Water Microgrids as a Hybrid Water Supply System: Review of Definitions, Research, and Challenges

Abstract

Hybrid water supply systems (WSSs) integrating centralized and decentralized water systems have gained increasing interest in recent years to enhance water service sustainability and system resilience. An example of implementing hybrid WSSs is water microgrids, inspired by energy microgrids. Water microgrids can be depicted as a network (grid) of localized networks (sub-grids) comprising local water sources and their storage and distribution systems that operate in conjunction with a central WSS. They can operate in both ‘grid-connected or ‘islanded’ mode and support interaction and demand trade-offs with centralized WSSs at varying degrees of decentralization, providing flexibility and increased control over water resources. However, the concept of water microgrids is still in its infancy, and their application is limited due to a lack of design guidance and frameworks. This paper provides a comprehensive review of water microgrids, discussing the concept, design, benefits, and potential challenges by drawing insights from energy microgrids, and also discusses the standpoint in comparison with centralized, decentralized, and hybrid WSSs. It also explores integration of decentralized and hybrid infrastructure within existing WSSs, highlighting the balance between localized optimization and systemwide sustainability. The findings aim to broaden understanding of water microgrids, assessing their applicability and operational strategies in urban settings.

1. Introduction

Centralized water supply systems (WSSs) have traditionally provided cities with essential water services, effectively supporting urban development for over a century [1,2]. However, these systems are now under strain due to a variety of challenges coming from climate and socioeconomic changes, aging infrastructure components, energy and food security, growing cyber-physical threats, and financial constraints [3,4,5,6]. In this context, many studies and reports have raised concerns that current centralized WSSs are limited in addressing these service challenges, responding to localized needs and opportunities, particularly under complex and uncertain circumstances in climate and socioeconomic changes [7,8,9]. As urban environments continue to evolve with increasing interdependency across critical infrastructures, the existing centralized water infrastructure is becoming inadequate for addressing future challenges due to the lack of flexibility, resiliency, and sustainability [10,11,12].

The centralized supply model in the energy sector also faces similar challenges to those faced by WSS. Historically, energy systems have been designed to operate through large, centralized power stations that distribute electricity across large networks. Like water systems, these centralized energy systems have encountered significant issues such as inefficiency in energy distribution, vulnerability to failures from natural disasters or targeted attacks, and difficulties in integrating renewable energy sources [13,14]. After years of research, the concept of energy microgrids has emerged as a solution aimed at enhancing the resilience, sustainability, and efficiency of energy supply, combating the rising challenges [15,16,17]. Energy microgrids, which function as localized energy generation and distribution networks, can operate independently from the centralized grid, offering communities greater control over their energy sources and fostering a more adaptive response to environmental and economic changes [18,19,20]. This shift towards decentralization in the energy sector provides a valuable framework for reconsidering water supply strategies, particularly through the adaptation of similar microgrid concepts to create water microgrids. Thus, the concept of the water microgrid comes to light through [9] in 2015, and then two consecutive research projects by the U.S. Department of Energy at the PNNL that published in 2021 [21] and 2023 [22]. Then [23] did a laboratory-based operational experiment and found this approach compatible and a good fit compared to the existing WSSs in terms of operational flexibility, resilience, and sustainability.

A water microgrid can be understood as a localized, ICT-enabled network that integrates multiple water sources, treatment units, storage, and distribution systems in a coordinated manner. It is built on top of existing centralized, decentralized, and hybrid types of WSSs. A centralized WSS relies on large treatment plants and long-distance transmission pipelines feeding entire cities. A decentralized WSS consists of localized systems such as rainwater harvesting tanks, greywater reuse, or small-scale treatment units operating independently of central networks. Hybrid WSS represents intermediate arrangements where consumers combine utility-supplied water with local sources, but without integrated management or control and interaction with centralized WSS. Finally, a water microgrid is more of a hybrid WSS, which can be defined as a network (grid) of networks (sub-grids) that can also be operated interdependently or independently under normal and abnormal conditions [23]. Thus, the water microgrids can secure additional water resources through sustainable ways with water reuse, recycling, and conservation approaches, and, in turn, increase access to water, especially in regions facing water scarcity and infrastructure challenges [9,22]. In addition, each microgrid can have flexible strategies (e.g., island mode) depending on WSS failure conditions to minimize water service losses from the system failures and rapidly recover the disrupted microgrid, enhancing WSS resiliency [22]. Recent advancements in Information and Communication Technology (ICT) also enable the water microgrids to more effectively achieve their sustainable water services with system resilience through real-time monitoring, decision-making, and control [9,23].

The decentralization or hybrid infrastructure approach has been proposed and practiced for the last couple of decades as a more resilient and sustainable way to design and manage water systems against the uncertainty and complexity of drivers [3,4,24,25]. However, despite the growing interest, water microgrids remain underexplored in both research and practice. A systematic literature review shows that, while thousands of studies exist on energy microgrids, only a handful explicitly examine water microgrids (WMs), and even fewer provide empirical evaluations [24]. Key issues remain insufficiently addressed, including quantitative comparisons of water microgrids and traditional systems using indicators such as levelized cost of water (LCOW), non-revenue water indices, greenhouse gas emissions, and resilience metrics. The hydraulic complexities of water microgrid operation—such as water age, storage optimization, and mixing dynamics, or its impacts on existing centralized systems and the integration and long-term management of both systems [26] are rarely studied in depth. Although the dual water systems in places like Japan, Singapore, and university campuses provide valuable lessons, they are seldom framed as precursors of water microgrids. Also, the dissimilarities between energy and water microgrids in terms of variability in quality, microbial safety, and physical properties are not addressed explicitly in previous studies. So, compared to energy microgrids, there is a limited understanding of the integration, configuration, and operation specific to water microgrids at the system level, and discussion on its resilience and sustainability effects, and implementation challenges.

Given these gaps, the purpose of this paper is fourfold. First, it seeks to clarify the definition of water microgrids by situating them in relation to centralized, decentralized, and hybrid systems. Second, it critically analyzes the technical, economic, and social dimensions of water microgrids, including the unique challenges posed by water quality, hydraulic requirements, and infrastructure costs. Third, it synthesizes insights from existing case studies and dual water systems, identifying both the benefits and the practical barriers to implementation. Finally, it outlines future research directions, highlighting the need for techno-economic assessments, hydraulic modeling, and incremental adoption strategies.

2. Methods

Because the concept of water microgrids is still in its infancy and limited research has been conducted on this topic to date, a comprehensive understanding of their concept, design, architecture, resiliency effects, sustainability aspects, and challenges, especially in comparison to traditional WSSs and energy microgrids, was necessary. To achieve this, the authors followed the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines [27]. A systematic five-step framework was developed to conduct a comprehensive literature review and collect existing evidence relevant to the study. The five steps included (i) online database searching and information clustering, (ii) citation and sample refinement, (iii) abstract screening and refinement, (iv) full-text review, and (v) final selection. The finally selected papers were then reviewed in depth, and the findings were analyzed and discussed in the subsequent sections of this paper.

2.1. Search Strategy and Screening

Following the PRISMA review protocol, a comprehensive literature search was conducted using multiple databases and platforms, and the PRISMA flowchart for review was employed to guide the review process through four main phases: identification, screening, eligibility assessment, and inclusion; the chart is presented in Appendix A [16,27,28,29,30,31]. Several eligibility criteria were applied to filter the literature, including publication domain, keyword frequency (i.e., how often specific keywords appeared within the selected literature), study design, year of publication, country/region of origin, and journal of publication. Relevant keywords were used to identify studies related to the concepts, designs, configurations, challenges, and impacts in various fields, including engineering design, supply chain, infrastructure systems, resilience, sustainability, physical networks, and non-engineering fields such as enterprises/organizations, social networks, and economics. Articles were further filtered using keywords such as “microgrid”, “energy microgrid”, “water microgrid”, “water-energy microgrid”, “hybrid water supply system”, “decentralized water supply system”, “water supply resilience”, “water supply sustainability”, and “engineering resilience”. The abstracts of the articles were assessed for relevance before retrieving full texts, and the Morris Library of Southern Illinois University was used to obtain access to the selected full-length articles.

2.2. Selection and Review

A search using the term “energy microgrid” in the Web of Science and Scopus databases returned 46,802 unique articles as of June 2025, primarily within the engineering field. On the other hand, a search for “water microgrid” yielded 747 unique articles. However, except for four papers [9,21,22,23], the majority focused on applying energy microgrid technologies to water supply systems as a power source rather than exploring water microgrids as an independent system concept. According to Figure 1, no domain or keyword for “water microgrid” was found, which indicates that the microgrid concept or approach has not been widely adopted in water systems yet.

Decentralization emerged as a central theme in both energy and water microgrids. For instance, a search for “decentralized water supply” identified 1066 publications mostly within Environmental Sciences, Water Resources, and Energy Fuels disciplinary fields, predominantly in Engineering and Environmental Sciences areas of publication. Similarly, “hybrid water supply” returned over 4000 publications within the same categories, and “centralized water supply” yielded approximately 900 publications. These findings confirm that, although direct studies on water microgrids are limited, substantial work exists on decentralized and hybrid systems—key building blocks for water microgrid configurations. Thus, conceptual similarities between decentralized/hybrid systems and water microgrids can be taken into account in future WSS design and implementation efforts.

