author(s), Year	Country	System Provided Water End-Uses	Type of the Building	Residents	Catchment Roof Area Size m2	Rainfall Yield Coefficients	Payback (Years)
Kasipiyawong et al., 2024 [36]	Germany	Toilet flushing, garden irrigation	Multi-residential buildings	5700	49,734	0.9	-
Ghisi and Freitas, 2024 [20]	Brazil	Washing machines (rainwater), toilet flushing (graywater)	Multi-family residential building (120 flats, 2 blocks)	276	-	-	7.4 to 11
Ferreira et al., 2023 [23]	Portugal	Toilet flushing, garden irrigation, washing machine, washing taps	Single-family house	4	158	0.9	-
Chen et al., 2023 [39]	Japan	Toilets, irrigation, emergency water tanks, cooling towers	Educational building	-	53,214	0.8	-
Gómez-Monsalve et al. 2022 [40]	Colombia	Internal tap, external tap, washing machine, sink, toilet flushing	Single-family house	4	101	0.85	-
Rosa and Ghisi, 2021 [27]	Brazil	Washing machine (rainwater), toilet flushing (graywater)	Single-family house	4	120	0.9	-
Stang et al., 2021 [41]	USA	Toilet flushing, irrigation	Residential buildings (single- and multi-family)	-	269	0.9	-
Zhang et al., 2021 [42]	South Africa	Toilet flushing, irrigation (non-potable use)	Single-family house	-	100	0.9	4.39
Chen et al., 2021 [18]	Japan	Washing machine (RW), toilet flushing (GW)	Single-family house	4	120	0.9	5.3
Maskwa et al., 2021 [43]	USA	Toilet flushing, irrigation	Single-family (SF) and multi-family (MF)	3 (SF), 15 (MF)	(SF = 116), (MF = 427)	0.8	-
Coutinho Rosa and Ghisi, 2020 [44]	Brazil	Washing clothes (rainwater), toilet flushing (graywater)	Single-family house	4	121	0.9	5.3
Marinoski and Ghisi, 2019 [47]	Brazil	Toilet flushing, garden irrigation, washing machine	Single-family house	4	78	0.81	-
Wanjiru and Xia, 2018 [48]	South Africa	Toilet flushing, garden irrigation	Single-family house	3–5	50	-	-
Leong et al., 2018 [49]	Malaysia	Toilet flushing, irrigation, laundry	Domestic (D), commercial (C)	-	(D = 487), (C = 2089)	0.8	-
Oviedo-Ocaña et al., 2018 [16]	Colombia	Toilet flushing, patio, garden, laundry, washing machine	Single-family house	4	101	0.9	23
Marinoski et al., 2018 [50]	Brazil	Toilet flushing, laundry tap, outdoor tap (non-potable)	Single-family house	3	80	0.8	-
Domínguez et al., 2017 [52]	Colombia	Toilet flushing, house cleaning, watering plants	Single-family house	5	30.5	0.9	30
Stec et al., 2017 [28]	Poland	Toilet flushing, garden irrigation (non-potable uses)	Single-family house	3 to 5	150	0.9	-
Vieira and Ghisi, 2016 [54]	Brazil	Toilet flushing (GW + RW), laundry (RW only)	Single-family house	4 to 5	87	0.78	-
Stec and Kordana, 2015 [24]	Poland	Toilet flushing, washing, green area irrigation	Multi-family residential buildings	72	500 m2	0.8	-
Ghisi and Freitas, 2007 [63]	Brazil	Toilet flushing, clothes washing, laundry trough, and cleaning (non-potable uses)	Multi-family residential buildings	118	324 m2 per block	0.85	>37
Ghisi and Mengotti de Oliveira, 2007 [64]	Brazil	Toilet flushing and washing machine (non-potable uses)	Single-family house	2 to 3	House A: 203.8—House B: 212.4	0.8	>17

summarizes key characteristics of HRGS implemented in various countries and building types, highlighting water end-uses, building occupancy, catchment roof area, rainfall yield coefficients, and estimated payback periods. Meanwhile, Figure 7 reflects the technical characteristics of HRGS. The studies cover a diverse range of climates and urban contexts, and HRGS configurations vary based on local water needs, building typology, and technical design. It is important to consider that the construction, operation, and treatment requirements of HRGS can differ substantially depending on their intended functions and setting [42,58].

By leveraging rainwater during wet seasons and graywater during dry seasons, these HRGSs ensure a more reliable and continuous non-potable water supply [17]. Potable water savings are directly affected by the volume of non-potable water utilized within a building, which in turn depends on the available storage capacity of RWH and GWR. Consequently, reductions in water bills are related to the volume of potable water displaced, with greater savings achievable in systems designed with larger storage volumes [23]. It is important to consider that HRGS can vary in terms of local climate, construction, operation, and treatment requirements, depending on system objectives and building context [42,58]. In a spatial analysis, Stang et al. [41] evaluated RWH and GWR systems across Boston, United States of America (USA), and found that life cycle cost, energy use, and demand fulfillment varied based on the performance of these systems, which are dependent on geographic and urban context.

Estimating non-potable water demand is essential for the effective sizing and operation of HRGS decentralized systems. The total daily non-potable demand per household $D_{N,d}$ $\left[\mathrm{L}·{\mathrm{d}\mathrm{a}\mathrm{y}}^{-1}\right]$ is estimated as shown in Equation (1).

$$D_{N,d}=n·D_{p,d}$$

(1)

where $D_{p,d}$ is per capita daily non-potable demand $\left[\mathrm{L}·{\mathrm{p}\mathrm{e}\mathrm{r}\mathrm{s}\mathrm{o}\mathrm{n}}^{-1}·{\mathrm{d}\mathrm{a}\mathrm{y}}^{-1}\right]. n$ is the number of occupants in the building.

