3.1. Urban Resilience and Ecosystem Services
The findings of this extensive literature study indicate that the resilience of urban ecosystems determines the sustainability and resilience of the urban environment. In other words, urban sustainability and resilience depend on the resilience of connected urban ecosystem services provided by different environmental and economic sectors [13
]. Ecosystem services are benefits provided to humans through the transformations of resources into a flow of essential goods and services, such as provisional (food and water supply), cultural (recreation aesthetic values), regulating (water purification, erosion control), and supporting services (nutrient cycling, water cycling) [14
]. The resilience of urban ecosystems depends on the ecosystem’s biodiversity quality and quantity, including plants, animals, and microbial organisms [15
]. Further, the diversity of urban organisms is the direct source of urban ecosystem services. It must be noted that an urban ecosystem is a human-modified ecosystem that requires continuous maintenance and management intervention to provide the intended services [16
]. In planning urban green infrastructure development, the significance of the network and multifunctional connectivity of the green infrastructure is highlighted in recent studies [17
This implies that strategic integration of ecosystem services is needed in urban planning related to urban stormwater management using green infrastructure. The need for strategic integration has been recognized to reduce inequality in the distribution of green infrastructure and associated ecosystem services in the disadvantaged socio-demographic and socio-economic areas of cities [19
]. In addition, urban ecosystem functioning changes over space and time; this is also the case for the quality and quantity of ecosystem services, in addition to their demand and production [20
]. Many researchers have recognized the importance of careful consideration of ecosystem services in urban planning, design, policy, and management [18
]. Nevertheless, there is a lack of studies about the integration of expanding urban farming with stormwater green infrastructure in a circular economy in a manner that maintains nutrient and water circularity while providing synergetic effects of ecosystem services. In this study, we explored means of enhancing locally produced ecosystem services, such as the provision of fresh food and supporting services of stormwater retention and water purification, through the integration of sustainable urban agriculture and stormwater green infrastructure. In sustainable cities with a circular economy, assessing the need and mismatch of ecosystem services must be considered in urban planning and management. This shows the importance of identifying an approach to the strategic integration of urban agriculture and stormwater management.
3.2. Urban Agriculture in a Circular Economy
Sustainable urban agriculture can play a significant role in the circular economy and circular cities in various ways. For example, sustainable urban agriculture provides foods and fibers to the city while reducing stormwater runoff and the combined overflow of sewers in a circular economy. Sustainable urban agriculture is considered to be a sustainable solution to address not only urban food insecurity, but also to reduce stormwater runoff [23
]. In addition, converting impervious areas to urban gardens reduces surface runoff and flooding [2
]. Finally, rainwater harvesting can be used to supplement urban irrigation water demand [24
]. Due to this ecological benefit, urban agriculture is expanding and is considered to be a form of green infrastructure [27
Sustainable urban agriculture is consistent with circular economy principles. The circular economy requires the utilization and reuse of natural resources and the release of by-products into the environment after treatment of waste. Sustainable urban agriculture practices include environmental wellbeing and social equity while benefiting the ecosystem. In these sustainable practices, many researchers have documented the use of organic fertilizer, compost, and biosolids to reduce nutrient runoff from agricultural sites while reducing the load on landfills by using organic waste [29
]. Further, avoiding synthetic pesticides is imperative to reducing the tradeoff of chemical pesticide control. Finally, safe urban farming practices can be applied to the greening and revitalization of brownfields [33
It should also be noted that the circularity of urban agriculture relates to resource recovery or waste reuse. Figure 3
shows the conceptual circularity of urban agriculture systems, including soil-based and soilless agriculture [34
]. Water lies at the center of both soil-based and soilless agricultural systems, including aeroponic, hydroponic, and aquaponic. In this sustainable model, fresh resources are reused via resource recovery, including that of water and plant nutrients [36
3.3. Stormwater Management in a Circular Economy
By 2050, two-thirds of the global population is expected to live in cities. This will significantly increase the consumption of natural resources and waste production, which may damage ecosystems [37
]. Rapid urban development, in addition to the increase in impervious areas, can make cities vulnerable and prone to stormwater-related damages, including flooding and disruption of aquatic ecosystems [38
]. This problem may exacerbate the challenge of conventional urban stormwater infrastructure, also known as gray infrastructure [39
]. The gray approach is designed to move stormwater away as quickly as possible from urban areas via pipes and conduits with a limited capacity, which can be overwhelmed during the wet season, thus contaminating local waterways. This centralized stormwater infrastructure applies a “Take-Use-Dispose” strategy, which is similar to the “Take-Make-Dispose” strategy of the linear economy. Traditional gray stormwater management follows a linear economic model and does not provide as many benefits as green infrastructure. This externality of stormwater gray infrastructure can be addressed by stormwater green infrastructures, such as bioretention cells, green roofs, tree trenches, retention ponds, or pervious pavements.