VOSviewer (version 1.6.20) [32], a widely used program for bibliometric mapping and a visualization tool, was used to analyze trends and co-occurrence analysis (frequency with which two or more keywords appear together within the same document or section of text), focusing on identifying the most dominant keywords in the title and abstracts, as shown in Figure 1 [32]. Metrics such as citation counts, co-authorship patterns, and keyword co-occurrence were examined to highlight influential authors, key research themes, and emerging trends [33]. This approach revealed the main research themes, the interconnection between keywords such as decentralization, hybrid, and microgrid, showing how these terms relate in the literature on energy and water microgrids. In the co-occurrence network constructed, nodes represent keywords and edges represent their co-occurrence frequency. The node size reflects keyword frequency, while colors indicate clusters of related terms. The network provides an overview of the microgrid research landscape, with “microgrids” positioned as the central theme. Larger nodes indicate more frequent keywords, and thicker edges represent stronger co-occurrence. Nodes with many strong connections signal keywords closely linked to multiple research areas, while clusters of interconnected nodes highlight specialized topics of focus within the field (see Figure 1 and

Table 1. Analysis of keyword occurrences in microgrids research.

Thematic Layers	Keywords	Description
Central Theme	Microgrids	The largest node in the network is “microgrids”, highlighting its centrality and significance in the research landscape.
Cluster 1	Energy Management	Keywords such as “energy management”, “control”, and “optimization” form a prominent cluster, indicating a strong research focus on managing and optimizing energy within microgrids.
Cluster 2	Renewable Energy Integration	This cluster includes keywords like “renewable energy”, “generation”, and “storage”, reflecting the integration of renewable energy sources as a critical aspect of microgrid systems.
Cluster 3	Performance and Reliability	Keywords such as “performance”, “reliability”, and “resilience” are grouped together, signifying research efforts aimed at enhancing the performance and reliability of microgrids.
Cluster 4	Demand Response and Economics	Terms like “demand response”, “economic analysis”, and “market” form a cluster, suggesting an emphasis on the economic aspects and demand-side management in microgrids.
Cluster 5	Technological Developments	This cluster includes keywords related to technological advancements such as “smart grid”, “IoT”, and “automation”.

).

As observed in Figure 1, the visualization illustrates active research in the energy sector related to microgrids, as indicated by the dense network of keywords clustered around “microgrid”, “optimization”, “renewable energy”, “management”, and “power”, shown as large, central nodes. These terms appear in larger font sizes and more central positions, reflecting their high frequency and strong co-occurrence in the literature. These clusters highlight dominant research directions, including energy management and renewable energy integration, underscoring ongoing efforts to enhance the system efficiency and sustainability of microgrids. Additionally, the prominence of keywords related to performance, reliability, and economic analysis suggests a multidimensional approach to improving microgrid systems.

The interconnection analysis also indicates numerous connections among components of energy microgrids. However, significantly fewer connections are observed for direct water microgrids, with only four publications. Nonetheless, research on leveraging energy microgrids for smart WDSs, particularly focusing on renewable energy to enhance operational efficiency, is emerging. This approach, applying energy microgrid principles to improve smart WDSs through renewable energy, is increasingly recognized as a promising emerging technology.

3. The Microgrid Approach

3.1. Concept

The concept of microgrids has been extensively studied and applied in the energy sector as a response to the limitations of large, centralized supply models. It is defined as a localized networks that integrate distributed resources and can operate either in connection with the central grid or in isolation [34,35,36,37]. The U.S. Department of Energy (DOE) [38] defines the microgrid as “a group of interconnected loads and distributed energy resources within clearly defined electrical boundaries that acts as a single controllable entity with respect to the grid, capable of operating in both grid-connected and islanded modes” [39,40,41]. While the definitions of a microgrid vary slightly depending on the target systems in the previous studies, overall, they are similar to the definition provided by the DOE. In this context, the microgrid approach can be characterized by the features of (1) distributed/decentralized resources, including existing central and local sources; (2) clear service boundaries in the supply of local resources; (3) a local controller to control the supply of distributed local resources; (4) convertible operation between grid-connected and island modes; and (5) a master controller for adaptive and flexible control in interactive operations between existing (centralized) and local systems. With advances in Information and Communication Technologies (ICTs), reliable sensing/metering, monitoring, and control capabilities are also considered as emerging features [9,22].

Building on this foundation, previous studies [21,22,23] have defined water microgrids by drawing analogies from the energy/power sector. These definitions highlight clear differences from existing centralized or decentralized water systems—which generally operate independently of legacy water supply infrastructure. However, those earlier definitions of water microgrids considered limited features of the microgrid approach, focusing mainly on localized optimization rather than capturing broader system-level interactions. For example, Ref. [42] describes a microgrid as “a platform that integrates supply-side resources like micro-generators with demand-side resources such as storage units and controllable loads within a local distribution grid.” Refs. [43,44,45] views a microgrid more broadly as subsystems that combine generation and associated loads. While these perspectives are useful starting points, they fall short of reflecting the unique physical, hydraulic, and quality-related challenges that distinguish water microgrids from their energy counterparts.

In this regard, the definition of water microgrids can be improved as “a network (grid) of interconnected networks (microgrids) that produce and supply both potable and non-potable water to corresponding end-users from central (existing) and local water sources. They optimally manage operational tradeoffs between central (existing) and local WSSs to sustainably meet water demands and minimize water service disruptions, with local WSSs capable of operating either interdependently or independently”. That means a (single) water microgrid is a localized water network responsible for the supply, treatment, and distribution of water. Its main objective is to fulfill essential water needs during interruptions to the main supply or to complement the existing centralized system. It is also designed to function independently from the centralized water system and is equipped with advanced sensory technology for necessary monitoring and control to manage the system effectively.

Among the technologies and system-level thinking perspectives, studying the interactions between (existing) centralized and decentralized (local) systems is highly important but complex. The impacts of such interactions on the quantity and quality of local water are not well understood [46,47,48,49]. Uncertainties associated with available data, climate, and socioeconomic changes further complicate the understanding of these interactions [50]. Water microgrids address these interactions by strategically considering the trade-offs between centralized and local systems, optimizing both in various scenarios to reduce energy consumption, improve hydraulic efficiency, simplify operation and maintenance, and ultimately enhance resilience. For example, in a pipe burst event in a water distribution system (WDS), conventional decentralized systems can isolate the disrupted District Metered Area (DMA) and maintain water supply within that DMA using local sources, without affecting the rest of the centralized system. This approach can also minimize energy consumption and operational challenges by adjusting the operations of the existing and local water systems—e.g., modifying the water supply portions of local water sources, depending on the severity of system disruption, the availability of local water, and the coordination of existing centralized and local systems. However, such systems also affect the operational performance of downstream infrastructure and existing treatment processes due to reduced flows and increased concentrations of effluents [51]. For instance, lower water consumption may lead to higher pollutant concentrations in sewage effluent, increasing sedimentation in the sewerage system and impacting the efficiency of wastewater treatment plants. However, water microgrids can mitigate these challenges by intelligently coordinating water supply and reuse in ways that balance localized conservation with maintaining adequate system performance. This includes managing flows to reduce excessive stagnation, optimizing reuse strategies, and supporting sewer functionality without creating additional treatment or financial burdens.

3.2. Design Criteria

For any system designed to serve people, it must consider relevance, functionality, efficiency, effectiveness, resilience, and sustainability, with design criteria encompassing technical, environmental, economic, and operational aspects [52]. In this study, the design criteria were defined using the Axiomatic Design (AD) principles [53]. The AD has been widely adopted method in the comprehensive processes of mechanical design, software design, manufacturing, and evaluation indicator framework in engineering and non-engineering fields [54,55]. The AD is a systematic design approach with logical and reasonable thinking to transform customer needs into functional requirements and then into design parameters, as shown in Figure 2, while ensuring functional independence (Independence Axiom) and minimizing information complexity (Information Axioms) [56]. The transformation process involves a zigzagging mapping between the customer, functional, physical, and process domains, defining the design requirements (or needs) in the former domain and refining the corresponding design elements (or solutions) in the latter domain [57]. Thus, the AD process determines design criteria for a product or system based on the customer’s needs.

In this context, the application of AD based on water supply sustainability and system resilience offers a set of design criteria for a sustainable and resilient water microgrid system. The criteria can be used to evaluate the configuration and operation of water microgrids. First, customer needs were identified as the features of a sustainable and resilient system, based on the definitions of sustainability and resilience—minimizing social, economic, and environmental impacts for sustainability features and minimizing system losses and recovery time for resilience features. These needs were then translated into independent functional requirements (FRs) that outline the system capabilities necessary to achieve sustainability and resilience features. Subsequently, the design parameters (DPs) were defined as the design criteria to address the FRs, as they directly represent the physical and functional attributes of sustainable and resilient water microgrids. Process variables (PVs) were not considered in this study, as they pertain to specific implementation methods based on system purposes, rather than intrinsic design attributes, which can be inappropriate for evaluating the configuration and operation of sustainable and resilient water microgrids.

Table 2. FRs and DPs for a sustainable and resilient water microgrid system.

