Advancing stormwater harvesting: a comprehensive review of current drivers, implementation advancements, and pathways forward

ABSTRACT

This review paper examines stormwater harvesting (SWH) as a solution to address urban water management challenges caused by urbanization, climate change, and population growth. Despite the potential benefits of SWH in urban areas, barriers related to planning, technological, and societal aspects have limited its widespread adoption. To address these challenges and move forward, this paper presents a scoping literature review on SWH, identifying key drivers for studying SWH and exploring various factors influencing its implementation, including planning considerations, technical and non-technical aspects, and benefit and risk assessments. These factors are categorized into three phases: pre-implementation, implementation, and post-implementation. The study findings demonstrate the critical importance of considering a multidisciplinary approach and various factors in designing and implementing SWH systems, such as regulations, decision-making tools, sustainable technology, end-use, and social and economic impacts. While there is diverse motivation for studying SWH, the focus has predominantly been on technology development and testing, with limited attention given to socio-economic analysis of SWH practices. Similarly, although there is a wealth of literature on system design optimization, there is a lack of research on regulations, post-implementation monitoring, and social analysis. However, this review highlights the substantial research in the field, which can contribute to the wider adoption of SWH practices for more sustainable urban water management in the future. The proposed comprehensive roadmap is a valuable tool for guiding future research efforts and practical implementation of SWH systems, ensuring a thorough understanding of the entire SWH system before embarking on new projects.

GRAPHICAL ABSTRACT

KEYWORDS:

• Integrated urban water management
• nature-based solutions
• stormwater treatment
• stormwater practice
• sponge city

1. Introduction

The growth of the world's population and the expansion of urbanization are putting a lot of pressure on the world's freshwater resources. This is observed in the increase of freshwater demand for human consumption, agriculture, irrigation of urban greenery (as a way to battle the urban heat island effect [1]), and in the rapid change of urban landscape creating flooding and pollution issues in the urban streams [2]. To tackle these challenges traditional rainwater and stormwater management in cities have tried to capture and release incoming rain events as quickly as possible. Historically, this approach worked well in regions with predictable climate patterns, however, as we face ongoing climate change and extreme weather events, traditional pit-and-pipe approaches are starting to fail [3]. Too often there is either too much water in urban drainage systems, causing flash floods, or too little water for minimum ecological stream flows, causing environmental concerns. In the last two decades concepts such as Low Impact Development (LID), Water Sensitive Urban Design (WSUD), Integrated Urban Water Management (IUWM), Nature-Based Solutions (NBS), Sustainable Urban Drainage Systems (SuDS),and recently Sponge City have emphasized the need to capture rainwater and stormwater when it is created in our cities and repurpose it when and where it is needed [4]. To be precise, LID (most commonly used in US) aims to mitigate urban environmental impacts, focusing primarily on stormwater management through sustainable, small-scale approaches like bioretention systems and green roofs [5]. Similarly, WSUD (coined and used in Australia) integrates urban planning with water cycle management. This approach not only aims to reduce hydrological impacts but also strives to seamlessly incorporate infrastructure within the water cycle, thus ensuring more cohesive and environmentally conscious urban planning [6–9]. An integral part of this approach, IUWM, is a comprehensive strategy that combines water supply, wastewater, stormwater, and environmental management, (developed in Australia) to promote water sustainability [10]. Likewise, SuDS (which emerged in the U.K.), aims to enhance stormwater management by replicating natural drainage processes [5]. In line with these concepts, NBS (dominant in EU countries and legislation) utilize nature's patterns to address various societal challenges. NBS provides multiple benefits, encompassing economic, social, and environmental aspects, by promoting flexible and sustainable resource use [11,12]. Building on these principles, the concept of Sponge Cities (most widely used in China) employs natural processes for water management, focusing on eco-friendly practices. They effectively blend green and grey infrastructure to enhance runoff management, illustrating a practical application of these interconnected environmental strategies [13,14].

This is further specified in terms of Rainwater harvesting (RWH) and Stormwater harvesting (SWH), which explore technological, planning, and societal considerations around repurposing these alternative water sources to protect and resilient our cities to future climates.

Rainwater harvesting (RWH) represents an old practice of collecting relatively unpolluted rain from roofs and elevated surfaces for storage and subsequent reuse. In underdeveloped countries and remote communities, RWH is often a primary source of freshwater [15], while in urban communities, it represents an additional source of non-potable water to supplement the reliance on centralized water supply systems. With over 400 documents published just in 2022 [16], RWH approaches are well documented, and several recent literature reviews cover novel findings (e.g. [17–19]). However, SWH has received much less attention due to higher pollutant contamination and treatment complexity, which challenges planning, technological, and societal aspects. SWH considers capturing, treating, storing, distributing, and reusing stormwater runoff from roads, pavements, and other impermeable, mostly urban, surfaces. Pollution accumulation on these surfaces between two rain events is often significant [20], and stormwater would dissolve and wash away the pollutants in the ‘first flush’ phenomenon [21] contributing to urban stream pollution. SWH can also be linked with traditional pit-and-pipe systems to intercept and capture stormwater flowing through stormwater drains. Due to high pollutant uncertainty in underground structures caused by cross-connections, leaching, etc. [22,23], SWH should be able to cope with different levels of stormwater pollutants, often through on-site treatment prior to storage. Despite these challenges, SWH has been widely considered in the literature as a promising solution for urban water management, particularly in areas with water scarcity [24]. Compared to RWH, SWH has the advantage of capturing a larger volume of water [25], which can be reused for a range of purposes, greatly reducing pollutant load in receiving streams [26]. Additionally, by storing initial runoff (or sometimes whole events), SWH reduces flood peaks and flash-flood damage [27], especially in the downstream areas, while also contributing to a wide range of ecosystem services [28]. With proper treatment and storage, stormwater can be used as an alternative for drinking water in various end-uses, helping in potable water saving to a great extent [29]. However, SWH also presents some unique challenges, such as the need for more advanced treatment technologies to address pollution and contamination issues compared with RWH [30], as well as the need for effective governance and stakeholder engagement to ensure the sustainable management of harvested water [31]. Despite these challenges, the potential benefits of SWH make it a compelling area for further research and implementation, especially considering its ability to enhance urban resilience to climate change and future stresses [32]. Therefore, there is a need for more research and data on SWH to understand its potential and limitations fully, but particularly to develop effective policies and practices for its implementation and governance.

SWH has been researched and developed in recent decades to address a range of water-related challenges, with varying motivations and drivers. Typical drivers found in SWH literature are technological advancements for water treatment and increased storage volume in SWH systems [25,33], increased water security [34], urban flood control [35] and understanding of the long-term performance and adaptability of SWH systems in climate change [36]. Additionally, finding the best techniques to increase social acceptance [37], the cost of implementation [38] and economic outcomes [39] of SWH systems have been studied to facilitate government and decision-making [40]. There are several examples of the performance assessments for implemented SWH practices around the world (e.g. [41]). Due to the growing body of literature on this topic, and diverse and sometimes opposing drivers for its implementation there is a need for comprehensive literature review on current SWH knowledge across all implementation stages (from concept to application and management). This would clearly outline which drivers and implementation stages are underrepresented, requiring further research.