In contrast to the gray infrastructure, stormwater green infrastructure is a distributed model that treats stormwater runoff close to the source. It enhances locally produced ecosystem benefits by reducing stormwater runoff, recharging groundwater, reducing flooding, and increasing stormwater water filtration and source water protection [2
]. Despite these benefits, limited studies have addressed the role and challenges associated with expanding stormwater green infrastructure to support the mission of sustainable cities with a circular economy.
Several studies have recognized that water is central to the circular economy and that the circularity of water resource management is crucial for building sustainable cities [41
]. Stormwater green infrastructure addresses all three principles of circular economy [42
]: (1) designing waste externalities by optimizing consumptive use of stormwater for use in agricultural or evaporative cooling; (2) keeping resources in use by optimizing resource yields within water systems; and (3) regenerating natural capital by preserving and enhancing natural capital, such as river restoration, pollution prevention, reduction of energy use, and reduction of combined sewer overflows. Table 3
provides examples of stormwater green infrastructure that enhances water circularity. The result shows that stormwater GIs can address all three core principles of the circular economy.
Based on this review, a conceptual model of a circular water system is proposed. Figure 4
illustrates a conceptual model of an integrated circular water management (ICWM) system. The components of ICWM include source water and receiving water (rivers), water supply management (industrial, domestic, and agricultural uses), wastewater collection and treatment, and stormwater management and groundwater recharge. In the integrated system, each of the three main water resources categories is addressed, including potable water, stormwater, and wastewater collection and treatment. The wastewater collection and treatment systems include a municipal-separate-storm-sewer system (MS4), separate sewer system (SSS), combined sewer system (CSS), and wastewater treatment. The primary focus of the proposed ICWM is to reduce waste while maximizing reuse by linking all major components of urban water and wastewater systems within a single complex ecosystem. This proposed complex system provides the maximum benefit, including reduced sewer overflows, stormwater, or rainwater reuse, reduced demand for water supply, and grey and recycled water reuse. In the integrated approach, stormwater GIs plays an essential role in maintaining water circularity in the urban setting. This implies that stormwater GIs can serve as a tool for stormwater retention and treatment for runoff from UA and can also provide wastewater retention and treatment for combined sewer overflows (CSOs). The water treated by stormwater GIs can be reused for non-potable water uses, including irrigation, cooling, and groundwater recharge [52
This implies that stormwater GIs can have a multifunctional role in a circular wastewater management approach and can have a significant role in addressing the impact of CSOs on the global aquatic ecosystem. Combined sewer overflows have been reported to be responsible for degrading urban aquatic ecosystems globally by discharging untreated raw sewage directly into the receiving waters. Combined sewer overflow constructed wetlands (CSO-CWLs) have been proposed to address this challenge. According to several researchers, CSO-CWLs can effectively purify CSOs [52
]. According to Tonderaab [54
], this type of stormwater GI can remove 85% of ammonia and carbonaceous Biochemical Oxygen Demand (BOD).
In addition, CSO-CWLs can remove 70% of micropollutants and pharmaceutical products [56
]. According to Ruppelta et al. [57
], this type of stormwater GIs can remove 80–91% BOD, 60–85% Chemical Oxygen Demand (COD), and 80–95% Total Suspended Solids (TSS). Further, in areas where stormwater GIs is not possible, end-of-pipe systems can be applied, such as primary settling tanks, dynamic rotating belt filters, adsorption on granular activated carbon, and UV disinfection steps for further CSOs treatment [58
Furthermore, anaerobic digestion is a key part of wastewater management circularity because it uses waste to generate renewable energy and biogas and to provide wastewater stabilization for land application. Nutrient-rich biosolids can be used for urban agriculture, including urban trees. Food waste and organic matter from urban agriculture can provide carbon sources for anaerobic digestion. In addition, anaerobic digestion can be applied to treat solid waste generated by primary and secondary wastewater treatment systems designed to purify wastewater received from CSOs and SSS.