Functional Requirements (FRs)	Design Parameters (DPs)
FR1 Meet water demands in quantity	DP1 Central and local water resources availability
FR2 Meet drinking water quality	DP2 Safe water treatment and distribution
FR3 Meet adequate water pressure	DP3 Water pressure control
FR4 Minimize total capital, operation, and maintenance costs	DP4 Life cycle costs (system investment, energy use, labor, and rehabilitation)
FR5 Conserve water resources	DP5 Water use efficiency with water reuse and demand management
FR6 Save energy use in system operation	DP6 Energy use efficiency
FR7 Minimize greenhouse gas emissions	DP7 Carbon-free (renewable) energy resources
FR8 Minimize losses in water resources availability during disruptions	DP8 Multiple diversified water resources
FR9 Minimize the spread of disruptions to the entire system	DP9 Dynamic operation between island mode and grid-connected mode
FR10 Rapidly detect and identify disruptions	DP10 Monitoring system (SCADA)
FR11 Mobilize recovery resources quickly	DP11 Physical, financial, and community resources for system recovery
FR12 Secure information capacity	DP12 Information system with a user-friendly interactive database
FR13 Smartly and remotely control in real time	DP13 Real-time, optimal, and predictive control
FR14 Build cybersecurity	DP14 Cybersecurity framework with software and education

summarizes the FRs and DPs for sustainable and resilient water microgrids, derived from the sustainability and resilience features.

The AD employs two principles—i.e., the Independence Axiom (maintaining the independence of functional requirement) and the Information Axiom (minimizing information content of the design)—to find the best design of a product or system [56]. According to the Independence Axiom, each FR should remain independent of other FRs and be satisfied by a single corresponding DP. Whether the design found AD process satisfies the Independence Axiom can be evaluated using a design matrix, as shown in

Table 3. The design matrix that relates FRs and DPs for water microgrids.

	DP1	DP2	DP3	DP4	DP5	DP6	DP7	DP8	DP9	DP10	DP11	DP12	DP13	DP14
FR1	×	0	0	0	0	0	0	×	0	0	0	0	0	0
FR2	0	×	0	0	0	0	0	0	0	0	0	0	0	0
FR3	0	0	×	0	0	0	0	0	0	0	0	0	0	0
FR4	0	0	0	×	0	0	0	0	0	0	0	0	0	0
FR5	0	0	0	0	×	0	0	0	0	0	0	0	0	0
FR6	0	0	0	0	0	×	0	0	0	0	0	0	0	0
FR7	0	0	0	0	0	0	×	0	0	0	0	0	0	0
FR8	0	0	0	0	0	0	0	×	0	0	0	0	0	0
FR9	0	0	0	0	0	0	0	0	×	0	0	0	0	0
FR10	0	0	0	0	0	0	0	0	0	×	0	0	0	0
FR11	0	0	0	0	0	0	0	0	0	0	×	×	0	0
FR12	0	0	0	0	0	0	0	0	0	0	0	×	0	0
FR13	0	0	0	0	0	0	0	0	0	0	0	0	×	0
FR14	0	0	0	0	0	0	0	0	0	0	0	0	0	×

The symbol ‘×’ indicates that the FR relates with only its corresponding DP, while ‘0’ means they have no relationships and are independent.

. The design matrix maps the relationships between FRs and DPs. A diagonal matrix represents an uncoupled design, and an upper or lower triangular matrix represents a decoupled design. Both designs satisfy the Independence Axiom [57]. The design matrix in Table 3 demonstrates a decoupled design, which indicates that the design criteria satisfy the Independence Axiom. While these can be further decomposed into sub-FRs and corresponding DPs to explore more specified design criteria for a water microgrid system, addressing the detailed design criteria is beyond the scope of this study. Thus, this study accepted the DPs as design criteria to systematically identify challenges and engineering insights in transforming current WSSs into water microgrids, based on the concept of water microgrids and literature review, and discuss the challenges, insights, and opportunities for building sustainable and resilient water microgrids.

Meanwhile, the Information Axiom is typically applied to compare and find the most efficient design among multiple sets of design criteria that satisfy the Independence Axiom. Thus, the Information Axiom was not applied to a single set of design criteria in Table 3.

3.3. Architecture of Water Microgrids

Considering the similarity between water and energy supply systems, the architecture and main components of water microgrids can be envisioned by mimicking the structure and operational principles of energy microgrids [9,21,22,58]. A water microgrid system is a complex system (system of systems) that integrates decentralized local WSS into existing centralized WSS through operational interaction [22,23]. Figure 3 depicts the conceptual representation of the water microgrid system. The key components include water sources, water treatment units, water storage, water distribution networks, a monitoring system (SCADA), and hierarchical system control.

3.3.1. Water Sources

A microgrid is a hybrid or decentralized system that integrates diverse and distributed resources to provide services. Here, local resources can supplement the centralized system in grid-connected mode or take on a larger portion of the services when the centralized system is disrupted. In this context, energy microgrids incorporate renewable resources such as solar, wind, biomass, hydropower, and fuel cells, as well as traditional resources such as oil, natural gas, and nuclear power. This diversification enhances energy self-sufficiency, reducing dependence on a single, centralized source, which is often located far from end-users. It can also increase system efficiency in power supply services by reducing distribution and transmission losses and environmental impacts (e.g., greenhouse gas emissions) during disruptions [59,60].

Similarly, water microgrids adopt the microgrid approach by incorporating diverse, distributed local water sources into existing centralized water infrastructure to meet potable and non-potable water demands. These include conventional natural sources such as rivers, lakes, creeks, springs, and groundwater, as well as alternative water resources derived from human intervention, including harvested rainwater, stormwater, reclaimed wastewater, greywater, and desalinated water [61]. Local water sources are located within a microgrid, close to end-users. For example, the only experimental research to date on lab-scale water microgrids used four local water sources to supply water at varying degrees of decentralization in different disruptive events; the results proved increased resilience compared to existing centralized and decentralized hybrid systems [23]. Such diversification enhances water security by increasing available supply, supporting recycling and reuse, reducing reliance on a single system, and improving resilience to unexpected and uncertain system failures [23,62,63]. Local water sources include partly controllable or relatively uncontrollable sources, such as rainwater harvesting, which are intermittent in production due to the dependence on external conditions (e.g., weather conditions) [64]. Diversifying water sources in a water microgrid will help prevent all water sources from being disrupted by a single disturbance [62]. However, unlike energy systems, water sources differ not only in availability but also in safety and treatment requirements, making integration more complex and often costlier. Recognizing these constraints is essential when designing water microgrids to ensure that resilience benefits are not offset by operational and economic challenges.

3.3.2. Water Treatment Units

A microgrid system produces transported medium from distributed local resources as well as existing centralized resources. Thus, the system employs modular or distributed units to produce its transported medium (e.g., water and electricity) that meet specific users’ requirements. These production units correspond to power converters (generation) and water treatment units in energy and water microgrids, respectively. Power generated by distributed renewable energy sources is converted to match the voltage, current, and frequency requirements of end-user loads [65]. This conversion is necessary because multiple local energy sources and generators often produce electricity that does not a form meeting the specific users’ needs [65,66].

Similarly, locally sourced water is treated through water treatment units to meet specific quality standards for potable and non-potable water uses to meet the users’ requirements. The water treatment units in a water microgrid can vary in scale from large water and wastewater treatment plants (that are commonly operated in existing centralized WSSs) to smaller, onsite treatment or harvesting systems designed for alternative water production for potable and non-potable options. Various treatment options based on physical, chemical, and biological processes (e.g., membrane filtration, reverse osmosis, activated carbon-mediated adsorption, advanced oxidation, and UV disinfection) can be used in local treatment units [67]. Operational costs for local treatment units can be less expensive because not all local facilities require high-cost, advanced treatment technologies to produce water with various quality standards based on the end-users’ needs [38]. Local treatment units are typically smaller in scale compared to the wastewater treatment plants in municipalities—which address larger volumes of wastewater—and are located closer to end-users, offering benefits such as reducing costs and energy use in water treatment and transport, minimizing water losses in transmission and distribution, and improving water quality control [48].

3.3.3. Water Storage

As the microgrid system produces transported medium (e.g., energy or water) from various sources, the quantity, quality, and timing of production can vary depending on the source types. This also leads to gaps between the production and service demand timings. In this regard, storage units serve as a buffer system to balance the variations in quantity, quality, and timing of production and demand, and ensure a continuous and reliable service supply.

In energy microgrids, energy storage such as batteries and pumped-storage hydropower is a buffer to manage intermittent energy production from renewable energy sources and store excess energy to supply it during low generation of energy generation or high demand periods [68,69]. In water microgrids, water storage such as water tanks (elevated, ground, and underground) and reservoirs store potable and non-potable water produced from centralized and local water sources and treatment units. Analogous to energy storage, water storages store intermittently or continuously produced water to secure water availability during normal conditions, high demands, or emergency conditions—e.g., when the amount of available water or water pressure is insufficient to satisfy requirements [62]. Local storage units for alternative local water sources are relatively smaller and distributed in a water microgrid, compared to large-sized storage tanks in centralized WSSs.

The design of local water storage, such as its type, size, and location, can impact water supply performance, capital/operating costs, and water quality. In a water microgrid, water can be supplied via pumps or gravity, and gravity-driven systems typically offer lower energy consumption and operational costs compared to pump-driven systems. However, separate storage tanks for individual and centralized systems due to differences in water quality is needed, and the storage systems should be designed to provide sufficient energy to deliver water to target users considering varying degree of decentralization and available local water source, which may lead to increased capital costs due to the need for elevated structures or limited storage placement and energy consumption to pump water to the storage. Properly sized storage helps balance supply during peak and non-peak demand periods, while oversized tanks can lead to stagnation, loss of disinfectant residuals, and water quality problems. The strategic locations of local storage can help minimize excessive pressure, water leaks, pipe lengths, and energy losses during water delivery [70].