While there have been some useful review papers published addressing various stormwater management issues, to date, the most relevant and highest-cited review paper on SWH by Hatt et al. [42], was published in 2006 and limited to the systems in a few Australian states, which may not reflect the experience of other regions. Additionally, it did not cover a full range of drivers and implementation stages of SWH limiting its usefulness only to climates and practices that are similar to Australian scenarios. A more recent review paper has been published on WSUD and sustainable stormwater management [43] presents a systematic and critical review, but it does not fully address SWH challenges and benefits. Furthermore, some reviews only address specific issues of SWH, such as reviewing pollution or treatment systems (e.g. [36]), while neglecting other important aspects, like planning and societal aspects. Overall, a lack of comprehensiveness, outdated information, geographical limitations, bias, limited scope, and focus on specific aspects of SWH leading to subjective conclusions are some gaps in current review papers.

In this study we reviewed the state-of-the-art scientific studies related to the SWH practices and highlighted future steps addressing possible gaps. The overall aim of this paper is to perform a literature review on SWH to (1) identify all key aspects and drivers of SWH research and practice, (2) understand planning, regulative, technological, and social advancements across three distinct implementation stages of SWH practices (pre-implementation, implementation, and post-implementation), and (3) propose a roadmap of opportunities and challenges at each stage, which can serve as a guide for future research and implementation of SWH. This will offer a comprehensive overview of the entire life cycle of a SWH project, providing stormwater practitioners and researchers with a better understanding of the current state of research and practice.

2. Methodology

This paper serves as a scoping review (e.g. [44,45]) with an added element of systematic review to complete the knowledge of SWH practices around the world. We systematically mapped and identified gaps in the literature on topics by synthesizing existing or emerging research [46,47] and by incorporating elements of both critical and systematic review. The main research questions that this review is seeking to answer are: (i) what are the global and local drivers for SWH implementation and research advancement? (ii) how are SWH practices integrated in different phases of project’s execution? and (iii) what areas of SWH implementation and research are lacking around the world and need further advancement. These research questions are analysed on global literature with a focus on finding commonalities between worldwide practices, but also finding regional distinctions. The final aim (and method) of this scoping review is to produce a comprehensive roadmap for SWH implementation, with suggestions for generalized and site-specific approaches.

To achieve this goal, a literature search was undertaken from Scopus and Web of Science databases using the search terms ‘stormwater harvesting’, or ‘stormwater recycling’, or ‘stormwater reuse’. The bibliography of found articles was further scanned for possible missed references. In our initial search, 264 articles were found which were further screened for relevant publications on SWH using the limitation process illustrated in Figure 1. Firstly, broad title analysis was done to exclude duplicates, or those written in other languages or focused on harvesting rainwater and roof water. Secondly, by reviewing the abstracts and titles of the articles, all publications that were less relevant to SWH (e.g. rainwater and roofwater harvesting) were removed. We have also excluded unavailable publications such as conference papers and those that did not have a link to the full text (only abstracts). Finally, through in-depth analysis of the full text of the selected publications, articles not directly relevant to SWH were excluded. This process led to a total of 185 papers for final analysis. We found that 90 out of the 185 analysed articles gave specific locations. The analysis encompassed cities from various countries, including the U.S.A. (e.g. [48,49]), Palestine (e.g. [50,51]), South Africa (e.g. [52,53]), Colombia (e.g. [54]), China (e.g. [55]), Egypt (e.g. [24]), Denmark (e.g. [39]), Brazil (e.g. [56]), and Saudi Arabia (e.g. [57]), respectively. About half of these 90 articles explored cities in Australia, particularly Melbourne (e.g. [26,58]), Sydney (e.g. [28,34]) Brisbane (e.g. [25,59]), and Salisbury (e.g. [60,61]).

Figure 1. Screening process for systematic literature review.

The information contained in these publications was carefully classified according to implementation stages, drivers, and other criteria listed below (Table 1 and Figure 2). These publications include a wide range of topics, such as the nature of SWH systems, the advantages and disadvantages of SWH, maintenance, and the impact of SWH on study catchments. Hence, we have extracted data from each article based on the following aspects and categories: (1) drivers of SWH work, determining the main purpose of conducted study, (2) planning process for SWH systems, (3) technical aspects behind SWH including the structural detail of SWH systems, and (4) the non-technical assessments, such as economic, social, etc. These aspects led us to make a clear overview of the main subjects that were covered by different authors and realize important gaps that should be addressed in future studies.

Figure 2. Main drivers for SWH implementation and research.

Table 1. Different phases of a SWH life cycle and corresponding reference examples.

Phase	Phase aspect		Description	Reference examples
Pre-implementation	Regulations and legislation		Specific regulations and legislation for SWH to ensure long-term sustainability	[5, 10, 31, 49, 60, 62–69]
	Benefit assessment		Types of benefits that SWH can provide and how to evaluate them	[6, 7, 10, 24, 25, 28, 29, 38–41, 48, 53, 62, 66, 70–85]
	Planning and decision-making		Muli-criteria methods and tools used for planning and decision-making processes	[10, 24, 26, 27, 30, 50, 54, 58, 67, 68, 70, 72, 75, 78, 81, 86–104]
	Risk assessment		Health risks associated harvested stormwater	[51, 59, 79, 105–114]
Implementation	Technical and structural	Collection	Stormwater collection using nature-based solutions (NBS)	[7, 30, 56]
		Treatment	Stormwater treatment via NBS, and advanced treatment technologies for pollutants of concern	[29, 30, 33, 34, 36, 39, 55, 56, 59, 65, 68, 79, 85, 95, 100, 105, 107–109, 115–149]
		Storage	How to store stormwater and state-of-the-art related technology & system reliability	[25, 35, 36, 52, 54, 56, 77, 102, 110, 135, 150–158]
		Distribution	Distribution of harvested stormwater to the end-users	[95, 156]
	End-use		Different types of end-uses with harvested stormwater	[6, 7, 10, 28, 29, 34, 35, 52–54, 57, 60, 61, 70, 76, 81, 93, 95, 105, 108–111, 115, 143, 151, 155, 159–168]
Post-implementation	Operation, Maintenance and Monitoring		Plans designed to maintain the system (financial plans, support groups, technologies, etc.). Monitoring of SWH performance and environmental impacts	[6, 7, 29, 40, 41, 48, 53, 66, 71, 75, 77, 84, 125, 132, 140, 141, 155, 156, 169–173]
	Non-structural	Social	How the community deals with SWH programmes and the methods to direct it	[26, 31, 37, 61, 67, 69, 75, 95, 159, 160, 171, 174–180]
		Economic	The impact of the harvest programme on the economy of the site, financial programmes and financial benefits resulting from it	[6, 10, 25, 38–40, 57, 78, 80, 124, 139, 181–183]

Since we wanted to understand the whole life cycle of SWH, different aspects of SWH were organized into three distinct stages/phases (Table 1). These three stages represent the planning, construction, and maintenance phases commonly found in engineering projects [184–186]:

1. Pre-implementation. This phase describes the first step of providing a water management system. Studies mainly focused on planning for implementing SWH systems and managing water crises (e.g. [27,35,70]). The driver of these studies supported subjects related to regulation and legislation, risk-benefit assessment and decision making. In fact, articles that focused on studies related to realizing if the SWH is an answer for their site's problems before starting the implementing process or which they wanted to plan for the best SWH system with considering their site situation, were in this category. Articles in this category also focused on studies related to determining whether SWH is a suitable solution for specific site problems prior to the implementation process.

2. Implementation. This phase includes connecting different technologies to implement a SWH system (as suggested in [57]). This phase consists of different technical and structural aspects of SWH, as well as end-uses. It is in this phase that the state-of-the-art technologies (e.g. permeable pavement to collect runoff [115], stormwater tanks for drought control [71], and different filter media to improve the treatment process [33,116]) were analysed to find possible gaps for future studies.