This demand is influenced by multiple factors, including climatic conditions, projected usage patterns, occupancy levels, building characteristics, and socioeconomic status [36,65]. Toilet flushing is one of the largest non-potable end-uses (typically >30% of household demand) and can be effectively supplied with either RWH or GWR. Washing machines also account for a substantial portion (typically 20%) of residential water demand; however, the water quality standards for this reuse are comparatively higher than those required for toilet flushing. Ghisi and Freitas [20], when assessing the feasibility of HRGS in multi-occupancy buildings in Brazil, found that RWH potential was often constrained by limited roof area, while GWR was limited by space availability for treatment systems. These system constraints underscore the need to contextualize feasibility assessments within broader regional consumption patterns, which exhibit substantial variability across different climatic and socioeconomic settings. Figure 8 illustrates the distribution of average daily indoor water use by household end-use category, using global residential benchmarks [66]. In the United Kingdom, average residential water demand is approximately 142 L/person/day, attributable to temperate climate conditions, well-established distribution infrastructure, and longstanding conservation policies [67]. Conversely, GCC countries exhibit per capita residential consumption exceeding 550 L/person/day, driven by extreme arid conditions, energy-intensive lifestyles, and heavily subsidized water supply systems [68]. This disparity represents a fourfold difference in consumption levels between temperate and arid regions. The exceptionally high consumption rates in GCC countries exacerbate water scarcity challenges and underscore the critical importance of implementing alternative water supply strategies. Specifically, GWR and RWH systems offer substantial potential for demand reduction and supply diversification in water-stressed arid environments, making their adoption essential for sustainable water resource management in the GCC region.

4.1. System Components: RWH, GWR, Storage Tank

4.1.1. RWH

RWH is a widely recognized sustainable water management strategy that involves the on-site collection and storage of precipitation from rooftops, providing a decentralized, independent source of water for various non-potable purposes.

Economically, RWH is considered a cost-effective alternative to centralized supply systems, as it reduces the need for extensive infrastructure investments and lowers operational costs [69]. Additionally, RWH contributes to urban flood mitigation, thereby reducing infrastructure damage [70,71]. Leong et al. [46] further identified RWH as the most environmentally optimal solution among decentralized water strategies. The efficiency of the RWH system is highly context-dependent on regional precipitation levels, roof surface area and type, per capita water consumption, occupancy rates, economic capacity, and the life cycle performance of the installed system [20,72]. However, this RWH strategy alone may not be equally effective in all countries. For instance, a study conducted in Germany by Kasipiyawong et al. [36] indicates that the amount of rainwater collected is insufficient to meet non-potable water demands. Another study in Brazil by Rosa and Ghisi [27] emphasizes the need for additional treatment processes, such as first-flush diversion and disinfection, to remove coarse solids and other contaminants before use. Kasipiyawong et al. [36] found that extensive green roofs exhibited significantly lower RWH potential compared to smooth-surfaced roofs, primarily due to their limited drainage efficiency. Although rainwater is regarded as a relatively clean source, its quality can degrade during collection, storage, and subsequent household use. Moreover, the study highlighted that microbial contamination is often highest in the initial runoff, with pollutant concentrations declining as rainfall continues.

During the rainy season, frequent precipitation events help naturally rinse rooftop catchments, reducing microbial loads over time. However, to ensure water safety, it is essential to exclude the initial runoff from entering storage tanks. This requires the installation of automated first-flush diversion systems, which effectively redirect the first, potentially contaminated flow away from the tank, allowing only cleaner subsequent rainfall to be harvested for non-potable applications. In recent years, governments have increasingly introduced regulatory and policy measures to incentivize the adoption of RWH systems. These measures serve a dual purpose; they provide supportive frameworks that enable widespread deployment but may also introduce design and operational constraints that impact how systems are specified and applied. In the UK, compliance with these regulations requires that RWH systems be designed and installed in accordance with relevant British standards, notably BS 8515:2009+A1:2013, which details requirements for the design, installation, and maintenance of RWH systems [21,73]. In arid regions such as Saudi Arabia and Jordan, specific licensing requirements govern the installation and use of RWH systems. For instance, in Saudi Arabia, rainwater collection is strictly limited to rooftop catchments. The collection of runoff from the ground, parking areas, or pedestrian surfaces is prohibited, except in specific cases where the water is designated for non-potable purposes, such as irrigation, and only after appropriate treatment to remove contaminants [74,75]. The quantification of rainwater available for harvesting utilizes the standard hydrological approach based on the effective runoff coefficient method. The volume of harvestable rainwater is determined by multiplying the effective rainfall data by the catchment area and the surface-specific runoff coefficient, as expressed in Equation (2).

ERt = Rt × A × CR

(2)

where ERt represents the effective runoff volume available for collection during time period t, Rt denotes the effective rainfall during the specified time interval (typically measured in mm/day), A indicates the effective catchment area of the collection surface (m²), and CR represents the catchment runoff coefficient, which accounts for surface material properties and collection efficiency.

4.1.2. GWR

GWR involves the reuse of wastewater generated from domestic activities, including water from sinks, showers, washing machines, and dishwashers [17,76]. In households with a water supply system, graywater production typically ranges between 90 and 120 L per person per day [77], accounting for approximately 50–75% of total domestic water consumption. Its reuse has the potential to reduce household water demand by up to 50%, thereby alleviating stress on freshwater resources, lowering water bill costs, and minimizing the stress on municipal sewage systems [78]. Additionally, GWR offers a dependable source for non-potable applications, such as garden irrigation and car washing. The pollutant concentration in graywater is closely linked to its source and is commonly categorized into three groups: bathroom, laundry, and kitchen wastewater. This variation highlights the necessity for source-specific treatment strategies to ensure the safe and sustainable reuse of water [28,60]. Although graywater is more readily available than rainwater, it requires appropriate treatment to meet health and safety standards. Treatment is generally divided into three stages: physical, biological, and chemical. Physical treatment removes suspended solids and surfactants, while biological processes, such as Rotary Biological Contactors (RBCs), Biological Aerated Filters (BAFs), and Membrane Bio-Reactors (MBRs), target organic matter and pathogens. Chemical disinfection, using agents like hypochlorite, chlorine, and ozone, is also commonly applied to remove microbiological contamination [53,62]. Despite these advancements, the GWR system remains relatively energy- and capital-intensive due to the operational demands of treatment technologies [42]. Nonetheless, the GWR system is subject to several limitations. For example, Chen et al. [18] caution that when graywater is utilized for toilet flushing or outdoor irrigation within a 24 h timeframe, basic filtration is generally sufficient to ensure water quality. However, for applications involving longer storage durations or higher quality requirements, more advanced and intensive treatment processes are necessary to mitigate microbial and chemical risks and maintain safety standards. Moreover, public acceptance remains a significant barrier. A study by Kasipiyawong et al. [36] in Germany found that only 20.78% of surveyed residents were willing to adopt GWR systems, with concerns over health risks and water quality cited as the primary deterrents. Levels of Total Organic Carbon (ToC), Chemical Oxygen Demand (COD), Biological Demand (BOD), and Dissolved Oxygen (DO) are important indicators therein.