3.3.4. Water Distribution Networks (WDNs)

Energy microgrids have power lines to supply power from traditional and renewable energy sources and storage units to end-users [45]. As local energy sources and storage are located close to end-users, the shorter distances in power distribution can contribute to reducing transmission and distribution losses and, in turn, increasing energy use efficiency [9]. In a water microgrid, distribution networks transport potable and non-potable water from centralized and local sources or storage units to residential, commercial, and industrial users. Here, a dual or multiple interconnected network of piping systems, valves, and pumps is needed that provides adequate pressure and prevents cross-contamination [71].

Local WDNs within water microgrids typically operate alongside centralized infrastructure. The potable water would be prioritized for drinking and cooking through a centralized or treated local supply, while non-potable water (e.g., reclaimed wastewater, rainwater, or stormwater) would be distributed via separate pipelines or storage for uses such as irrigation, toilet flushing, or industrial processes. Their integration and interactive operations can improve redundancy and connectivity in water distribution, which are the features of a resilient system. However, it can also increase the complexity of WDN operation and management with potential risk of cross-connection between potable and non-potable water pipes [62,63]. Thus, the design of the piping system and materials should comply with local piping standards and guidelines. Water flow in WDNs is driven either by gravity from a higher elevation source to lower elevation areas or by pumps that pressurize the supply from local sources or storage tanks. These pumps must be supported by reliable power sources to ensure their operation under both normal and abnormal operating conditions [72]. Also, similar to the static switches or circuit breakers in energy microgrids, isolation and control valves in water microgrids should function in both the grid-connected and/or islanded mode, depending on the disrupted conditions of the system.

3.3.5. Monitoring System

Because the distribution system in the microgrid approach is a network (grid) of interconnected networks (microgrids), it is a “system of systems” that is inherently complex, for both the energy and water domains. The integration of multiple microgrids suggests a hybrid system with complex operational interactions between centralized (existing) and local systems to strengthen system resilience to unexpected and uncertain disruptions and sustain service supply to meet varying demands with higher system efficiency [16,63,73,74]. However, compared to traditional centralized systems (e.g., centralized water and power distribution systems), the microgrid system has higher complexity, making it more vulnerable to disruptions due to more difficulty in detecting, identifying, and localizing disrupted components and the potential for rapid cascading impacts from disrupted components to the entire system [75]. Thus, a reliable monitoring system is a critical component of a microgrid system.

In this context, energy microgrids have employed a cyber-physical system approach with sensors, smart meters, communication technologies, SCADA, and data analytics [76,77]. This enables the collection of real-time data on system performance (e.g., energy losses in transmission, energy use intensities) in energy production, storage, distribution, and load, the monitoring of system operations with anomaly detection, and the effective and timely implementation of system control [16,77,78].

Similarly, water microgrids can adopt a higher level of monitoring system (consisting of sensors/meters, communication technologies, SCADA, and data analytics) to enable real-time tracking and monitoring of water supply performance in water sources, treatment, storage, distribution, water/energy usage, and water quality parameters. For example, the monitoring system can monitor the water availability of each water source, the quality of treated water for users’ needs, water quality issues due to a high water age, water pressure, and flow, water losses due to pipe leaks/bursts, and contaminant intrusion in WDNs. This system can be called cyber-physical water microgrids or smart water microgrids. By coupling with AI-based analytics and hydraulic/hydrologic models, water microgrids can achieve predictive model control depending on operational conditions and adaptive management through dynamic reconfiguration of operational rules to sustain adequate water supply service in disruptions.

3.3.6. Control System

The microgrid system operates in two modes: grid-connected and islanded [41,79]. The operation modes switch between each other based on operational conditions. The system control for these two modes is not straightforward and should be designed with multiple different strategies for the two modes to various operational conditions [80]. A microgrid system can employ three control options: centralized, decentralized, and distributed structure [80,81]. The centralized control system has a central master controller. It determines all control actions based on the information received from meters and sensors on system components. The decentralized control system has local controllers managing each corresponding microgrid. It has no communication or operational connections between the microgrids. The distributed control system has a similar configuration to the decentralized control system; however, the local controllers are interconnected for information exchange to optimize the overall operation of microgrids. However, due to the complexity of microgrid operation, the entire microgrid system cannot be controlled through only centralized or decentralized control options [80]. Moreover, each control system has its strengths and weaknesses depending on spatial and temporal scales. For example, in energy microgrids, the centralized control system is more vulnerable to single-point failure (e.g., disruption of the central master controller) and requires extensive communication between the master controller and system components, while the decentralized/distributed control system needs to manage complex coordination between local controllers [80].

Considering the complexity in microgrid control, energy microgrids have widely accepted a hierarchical control system, which integrates centralized and decentralized control systems to utilize their strengths [80,81,82,83]. The typical structure of a hierarchical control system has three layers: primary control layer (decentralized control), secondary control layer (hybrid centralized and decentralized control), and tertiary control (centralized control) [80]. The primary control layer controls voltage and frequency locally in a single microgrid, ensuring rapid response to disturbances, and does not consider communications with other local controllers. The secondary control layer regulates and coordinates local controllers to restore voltage and frequency and maintain the energy supply balance with economic and reliable operation under normal and abnormal conditions. This control layer addresses the operational interactions between microgrids. The tertiary control system optimizes bidirectional power flow between microgrids and the main grid to ensure economic efficiency and activate energy trade and markets. This control layer focuses more on microgrid-main grid coordination.

Similarly, water microgrids can adopt the hierarchical control approach to implement diverse operational strategies for enhancing system resilience and ensuring sustainable water supply. Key control variables in water microgrids include water pressure, flow, and storage, which are managed to maintain water supply–demand balance under normal and disrupted conditions. At the primary control level, these control variables can be managed locally by local controllers to adjust local WSS operations (e.g., pump operation for water pressure) in each microgrid. At the secondary control level, local controllers are coordinated by a central master controller to optimize the operations of multiple water microgrids. This coordination aims to enhance water use efficiency and sustainable water supply services by addressing imbalances between intermittent water production and water demand. The tertiary control layer provides a higher level of control for global optimization objectives such as equitable water allocation, economic costs and gains, energy use, environmental impacts, and minimizing impacts on downstream WSS infrastructure. This layer requires extensive operational interaction between microgrids (local WSS) and the existing centralized WSS.

Although energy and water microgrids differ in important aspects such as treatment requirements, water quality standards, dual piping, and flow directionality, they share many similarities at the component, functional, and operational levels. In both domains, the microgrid framework emphasizes decentralization, resilience, and sustainability by strategically integrating local resources, advanced control technologies, and adaptive operational strategies to address contemporary resource management challenges [59,84,85]. At the same time, water microgrids are clearly distinct from other water supply options, as they represent a system-level approach that differs from purely centralized, decentralized, or hybrid arrangements. Evidence from a lab-scale water microgrid experiment [23] further demonstrated the feasibility of this approach, showing that the parallels with energy microgrids strengthen the potential for adopting the microgrid concept within existing WSSs. Accordingly,

Table 4. Comparative analysis of design parameters in centralized, decentralized, hybrid, and water microgrid systems.