3. Post-implementation. This phase analyses implemented projects based on on-site information, providing an understanding of new findings from already implemented projects (e.g. [40,61]). Importantly, this includes operation and maintenance of the systems, but also non-structural aspects, such as economic and social assessments. This phase shows if the performance criteria set out in the ‘pre-implementation’ phase have been met and how.

Literature corresponding to each phase and different aspects of the phases have been outlined in Table 1. The collected articles were analysed, and overlapping themes were consolidated into broader concepts to build the main framework. While individual studies provide considerable knowledge about SWH practices, their findings are often limited to a single driver and/or implementation phase. Hence, we used information and gaps gathered in the literature to create a roadmap for future SWH practice and implementation listing current needs and required advancements in the field. The roadmap presents an essential tool that outlines and prioritizes the key actions necessary to achieve specific goals and objectives related to SWH, ensuring that practitioners are considering all aspects of SWH, and researchers are investigating knowledge gaps.

3. Main drivers for SWH implementation and research

Analysing all the literature on SWH over the past decades, we identified six main drivers (Figure 2). The majority of the studies aimed to develop technologies and tools that can aid in SWH practices (Driver #1, Figure 2). For example, Kazemi et al. [126] assessed developing permeable pavement as one of the technologies in SWH for irrigation use, and there was an increased focus on the development of nature-based technologies (e.g. biofilters, wetlands, green roofs, etc. [61,107,187]) for pollution removal from urban stormwater for reuse purposes. Further to the developed technologies, the tools that can be used for different purposes were also a focus of previous studies, e.g. tools for master planning of stormwater management in the cities (e.g. [97]), tools for spatial placement of suitable technologies based on multi-criteria assessment (e.g. [58]), and tools for simulating and predicting the amount of pollution in run-off [96].

Various environmental crises have been caused by urban stormwater, and consequently, a large number of previous SWH research tried to solve these issues (Driver #2, Figure 2), e.g. environmental pollution control [85], environmental protection [7], and flood control [154].

As one of the key aims for SWH is to provide alternative water sources, a significant number of studies tried to analyse the reliability of water supply in SWH systems (Driver #3, Figure 2). For example, studies on choosing a reliable water supply among a wide range of water alternatives (e.g. [60,102]), finding if SWH is reliable enough to reduce potable water consumption [30], and reliability of SWH to meet the irrigation water demands following a storage-behaviour analysis method [36].

Despite the importance of economic aspects and concerns around the sustainability of SWH, little research has focused on exploring the cost-benefits of SWH (Driver #4, Figure 2). For instance, Hagare et al. [38] assessed the cost and benefit of all technical parts of stormwater systems from operation and maintenance and found that implementing these types of projects in larger developed areas can significantly reduce the cost and improve economic efficiency. There are also creative economic ideas about pollution trading that can be a solution to increase the financial efficiency of the project [139].

Regarding the assessment of operation and maintenance (Driver #5, Figure 2) of SWH systems, researchers focused on evaluating the implemented systems conditions and real site situation with the specific aims, e.g. optimization of system performance [41], water supply assessment [171], and environmental protection [48]. While crucial for the success of SWH, this driver has not received appropriate attention in the literature, especially considering modern practices for urban water management and sustainable water designs.

Ultimately, stormwater management technologies are often based on nature-based solutions, and are visible to the public. Thus, questions relating to social acceptance and public risk perceptions are of interest (Driver #6, Figure 2). A survey was done by Wu et al., [61] to investigate the attitude and intentions of residents to use stormwater following wetland and Managed Aquifer Recharge (MAR) system treatment. The study found that the proximity of the end-use to human contact was an important factor to people's emotions or perceptions of health risks. Other studies have focused on understanding what co-creation and co-design approaches for SWH would work the best to facilitate social awareness and community ownership (e.g. [67]). Recently, the incorporation of cultural features in stormwater practices has been discussed as a way to raise community buy-in further and promote better SWH management (e.g. [69,176]).

4. Analysis of SWH implementation stages

4.1. Pre-implementation phase

Prior to implementing SWH practices, several regulatory and planning challenges should be considered to ensure the success of the project. Questions often arise regarding local regulations and legislation that may impact the site, as well as whether stormwater is the most beneficial alternative water source. Additionally, it is important to assess the potential risks associated with SWH implementation. Although there is substantial research on planning-support tools for SWH, it remains unclear how to make well-informed planning decisions that consider multiple criteria (some of which may be conflicting) to minimize risks related to water supply and water quality.

4.1.1 Regulations and legislation

On a global scale, stormwater management has been extensively promoted in different parts of the world, under various concepts, e.g. water sensitive urban design (WSUD) in Australia, low impact development (LID) in North America, nature-based solutions (NBS) in Europe, Sponge City in China, etc. [5]. Nevertheless, specific regulations and legislations relating to SWH are still lacking, and mostly just focus on the water quality requirement of yield after the treatment process [65,66,68]. Other requirements such as planning, construction and maintenance could be of particular importance to the success of such projects [169] are still underrepresented. Studies have shown that the uptake of SWH practices may be challenged by strict regulation and approval processes for water harvesting schemes, which could be further hindered by the lack of coordination between planners and legislators [10]. In addition, the lack of regulation to attract incentives and financial support for SWH adoption and implementation has been identified as a barrier to the widespread uptake of this concept. It is notable, in some studies, the critical organs in SWH practices, such as local governments and water utility, are acknowledged (e.g. [31,63]), however, there is a need for greater clarity on the legal ownership and rights to harvested water, as well as the responsibilities of different stakeholders in the management of SWH systems [64,67].

While SWH is often a local issue, constrained by local regulations, climate, level of development and culture, it is clear that there is a need to compare the legislation of different sites, regions or countries to explain the disadvantages and weaknesses of the regulations, and address their strengths [49]. For example, in Australia, councils promote WSUD in the private sector through regulations and incentives, emphasizing the protection of unique ecosystems. In China, rapid urbanization has led to significant investments in ‘sponge cities’, which utilize natural and engineered solutions, focusing on large-scale engineering projects [13,26]. It could help planners to identify possible needs in their different site conditions and consider various factors that may not be addressed in their local frameworks. In creating regulations, and later stages of project implementation and care for SWH systems, it would be beneficial to consider the involvement of stakeholders and local community groups [69]. Such involvement can help ensure that regulatory frameworks are responsive to the needs and priorities of these groups. Additionally, it can help to build support and buy-in from the regulators, increasing SWH chances of success and long-term sustainability.

4.1.2 Benefit assessments

Extensive research has been done to provide evidence of the multiple SWH benefits, including supplementation of water supply [24,71], pollution reduction [41,85], flood control [48,53], mitigation of the urban heat-island effects [6,7], restoration of the natural flow regime [62,72,80], reduction of energy consumption, protection of eco-system service [40,82],creation of recreational areas with the natural SWH assets [53], as well as reduction of greenhouse gas (GHG) emissions [29,40,75]. Several studies have suggested that SWH can also be beneficial to many industries. For example, Kandulu et al. [66] found that treatment of raw stormwater that discharges into coastal waters can increase the number of fish and thus can be profitable in the fishing industry. In a study with irrigation end-use of stormwater, it was shown that stormwater can be useful for the growth and blossoming of trees, especially in arid seasons [71]. Pimiento et al. [139] designed a sediment trap exclusively for the target site, and showed the sediments collected from stormwater in the collection process can be profitable in industries such as concrete manufacturing. Attwater & Derry [28] found that stormwater management for agricultural use can reduce the environmental risk in the study area, such as bushfires and the risk of plant and animal species loss in the area. Using stormwater for agricultural irrigation is also found to be economically beneficial [76].