International graywater reuse regulatory frameworks in arid countries, specifically Saudi Arabia and Jordan, demonstrate a clear pattern of convergence around key principles aimed at ensuring system safety, sustainability, and regulatory compliance, despite differences in institutional structures and enforcement mechanisms. Both countries consistently prohibit the connection of kitchen and dishwasher wastewater to graywater systems, recognizing that the high concentrations of oils, fats, and organic solids in these sources pose significant challenges to decentralized treatment. Moreover, they increase the likelihood of odor, clogging, and pathogen proliferation. This shared regulation reflects an internationally aligned concern for treatment efficiency and public health protection in system design. Additionally, both Saudi Arabia and Jordan mandate the separation of graywater from blackwater at the source through dedicated dual-pipe networks.

The installation of parallel piping systems enables effective collection, treatment, and targeted reuse of graywater and blackwater, institutionalizing source separation and facilitating sustainable water management. These regulatory pillars highlight a consistent and internationally convergent approach toward safe, efficient, and sustainable GWR in these arid environments [79,80].

4.1.3. Storage Tank

Storage tank sizing plays a critical role in the performance and economic viability of HRGS. Maskwa et al. [43] demonstrated that when storage tanks are sized for the shortest payback period, the proportion of water demand met is typically about 10% lower than the maximum achievable, illustrating a fundamental trade-off between economic efficiency and systems performance. This underscores the requirement of balancing financial returns with water-saving objectives in system design. Marinoski and Ghisi [47] further emphasize that accurate tank sizing is a significant implementation factor, as oversizing may lead to inflated capital costs, while under-sizing can limit water saving potential and reduce overall system efficiency. In the case of RWH, accurate rainfall data is critical for determining the appropriate sizing of RWH tanks, which directly affects the supply–demand balance of harvested water. The volume of water available each day depends on the inflow to the RWH tank from rainfall. When daily inflows are insufficient to meet non-potable water demands, the water level in the tank will decline. Conversely, when inflows exceed demand, the tank is topped up through mains water. It is important to note that the RWH tank never fully empties. This is to prevent pump cavitation and mechanical damage under such dry conditions. RWH systems can contribute positively to flood risk reduction and water supply sustainability; their effectiveness depends on the dynamic patterns of tank filling and emptying, specifically the available stored water volume and spare storage capacity, the latter aspect being critically important during peak flow or extreme rainfall events [21].

Tank sizing, which estimates storage capacity, is undertaken according to BS 8515:2009, where the recommended RWH tank size is determined as the lesser of 5% of the annual rainwater yield or 5% of the annual non-potable water demand. This guideline optimizes storage capacity by balancing water availability with consumption requirements, enhancing both system efficiency and cost-effectiveness [73,81]. Tank sizing for HRGS is commonly determined through a mass balance modeling approach, which accounts for inflow, outflow, storage dynamics, and final volume to achieve an optimized balance between water availability and performance. To improve precision, a range of simulation tools, such as Netuno (version 4), EPA SWMM (available at: https://www.epa.gov/water-research/storm-water-management-model-swmm (accessed on 3 June 2025)), RainTANK (Excel based spreadsheet tool (accessed on 3 June 2025)), and Aquacycle (available at: https://toolkit.ewater.org.au/Tools/Aquacycle/history (accessed on 3 June 2025)), have been employed. These tools facilitate evaluations at various temporal scales, most frequently daily, while also accommodating weekly and long-term historical rainfall data [17]. For instance, Netuno software (version 4) requires input parameters including daily precipitation, first flush volume, catchment area, per capita demand, household occupancy, total rainwater demand, and runoff coefficients. It then evaluates system performance across various tank sizes to determine an optimal volume tailored to site-specific conditions [20]. Additionally, Zhang et al. [42] note that larger tanks can improve energy efficiency by shifting pump operation outside peak electricity demand periods, whereas smaller tanks offer the benefit of lower upfront costs. These findings collectively highlight the importance of integrating technical, economic, and operational criteria when determining appropriate tank capacities for HRGS applications. The performance of the RWH and GWR systems is a measure of whether demands are met year-round. As such, this is highly dependent on the dynamics of tanks both filling and emptying, hence obtaining correct sizing is vitally important.

4.2. Water Quality and Treatment

The selection of an appropriate HRGS treatment process depends on a comprehensive characterization of the influent’s physical, chemical, and microbiological pollutant loads, as well as the intended end-use activities, to ensure that treatment efficacy meets the required water quality standards for safe reuse [23]. RWH treatment systems commonly incorporate first-flush diversion and activated carbon or sand filtration, while GWR treatment may also include sand filters and aeration units [17,23]. In systems with differentiated treatment, rainwater is collected and stored before use, while graywater undergoes treatment and is then combined with rainwater and mains water in a secondary non-potable storage tank, often with an additional disinfection stage. Alternatively, joint treatment designs collect and treat both waters together, typically requiring dual filtration and disinfection stages. Xu et al. [45] evaluated a hydroponic green roof system capable of treating both rainwater and graywater, achieving up to 90% runoff recovery during storm events. A study in Malaysia by Leong et al. [51] found that untreated graywater from full-scale systems exceeded Malaysian water quality standards, with pollutants categorized into organic matter, detergents, fecal indicators, and ammonium salts, reinforcing the necessity of comprehensive and multi-stage treatment before reuse.

4.3. Type of Building and Number of Occupants

HRGSs have been implemented in residential and commercial building settings. Leong et al. [49], in a study conducted in Malaysia, demonstrated that building type plays a critical role in optimizing HRGS performance.

For domestic buildings, the most effective strategy involved prioritizing rainwater reuse, with graywater serving as a supplementary source to meet the remaining non-potable demand. Conversely, in commercial buildings, greater efficiency was achieved by utilizing graywater as the primary supply and rainwater as a secondary top-up source. Across various sites, maximum main water savings ranged from 25.1% to 57.1%, depending on climatic and operational conditions. A subsequent performance evaluation of HRGS across four distinct building typologies, a single-family house, a multi-occupancy building, and two service-oriented facilities, using a decision support tool, revealed that system effectiveness is chiefly governed by two factors, which are the intended non-potable water end-uses and the available storage capacity. This aligns with broader findings indicating that the adaptability of hybrid systems, alternating rainwater reuse during wet seasons and GWR during dry seasons, enhances water reliability and reduces dependence on climate-sensitive sources. Ghisi and Ferreira [63] reported that commercial buildings equipped with HRGS achieved potable water savings between 36.7% and 42.0%, while domestic households demonstrated savings of 33.8% to 36.4%. These outcomes consistently affirm that hybrid systems outperform standalone RWH or GWR setups across multiple building contexts when properly aligned with building function, storage design, and seasonal water availability [64].