Sl.	Design Parameters of WSSs	Centralized WSS	Decentralized WSS	Hybrid WSS	Water Microgrid (WM)
1.	Availability of central water resources	Available; Primary source of water [3,86].	Available; Used alongside local sources [26].	Used when local sources are insufficient [63].	It incorporates centralized water sources while enhancing control and resilience at the local level, offering a flexible response to supply disruptions.
2.	Utilization of local water sources	Limited use; Focus on central sources.	Extensively used; Primary sources for the system [87].	Complementary to central resources.	WM maximizes the utilization of local water sources and strategically combines them to optimize resource use and enhance system adaptability.
3.	Diversification of water resources	Low; Relies mostly on singular large sources [3].	High; Uses multiple small-scale sources [88].	Moderate; Combines features of both systems [4,63].	Connects a diverse array of water sources to improve system entropy and resilience, making it robust against environmental and demand shifts [9,22].
4.	Advanced water treatment facilities	Standardized large-scale facilities.	Smaller, localized treatment systems [89].	A combination of both approaches.	WM integrates scalable water treatment facilities, considering single or multiple grids that are adapted to local conditions, enhancing water quality management across diverse sources.
5.	Robust storage facilities	Large, centralized storage.	Smaller, decentralized storage solutions.	Both centralized and decentralized storage.	It considers both short-term and long-term storage strategies that are customized to local demand, balancing supply and enhancing system responsiveness. It works as a function bridge between the supply side and demand side [22].
6.	Efficient water pressure control mechanisms	Consistent pressure is maintained centrally [1].	Locally managed, varying pressures [90].	Hybrid systems adjust pressures as needed.	Because of the demand-based supply system, the control mechanism optimizes water pressure dynamically across the network to ensure addressing fluctuating demand while minimizing losses [22].
7.	Water use efficiency with water reuse strategies	Minimal; focus on supply rather than reuse.	Emphasized; critical for sustainability.	Incorporates strategies from both systems.	WM’s core concept is the utilization of local water sources at the maximum level possible and also collecting the utilized water through local water resources such as lakes, ponds, storm drainage, etc., and then returning the water to supply again in the localized microgrid distribution system [22].
8.	Life cycle costs consideration	Focus on long-term operational efficiency.	Prioritizes upfront cost savings.	Balances both perspectives.	Although WM requires more initial investment than traditional systems, its long-term savings and functional economic benefits are expected to surpass overall economic returns [91].
9.	Fluctuating demand management	Generally inflexible to local rapid changes [92].	Highly adaptable to local demand fluctuations [26].	Moderately adaptable.	Unlike centralized systems, WM addresses fluctuating demands by utilizing local water sources through a demand tradeoff mechanism.
10.	Demand tradeoff/Interaction between centralized and decentralized systems	N/A	N/A	N/A	This aspect is distinctly characteristic of water microgrid systems and is notably absent in hybrid systems. Water microgrids are designed to facilitate continuous interaction between centralized and decentralized local supply systems, adapting to various operational scenarios such as 90% centralized supply and 10% local supply, or other ratios like 80–20, 70–30, or 60–40. This dynamic balance enhances the economic efficiency, environmental sustainability, and functional flexibility of water microgrids.
11.	Integration of green energy	Limited integration.	Focus on sustainable energy sources.	Combines both approaches.	WM integrates green energy into its operations, mirroring sustainable practices found in other WDSs. However, the usage of an energy microgrid with decentralized or hybrid WSS has been found to be more efficient in previous studies.
12.	Dynamic operation between the island mode and the grid-connected mode	N/A	N/A	N/A	The decentralization system used in WM is functionally different from a traditional decentralized system. In a traditional decentralized system, the distribution system is divided into a fixed number of DMAs, but here one microgrid can function as a micro level of the grids, which means one DMA can function as a smaller unit of the DMAs, based on the demand and scenario.
13.	Monitoring system (SCADA)	Advanced central monitoring.	Localized monitoring solutions.	Integrated monitoring approaches [93].	Different monitoring systems have been used in different types of WDSs. However, because a water microgrid is more of a system-level operation model, a more advanced level monitoring system needs to be developed, which will integrate the functional management of demand and supply side operation, both for centralized and decentralized operation.
14.	Physical, financial, and community resources for system recovery	Centralized recovery strategies [92].	Community-driven recovery efforts [47].	A combination of centralized and community resources.	Because the WM includes local water resources that belong to community-level resources, building a new WM or transforming an existing system into a WM requires community-level acceptance. System recovery depends on the willingness and participation of the community; therefore, collaboration with the community is essential before the project begins [85,94].
15.	Considerations for system expansion or modification	Structured for gradual expansion.	Designed for flexibility and scalability [89].	Adaptable to changing needs [91,94].	WM is by definition open to system expansion or modification due to the necessary changes in local water resources’ availability and quality, and with the demand and situation-based changes.
16.	An information system with a user-friendly interactive database	Centralized data management [1].	Local data handling and access.	Hybrid data management approaches [85].	The advanced SCADA-based control station should continuously save and analyze user data and their usage patterns, along with supply and demand responses. Additionally, real-time optimization of the system should always be in place based on this analysis.
17.	Cybersecurity framework with software and education	High priority; extensive protections in place.	Varied; depends on local capabilities [5].	Integrated security measures [94,95].	WM is equipped with a robust cybersecurity framework in the control center that protects against threats, ensuring the safety and reliability of the water supply and management systems.

summarizes the position of water microgrids in relation to traditional centralized systems and the more recently emerging decentralized and hybrid configurations.

3.4. Sustainability Effects of Water Microgrids

Resilience and sustainability are interconnected concepts that enhance a system’s adaptability and endurance in the short- and long-term periods [74]. Resilience refers to the capacity to quickly recover from disruptions, which addresses short-term problems [62]. Sustainability focuses on long-term viability, meeting current needs without compromising future generations [96]. Previous studies have presented the sustainability benefits of the microgrid concept across energy, information, financial, and social fields, providing the essential insights for a systematic understanding of the benefits [7,48,97]. This section will discuss how microgrid configuration and operations can improve water supply sustainability in terms of techno-economic, environmental, economic, and social aspects.

3.4.1. Techno-Economic Aspect

While the economic aspect of sustainability is purely financial, for example, job creation, investment growth, and economic development, the techno-economic aspect emphasizes technological efficiency, optimization, and cost savings associated with water microgrids—such as reducing transmission and distribution costs and improving demand response flexibility. Regarding the sustainability aspect of energy microgrids, the transmission and distribution costs can be reduced or avoided by utilizing distributed resources closer to load centers within individual microgrids, which displace or defer large, centralized generation and bulk transmission/distribution systems [97]. Similarly, the strategic siting of local water sources or tanks and optimized local operation can reduce Total Cost of Ownership (TCO) in the long term [9]. Also, the microgrids’ “option value” allows their component infrastructure to adapt modularly to changes in loads, lead times, and/or renewables targets; this means the adjusted sectorization according to demand and hydraulic parameters results in lowering one-time investment costs [98]. Here, the option value is based on the concept of “real options”—an approach to valuing alternative investments that considers the options to continue, adapt, or abandon investment in the future based on forthcoming information [97].

While water microgrids enhance flexibility in demand response by reducing inefficiencies in distribution or water wastage and meet future service needs in line with demand growth, the requirement for dual piping systems, localized treatment, along with sensors and monitoring systems, further increases capital and operational costs [23]. The value of this flexibility, therefore, depends on the strategic use of local water sources and the degree of redundancy available at the system level.

3.4.2. Environmental Aspect

The environmental sustainability of the water microgrids lies in their ability to optimize local water resource management, reduce energy consumption, and lower greenhouse gas (GHG) emissions [21]. While local water treatment and supply may require additional energy, water microgrids offset this through reduced long-distance pumping, integration of renewable energy sources, and efficient demand management. Unlike centralized systems that rely on extensive transmission networks, water microgrids utilize localized water sources, minimizing energy-intensive transportation and associated losses. For example, power microgrids can use storage and demand-side resources to mitigate the risks of frequency and voltage fluctuations associated with intermittent renewables, at levels of locational granularity that centralized generation and transmission and distribution (T&D) systems cannot easily accommodate [97]. Similarly, water microgrids use diverse local water sources and aim to meet demand based on the average consumption within a specific microgrid, which helps mitigate overload on the central water supply system and reduces the risk of water service interruptions or distribution system failures. Previous studies have shown that energy microgrids reduce environmental damage, with regulatory authorities in the US and Europe measuring impacts on CO₂, SO_x, NO_x, and particulates [74,97]. Similarly, reduced energy consumption for operating water microgrids and utilization of green energy at both the centralized and local levels can contribute to these environmental parameters.

A key environmental impact of water microgrids is water recycling and reuse, which helps reduce freshwater extraction and wastewater discharge. This can enhance water use efficiency within the limited, available water resources under changing climate and socioeconomic conditions [22]. For example, in regions facing water scarcity due to drought or contamination, water recycling and reuse can significantly reduce dependence on the existing, centralized water system and reduce the need for energy-intensive water extraction and treatment processes (e.g., large-scale purification and pumping) [9,99]. This approach can mitigate environmental impacts from the overexploitation of existing water resources and contribute to the long-term sustainability of water resources under current and future water stress.

3.4.3. Economic Aspect

Several countries and local authorities exploring the development of microgrids are examining their economic impacts, particularly regarding the growth of primary and secondary employment opportunities and regional products [100]. According to an economic analysis on energy microgrid in the U.S by Guidehouse, for every USD 1 million invested here, 3.4 skilled jobs are created in addition to USD 500,000 in additional economic benefits across all U.S. states, which will also create nearly 500,000 new jobs over the next decade. Even in 2021, 4670 MW of renewable capacity resulted in 17,290 jobs and contributed USD 2.8 billion in GDP and USD 5.6 billion in business sales [101]. Direct investments in energy microgrid development often include energy efficiency improvements, grid upgrades, and the expansion of microgrid technologies into research and development of other interconnected “smart” technologies [97].

Water microgrids have the potential to contribute to regional economic development by serving as benchmarking models for sustainable urban water systems. Their implementation can enhance the reputation of cities committed to environmental stewardship, while also reducing costs associated with water abstraction, treatment, and long-distance transportation in centralized WSSs. Additionally, the planning, construction, and maintenance of decentralized infrastructure within water microgrids can also stimulate local job creation and support green economic growth [63,97]. Although transforming an existing WSS into a water microgrid-based system through direct integration of local water systems with centralized networks could be technically complex, economically unjustified, given the aging infrastructure and upcoming renovations, global resource crises, and the need to build resilience and sustainability, a robust and future-ready framework that strategically integrates local water sources and mainstreams them into system-level operations is needed [9,100,102]. For example, the 2014 chemical spill in West Virginia’s Elk River—which affected drinking water for 300,000 people and caused a USD 61 million economic loss—could be tackled by enabling the islanding and demand tradeoff mechanism of water microgrids while maintaining a continuous water supply [9]. However, while existing research has well-established the economic gains of energy microgrids—also forecasting that national renewable asset microgrid capacity will grow 3.5 times over the next 10 years to reach 32,470 MW by 2030, creating over 496,700 jobs, contributing USD 72.3 billion to GDP, and generating USD 146 billion in business sales—no comparable study has been conducted yet on the economic benefits of water microgrids [101].

3.4.4. Social Aspect

Social gains can be expressed as improved access to generally accepted reliability and quality levels relative to current levels, and the value of such improvements. Several methods, for example, WUSI, DPSIR, MuSIASEM, have quantitatively proved that decentralized WSSs achieve higher sustainability than centralized systems [103,104,105]. Another relevant sustainability evaluation in Bandung, Indonesia, compared communal networks (CNs)—small-scale, community-managed WDSs—with public wells (PWs), which are publicly accessible shared wells without formal management structures. CNs exhibited higher sustainability due to structured community involvement, formal management, and consistent financial frameworks, such as regular tariffs [106]. CNs benefit from active community participation and organized oversight, ensuring their operational and financial sustainability. In contrast, PWs rely on informal practices like voluntary contributions and ad hoc conflict resolution, limiting long-term viability. Water microgrids build on decentralized, community-managed WSSs like CNs but require even stronger community ownership, since operating in both grid-connected and islanded modes during disruptions demands active local participation, which enhances social sustainability.