Integrated assessments of multiple benefits associated with SWH were conducted using various methods. For example, Dandy et al. [78] presented an integrated framework to assess the benefit of SWH using multi-criteria analysis covering economic, social, and environmental benefits. This study noted that SWH can have economic benefits by decreasing flood damage costs and potable water consumption. Financial benefits from other aspects of SWH, such as financial gain from social and environmental benefits as well as energy conservation, are also recognized [38,39]. For example, SWH through green technologies has not only increased the value of properties near these green spaces compared to other properties [73,74], but also reduced heat islands and improved air quality, leading to a decrease in energy consumption required for air conditioning [74]. In another study by Kandulu et al. [40], a methodological framework was proposed to enable systematic consideration and quantitative assessment of broad environmental impacts associated with water supply investments. These impacts include urban recreational amenity, regulation of coastal water quality, salinity, greenhouse gas emission and support of estuarine habitats. The framework is applied to a SWH scheme in Australia, providing further evidence of ecosystem services benefits. This could help the stakeholders to identify environmental values of the systems and estimate economic benefits [40]. Tools and methods for SWH cost–benefit assessment presented in these studies show the possible profitability of multiple SWH aspects.

Additionally, the benefits of stormwater harvesting significantly depend on its integration with the overall system. Different scenarios, such as combining stormwater harvesting with centralized supply or greywater tanks, demonstrate varied impacts on reducing potable water demand, stormwater flow, and wastewater contaminants. Each hybrid approach offers unique benefits and challenges, highlighting the need for careful consideration of how these systems are implemented to optimize their effectiveness [188,189].

Despite all these well-identified benefits relating to SWH, the economic benefits of SWH are difficult to quantify, especially in the social and environmental aspects. Consequently, the business case of SWH is not easily justified, inhibiting the uptake of SWH practices [85]. Therefore, further studies and efforts should be encouraged towards the development of quantitative assessment tools that can monetize various SWH benefits for easier communication with relevant stakeholders and communities. Given the challenge of obtaining comprehensive data, another approach could be to conduct trials combined with monitoring and assessment. This would help gather real-world evidence on the benefits of SWH, making it easier to justify their adoption and demonstrate their effectiveness (e.g. [124]).

4.1.3 Planning and decision-making

Planning for optimal potable water alternatives presents an important stage for any water management scheme. Practitioners can stay up to date with best practices by utilizing several strategies. For instance, if local guidelines are not available, it is advisable to find guidelines from other countries or regions that are freely accessible and adapt them to local conditions [180,190]. They can benefit from online resources, such as freely available case studies on websites of environmental organizations, government agencies, and international water management bodies (e.g. [114,191]).

Moreover, planners tried to develop planning tools such as framework [90], planning matrix [86] and flowchart [103] to provide guidance to the SWH planning processes. Almost all of them emphasized the multi-benefit aspects of SWH schemes as mentioned above. Consequently, there are a large number of studies that adopted multiple criteria assessment (e.g. [27,70,101,102]) methods across these aspects, to aid in selecting the most appropriate and sustainable water source. For example, Godskesen et al. [81] used ‘Assessing the most SusTainable Alternative'’ (ASTA) model to evaluate different water source alternatives, including centralized systems, RWH, SWH, treated greywater, and recycled water via third pipe options. The analysis considered various criteria, such as supply reliability, potable water demand, runoff volume, and wastewater generation volume, to determine the best alternative for specific sites. They demonstrated that while SWH may not be the most economically sustainable option, it still presents itself as one of the most environmentally sustainable water sources for Copenhagen. The results showed that the hybrid SWH system (SWH combined with centralized supply) was ranked the highest and identified as the most favourable scenario.

Decision-making is not limited to high-level planning for various water resources, but also specifically to different components of SWH systems, such as the size, capacity, type of collection surface, storage tank, and treatment technology, etc. [87,90]. These factors are critical to the success and sustainability of SWH systems, since they can impact the safety, quantity, and quality of harvested water and eventually the feasibility of the plan [78].

Previous studies also highlight key geographic factors influencing stormwater harvesting (SWH) design. Variations in river basins, watersheds, and groundwater were found to impact the SWH feasibility [192]. For instance, in small catchments, capturing runoff can balance high and low flow periods, but as imperviousness increases, managing these flows becomes more challenging [72]. Climate, including tropical, temperate, and arid zones, significantly affects SWH efficiency, influencing variables like temperature crucial for biodegradation. Dry periods’ length impacts nitrogen removal and micropollutant inadequacies. Water volume per event and extreme wet conditions challenge system reliability [68,140]. Region topology and the vegetal cover variety are other geographic impact factors that need to be addressed in further studies [193]. Considering these factors is essential for effective SWH system planning and design.

Furthermore, Geographic Information System (GIS) software, as a multi-criteria analyzer, was one of the most common tools used during the planning of SWH practices. Researchers from Australia, Saudi Arabia and India investigated the best geographic sites to implement SWH systems [58, 70, 89]. For this particular purpose, data layers and maps such as runoff volume and depth, water demand, and catchments, urban land use, geo-morphology, and recharge conditions were used. Spatial planning tools have also been developed. For example, the urban planning tool UrbanBEATS [88], which has many advantages in addition to the possibility of complex modelling, was able to introduce the most appropriate locations for stormwater treatment and harvesting systems in Melbourne by modelling the need, pollution emission, and flood management in Deletic et al. ‘s [27] research. Often, these studies investigate limited mostly local cases, and thus there is a need to explore the application of large regional or global scales across multiple different case studies [10].

Apart from the tools for planning for SWH, cooperation and coordination between different stakeholders, as well as final decision-makers is essential. For example, land use planners could coordinate with water engineers and planners to make well-informed water-centric planning decisions and for this particular purpose development of tools is very important to allow communications [10, 92].

4.1.4 Risk assessment

In the context of SWH, the primary risk concern is on addressing the presence of metals, organic chemicals, and pathogens due to their direct impact on human health [55,109]. Various studies have investigated methods to improve the elimination of these contaminants [106,109,116]. However, the relevant studies on emerging contaminants and real pathogens are relatively limited. Although SWH is a feasible approach, the emphasis of studies is on enhancing pollution reduction and performing thorough risk evaluations to guarantee the safety of the treated stormwater for end-users [59,105]. Many studies have found that stormwater treatment systems or treatment trains may not provide adequate pollutant removal performance, making stormwater unsafe for a wide range of potable and non-potable end-uses [79,109]. Literature highlights several methods of assessing and quantifying these risks, and the risks from pathogens are of particular concern. Chong et al. [59] have made efforts to assess the public health risk in urban stormwater from a medium-density residential catchments, by using a combined chemical-toxicological methods. Murphy et al. [109] assessed the risks from a pathogen Campylobacter present in treated stormwater following seven treatment scenarios, by using Quantitative Microbial Risk Assessment (QMRA). The findings suggested that only two scenarios were able to provide water of adequate quality for toilet flushing and irrigation end-uses. These two scenarios involve the use of biofilters combined with UV treatment as well as a more conventional coagulation, filtration, UV, and chlorination treatment. Using the same QMRA method, other studies have suggested that choosing a proper treatment system can strongly reduce the human health risk even for potable end-use [79,109]. Using QMRA, Lim et al. [108] quantified the risk from microbial hazards (adenovirus and norovirus as target pathogens) in harvested stormwater for domestic non-potable uses, and found that the risk of using stormwater for irrigation of agricultural crops for human consumption is much higher than other non-potable uses investigated. Page et al. [110] published a risk assessment framework based on the aquifer storage transfer and recovery (ASRT) treatment plan and suggested that pathogenic hazards were acceptable if further supplementary treatment was included.

