Mitigating the Global Potable Water Crisis: A Systematic Review of Emerging Urban Stormwater Conversion Technologies ^†

Abstract

The wide-scale management and treatment of urban stormwater is a promising technological advancement to address the ongoing global potable water crisis. About 26% of the global population have unsafe drinking water and 46% have no safely managed water for sanitation. However, the lack of a regulatory framework for urban stormwater usage and uncertainty in water quality pose significant threats to its wide-scale application. A total of 76 articles were gathered through the Scopus database, 63 of which were screened individually for their eligibility, and only 35 articles were selected as the most compatible with the study. Emerging conversion technologies provide a more efficient and cost-effective means to convert urban stormwater to potable water. Urban stormwater management, such as capture in large cisterns and run-off capture, provides the necessary means to properly collect and manage stormwater, while engineered stormwater treatment systems, such as stormwater biofilters, provide high reliability and performance while having zero energy consumption. By utilizing an efficient urban stormwater management system and appropriate treatment technologies, urban stormwater can be used to alleviate the problem of potable water scarcity. This study delves into utilizing urban stormwater for the generation of potable water, evaluates conversion technologies and presents its applications to mitigate the global potable water crisis. Urban stormwater management systems are thoroughly examined and treatment processes are investigated, highlighting the importance of using appropriate technologies in potable water generation. Through conversion technologies, high-volume urban stormwater can be transformed into potable drinking water, addressing water resource management problems and the ongoing global potable water crisis. This systematic review will identify existing conversion technologies and research gaps and pave the way for more efficient and cost-effective conversion technologies that will use high-volume urban stormwater for the production of water.

1. Introduction

The world’s population has grown significantly fast, up to more than three times that of the population during the mid-twentieth century, and it is predicted to grow from the current 8 billion to 9.7 billion in 2050 and 10.4 billion in the mid-2080s [1]. This population growth has led to an increase in water usage, water pollution and climate change, threatening the conventional approach to using freshwater sources of water [2]. The percentage of the population that experienced urbanization increased rapidly from only one-third in the 1950s to more than half of the population [3]. The current 55% of the population residing in urban areas is predicted to increase to 68% in 2050, with 90% of the increase occurring in Asian and African regions [4]. Rapid urbanization, which is an effect of the increased population, pushes the need for supplementation of conventional water sources through the use of modern stormwater management systems, such as the use of aquifer storage and recovery systems [5]. This rapid growth of urbanization and climate change has exacerbated water stressors [6]. As a result, about 26% of the global population have no access to safe drinking water and about 46% have no safely managed water for sanitation [7].

This ongoing global potable water crisis, together with the increasing demand for potable water supply and increasing calls for sustainability, has resulted in the need to find alternative sources of water. Additionally, there has been an increase in interest in the utilization of unconventional water resources, such as reclaimed water and urban stormwater, to augment drinking water supplies [8]. This is because urban stormwaters are traditionally only managed to avoid adverse effects on properties, making them an underutilized resource [9]. Urban stormwater has the potential to serve as a valuable alternative source, as it requires less treatment as compared to municipal wastewater [10]. Its use can reduce pressure on fresh water sources and reduce the impact of runoff [11]. As an example, aquifers can be used as a means to recycle water to augment urban water supplies [12].

However, the main problems with regard to the use of urban stormwater for the production of potable water are the lack of a regulatory framework and the uncertainties with regard to the proper assessment of its quality. These cause hindrances to its wide-scale adoption [13].

2. Methodology

This study utilized the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines for the selection and identification of relevant publications in the literature on the study of urban stormwater-to-potable water conversion technologies. All relevant studies were obtained from the SCOPUS database. Figure 1 summarizes the process of selection of relevant studies using the PRISMA guidelines.