Water microgrid, through its participatory model of local operation, encourages community involvement in both planning and operational phases, fostering social cohesion and enhancing the social fabric of communities [107]. This engagement promotes local employment and skills development, as community members are often involved in the implementation and maintenance of microgrid systems, leading to job creation and improved local economic conditions.

3.5. Resilience Effects of Water Microgrids

The microgrid approach offers a potential infrastructure solution to enhance the resilience of the whole WSS [73]. The concept of resilience emerged from concerns that conventional protection and prevention strategies—based on prediction, average, and risk—may fail catastrophically due to the uncertainty of disturbances [62]. Resilience strategies, therefore, focus on addressing disturbances and their uncertainty (which may exceed predicted levels) by minimizing damage and functional loss from their disruptions, and rapidly recovering to normal conditions [5,62,73]. In this context, many engineering and non-engineering fields have adopted diversification and decentralization as key resilience strategies to address uncertain environments. For example, military forces employ diverse training programs and strategies to adapt to various missions [13,14]. Financial managers emphasize asset diversification for higher returns and lower risks in unpredictable markets [15,16,17]. In ecosystems, species diversity and functional variety are well-recognized as essential for survival [18,19,20]. In modern energy systems, decentralized renewable installations like solar panels and wind turbines spread throughout a region ensure that if one unit fails, others can continue to operate, thereby maintaining service continuity. Similarly, for the urban infrastructure, decentralized water management systems—comprising localized treatment plants and community-scale water recycling—allow cities to sustain water supply even if a centralized system is disrupted.

The water sector has also been extensively studied in terms of diversification and decentralization strategies, highlighting their contributions to enhancing resilience. For example, Ref. [61] introduced the concept of cost-effective diversification in water resources systems using Modern Portfolio Theory and applied it to find optimal sets of diverse water resources to address the water shortage due to droughts. The diversification refers not just to having different types or shapes of options but to ensuring that each option responds differently to the same disruption. For example, a drought may cause water shortages in a dam reservoir, while it will not directly impact the production of reclaimed wastewater. Thus, a WSS supplying water from a dam reservoir and reclaimed wastewater storage has diversified water resources that are not affected by the same disturbance. In this context, diversification strategies such as incorporating diverse water sources, supply routes, and treatment technologies can enhance redundancy and adaptability to changing conditions of disturbances. If one option fails due to a disturbance, alternative options that remain unaffected can sustain system functionality. In addition, diversification allows for a flexible combination of diverse options to dynamically adapt to changing and uncertain disturbances

Decentralization strategies can enhance resilience by reducing dependence on a single component and uniformly distributing functions across multiple units [108]. This strategy aligns with the concept of risk transference, where potential failures are shifted or redistributed from one highly dependent component to several distributed ones, limiting system-wide losses [4,109]. Centralized systems benefit from economies of scale and broad control but are slow and costly in addressing local problems; unresolved failures may even propagate into systematic collapse. In contrast, decentralized and distributed components are closer to the local issues, enabling faster detection, lower repair costs, and localized optimization without disrupting the entire system. The principle that diversification and decentralization strengthen resilience against uncertain disturbances is well established in the field of Cybernetics—the “Law of Requisite Variety: only variety can destroy variety” [110].

In this context, water microgrids represent an infrastructure model that combines both diversification and decentralization to enhance WSS resilience. As described in Section 3.1, they operate by integrating multiple small-scale, locally distributed water sources with centralized systems to maintain service continuity even during disruptions. This structure reduces dependence on a single source and allows redistribution of supply between central and local systems. By incorporating both traditional and alternative sources, water microgrids increase adaptability to disturbances, minimize service losses, and enable rapid detection and recovery. For example, Ref. [23] developed the first lab-scale water microgrid and tested its performance under disruptive scenarios such as pump shutdowns, pipe bursts, cyberattacks, and service interruptions. Using quantitative resilience measures—including robustness, loss rate, and recovery rate—the study found that the water microgrid system consistently outperformed centralized and decentralized WSSs, demonstrating higher resilience across all attributes (above 99%) under uncertain and unpredictable disturbances.

4. From Research to Practice

With some functional dissimilarities, water and energy microgrids are almost similar in their design, operations, and the challenges they face [9]. This section explores the transition from research to practical applications by comparing energy and water microgrids in terms of their design, key components, functional roles, operational strategies, and implementation across various domains.

4.1. Lessons from Microgrid Research

There have been many studies on various components of energy microgrids—such as service provision [59,84,85], storage systems [83,111,112], distribution strategies [42,75], design options [113], efficient operations [114,115], and smart monitoring and control [116,117]. On the other hand, there is still no solid framework available for water microgrids. Instead, the latest research in the WDSs has largely focused on the decentralized and hybrid WDSs, which share some properties with water microgrids and have been implemented at varying scales and forms in recent years. Parallel efforts have also investigated the integration of alternative water sources, resource optimization strategies, and localized treatment and storage, all of which represent building blocks of a potential microgrid framework. That means the concept of water microgrids brings together these individual developments into a more holistic vision for resilient and sustainable water management. To date, only four studies have explicitly examined water microgrids: two reports from the Pacific Northwest National Laboratory (PNNL) and one conference proceeding in 2015, and one laboratory-based experiment, conducted in 2025. That means there remain substantial opportunities for advancing knowledge in this novel approach compared to energy microgrid research.

Still now, for example, no studies have been found on how advanced water storage options—such as elevated, modular, automated, or pillow tanks—could support water microgrids in balancing water supply during demand fluctuations. Similarly, the role of SCADA, PLCs, AMIs, VFDs, and cloud-based systems in monitoring and control has yet to be fully framed. In terms of operational efficiency, research on dynamic operation, including how environmental and demand changes could be controlled using variable speed pumps, pressure-reducing valves, on-demand water storage, smart water meters, GIS, and remote sensing, is needed for water microgrids. Most importantly, water quality, mode of delivery to the network, pressure requirements, and microbiological safety differ significantly from the transmission of electricity. While in energy systems the final product—electricity—is of uniform quality and can be directly injected into the grid, in water microgrids it is necessary to ensure appropriate water quality and eliminate the risk of secondary contamination.

4.2. Application of Water Microgrids

The energy microgrid approach has been successfully implemented in various contexts, including university campuses, industries, and military bases [118,119,120,121]. The U.S. Department of Defense has described some of these efforts as “enormously successful”, particularly in relation to islanded operations, positioning these bases among the most energy-forward defense installations in the country [117,118,119]. Although establishing a full-scale water microgrid may be complex and costly, pilot applications can be developed by building on existing decentralized and diversified practices already present in many water distribution systems. These practices provide a practical foundation for testing and refining the concept of water microgrids. The following sections outline sectors and settings where water microgrid systems could be effectively integrated with existing infrastructure.

4.2.1. Urban Water Systems

Water microgrid systems can be integrated with existing decentralized or hybrid systems of urban areas at small, medium, or large scales, both for basic potable water needs and non-potable uses. For example, in Cedar Valley, Utah, the decentralized system utilizes a combination of local groundwater, surface water from nearby reservoirs, and reclaimed wastewater [122]. The WaterHub at Emory University in Atlanta treats and reuses wastewater onsite, reducing dependence on municipal supply and promoting water resilience [123]. In Singapore, a hybrid decentralized system in the Marina Barrage combines rainwater harvesting, stormwater management, and treated wastewater to form a comprehensive, resilient urban water management system [124]. This system ensures a continuous water supply by dynamically adjusting operations based on real-time water quality and demand data, showcasing a level of sophistication and integration. All of these example benefits are also associated with a water microgrid system by properties, and since these decentralized and hybrid WSSs share key characteristics and meet several configuration requirements, they are well-suited for integration with standard water microgrid frameworks to enhance their current performance.

4.2.2. Rural Water Systems

While urban areas may benefit most from the grid-connected mode of water microgrid systems, rural regions can particularly benefit from the islanded mode. In remote or sparsely populated areas where centralized systems are either unfeasible or economically impractical, the islanded mode can operate independently by integrating local water sources functioning within each smaller microgrid [22,125]. Regions prone to natural disasters—such as coastal areas vulnerable to hurricanes, drought-prone areas, or towns near earthquake faults—can utilize both modes, switching based on forecasts, real-time demand, and risk conditions. In places like Flint, Michigan, where traditional water infrastructure has failed to deliver safe and reliable water [126], water microgrids could offer decentralized, resilient alternatives that are less vulnerable to pollution and systemic breakdowns. Similarly, hurricane-prone states such as Florida, Texas, Georgia, and Louisiana could greatly benefit from the implementation of such systems. In Australia, the decentralized separation (e.g., island mode of water microgrid) approaches have been explored as sustainable solutions for drought-affected communities by integrating rainwater harvesting, groundwater management, and localized treatment units [127].