On the other hand, the methods for assessing human health risks from chemicals are less developed. Therefore, only a few studies have specifically assessed the risks by using the risk quotient (RQ) approach, for heavy metals [79] and organic chemicals [113] in stormwater. Fang et al. [79] found that raw stormwater contains Cd at levels that can cause human health risks when used directly for drinking, but low risks are expected for non-potable uses. Ma et al. [113] showed that the combined risks of heavy metals in stormwater can exceed recommended values, with higher risks in high-traffic and industrial areas. The same work showed that higher molecular polycyclic aromatic hydrocarbons (PAHs) posing higher risks in stormwater. Nevertheless, the risk mitigation by stormwater treatment systems has rarely been tested. Fang et al. [79] evaluated the potential of biofilters in reducing risks from heavy metals and found that risks can be mitigated but might not be sufficient to deliver potable end-use. Page et al. [110] found that iron concentrations increased after underground storage but would be acceptable after post-recovery aeration treatment. With the monitoring data of two urban catchments, Brodie [106] used a screening-level risk assessment, which showed that showed that heavy metals pose a low-level risk, with some detected below the local drinking water guidelines. Due to the highly limited data, the ‘Australian Guidelines for Water Recycling (Phase 2): SWH and Reuse’ [114] simply states that ‘stormwater is not expected to be particularly chemically hazardous’. This is concerning as the data underlying the guidelines is very limited. To address this concern, recent efforts have been made to identify the occurrence of organic chemicals detected in stormwater and their risks [194,195]. For example, hang et al. (2024) found a total of 629 organic chemicals have been detected in urban stormwater, with 82 ones posing high ecological risks and 3 with high health risks.

In summary, there are a significant number of studies assessing the risks of SWH for various end-uses, but mostly focused on pathogens, and results are still inconclusive as to the level of risks for human health, under different catchments and climates [107,109]. While it seems that treated stormwater has generally low risks from chemicals for non-potable use, the potential for cross-contamination from different sources is still high, and there is a lack of monitoring data to support risk management actions [112].

4.2. Implementation phase and technology development

SWH usually takes a treatment train approach, consisting of four technical and structural aspects: collection, treatment, storage, and distribution [90]. In fact, these components connect to and interact with each other [196] in order to deliver water to specific end-use planned for that SWH system. With the development of technologies, there is often a wide range of choices for each technical part of the system, opening various possibilities for stormwater treatment to meet different end-uses.

4.2.1 Technical and structural aspects

Collection: Commonly used stormwater collection approaches are based on either traditional approach (e.g. gutter, pipes, channels, ditches) or nature-based systems (NBS, e.g. buffer strips, grass swales, porous pavements, wetlands, etc.). The latter one was more interesting as they could also provide a certain level of pre-treatment (e.g. [52,109,126]). The collection component is usually not tested individually for its collection behaviour but is often studied in connection with the other technical parts. For example, Hammes et al. [56] worked on the quantity of water absorbed from porous asphalt pavement collectors and the quality of collected water, as well as storage capacity. Another example is the study by Broadbent et al. [7], who assessed the environmental benefit of artificial water bodies such as wetlands and basins that work as both as collection and storing systems.

Treatment: An extensive focus of previous studies was on developing and optimizing treatment technologies, to improve the treatment performance and thus increase the safety of treated water for different end-uses. This covered a range of different NBS such as vegetated swales [133,140], biofilters [107,119,134], constructed wetlands [129,140], sediment basins and ponds [141], aquifers recharge and recovery [135] or nature-based technologies such as bioretention systems [127], permeable pavement [126] and filtration systems [33,142].

NBS for stormwater treatment demonstrates varying levels of performance based on the technology used and the targeted pollutants. Most of these studies have shown sediment can be effectively removed through the use of sediment basins [139], swales [133], and vegetated buffers [105], while more complicated treatment of sediment, nutrients and sometimes metal [143] could be targeted through the use of constructed wetlands, bioretention [117], and permeable pavements [56]. Other pollutants, such as pathogens and organic chemicals, have received less attention in natural-based stormwater treatment research [55,65]. Some filter materials such as enviss™ and zeolites have been proven to remove a wide range of pollutants, including harmful microorganisms, that exist in urban stormwater [33,116]. Notably, Murphy et al. [109] suggested that the quality of water after treatment in single wetland or biofilter systems is not reliable enough to remove all types of infections, so there is a need to combine these systems with advanced technologies, for toilet flushing and irrigation end-use. UV radiation was found to be effective for most bacteria (Salmonella spp., Campylobacter spp., and E. faecium) [197], protozoa (Cryptosporidium oocysts) as well as viruses (rotaviruses) [144]. Electrochemical oxidation (ECO) was also used to remove dangerous infections further post biofilter treatment [34]. Although relatively less work has been done for organic chemicals, many feasibility studies also confirm that advanced oxidation technologies are able to further reduce chemicals that are not well removed in NBS, e.g. UV/H₂O₂ for small aromatic compounds [146], solar driven advanced oxidation process (e.g. photoelectrochemical oxidation) for atrazine and diuron, etc. [145]. Membrane technologies in recent studies were also identified as a new and capable advanced system to eliminate a broad range of pollutions, including the majority of dissolved organic matter (DOM), turbidity, endocrine disrupting compounds (EDCs) and pharmaceuticals [95]. These advanced technologies are often placed at the end of stormwater treatment train to remove any residual contamination from chemicals and pathogens [109,128], ensuring higher level of safety during reuse.

Storage: There are diverse types of storage systems, such as constructed underground tanks and NBS such as ponds, wetlands, and aquifers. Using a combination of storage systems could have many benefits, e.g. increase the volume storage and the balance of the system yield with the demand to provide a constant source of water [77,150]. Among the examples of these combined storages is a storage plan with three steps of (1) basin storage for holding the harvested runoff before starting any treatment process, (2) wetland for treating and storing, and (3) the aquifer storage system that can store treated water before sending it to the distribution system [152].

Storage volume is one of the most important research subjects related to storage systems since it has a direct effect on system reliability to meet the demand [56]. Reliability can directly affect on the system performance, e.g. a large storage system without considering demand and environmental situation can destroy the environmental cycle by releasing the extra water to the environment [156], while a small capacity of storage cannot meet the demand. Most studies have also incorporated climate change scenarios into GIS or statistical models to test the reliability of different storage systems under varied climate conditions, considering appropriate urban locations for storage placement [25,151, 152,155]. To evaluate the reliability, these studies used a variety of parameters like demand pattern, rainfall-runoff statistics, infiltration rate, evaporation rate, and the capacity of the storage systems. Their application can provide valuable information to guide decisions related to the planning, operation, and maintenance of storage systems, guaranteeing their sustainability and efficiency in meeting water demands over the long term [25,150].