4.4. Water End-Uses

Water end-uses in urban environments are generally classified into two categories, potable and non-potable applications. Potable uses involve direct human contact and consumption, including drinking, cooking, and personal hygiene activities such as toothbrushing. In contrast, non-potable uses, such as toilet flushing, laundry, floor cleaning, and car washing, do not require high-quality potable water. Among these, toilet flushing and washing machines represent the most significant components of non-potable household water demand. Non-potable water should conform to bathing water standards (Bathing Water (Classification) Regulations 1991 are based upon EC Bathing Water Directive 76/160/EEC.) [73].

These high-consumption end-uses present considerable opportunities for substituting potable water with alternative sources, particularly through the implementation of RWH and GWR systems. By addressing these non-potable needs, decentralized RWH and GWR solutions can significantly reduce dependence on municipal water supplies and contribute to improved water-use efficiency in urban buildings [20,61].

5. Sustainability Dimensions and Embedded Indicators for HRGS

In this study, the proposed HRGS framework is structured based on three core sustainability dimensions, including social, economic, and environmental, each encompassing a set of context-appropriate indicators tailored to the unique challenges of ASAR. By addressing issues such as water scarcity, inequities in water distribution, and the limitations of centralized infrastructure, the framework offers a comprehensive tool for evaluating and guiding the implementation of decentralized systems. The analysis and classification of literature formed the core of this systematic review. Therefore, following the selection of relevant articles, three major sustainability criteria of the framework are identified. These are discussed in turn in the text that follows, and potential indicators and their associations are provided.

5.1. Environmental Sustainability

The environmental dimension of HRGS focuses on the ability to support sustainable urban development through improved water resource management and ecosystem protection. These decentralized systems provide an alternative to conventional centralized water supply and wastewater infrastructure, reducing environmental impacts such as water extraction, energy consumption, and contamination. Key environmental performance indicators include their integration into green infrastructure, potential for potable water savings, contributions to climate change mitigation, and alignment with existing environmental policies and regulations. Together, these factors provide a comprehensive framework to assess the ecological benefits and challenges associated with the adoption and operation of HRGS in various urban contexts.

5.1.1. Green Infrastructure Integration

HRGS are increasingly recognized as essential components of green infrastructure, offering localized solutions for water sustainability by facilitating on-site treatment and reuse. These systems significantly reduce the dependence on energy-intensive, centralized water supply networks by minimizing the need for long-distance water transport and associated infrastructure. However, despite their demonstrated environmental advantages, such systems are frequently implemented in isolation rather than as part of a coordinated, integrated water management framework, highlighting a gap in systemic urban planning [39].

Indicator: Percentage of Site Area Covered by Decentralized Systems (% Site Area Connected)

Leong et al. [46] examine decentralized systems at both domestic and commercial scales in Malaysia. The study highlights a significant disparity in the extent of decentralized system coverage relative to the site area between domestic and commercial buildings. In commercial buildings, although the total roof area is large, only a small portion of it is typically connected to the decentralized water systems; for instance, Site 1 had a total roof area of 2477 m², but rainwater was collected from only 177 m², indicating limited proportional coverage. Conversely, domestic sites featured connected roof areas that more closely matched the total roof area, such as Site 2, where 410 m² out of 487 m² roof area was connected, representing a higher percentage coverage of the available site.

This difference is critical because it directly influences the volume of rainwater and graywater that can be captured and reused, thus affecting the system’s overall efficiency and potential to reduce mains water demand. The smaller connected roof proportion in commercial buildings, combined with higher water demand, results in lower mains water savings compared to domestic setups, where a larger percentage of the site area is serviced by the decentralized systems. This contrast illustrates that decentralized water system design and implementation must be customized to the specific site characteristics, including roof connectivity and building scale, to optimize water conservation outcomes in different urban environments. The study’s findings emphasize the importance of maximizing connected roof areas and system capacity to improve the sustainability and effectiveness of decentralized water reuse systems in both commercial and domestic contexts [46].

Indicator: Reduction in Stormwater Runoff (% Reduction)

Evidence from Brazil reinforces the hydrological benefits of decentralized systems. Ghisi and Freitas [20] reported that implementing a RWH system in an urban setting reduced stormwater runoff to the municipal drainage network by approximately 11.8%, thereby alleviating pressure on conventional stormwater infrastructure and enhancing overall urban flood resilience.

Indicator: Reduction in Stress on Water Supply System (% Reduction)

Supporting the environmental efficacy of integrated systems, Gómez-Monsalve et al. [40] conducted a comparative life cycle assessment of an HRGS versus a conventional centralized water supply in a high-demand household in Bucaramanga, Colombia. Utilizing GaBi software GaBi database version 2019, their analysis demonstrated that the HRGS achieved a 42.5% reduction in potable water consumption, equating to 131 m³ annually, and a 20% decrease in wastewater discharge to treatment facilities.

Indicator: Green Roof for Rainwater Harvesting (% Adoption and % Recovery)

Xu et al. [45] developed a hydroponic green roof system capable of treating both RWH and GWR, achieving up to 90% runoff recovery during storm events. Furthermore, reducing land use and energy demand positions it as an effective form of green infrastructure. These findings collectively underscore the environmental superiority of hybrid decentralized systems. By reducing both resource consumption and environmental impacts, they serve not only as effective solutions but as critical components of resilient, resource-efficient urban water infrastructure.

5.1.2. Potential Water Saving

In the context of water-scarce urban environments, HRGS have emerged as highly effective strategies for potable water savings, particularly in residential and commercial buildings. Ghisi and Freitas [20] evaluated the financial and water saving performance of such systems in a multifamily residential building comprising 60 flats in Florianópolis, Brazil. Their findings indicated that the use of RWH for washing machines resulted in a potable water savings of approximately 6.9%, while reusing GWR for toilet flushing yielded an additional 5.7% savings. These results not only demonstrated the operational feasibility of HRGS in compact urban developments but also emphasized their potential as sustainable solutions for water conservation.