4.2.3. Industrial Water Systems

Industrial zones also represent a prime application for water microgrids, where high-quality and quality water is critical for manufacturing processes, and fostering sustainability is a responsibility; the reuse of treated wastewater can lead to significant cost savings there. The Frito-Lay water reuse facility in California has implemented closed-loop water recycling systems to minimize freshwater intake and wastewater discharge [128]. Water microgrids can be scaled and adapted to meet this differential demand of various environments—from small residential scales to large industrial areas. Water microgrids are needed in industrial areas where traditional centralized water systems are under stress from overpopulation, infrastructure aging, and increased water demand [22]. In such settings, this system helps manage the supply dynamically, adjusting to daily or seasonal fluctuations in water usage and providing a buffer against disruptions by its grid-connected mode. Even the Brooklyn Energy Microgrid in New York or Borrego Springs, California, allows residents to buy and sell locally generated solar energy after fulfilling their local demand (which could be a successful scenario for water microgrids also) [129]. However, no comparable implementation currently exists for water microgrids yet, highlighting a significant opportunity for innovation in industrial water management.

4.2.4. Agricultural Water Systems

Agriculture, as one of the largest consumers of freshwater globally, stands to benefit significantly from water microgrids. The decentralization here allows farms to collect, store, treat, and redistribute water locally, forming a closed-loop network tailored to site-specific water needs and crop requirements. In water-stressed agricultural regions such as California’s Central Valley, precision irrigation systems are being integrated with local water storage and treatment units, forming a decentralized approach to agricultural water management. Furthermore, integrating renewable energy sources such as solar-powered pumps and treatment units can reduce operational costs and carbon footprints, aligning with broader sustainability goals.

5. Challenges and Ways Forward

5.1. Water Supply Reliability Under Disruptions

Reliability is a measure of the ability of a WSS to meet the customers’ needs in quantitative and qualitative aspects under any conditions at any time [130]. This can be classified as mechanical, hydraulic, and water quality reliability [131]. With the vulnerabilities in valves, pumps, storage tanks, smart meters, and pipes, failure in these components can undermine the three reliability components of a WSS. Thus, it is a crucial design factor for any WSSs. As a water cyber-physical system, the water microgrids integrate existing and local water system components and smart and controllable components, making it essential to prioritize the reliability of these elements to ensure water supply performance.

Typically, a WSS is designed to consider available water sources, treat the water, and deliver it to the customers over its design period. The system capacity and operation are designed based upon the predictions of future climatic and socioeconomic (e.g., population, capital water demand) conditions. Uncertainties introduced by population growth, water resource availability, and changing social conditions need robust and dynamic decisions, such as the use of alternative water resources to accommodate the changes over time. Because the water microgrids supply water from both the existing centralized and local water sources, decisions on their design and operation must account for the inherent uncertainties associated with climate variability, socioeconomic changes, and limited resource availability. As such, both the deterministic and non-deterministic (e.g., analytic, systemic-holistic, and heuristic methods) approaches are essential [132]. Designing microgrids should also consider the reliability aspects over an extended time period to accommodate future changes in water supply and demand [130].

Moreover, water microgrids operate as integrated water cyber-physical systems, characterized by complex operational interactions among diverse water sources—both centralized and local—as well as treatment, storage, and distribution components. This operational complexity can increase the system’s vulnerability to external disruptions, underscoring the need for robust design and operational strategies that enhance resilience and ensure sustainable water services under the disruptions [133]. While the microgrid or decentralization approach offers flexibility to address disruptions—e.g., reliable power supply during the Texas blackout in 2021 [134] and hurricanes Irene and Sandy in New York [135]—water microgrid design remains challenging due to limited investigation into their multifaceted configurations. Effective design must consider both discrete and continuous failures, the combined impacts of concurrent natural and man-made events, and various sources of uncertainty, including climate and socioeconomic changes and operational variability [132,136,137]. The reliability of water microgrids can be further affected by their scale and operations, such as intermittent supply from local sources, dynamic transitioning between grid-connected and islanded modes, and component placement (valves, smart meters, storage tanks), all of which require detailed reliability studies.

5.2. Sustainable Implementation of Water Microgrids

Uncoordinated implementation of water microgrids poses challenges across economic, social, and environmental dimensions, i.e., the triple bottom line approach (Figure 4a) [138]. For example, the implementation of water microgrids can be of two types: constructing a new water system as a water microgrid and transforming an existing centralized or decentralized system into a water microgrid. Both types require substantial investment for infrastructure such as dual piping systems, distributed treatment facilities, and localized storage units [71]. These capital-intensive requirements will hinder their economic feasibility compared to conventional centralized systems. The success of water microgrids, particularly those that rely on local water sources located within urban areas (residential and commercial areas), will also require community engagement in system management and operation. Effective implementation will depend on the communities’ understanding and acceptance of the system and their active participation in the system management and operation, which could be a challenge in the social dimension. Environmentally, there is a lack of comprehensive understanding of the potential impacts of implementing water microgrids, e.g., reduced downstream flow from distributed rainwater harvesting. These challenges highlight a need for holistic evaluation frameworks that capture the full scope of water microgrids’ implications and engineering strategies that satisfy economic, social, and environmental objectives—e.g., minimizing their impacts.

Meanwhile, while the TBL approach also suggests a limited development to minimize economic, social, and environmental impacts, which does not always imply the maximum beneficial impacts from the implementation of water microgrids—”not always being less bad is good” [139]. In this context, adopting a triple top line (TTL) approach will be helpful in simultaneously addressing TBL-based sustainable development goals while creating new economic and social values [138]. Rather than considering the economic, social, and environmental impacts as constraints to be minimized, the TTL approach emphasizes maximizing or creating new (emerging) benefits across the three pillars of sustainability from the implementation of a system (Figure 4b). Thus, this model could emerge as a new water services and business model leveraging both existing centralized and decentralized local water systems. For example, locally water-reuse and recycling systems within microgrids can support “water as a service” or “recycled water sales” models, where excess local water can be sold to neighbors, commercial buildings, or utilities, which creates new revenue [140]. The design, construction, and ongoing maintenance of water microgrids can produce local employment opportunities in engineering the decentralized, local water systems and their connections to the existing water systems. In addition, by integrating energy harvesting (e.g., biogas recovery) from wastewater or capturing stormwater, water microgrids can contribute to resource use efficiency, enhanced energy security, and flood control.

5.3. Water–Energy Nexus in Microgrid Systems

Water microgrids, including localized water collection, treatment, storage, and distribution, inherently consume more energy than centralized systems (that benefit from economies of scale) due to their decentralized and localized nature. They often rely on small-scale water treatment processes for local sources, such as rainwater, reclaimed wastewater, or groundwater, which require additional treatment to meet water quality standards. This is energy-intensive, as it involves processes that centralized systems can manage more efficiently due to economies of scale [92]. Furthermore, water microgrids require multiple pumping stations for transporting water between storage tanks, treatment facilities, and end-users, further increasing energy consumption [22].

This elevated energy demand presents challenges, particularly when energy is fossil fuel-based. Increased reliance on fossil fuels contributes to higher greenhouse gas (GHG) emissions, undermining environmental benefits and escalating operational costs, burdening municipalities and raising consumer water bills [141]. Communities with limited financial resources could face the greatest difficulties, as higher energy costs can render water microgrid solutions less accessible [21]. To address these challenges, integrating renewable energy sources, such as solar, wind, and battery storage, in each microgrid could provide a viable solution [102,141,142]. Hybrid energy configurations, combining renewables with small-scale generators and advanced battery systems, can balance intermittent renewable energy supplies, reduce reliance on fossil fuels, and lower overall emissions [141,143]. This approach not only improves energy security but also enhances system resilience in an environmentally sustainable way [141,143].

Improving energy efficiency is another strategy. A water–energy microgrid framework can optimize operations by aligning water and energy management. For example, minimizing pumping pressure through advanced control strategies, such as Model Predictive Control (MPC), can significantly reduce energy consumption while ensuring effective water delivery [72,143]. Strategically placing local storage tanks at elevated locations allows gravity-driven water distribution, reducing pumping needs [21,102]. While energy challenges are substantial here, they can be mitigated through a combination of sustainable energy sourcing and efficient operational strategies. Integrating renewables, adopting hybrid configurations, and implementing energy-efficient designs can ensure its environmental and economic viability [22,38,141].

5.4. Cyber-Physical Water Microgrid System

The integration of renewable/alternative sources, distribution path, treatment, and storage into the main system with effective communication links gives the potential to enhance the operational efficiency of a water microgrid system. A cyber-physical water microgrid model operates through four independent layers:

• Physical component (pumps, valves, pipes, tanks);
• Sensor and actuator (monitoring pressure, flow, water quality; operating valves and pumps);
• Communication (wired/wireless protocols ensuring data exchange);
• Management and control (operation, monitoring, and decision-making under varying scenarios).

Smart communication technologies can improve these operational components and coordination, but also unintentionally expose the microgrid systems to cyber-physical threats when there is a lack of proper security management. Due to the tight interdependence of cyber and physical systems, water microgrids can be vulnerable to a range of cyber threats that compromise communication reliability, data safety, and operational continuity [144].

The cyber-physical microgrid system gathers, transmits, and processes operational data, which must satisfy three fundamental requirements: availability, integrity, and confidentiality [145]. False data injection attack targeting data integrity has been one of the most challenging smart grid threats [146]. Complex communication networks can provide entry points for attackers, allowing both logical (remote manipulation of data) and physical (tampering with equipment) intrusions [147,148]. Common threats include the following:

• Unauthorized physical access to pumps, valves, or treatment plants;
• Cyber-attacks on digital assets;
• Communication interruption between layers.

Threat agents range from cyber criminals and insiders to state actors and terrorists [149]. Exploiting these vulnerabilities, they can disrupt pressure and flow control, demand management, and energy coordination, often without immediate detection. Consequences may include damaged infrastructure (e.g., water hammer from valve tampering), costly repairs, disrupted manufacturing or agriculture, and compromised firefighting capacity, leading to property damage and financial losses.