Distribution: As Mitchell et al. [156] mentioned, distribution systems for harvested stormwater are divided into two broad categories: open-space irrigation systems and non-potable distribution systems (‘dual’ reticulation). Furthermore, in built-up areas, particularly in locations with dense construction, there are two distribution system options available for implementing a new SWH system. The first option is to integrate the distribution system with the existing potable water system, which necessitates advanced treatment measures but offers cost reduction through shared infrastructure. The second option is to install a separate third-pipeline system, which entails higher initial costs but provides a safer alternative. Examples of such systems are implemented in north of Adelaide, South Australia [95,198]. Combined distribution systems have not been presented in the literature, however, similarly, Windhoek in Namibia has been successfully using recycled wastewater for potable end-use for years [95,174].

While significant research was done on the quality of stormwater post-treatment (e.g. [56,131]), no studies have been found to test the quality of water at the end-use point (i.e. end of the distribution system) and how it differed to the freshly treated stormwater or in the storage systems. More specifically, the subjects such as the possibility of pollution re-entering the final yield due to distribution pipes rusting [199], the growth of microorganisms within the distribution system [200]were not included in the reviewed articles, and should be considered important, especially for close-contact end-uses.

Recently, there has been an emergence of studies exploring the application of Internet of Things (IoT) techniques in urban water systems, including stormwater [201,202]. Relevant studies include the development of low-cost sensors for real-time monitoring [22], data-driven methodologies for predictive control [96], real time control strategies for enhancing the quality of harvested stormwater [134], etc. All of these have immense potential to simplify the operation of SWH systems and improve their performance and therefore should be the focus of future research.

4.2.2 End-use

Various end-uses have been practiced with harvested stormwater (Table 2). The use of harvested water for close human contact end-uses was found to pose a higher risk of exposure to pathogens and contaminants that can cause illnesses [160]. It is one of the main reasons why people may not support this end-use for SWH projects [159]. However, by having an appropriate treatment plan, SWH can produce appropriate water quality suitable for potable end-uses [95], even though this practice might not be financially viable for most cases [52]. In a study published by Dobbie & Brown [177], some of the key concerns and risks associated with potable end-use were highlighted as compliance with regulations, high capital and maintenance costs, potential reputation loss in case of system failure, and the need for commercial-scale treatment. Consequently, when the acceptance of end-uses with close human contact in society is very low [61], practitioners should be cautious in introducing the SWH. It is why, in most studies, the main objective is to reduce traditional potable sources consumption by providing alternatives for the other non-potable end-uses [29,49,81].

Table 2. Various end-use categories.

End-use category	End-use
Close contact end-use	Potable use (or cooking) [95], swimming [109], showering [108], washing pets and washing fruits [61]
Household use	Flushing toilets [10, 108, 160], washing clothes [81], lawn and garden irrigation [52, 61], washing cars [93, 115]
Urban use	Golf course irrigation [34], public open space irrigation [10, 111, 160], recreational area creation [7], flood control [70], cooling [6, 7], amenity/art [161, 162, 164]
Industrial use	Agricultural irrigation [28, 167], construction applications [105], food production [76], electricity production [163], industrial cooling [54]
Environmental use	Aquifer/groundwater recharge [60, 117, 155], supporting ecosystem (biodiversity) [28, 53]

SWH for household use (e.g. toilet flushing, washing clothes, etc., Table 2) is generally more accepted in the community compared to potable end-use. For instance, in a study conducted by Wu et al. [61] more than half of respondents agreed that using treated stormwater for non-potable household use can protect water sources and the environment. Notably, using SWH for toilet flushing had over 80% acceptance rate. Furthermore, for stormwater treatment for household end-uses, it is highly recommended to use passive NBS technologies such as biofiltration and ponds followed by advanced technologies such as ultraviolet (UV) and/or chlorination [52,109] since there is a some human health risk from the exposure to airborne droplets and aerosols [108].

Urban uses, especially green space irrigation, were one of the most widely desired in reviewed articles (e.g. [126,143,151,166]). Not only that they reduce the demand for freshwater resources, but also they could improve the visual appeal and increase biodiversity of these spaces [34]. SWH has also been used to supply water for urban cooling [6,7] and water art features [161,164]. The treatment requirements are less strict compared to close contact end-uses [119,126]. However a recent study [34] has found that some green systems, like wetlands, may not meet the required treatment standards (e.g. microorganism treatment) for watering green spaces. Therefore, it is also important to ensure appropriate treatment is implemented.

Stormwater is also a promising alternative for agricultural water use. Specifically, by utilizing SWH, farmers can retain their crop yields while simultaneously minimizing their water expenses [76]. This reduction in costs and the decrease in drinking water usage has been noted in other industries as well (e.g. metal and cement manufacturing industries and wool processing facility [165]), which could ultimately lead to lower product prices [54]. It is known that these end-uses are economically viable, yet there is limited information related to the other kinds of industrial end-uses, e.g. electricity production [163], agricultural irrigation [28], construction applications [105], food production [76], industrial cooling [54], etc.

SWH systems can also be designed with environmental protection in mind. Treated stormwater can be harvested and used for groundwater aquifer recharge [60, 117, 155], especially in areas with heavy agricultural activity that can deplete and pollute groundwater reserves (e.g. Israel [203]). However, MAR is best suited for areas with favourable hydrogeological conditions such as high permeability aquifers, permeable soils, and good groundwater quality, like Salisbury, Australia where MAR has successfully augmented water supplies and mitigated drought impacts [110]. Harvested stormwater has also been used to establish natural water corridors (artificial wetlands, creeks, etc.) for flora and fauna biodiversity protection (e.g. Lynbrook Estate, Australia [181] and Melbourne Botanical Gardens [204]).

Despite the variety of possible applications, SWH provides the best value for non-potable end-uses such as household applications and urban uses, which have higher community acceptance [61] and require less intensive treatment [119, 126]. Implementing SWH for these purposes not only helps to alleviate the demand for conventional freshwater sources but also contributes positively to environmental outcomes [151].

4.3. Post-implementation phase

After the implementation of SWH systems, it is important to maintain the performance over time through routine operation and regular maintenance [125]. It is also essential to assess the impacts of SWH on different non-structural aspects, e.g. social and economic [78]. Not paying attention to these factors can lead to failure in delivering on the expected SWH outcomes [10, 41].

4.3.1 Operation, maintenance, and monitoring

System operation and maintenance can guarantee the system's longevity and consistent performance [125]. Routine monitoring of SWH system’s treatment performance is suggested to assess the impacts of climate change and aging of the system’s components [106,107,125,205]. Burns et al. [41] suggested that a maintenance plan, with sufficient resources allocated for monitoring, can identify weaknesses and potential for the system’s future improvement or novel system development. Nevertheless, although a considerable amount of research has been done to suggest optimum designs of NBS and other technologies for SWH, ongoing maintenance has always been lacking, which is largely due to related cost [40,125,155].

To solve these challenges, a possible solution could be to develop an automatic low-technology, self-managed system, as suggested by Nicles and Lucke [71]. Employing low-cost sensors for real-time monitoring data could also be one promising approach to inform the need for maintenance and reduce costs. However, such smart systems may also face limitations due to the system's dependency on unpredictable environmental conditions [172], as well as the additional need to maintain the low-cost sensors [132]. Other suggested solutions could include the development of technical guidelines and better identification of responsibilities for maintenance [169]. For example, a guideline for disposal of the collected pollution from treatment systems such as biofiltration systems and understanding the clear connection between the maintenance process and the quality of yield is essential, as suggested by Payne et al. [132]. At the same time, issues around consistency of education for maintenance staff have been noted, as well as responsibility for SWH system monitoring and ongoing maintenance [170].