3. Bibliometric Analysis

Recent and emerging areas in the field of urban stormwater-to-potable water conversion were identified using bibliometric analysis. All relevant studies were summarized in terms of all the keywords chosen by the authors of the studies (co-occurrence) or in terms of the authors involved in the publication of these studies (co-authorship). Relevant studies obtained from the SCOPUS database were subjected to VOSviewer version 1.6.20 to generate a map of them. These studies were mapped in terms of the year of publication (overlay visualization), interconnectedness with other relevant studies (network visualization) and how densely concentrated these studies are (density visualization).

3.1. Co-Authorship

Network visualization was used to establish the interconnectedness of authors through their collaborations. Out of the 270 authors identified in the Scopus database, the largest set of connections involved 36 authors, as presented in Figure 2a. The colors represent the different research groups or clusters based on their collaborative relationships. Additionally, the size of the nodes represents the weight of the authors in terms of the number of collaborations, larger nodes means more collaborations with other researchers.

Overlay visualization was used to identify relevant authors in terms of their years of publication. As shown in Figure 2b, the study entitled “Urban stormwater to enhance water supply”, authored by Richard G. Luthy, together with Sybil Sharvelle and Peter Dillon [13], served as the link between past and present-day studies related to the use of urban stormwater for potable water generation. Richard Luther became the cornerstone of present-day studies, based on his collaboration with several authors in the 2020s, while Peter Dillon served as the backbone of past studies, having published papers in the early 2000s.

Density visualization was used to identify the authors who published the most about the utilization of urban stormwater for potable water conversion. The colors show the density of the authors in terms of the number of their publications. Yellow areas have high density, green areas are moderate density, and blue areas represents low density. As presented in Figure 2c, Declan Page, Karen Barry, Peter Dillon and Richard Luthy are the authors who published the most.

3.2. Co-Occurrence

Co-occurrence analysis analyzes the keywords used by authors in their publications. Network visualization provides an insight into the different keywords used by authors and the interconnectedness of the keywords with each other. For the network visualization to truly represent the studies, several keywords were removed and only the top 1000 keywords were used. The keywords that were removed were the following: ‘article’, which was used 27 times; ‘nonhuman’, used 10 times; ‘controlled study’, used 10 times;; and ‘priority journal’, used 8 times. The colors represent thematic groupings of the keywords used in the study which groups interrelated keywords together.

The overlay visualization was used to identify keywords in terms of year of publication. This showed future trends of studies based on keyword use. As shown in Figure 3c, future trends for studies will involve the use of machine learning and artificial intelligence for higher data accuracy. Additionally, treatment of emerging pollutants using emerging treatment technologies is another future trend to be looked into. Climate conditions, best management practices and city management are also fields that will potentially be studied in the future.

The density visualization was used to identify the keywords most used by authors in their publications. The colors represent the density of usage of keywords in publications. Yellow areas are high density areas where keywords are used the most across all of related studies, green area with moderate density which could represent significant but less frequent occurring keywords, and blue are having the lowest density which would indicate keywords that are rarely used or could potentially indicate emerging or specialized fields of studies or topics of interest. Based on Figure 3c, the most-used keywords were potable water, water quality and runoff. This highlights the key concept involved in this study, which is the importance of proper management of stormwater runoff and the establishment of water quality for stormwater to be used as potable water.

4. Emerging Technologies

Current and emerging technologies for the conversion of urban stormwater into potable water involve two main aspects: urban stormwater management and collection, and the treatment of stormwater. As summarized in Figure 4, urban stormwater management and collection can be performed using green stormwater best-management practices and emerging urban stormwater collection or harvesting systems, while the treatment of stormwater can be performed by treating pollutants and contaminants and through engineered stormwater treatment technologies.

Due to the increase in demand for water supply, together with limited sources and the increasing population growth, alternative strategies such as stormwater harvestry have been introduced [14]. Innovative stormwater management strategies utilize rainwater harvest systems for water reuse, which reduces runoff, which is typically managed through urban stormwater management systems [15]. The capture of stormwater is considered to be the new standard when it comes to sustainable urban stormwater management, since it supplements water supplies for water-stressed cities [16]. Additionally, important water management systems should include the utilization of urban stormwater or recharge of aquifers [17].