Indicator: Percentage of Potable Water Replaced per Household (% Replacement)

This indicator quantifies the daily volume of a dwelling’s potable demand that is offset by on-site non-potable supplies such as RWH and GWR systems delivered at reuse quality [45]. This study by Maskwa et al. [43] conducts a comprehensive spatial life cycle cost comparison of household GWR and RWH systems across diverse climatic and geographic settings in the United States. The research evaluates optimal system tank sizes, water saving performance, and economic viability for typical single-family and multifamily residences. The results reveal that GWR systems generally outperform RWH in meeting non-potable water demand, with GWR achieving 70% to 90% of the designated non-potable uses, while RWH meets 50% to 70%. The volume of potable water replaced varies according to household type and system design, with average daily graywater inflow, including showering, bathroom sink, and laundry, accounting for approximately 0.144 m³/day per person, and toilet flushing water demand around 0.072 m³/day per person. Variations among cities in terms of precipitation, utility rates, and household sizes influence the volume of potable water replacement. Moreover, a study by Coutinho Rosa and Ghisi [44] evaluates a combined RWH and GWR system designed to reduce potable water consumption in a residential building. The system utilizes rainwater for laundry washing, subsequently reusing the graywater from this process to flush toilets. This integrated approach was modeled for a house in Florianópolis, Brazil, and revealed significant potable water savings. Specifically, the washing machine accounted for the largest share of potable water use at 44.4%, and incorporating rainwater use for this purpose could reduce potable water demand by an average of 40.4%. Additionally, by reusing graywater for toilet flushing, an extra 11.2% reduction was achieved, leading to an overall potable water savings potential of 51.6%. This combined system decreased average monthly potable water consumption from 18 m³ to 8.71 m³, representing a substantial replacement of potable water with non-potable sources. These results underscore the effectiveness of integrated RWH and GWR systems in achieving considerable water savings at the household level, highlighting their potential for sustainable water management in urban residences.

5.1.3. Climate Mitigation Potential

HRGS holds considerable potential for enhancing urban climate resilience by reducing dependency on centralized water supplies, especially during drought conditions, and by mitigating flood risks during periods of intense rainfall [17]. Marinoski and Ghisi [47] conducted a case study in southern Brazil to evaluate the environmental performance of an HRGS in single-family residences. The study compared two scenarios, one utilizing a conventional centralized water supply system and the other implementing a decentralized HRGS. Results revealed that the HRGS achieved an average potable water savings of 41.9%, a 40% reduction in domestic sewage generation, and a 36.1% decrease in total energy consumption relative to the centralized system.

Indicator: Reduction in Energy Use (kWh/Year)

Malinowski et al. [56] conducted a national-scale study in the United States to assess the potential energy and cost savings associated with decentralized water reuse strategies. The study estimated that the substitution of potable water for landscape irrigation and other outdoor uses with RWH alone could result in annual savings of up to 3.8 billion kWh of electricity and USD 270 million in utility costs. When united with GWR, the savings potential increases substantially, reaching up to 14 billion kWh and USD 950 million annually.

Indicator: GHG Emissions (kg CO₂e/Year)

The Greenhouse Gas (GHG) emissions indicator (kg CO₂e/year) played a central role in the life cycle assessment of RWH and GWR systems compared to a conventional centralized water system in Colombia. Gómez-Monsalve et al. [40] found that decentralized systems achieved a climate change impact of 1.99 kg CO₂e per cubic meter of delivered water, whereas the centralized system resulted in 2.44 kg CO₂e per cubic meter, demonstrating an 18% reduction in GHS emissions with the decentralized approach. This improvement is primarily attributed to a reduced demand for potable water and a lower volume of wastewater requiring treatment, both of which are major contributors to energy use and emissions in urban water cycles.

5.1.4. Environmental Regulation Alignment

The implementation of decentralized systems demonstrates clear potential to support the objectives of environmental regulations that promote water conservation, sustainable urban infrastructure, and reduced pressure on centralized water supply and wastewater systems [39].

Indicator: Compliance Level with Local or National Reuse Standards (% Compliance)

Despite the recognized importance of sustainable water management in achieving long-term environmental objectives, the widespread implementation of water reuse and recycling practices, particularly RWH and GWR, remains limited in many developing countries. This implementation gap is largely attributed to the absence of comprehensive environmental policies, regulatory frameworks, and enforcement mechanisms. Without clear guidelines and institutional support, the integration of decentralized reuse technologies into national water strategies remains fragmented, thereby hindering progress toward water security, climate resilience, and sustainability goals. Chadee et al. [37] highlighted these challenges through a case study in Trinidad and Tobago, where they applied the Analytical Hierarchy Process (AHP) and Multi-Criteria Analysis (MCA) to assess the effectiveness of RWH and GWR systems. Their analysis identified public awareness as the most influential factor in advancing reuse practices. The strong correlation between public engagement and system feasibility underscores the urgent need for policy interventions that go beyond infrastructure provision. A leading example of progressive regulatory adoption is found in the Metropolitan area of Barcelona, where several municipalities have mandated the inclusion of RWH and GWR systems in new building developments [55]. These policies were characterized by their institutional flexibility, allowing adaptive regulatory responses to practical challenges encountered during implementation. In parallel, the imposing surcharges for excessive water consumption further incentivized the adoption of decentralized systems by improving their economic viability. This multi-level governance approach exemplifies how adaptive, responsive policy instruments can drive the mainstreaming of sustainable water management technologies.

5.2. Economic Sustainability

The economic viability of HRGS remains a critical factor influencing its broader adoption and integration into urban water management. Realizing the full economic benefits of HRGS requires more than technical efficiency; it also demands enabling institutional frameworks.