After SWH implementation using NBS there is an expectation of beneficial microclimate changes in urban environments [6, 7]. Monitoring the effects of SWH on microclimate can be beneficial in increasing awareness of these systems and quantifying benefits, which increasees capacity for ongoing funding for maintenance. Furthermore, with the introduction of SWH the flow regimes in urban creeks and streams can significantly change, which can lead to adverse effects on local biodiversity. Monitoring and management (possibly through period release of harvested stormwater) should be taken into consideration after system implementation, to secure biodiversity and livability outcomes [28].

4.3.2 Non-structural aspects

Social: To serve as an alternative water resource, SWH faces challenges in gaining acceptance from the general public [61]. After (but also before) SWH implementation focus should be on professional and community education about the systems’ performance, operation, and community expectations [179].

Although mobilizing community networks may enhance policy support, some social groups may need additional targeting to build support [160]. Their involvement is positively associated with support for alternative water sources, influenced by stronger water-related norms, knowledge, and increased recall of water-related information [37]. Community education programmes have been implemented to raise awareness about the benefits of SWH and encourage adoption, often involving educational campaigns in schools, community events, and outreach programmes [174, 179]. So that an effective communication and education campaign can help dispel misconceptions and concerns about SWH [159, 178]. Askarizadeh et al. [160] demonstrated that by altering the community's perception of SWH and enhancing their understanding of its advantages, it is possible to influence decision-making processes and implement appropriate policies. Consequently, co-designing or planning stormwater infrastructure with community involvement has become a subject of interest in recent studies, with Yu et al. [67] proposing involving the community in SWH management planning to ensure its adoption and widespread use of the plan. Recently, culturally inclusive water urban design (CIWUD) has been promoted by Coyne et al. [176] and Naserisafavi et al. [69], emphasizing the need for increased stormwater awareness among the general public and highlighting cultural features in SWH practices to achieve better management outcomes.

On the professional side, research has demonstrated the impact of years of experience on experts’ comprehension of a system, highlighting the significance of education and knowledge level in SWH [177]. Through specialist training programmes, it is feasible to steer the design and planning of systems toward greater benefits and fewer adverse effects. Professional education for specialists can also enhance their efficiency in system installation, operation, and maintenance [175]. Additionally, in developing countries like the South African context, it is essential for water practitioners to engage with local communities and consider racialized histories to ensure the successful implementation and completion of projects [180]. This combined approach of professional education and community engagement can significantly improve the outcomes and acceptance of SWH systems.

Economics: To ensure the long-term feasibility of SWH practices, all costs and benefits associated with SWH should be considered in the post-implementation stage and compared to the pre-implementation assessments. Cost estimations may include various aspects such as the construction of stormwater treatment systems, collection and distribution systems, operation and maintenance, staff training, and the cost of reduced system reliability and efficiency over time. The implementation costs of SWH systems differ depending on their end-use [78,183], which would require different levels of treatment depending on the target population. Hagare et al. [38], found that in such systems, although the costs for treatment are lower than those for storage, it is the distribution system that incurs the highest expenses, highlighting it as the main financial challenge [38]. This can be offset by planning small-scale decentralized SWH systems with end-users close to the point of stormwater collection and treatment. Thus, assessing ongoing economic feasibility of fit-for-purpose SWH plans in management decisions is significantly important [57].

Kandulu et al. [40] conducted an economic assessment of ecosystem service benefits gained from the applied SWH system. The primary benefit identified was the significant cost savings resulting from the water quality improvement. The study also found that other eco-economic income, such as recreational amenities, greenhouse gas regulation, and habitat support services, were relatively insignificant. The net benefit of SWH ranged from A$1.10/kL to A$1.36/kL, depending on the alternate source to which it was compared. Interestingly, if a single utility provides both supply and treatment, more economic profit was found after implementation, as it would directly reduce their bottom-line operating cost. This highlights the potential for SWH systems to reduce system costs in addition to providing other benefits. In another study on an implemented case, it was shown that the use of custom parts and designs in a SWH project resulted in an increase in the final budget. This suggests that streamlining SWH designs and products by familiarizing the contractors with the infrastructure of SWH systems and providing prefabricated systems can greatly reduce the budget of future projects [181]. It is noted that ongoing maintenance costs are mostly ignored in the literature when considering cost assessment. Additionally, outdated pricing may contribute to unexpected costs during plan implementation [182].

While extensive efforts have been dedicated to analysing benefits across various aspects (as outlined in section 4.1.2), quantifying economic benefits remains challenging. This difficulty hinders the feasibility of a realistic cost–benefit analysis. Consequently, further studies and initiatives aimed at creating quantitative assessment tools capable of monetizing diverse stormwater harvesting (SWH) benefits are needed. This approach will facilitate a more comprehensive and accurate cost–benefit analysis.

5. A roadmap for practice and future research needs

In a comprehensive and optimized SWH practice, planning, design, application, and ongoing care must always be in line with the system’s main objectives, such as technical, environmental, social, and economic goals [78]. Despite the complex non-linear relationships among the various components of the system, planning to build SWH systems requires that all these components are considered in the localized, site-specific context. These components should be appropriate to the needs of the site, local climate, and regulations, but also social and environmental awareness of the communities of practice in the application catchment [125] to ensure the success and dissemination of positive SWH examples.

In the proposed roadmap for the application of SWH systems (Table 3), we provide guidance to practitioners based on the different stages of SWH implementation discussed in previous sections, as well as suggest future research needs. When embarking on a new SWH project, practitioners should first assess the social, environmental, and economic conditions of the site, while also considering potential end-use and technical requirements. Understanding relevant regulations and legislation is crucial, as practitioners need to navigate the requirements and seek necessary approvals and permissions for project execution. To promote the benefits of SWH in broader water management plans, it is essential to explore strategies for quantifying these benefits and attracting collaboration and funding from local or state organizations and industry bodies. Furthermore, conducting comprehensive risk and benefit assessments of SWH systems using robust tools and methods is vital to ensure their feasibility, reliability, and longevity. Future research could focus on reviewing global regulations, developing assessment tools that accurately quantify SWH benefits, creating effective planning tools, and collecting data for well-informed risk assessments, particularly concerning emerging contaminants. Integrating these aspects into decision-making processes will facilitate the selection of optimal water management systems, including SWH, and the strategic combination of system components within a robust spatial plan.

Table 3. Roadmap for practice and future research needs.

	Stages	Implementation considerations	Future research needs
Pre-implementation	Analysing site situation	o Understand site characteristics (social, environmental, economic) o Site opportunities and constrains o Define SWH aims, objectives and end-uses	o Global regulations review o Benefits assessment tools o Planning tools o Risk assessment data (emerging contaminants, pathogens, heavy metals, etc.)
	Understanding site regulations and legislations	o Find legislative bodies (local, state, and/or federal) o Discover adaptable regulations o Establish collaborations o Secure funding
	Assessing benefits and risks of SWH system	o Comprehensive evaluation of all risks and benefits o Explore the best methods for managing risks
	Multi-criteria decision-making	o End-use decision-making o Plan water management o System component combination o Spatial planning o Analysing geographical impact factors.
Implementation	Collection	o Consider the rate of water collection based on the site character o Careful placement of water collection equipment.	o Real-time monitoring & control (at every SWH stage) o Technologies for treating emerging contaminants o Risks associated with various end uses
	Treatment	o Consider nature of pollution o Reduce energy consumption and environmental impacts o Treatment plan selected by end-use requirement o Treatment rate based on site condition in peak events
	Storage	o Consider storage capacity based on end-use demand o Consider storage location o Consider possibility of infection in long-term storage and find a treatment solution
	Distribution and end-use	o Understand end-use requirements (quantity and quality) o Consider periodic water quality checks
Post-implementation	Operation and maintenance	o Maintenance plan: people and their roles, cost analysis, knowledge sharing, system and operation monitoring, performance assessment o Consider process automation (remote sensing, IoT, etc.)	o Maintenance guides o Monitoring & operational automatization o Community studies o Economic feasibility studies
	Social assessment	o Consider cultural and heritage values o Implement methods to increase social acceptance. o Consider the effect on different community groups and find solutions for fair distribution of benefits
	Economic assessment	o Find methods to assess economic benefits of other aspects (e.g. environmental benefits, water savings, etc.) o Find the most sustainable cost for the system (cost-benefit re-estimation)