Rainwater harvesting systems (RWHSs) are alternative solutions that have the potential to increase water supply, which can increase water supply security and reduce the pressure on conventional water resources and urban stormwater drainage systems [18]. Water sensitive urban designs (WSUDs) are systems that incorporate the hydrologic cycle by managing water where it falls, which optimizes and enhances infiltration capacity through the design of infiltration systems where rainfall is often observed or through the rerouting of water towards pervious areas [19]. Aquifer storage and recovery (ASR) is a system that stores and harvests rainwater for its reuse, which can improve the quality of water through its passage in aquifers. This could improve the management of recovered water quality [20]. Water that is produced through harvest systems can be considered to be near potable water standards [21]. As an example, the Orange City Council in central NSW was the first council in Australia to implement urban water harvestry, utilizing water harvestry to increase drinking water supplies, which was achieved through engineered treatment processes and utilization of an existing water supply dam [22].

Aside from its collection, appropriate treatment technology is another crucial aspect for the conversion of urban stormwater to potable water. This includes the development of passive stormwater harvestry systems, such as stormwater filters, examples of which are copper-modified zeolite and granular activated carbon filters [23]. The utilization of aquifers as an added level of treatment alongside engineered water treatment can make urban stormwater more suitable for drinking [24]. Due to the Australian guidelines for water recycling, managed aquifer recharge (MAR) has been developed as a treatment barrier to assess risk for recycled water systems and ensure the safety of recycled water for use [25].

One real-life example where urban stormwater is properly managed is Singapore’s lower Seletar/Bedok water scheme, where urban stormwater is collected using urban catchments and serves as a significant drinking water supply. However, it can only ideally be used if the catchment system is effective [26]. Another example is the primary aquifer at Atlantis, which is suited to providing a water supply for indirect recycling of urban stormwater and a site for the treatment of domestic water, which can be used for the generation of potable water [27].

5. Trajectories of Research

Having the ability to predict levels of heavy metals in stormwater is crucial in the assessment of stormwater quality and provision of the most effective treatment methods [28]. Hence, one future area of research is the application of machine learning and artificial intelligence for optimization.

Effective collaborative efforts between government agencies and the scientific community will also be a future trajectory to further optimize urban stormwater conversion technologies, an example of which is the Atlantis Water Resource Management Scheme, which is successful through effective collaborations between hydrologists, engineers and urban managers [29].

Future trends will also include the construction of multipurpose facilities. This includes the construction of stormwater treatment systems that retrofit existing urban runoff. This would increase water quality, decrease costs and maintenance, provide potential wildlife habitats, and mitigate adverse effects of rapid development [30]. Additionally, economic analysis of treatment technologies, such as rainwater harvesting systems, should also be a priority. This would maximize resource efficiency while minimizing urban stormwater runoff and optimizing resource allocation [31].

Future studies could also explore the development of new or alternative treatment technologies. These could include low-cost natural water treatments, such as RBF [32]. Disinfection of stormwater, which is typically conducted only for drinking water and reclaimed water, could also be studied [33]. Furthermore, in the future, the combination of both conventional and decentralized urban stormwater management, which are employed in Singapore and Berlin, respectively, can be the most practical method, combining both of their strengths to produce an optimized and practical system [34]. This is because, in the long run, the utilization of centralized water infrastructures is not sustainable [35].

6. Conclusions

The utilization of underutilized urban stormwater as a potential alternative source of potable water can be a means of addressing the ongoing potable water crisis. Utilization of urban stormwater involves proper collection and management systems and appropriate treatment. The application of artificial intelligence or machine learning in management systems or treatment technologies, as a future trajectory of research, can further optimize the process of urban stormwater-to-potable water conversion and can help minimize the amount of resources used while maximizing its usage.

Previous review articles involved management systems, collection systems or treatment technologies singly and not in combination for the conversion of urban stormwater to potable water, which was explored in this study. Wang et al. (2022) only included pollutant removal technologies for stormwater runoff [36], while Jusić et al. (2019) explored new technologies for urban stormwater management [37].