5.2.1. Indicator: Operation and Maintenance Cost (USD or Local Currency)

Operational and maintenance costs are key components in evaluating the long-term financial viability of HRGS. These costs not only influence system affordability but also affect stakeholder confidence and the likelihood of widespread adoption. Rosa and Ghisi [27] assessed an HRGS in southern Brazil, comparing potable water and sewage bills before and after system implementation. Their analysis incorporated installation costs, operational expenses, and applicable tariff structures. The system achieved a 38% reduction in monthly potable water consumption, equivalent to 7.00 m³ per household per month. Chen et al. [18] highlighted that inadequate design assessment and performance forecasting can result in increased operational failures and elevated maintenance costs, undermining economic returns and system reliability. In support of HRGS efficacy, Wanjiru and Xia [48] carried out a case study in South Africa; their proposed hybrid system achieved significant savings, 32.3% in water costs, 29.5% in wastewater costs, and 35.7% in energy expenditures. Overall, the system delivered a 31.5% reduction in total monthly operational costs, affirming the financial and environmental advantages of integrated water reuse systems in resource-constrained contexts. Capital cost optimization is also critical found that sequential installation, adding a graywater system after an existing rainwater system, incurred a 21% higher capital cost (USD 2448) compared to a fully integrated installation from the outset (USD 1929) [63,64]. Chen et al. [18] evaluated the operational performance of HRGS implemented on a university campus in Japan. The system, equipped with water distribution pumps, faced significant challenges related to operation and maintenance. The study found that there are high maintenance costs. The study concluded that reducing maintenance costs is a more effective strategy for enhancing the long-term economic performance of HRGS. This case underscores the importance of aligning system design with demand and optimizing operational efficiency to ensure financial sustainability. Collectively, these findings underscore that the economic success of HRGS depends not only on the system’s performance in saving water and energy but also on strategic planning, tariff alignment, and optimized installation to reduce both upfront and recurring costs. A practical, integrated approach to design and maintenance is essential for realizing the full economic potential of decentralized water reuse systems.

5.2.2. Financial Viability

The financial efficiency of HRGS is a critical consideration for both developers and policymakers pursuing cost-effective solutions for sustainable urban water management. Recent studies have demonstrated that, despite varying investment and return periods, these systems offer promising financial performance, particularly when evaluated through the lens of long-term savings, energy offsets, and user priorities.

Indicator: Payback Period (Years—the Lower the Better)

In a comprehensive study conducted in Brazil, Ghisi and Freitas [20] evaluated the financial viability of decentralized water reuse systems in a multifamily residential complex. Their analysis estimated payback periods ranging from 57 to 76 months for a standalone RWH system, and from 127 to 159 months for a GWR system. When implemented as an integrated hybrid system, the payback period ranged from 89 to 132 months.

An additional study from Zhang et al. [42] supports the economic promise of integrated systems. In a simulation-based study conducted in South Africa using five years, the proposed hybrid system achieved notable reductions in potable water demand and operating costs. With an estimated payback period of 4.39 years, the system demonstrated strong potential as a financially viable and sustainable household-level solution for decentralized water management in variable climatic conditions.

Indicator: Life Cycle Cost (USD or Local Currency)

Life cycle cost (LCC) analysis confirmed that such multifunctional configurations yield both environmental and financial benefits, offering a viable alternative to conventional resource-intensive systems. The cost effectiveness of implementing an HRGS relies on a thorough assessment of all associated costs and revenues throughout the system’s lifetime. The LCC methodology provides a comprehensive technique to account for all acquisition, operating, maintenance, and disposal costs over the project’s life. Equation (3) for LCC contains the capital cost (C_c), cumulative discounted operation cost (C_o), and the salvage cost (C_s) at the end of life of the system:

$$LCC=C_c+ C_o+C_s$$

(3)

The (C_c) covers all expenses related to purchasing, installing, and providing labor for setting up the system. (C_o), incurred throughout the system’s use, encompassing expenses for water, wastewater, energy, and maintenance. Moreover, the end of the system’s life (C_s) accounts for removing and disposing of the system [48,82]. The results of a study in Boston, USA, by Stang et al. [41] showed that RWH systems could only achieve positive life cycle cost savings in 12% of residential buildings, with an average increase in life cycle cost of USD 33 to USD 57 per year and an average increase in life cycle energy consumption by 362 to 417 MJ per year. Meanwhile, GWR systems enable nearly all households to achieve positive cost savings, with average life cycle cost savings of USD 909 to USD 948 per year and average energy savings of 586 to 622 MJ per year. Moreover, GWR was able to meet approximately 82% of total toilet flushing and irrigation demands per year [41].

5.3. Social Sustainability

Over the last 30 years, RWH and GWR have gained increasing attention in the research community, not least with respect to sustainability. Research in the field has predominantly addressed the technical, economic, and social dimensions associated with the deployment of these systems. Among these, social aspects are often identified as the most significant barriers to widespread adoption, underscoring the importance of public acceptance, user behavior, and awareness of health and hygiene considerations in the successful implementation of decentralized water reuse technologies [38].

5.3.1. Indicator: User Behavior and Water Use Patterns (Various—e.g., Minutes/Shower Reduced)

User behavior is a key determinant in the performance and efficiency of decentralized water reuse systems. Household water consumption patterns vary significantly based on user routines/habits, cultural influences, appliance ownership and usage, and occupancy levels. All these factors must be carefully considered in the design and implementation of HRGS. In Germany, Kasipiyawong et al. [36] analyzed domestic water use patterns in the Bahnstadt area district through survey data. Most respondents reported taking one daily 10 min shower, typically using standard showerheads alongside dual flush toilets. Laundry was generally performed once weekly per household. Meanwhile, gardens were watered approximately once per week.

In contrast, the adoption of water-efficient appliances, strongly influenced by user preferences and habits, achieved similar levels of potable water savings. However, the greatest performance gains were observed when RWH and GWR were applied with user-driven efficiency measures. This integrated approach led to significant reductions in both potable water consumption and sewage discharge, underscoring the synergistic potential of technology and behavior.

5.3.2. Indicator: Health and Hygiene Perception (To Be Defined—Likely Improved Perception)

Public perception of health and hygiene plays a pivotal role in shaping the adoption and acceptance of HRGS. Despite the technical viability and environmental benefits of these systems, concerns over water quality, potential exposure to contaminants, and associated health risks often present significant barriers to their widespread implementation, particularly in regions where water scarcity is not perceived as an immediate threat. In Germany, Kasipiyawong et al. [36] found that although RWH systems were widely accepted for reducing potable water demand, their limited supply capacity necessitated the supplementation of treated graywater to meet full non-potable water needs. However, only 20.78% of respondents expressed willingness to adopt graywater systems, primarily due to concerns regarding hygiene and potential health risks. This hesitancy illustrates the persistent stigma surrounding the use of non-traditional water sources, particularly graywater, in domestic settings. Stec [38], in a cross-national survey spanning twelve countries, including both water-rich countries such as Poland and Sweden, and water-scarce regions like Egypt and Iraq, revealed significant disparities in public perceptions of RWH and GWR systems. The study found that the safety of these systems was most pronounced in countries with abundant freshwater resources, where the absence of acute water scarcity diminished the perceived need for conservation. Limited public education and outreach further contributed to low awareness, with only 50% of respondents aware that rainwater and graywater could be safely reused for non-potable purposes such as toilet flushing and landscape irrigation. In contrast, participants from water-stressed regions expressed greater acceptance and demonstrated fewer hygiene-related concerns, suggesting that direct exposure to water insecurity can significantly influence public support for alternative water sources. In Colombia, Domínguez et al. [52] explored health-related decision making in a social housing project; 20.6% of participants rejected GWR specifically due to hygiene concerns. This duality highlights the tension between economic limitations and perceived health risks. Even among economically vulnerable populations, water quality and hygiene remain high-priority considerations that can override cost-based decision making. Collectively, these findings underscore the importance of integrating health and hygiene considerations into both system design and public engagement strategies.