During the implementation of the SWH (Table 3), practitioners should strive to stay informed about state-of-the-art technologies for stormwater collection, treatment, storage, and distribution, tailored to specific end-uses. Site characteristics are crucial for designing appropriate stormwater collection systems that capture and convey certain volumes of stormwater. Designing appropriate stormwater collection systems requires understanding site characteristics and their impact on water quality. Regulatory requirements guide the treatment of stormwater in SWH systems, necessitating solutions that meet quality standards while minimizing energy consumption and environmental impacts. Green SWH technologies, such as biofilters and wetlands, offer sustainable solutions but need reliability improvements during extreme weather events, i.e. floods and/or pollution spikes [57, 140]. Storage capacity should align with end-user demand, considering alternative methods in space-constrained urban areas. Long-term storage must address water quality degradation risks, with active treatment (e.g. UV, ozonation, etc.) or recirculation options that can present re-emergence of pathogenic organisms [206]. Testing water quality at end-use sites and understanding diverse end-user requirements are vital for system effectiveness and sustainability. Future research could focus on real-time monitoring and control of SWH systems, addressing water quality deterioration during storage and distribution, developing technologies for treating emerging contaminants, and assessing risks associated with various end uses.

In the post-implementation phase of the SWH roadmap (Table 3), a well-executed operation and maintenance plan is essential to ensure the system functions as intended. This includes not only personnel-related matters (e.g. their roles, knowledge, and capacity building), but also technical components (e.g. accessibility for maintenance), components that are prone to fail, performance monitoring, and the costs associated with all these activities (and who pays for that) [64]. For smart SWH systems, factors such as ease of use, costs, safety, security, and maintenance of monitoring devices need careful consideration [71,207]. Additionally, assessing environmental benefits (e.g. biodiversity, climate, greenhouse gas emission, energy consumption, etc. [40,75]), social acceptance, cultural inclusion, and economic feasibility are important. The involvement of the local communities in the project could reduce economic and social costs and influence further societal change, e.g. reducing water consumption, promoting urban greening, promoting environmental education, and even creating job opportunities [67]. Future research could focus on providing technical guidance for maintenance, defining maintenance responsibilities, implementing automatic monitoring and operational systems, conducting community studies, and assessing economic feasibility to encourage investor participation in SWH projects.

Eventually, we express the hope that the proposed roadmap will offer a holistic approach to implementing SWH. Simultaneously, it serves as a framework for transdisciplinary experiments, aimed at demonstrating the ability of SWH systems to achieve expected benefits at an affordable cost. This approach is anticipated to effectively highlight SWH's practicality and efficacy across different scenarios, offering extensive insight into its role in enhancing sustainable water management practices.

6. Conclusion

This review of the literature on SWH aimed to provide a comprehensive understanding of current research and practical studies in this field and the key drivers of interest in SWH. The review was structured according to the three main phases of a SWH system's life cycle: pre-implementation, implementation, and post-assessment. This approach allowed for a detailed analysis of various aspects of SWH and the identification of potential research gaps. The results of this review demonstrate the crucial importance of considering various factors and multidisciplinary approaches to SWH, such as regulations, decision-making, technology, end-use, social, and economic impacts, in the design and implementation of SWH systems. While drivers for SWH are diverse, they are mostly centred around technology and tool development and testing, while socio-economic analysis of SWH practices is lacking. Similarly, while SWH literature around system implementation and optimization is abundant, exploring regulations around SWH practices and post-implementation monitoring and social analysis is often lacking. Nevertheless, this review proves that there is significant research on this topic, which can help to popularize SWH practices around the world for more sustainable urban water use in the future. The roadmap developed in this study serves as a valuable guide for the practical implementation of SWH systems, as well as future research endeavours, ensuring a comprehensive understanding of the entire SWH system before undertaking new projects. Additionally, the findings provide valuable insights for researchers, practitioners, and decision-makers in the SWH field, contributing to the advancement of knowledge and promoting the widespread adoption of SWH practices.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Data availability statement

The data that support the findings of this study are available upon request. Raw data were generated at the Water Research Centre, School of Civil and Environmental Engineering, UNSW Sydney. Derived data supporting this study's findings are available from the corresponding author [KZ] on request.

Additional information

Funding

This work is funded by the Australian Research Council Discovery Early Career Researcher Award – ARC DECRA (DE210101155).

Notes on contributors

Niloofar Shoja Razavi

Niloofar Shoja Razavi is an urban designer and researcher with a background in sustainable development and resource management. She graduated from Imam Khomeini International University, Iran, with a master's degree in Urban Design and Engineering and is currently pursuing a master's in public policy at the University of California, Riverside (UCR), specializing in environmental policy. Her academic journey includes pioneering research on urban regeneration and the Sponge City concept. With professional experience in urban and interior design, Niloofar has contributed to various projects emphasizing sustainability. Niloofar's key interest is in water and environmental protection in a sustainable way. She focuses on the relationship between public policy and sustainable urban development, aiming to influence environmental policy globally. Her vision is to apply insights from her studies to drive sustainable urban planning and policy-making, bridging the gap between theoretical research and practical environmental policies.

Veljko Prodanovic

Dr Veljko Prodanovic is a research associate professor in Serbia, working at the Institute for Artificial Intelligence (part-time) and the University of Belgrade - Institute for Multidisciplinary Research (part-time) while maintaining an adjunct position at the UNSW's Water Research Centre, in Australia. Veljko has been working on modelling and validating the effectiveness of urban stormwater mitigation options (infrastructure and policies) on pollution levels and flooding, through exploratory and integrated model approaches and has done extensive research on different modelling approaches for urban water management (including AI-based methods). His key interests are Integrated multi-functional urban water systems, focusing on the interdisciplinary challenges in delivering green sustainable urban nature-based solutions (NBS) capable of treating various sources of wastewater and their social acceptance. Veljko is a recipient of the prestigious EU Horizon MSCA Postdoctoral Fellowship and is involved in different national and international projects around urban water design, planning and application with a recent focus on the application of AI tools and methods to support this process. Dr. Kefeng Zhang is a senior lecturer at the Water Research Centre, UNSW Sydney. He is a dedicated researcher who strongly focuses on stormwater water management and implementing nature-based green solutions (NBS) such as bioretention systems, wetlands, and green walls. His expertise lies in understanding the pollution and risks associated with various water sources, including stormwater, greywater, and pre-treated wastewater, and effectively utilizing NBS to manage these risks. Currently, Kefeng is engaged in developing frameworks for quantifying, controlling, and monitoring the risks associated with stormwater harvesting systems, which involves real-time control and monitoring of stormwater treatment systems (ARC DECRA project). By conducting this research, Kefeng aims to encourage the practical implementation of safe stormwater harvesting and water recycling while advocating for using NBS for water treatment.