The risk of cyber-physical threats cannot be entirely avoided but can be systematically reduced through two main approaches: reducing the likelihood of cyber contingencies and minimizing their physical impact on microgrid operations. Taking into account the unique features of hardware devices and software applications, as well as the characteristics of microgrid operations, can help choose and implement security measures for reducing cyber risk. Skilled human operators with an acute understanding of cyber systems and microgrid operations are needed, supported by comprehensive technical training and physical protection policies. Also, defensive measures, including firewalls, antivirus software, intrusion detection systems, additional security in the critical zones, using wired connections, real-time access monitoring, and incident response and recovery plans, can enhance cybersecurity [150,151,152]. These must balance the performance, cost, and protection, as excessive measures (e.g., double authentication) may introduce unacceptable latency, and time-dependent functions may be affected [138].

Also, the defensive strategies developed for energy microgrids can guide water microgrids here. Ref. [146] groups them into protection-based (safeguarding sensors and communication devices) and detection-based (analyzing data to identify intrusions). Ref. [149] further categorizes the implementation of security mechanisms into network-level (authentication, firewalls, encryption), device-level (monitoring integrity of field devices), and system-level cybersecurity (behavioral analysis and advanced mitigation).

With the application of smart technology, dealing with cyber-attacks can become more challenging with creative attack ways to overcome the standard security mechanisms [153]. Monitoring and analyzing the flow of data among cyber-physical components becomes challenging when the attack is made at the device level. Artificial intelligence (AI) offers strong potential by precisely predicting outcomes, analyzing large data sets, and identifying anomalies [141]. It can adapt to new threats and adjust system operation, while traditional software-based systems have been unable to recognize and adapt in response to the rapidly growing diversity of cyber threats. However, limited access to quality datasets remains a major obstacle. In this regard, synthetic data generation is widely used through repositories such as IMPACT (), KYOTO (), KDD’99CUP (), and DARPA ().

Machine learning [154,155], deep learning [156,157], and reinforcement learning methods [158,159] have also been proposed to strengthen cybersecurity. Even though AI is considered an adaptive system, the possibility of attackers using AI cannot be neglected [159]. Therefore, AI-based defense mechanisms should be designed with awareness of both opportunities and risks to effectively mitigate cyber-physical attacks in water microgrids.

5.5. The Role of Local Community

Water microgrids, by definition and design, encourage public participation, which is essential for leveraging communal knowledge, enhancing system transparency, and building trust; reducing delays and increasing project success [160,161]. Community involvement has long been pivotal in any decentralized and community-based WSSs. For example, in rural India, community-driven rainwater harvesting effectively addressed local water shortages [87]. In urban contexts, technological integration with smart water systems complements community initiatives by reducing consumption and waste, while individual meters provide direct feedback to households, lowering water use [162,163].

The importance of community engagement also extends to broader outreach and education. In California, social media campaigns during droughts improved professional and public communication, strengthened conservation advocacy, and increased community involvement [157]. Educational efforts also positively impacted user behavior regarding recycled water, increasing its acceptance and maintaining trust in its use, reinforcing the community’s support for sustainable water practices [164]. Acceptance of alternative sources improves when communities are well informed about treatment processes [165]. The effectiveness of rainwater harvesting, for example, depends significantly on social influence and confidence, which can be hindered by psychological and technical barriers unless there is active promotion and technological advancement to mitigate associated health risks [166].

Community–microgrid interactions can yield both benefits and challenges. On one hand, they enhance conservation and resilience during extreme events; on the other, they may encourage resistance to change or overconsumption, undermining functionality [167]. The successful implementation of water microgrids, therefore, requires consideration of the local societal context, which influences technology and system design to meet specific community needs regarding water consumption and production, and wastewater management [168].

5.6. Advances in Design and Operation of Water Microgrids

The complexity of designing and operating water microgrids may hinder their development if operational and management challenges are not effectively addressed. Managing variable water quantity and quality at efficient supply rates requires advanced control technologies and dynamic coordination of centralized and decentralized components. Additionally, the need for multifunctional storage, real-time data analytics, and adaptive control adds further operational complexity. To tackle these challenges, conducting situation-based, predictive simulations in advance and real-time optimization in system design and operation can contribute to effective solutions. For example, one major hurdle is the integration of diverse data sources, including SCADA, GIS, and AMI, into cohesive simulation frameworks. Digital twin implementations, while promising, face scalability issues and require substantial computational resources, particularly if the local systems within water microgrids are larger [169].

Another challenge is transitioning existing WSSs to water microgrids with balancing hydraulic and topological redundancy in local WSSs (individual microgrids). Subsystem-based optimization frameworks using graph-theoretical methods, such as st-numbering and spanning tree algorithms, can address this issue by ensuring reliable sectorization while minimizing resource consumption [170]. Additionally, renewable energy integration into water-energy microgrids highlights the potential of mixed-integer linear programming (MILP) to optimize pump scheduling and renewable resource allocation, reducing dependency on fossil fuels [171].

Simulation tools could also model and predict the dynamic behavior of water microgrids under various conditions, helping stakeholders understand how decentralized systems interact with centralized infrastructures [172,173]. For instance, digital twin models represent a transformative advancement by creating real-time virtual replicas of physical systems, allowing for predictive analysis, operational adjustments, and efficient anomaly detection. Optimization tools, on the other hand, enable stakeholders to balance complex trade-offs between demand, cost, resilience, and energy efficiency, critical for achieving sustainable and reliable microgrid configurations. These tools not only enhance operational decision-making but also facilitate long-term planning, ensuring efficient resource allocation and system resilience even under uncertain conditions. These tools can also enable the modeling of complex interactions between centralized and decentralized systems. EPANET, a widely used hydraulic simulation tool, when combined with Python-based frameworks such as USEPA/WNTR, serves as a foundational platform for analysis and model building. Furthermore, the use of graph-based approaches, such as Temporal Graph Convolutional Neural Networks (T-GCNs), can solve estimation challenges and system monitoring within digital twin frameworks, enabling more accurate and efficient management of water microgrids [174].

In addition, multi-objective optimization frameworks, such as Non-Dominated Sorting Genetic Algorithm II (NSGA-II), can solve sectorization challenges to transform existing WDS to the configuration of water microgrids. These frameworks identify optimal configurations of local WSS by minimizing costs, energy consumption, and average pressure, while maximizing the number of sectors and ensuring adequate node pressure. By utilizing Shannon’s entropy and graph metrics such as link density, average node degree, and meshedness coefficient, researchers have quantified the resilience of water microgrids under varying configurations [175,176]. Furthermore, optimization algorithms like stochastic programming could solve uncertainties in demand and operational constraints, enabling robust decision-making in real-world applications such as the Hanoi WDS [177]. For instance, these algorithms have been used for real-time system monitoring and anomaly detection, while machine learning algorithms like T-GCNs enhance predictive capabilities in digital twin applications [174].

6. Conclusions

From urban areas grappling with the pressures of climate change, aging infrastructure, and population growth to remote locations where traditional WSSs are untenable, water microgrids offer a promising complementary option among hybrid water supply options. Their ability to integrate both centralized and decentralized systems, leveraging local water sources through smart, adaptable infrastructure, demonstrates their versatility and alignment with contemporary needs for sustainability and resilience. A key distinction between conventional decentralized WSSs and water microgrids lies in the strategic trade-offs of demand between centralized (existing) and decentralized (local) water systems, and the activation of local water use to meet varying demand under normal and disrupted conditions by facilitating varying degrees of decentralization, such as demand sharing in different percentages, and dynamic interaction between the centralized and local systems. Water microgrids have the potential to operate in both grid-connected mode and purely islanded mode in disrupted conditions, supplying treated potable and non-potable water strategically using optimized sectorized centralized and local systems.

However, while the microgrid concept has become highly popular and proven effective in the energy sector, the concept of water microgrids is still in its early stages. To date, only four studies have examined this concept, with one lab-scale experiment demonstrating encouraging resilience compared to conventional centralized and decentralized systems under disruptive scenarios. Although hybrid or decentralized systems with local water sources have shown satisfactory performance in some contexts, the complete and sustainable implementation of water microgrids has yet to be realized in practice.

Water microgrids also face various challenges, including high capital costs due to economies of scale, spatial constraints—especially in urban areas—and limited community acceptance, which can hinder the implementation of distributed local treatment facilities. Thus, the design of local facilities must account for scale, location, treatment technology, system configuration, operational strategies, and monitoring and remote control to meet social, economic, and environmental objectives for sustainable development. Key design criteria should include minimizing capital and operating costs, optimizing energy use, reducing the carbon footprint, encouraging community engagement, adopting smart system approaches, ensuring public health and water quality for users’ needs, and maintaining operational reliability.

The foundational principles, system components, configurations, and operational challenges of energy and water microgrids exhibit significant parallels. The tools and approaches used to address challenges in energy microgrids and the long-term practices and experiences in the water industry will contribute to guiding the successful implementation of water microgrids. With the current emphasis in developed countries on managing aged infrastructure and the focus on new infrastructure in developing countries, now is the time to further explore microgrid development in response to the changing resilience and sustainability requirements of many cities. The discussions and insights in this study aim to guide the conceptualization and design of water microgrids, encourage stakeholders to consider the approach in future planning, and support the academic and engineering communities in developing decision-making tools for sustainable implementation.