5.3.3. Public Acceptance

Public acceptance is a significant determinant in the successful implementation and scaling of RWH and GWR systems. While technical performance and economic viability are essential, the perceived safety, utility, and cultural alignment of these systems often dictate their adoption at the community level. Societal attitudes toward alternative water sources are shaped by a range of factors, including gender roles, water security perceptions, environmental values, and public health concerns. In a study conducted in urban areas of the Philippines, Mason et al. [26] explored the social and economic dynamics influencing water reuse behaviors across wet and dry seasons.

The findings revealed that women were more likely than men to engage in GWR practices, indicating the importance of integrating gender-sensitive approaches into water reuse policy and outreach. Domínguez et al. [52] investigated public acceptance in a low-income social housing in Colombia. Their study reported high levels of user willingness to adopt decentralized systems, with 91% of respondents expressing readiness to implement RWH and 78% for GWR.

Acceptance of RWH was highest for toilet flushing (86%), house cleaning (75%), plant irrigation (61%), and laundry (45%), motivated primarily by environmental concern (66%) and cost savings (61%). While GWR was most accepted for toilet flushing (78%), with lower acceptance for house cleaning (34%) and plant watering (22%). These collective findings underscore that increasing public acceptance of decentralized water reuse systems requires more than technological innovation.

Indicator: Adoption Rate (Higher Adoption Rates Are Better)

According to Stec [38], countries with abundant freshwater resources, such as Poland, demonstrated lower levels of public support for RWH and GWR systems. In contrast, acceptance was significantly higher in water-scarce countries, including Iraq, Turkey, Egypt, and Spain. In these regions, experiences with water scarcity were linked to greater willingness to adopt alternative water solutions, indicating that public perception of water security plays a central role in shaping behavioral attitudes.

Indicators: Stakeholder Engagement Level (Higher Engagement Rates Are Better)

Kasipiyawong et al. [36] further emphasized that public acceptance of GWR could be significantly improved by designating treated water for applications involving minimal human contact, such as toilet flushing, laundry, and dishwashing. Their findings highlight the importance of targeted public engagement and education campaigns that clearly communicate the safety protocols, system design, and intended uses of nonportable water. This approach is particularly relevant in ASAR, where water reuse is critical for long-term water security. Global patterns of acceptance reveal further distinctions based on water availability. Chen et al. [39] highlighted that strategic cooperation between governments and municipal water utilities is essential for enhancing system integration and improving economic performance. Broader stakeholder engagement, particularly involving planners, engineers, and community organizations, can further amplify the financial and environmental returns of decentralized water reuse systems by aligning regulatory, technical, and financial resources. These studies emphasize that while economic feasibility is a central concern, its evaluation must be both comprehensive and contextual. When supported by targeted policy interventions and stakeholder collaboration, HRGS can serve as both an environmentally responsible and economically sound solution for sustainable urban water infrastructure.

6. Discussion and Conclusions

This study systematically reviewed 40 peer-reviewed articles published between 2007 and 2024, identifying key technological trends, performance outcomes, and contextual limitations associated with HRGS applications. Moreover, this study examined the extent to which HRGS has been implemented in ASAR and evaluated how comprehensively these applications address the environmental, economic, and social dimensions of sustainability. Figure 6 and the comparative assessment Table 2. In Section 3.3, we collectively reveal clear patterns in the research focus of RWH and GWR systems. A total of 40 studies were analyzed, showing a dominant emphasis on technical indicators, which accounted for 49% of all extracted themes.

Environmental indicators ranked second (24%). However, the social and economic dimensions remain less explored, accounting for only 15% and 12% of total theme frequency, respectively. This imbalance demonstrates that while the literature strongly confirms the technical and environmental feasibility of HRGS, successful mainstream adoption depends on stronger integration of social behavior and financial viability indicators. In response to RQ1 and RQ2, findings indicate that HRGS deployment in ASAR is experiencing growth but remains largely constrained to pilot projects and small-scale residential applications. In ASAR, implementation is still limited, with most projects serving as demonstration cases rather than being adopted at a commercial or municipal scale. This underscores the need for further research to evaluate the long-term sustainability and broader environmental benefits of HRGS in these contexts under present and future climatic and economic conditions. Among the reviewed case studies, only partial assessments of the proposed environmental, economic, and social indicators were observed. Notably, none of the studies examined addressed the full set of sustainability indicators within an HRGS framework in ASAR, underscoring a significant gap in the literature.

The dominance of environmental metrics demonstrates their centrality for climate resilience and water security, while the weak representation of economic and social criteria highlights existing research gaps. Studies from South Africa, including Zhang et al. [42] and Wanjiru and Xia [48], focused primarily on technical and financial aspects of HRGS in single-family homes. Zhang et al. emphasized system design optimization based on financial performance, while Wanjiru and Xia explored operational efficiency, particularly in relation to energy use. Both highlight system feasibility, yet neither addresses broader social or environmental integration. Similarly, Mexico (López Zavala et al. [19]) and Tunisia (Redwood et al. [57]) highlight the technical and economic viability of HRGS in reducing potable water consumption and alleviating stress on centralized infrastructure. However, these studies are often constrained by limited geographical scope and narrow stakeholder engagement. By contrast, Vallès-Casas et al. [55] in Spain offered a socially focused evaluation, highlighting the role of political will, stakeholder engagement, and public acceptance in the successful adoption of HRGS. The study emphasized that institutional support and community involvement are essential for long-term sustainability. However, the study lacked quantitative data on economic or environmental outcomes, limiting its contribution to a fully integrated assessment. Additionally, Stec [38] conducted a cross-national survey across twelve countries, offering broader insights into public acceptance of RWH and GWR systems. The findings revealed that acceptance was highest in ASAR countries, such as Iraq, Egypt, and Spain, where the urgency and awareness surrounding water scarcity were more pronounced. The reviewed literature demonstrates that while individual studies offer valuable insights into the feasibility and benefits of HRGS, there is a clear absence of integrated, multi-dimensional evaluations that address the economic, environmental, and social pillars of sustainability simultaneously to strengthen the evidence base and support policy in ASAR. In addressing RQ3, the most context-appropriate sustainability indicators for evaluating HRGS in ASAR are those directly tied to enhancing climate resilience and ensuring long-term water security. These indicators span three core dimensions: environmental, economic, and social. Within the environmental dimension, indicators include water saving efficiency, which is essential for quantifying potable water reductions in water-scarce environments, and climate mitigation, where HRGS contributes by attenuating stormwater runoff, reducing GHG emissions through decreased energy use, and increasing resilience to droughts.

The inclusion of green infrastructure highlights HRGS integration within broader urban sustainability strategies, while environmental policy and regulations reflect the institutional readiness necessary for widespread adoption. Moreover, Figure 9 supports the RQs by visualizing the distribution and frequency of sustainability indicators reported across HRGS case studies conducted in ASAR contexts. Regarding RQ1, the figure shows that HRGS applications vary geographically, with implementations in Tunisia, Spain, Mexico, South Africa, Iraq, and Egypt. This confirms that adoption exists but remains limited to case studies rather than broad implementation. For RQ2, it reveals clear disparities in indicator coverage. Environmental indicators, particularly potential water savings, appear consistently across most studies, indicating strong attention to water conservation benefits. In contrast, economic indicators such as financial viability and operation and maintenance costs are less frequently addressed, and social indicators (public acceptance, health perception, and user behavior) show even greater inconsistency. This evidence confirms that comprehensive sustainability evaluation remains limited, with most studies focusing on environmental performance indicators. According to RQ3, the figure supports identifying which indicators are most recurrent and therefore more context-relevant for ASAR evaluation, such as potential water savings, indicating their widespread relevance and emphasis in ASAR-related assessments.

The economic dimension prioritizes financial efficiency, crucial for evaluating feasibility in resource-constrained contexts. Operation and maintenance costs are equally critical, as the long-term viability of HRGS depends on affordable upkeep, especially in areas lacking consistent technical support or funding. From a social perspective, indicators such as user behavior, health and hygiene considerations, and public acceptance are fundamental for gauging societal readiness and ensuring the long-term success of HRGS. Despite these advances, the current literature still reflects several limitations. Many studies are geographically constrained and fail to adopt a comprehensive framework encompassing all sustainability dimensions.

Furthermore, governance and institutional policy, while acknowledged in studies like Vallès-Casas et al. [55], remain underexplored across the ASAR context. Several uncertainties and risks must be acknowledged according to the water-decentralized systems. Evidence from multiple studies demonstrates that public acceptance and adoption of RWH and GWR systems depend strongly on perceptions of hygiene, safety, and awareness of the users. For example, Kasipiyawong et al. [36] showed that only 20.78% of respondents were willing to adopt GWR due to concerns about hygiene and perceived health risks. Similarly, Stec [38] found substantial variation across different countries; in water-scarce regions (e.g., Egypt and Iraq), acceptance for GWS was higher, and hygiene concerns were less pronounced. Domínguez et al. [52] further reported that 20.6% of households in a Colombian social housing project rejected graywater reuse primarily because of hygiene considerations. Collectively, these studies indicate that public acceptance and adoption of HRGSs are shaped by perceived health risks, cultural attitudes toward non-traditional water sources, and the level of public awareness and education. In addition, increasing water scarcity across the ASAR region, particularly in the GCC, underscores the urgency of alternative water-supply strategies. As highlighted by Al-Zubari et al. [83] that GCC countries have recently begun transitioning toward decentralized alternatives to address growing supply and infrastructure pressures. The study emphasizes that cogeneration seawater desalination plants in the GCC have very high energy requirements, with energy expenses accounting for nearly 85% of total operating costs. Furthermore, desalination consumes 55% of Kuwait’s total national energy use, 30% in Bahrain, and 25% in Saudi Arabia, placing substantial pressure on national energy budgets and contributing significantly to greenhouse gas emissions. Consequently, the combined pressures of energy intensity, water scarcity, and infrastructure costs underscore the importance of diversifying water-supply strategies in the ASAR region. This includes expanding decentralized options, such as RWH and GWR, particularly to reduce both energy consumption and environmental impacts.

Although HRGSs demonstrate promising potential for water demand reduction, their application in ASAR remains associated with several uncertainties and implementation risks that require further investigation. These include performance variability under arid climatic conditions, high energy intensity in treatment systems, fluctuations in rainfall availability, and concerns over graywater quality and public health safety. Additionally, long-term maintenance responsibility, operational costs, and the absence of clear regulatory frameworks pose further challenges for system adoption. Public acceptance represents another influential factor, as willingness to invest in or operate such systems is often dependent on perceived hygiene, risk awareness, and socio-cultural attitudes. The lack of comprehensive sustainability assessments in ASAR highlights that current evidence is fragmented, reinforcing the need for more integrated research that evaluates environmental, economic, and social dimensions under real-world conditions. To bridge these gaps, future research should embrace multidisciplinary approaches that evaluate environmental, economic, and social indicators in an integrated manner. Such frameworks will be instrumental in developing resilient, context-specific HRGSs tailored to the specific climatic, infrastructural, and cultural realities of ASARs. These indicators were deliberately selected for their alignment with the most pressing challenges and contextual realities of ASARs, particularly with respect to climate resilience, long-term water security, and social adaptability. In conclusion, the proposed indicator-based framework serves as a robust evaluative tool to guide the planning, implementation, and scaling of HRGSs in ASARs. Its adoption can support evidence-based decision making, enhance stakeholder trust, and ultimately ensure the resilience and sustainability of decentralized water reuse systems in water-stressed regions.

7. Future Work

The next stage of this research will focus on developing a context-specific decision support tool for HRGS participatory indicator validation, involving structured engagement with end-users, policymakers, and technical experts to ensure that the selected indicators, spanning environmental, economic, and social aspects, are both meaningful and practical for ASAR contexts. The framework will be designed to align with integrated water resources management principles and climate adaptation goals, operationalizing the Dublin Principles through collaborative, stakeholder-informed processes. Ultimately, this participatory decision-support tool aims to deliver practical guidelines, regulatory pathways, and implementation strategies that account for the unique climatic and institutional realities of ASAR.