Rainwater harvesting is also important in the integrated cycle of water management and provides benefits such as reducing CSOs to improve wastewater treatment, reducing the cost of damage caused by floods, decreasing water demand, reducing the cost of drinking water production, aquifer recharging, and improving stream base flows [26
]. A recent study demonstrated that rainwater harvesting can provide up to 25% of the water that is used for washing and flushing, with no treatment, in India [45
]. In Australia, rainwater harvesting can satisfy up to 100% of toilet flushing and laundry use [60
] and 97% of drinking water demands [61
]. In China, the integration of rainwater harvesting, and drip irrigation can increase apple fruit production by 54% [25
]. In the State of Florida, rainwater harvesting can reduce flooding by 20% [26
]. It must also be noted that if rainwater harvesting is used to reduce flooding and CSOs, the water captured from impervious areas, including roofs in cisterns, should be used for some appropriate purpose before the next significant rain event [62
]. Overall, the literature study shows that rainwater harvesting is an important part of sustainable urban water resource management and urban farming in a circular economy.
3.4. Stormwater Management in a Circular Economy
Stormwater GIs can be designed to reduce runoff and flooding while providing local fresh foods. They are considered to be more adaptive strategies for both short- and long-term urban stormwater management planning than gray infrastructure [63
]. Nevertheless, integration of stormwater management and urban agriculture (ISMUA) is not part of conventional urban land use planning. In addition, there is also limited field data related to appropriate methods for implementing ISMUA. There is a knowledge gap and a lack of policy regarding the successful implementation of the ISMUA system. This study focused on possible means of implementing ISMUA for the maximum ecosystem benefit of the integrated system while highlighting approaches to reduce tradeoffs.
Based on an extensive literature study, three means of implementing ISMUA were identified: (1) use of stormwater GIs as food-producing systems; (2) use of urban agriculture as urban stormwater GIs; and (3) non-consumptive and consumptive use of harvested stormwater for urban agriculture and stormwater GIs. Figure 5
illustrates ISMUA. Previous studies have examined applying stormwater GIs for food production, such as green roofs or rooftop gardening, which are commonly used to retain stormwater and can be used for rooftop gardening. Recent literature studies [34
] extensively discussed how stormwater GIs can address urban food insecurity and energy conservation while retaining stormwater during peak flows. This approach focuses on the water–energy–food–ecosystem security nexus approach to contribute to the sustainable and resilient development of cities. The environmental benefits of green roofs are well recognized as mitigating the effects of urban heat islands and stormwater runoff management. According to Berardi et al. [66
], green roofs can reduce heat flow by up to 90% in summer and up to 30% in winter. In addition, green roofs can contribute to urban food production. For example, in Singapore, green roofs have produced about 35% of the domestic food demand [65
Nevertheless, the capacity of food-producing green roofs to mitigate stormwater quantity and quality must be verified using controlled field data. For example, adding compost to the growing media of GIs in the case of productive green roofs may have mixed effects on mitigating stormwater quality. Using growing media or soil mix with excessive plant nutrients and the application of mineral fertilizer could result in the potential leaching of nitrogen and phosphorus [67
]. In contrast, the addition of compost was reported to be a potential absorbent or treatment method for heavy metals from industrial and landfill stormwater [71
]. The substrate or growing media of green roofs is critical for the success of the system as a tool for stormwater quality and quantity management. In contrast to the most commonly used lightweight substrates for mitigating urban heat islands, and stormwater quantity and quality, sustainable food production on green roofs requires a greater quantity of organic matter and an optimum level of plant nutrients. This demonstrates the need to balance the effect of the green roof substrate by using stable compost materials. The addition of locally available recycled materials, such as biochar, perlite, and zeolites, are reported to reduce nutrient leaching [69
In addition, stormwater GIs can provide food production services while mitigating stormwater quality and quantity. Several studies have demonstrated that vegetable gardens can reduce stormwater runoff while providing good food yields [43
]. This indicates that characterization of GIs is important prior to using them to treat stormwater because stormwater carries a range of contaminants including, total suspended solids (TSS), heavy metals, polycyclic aromatic hydrocarbons (PAHs), nitrogen, and phosphorous compounds.
Furthermore, the benefit of stormwater for urban agriculture can be described in two ways. First, urban agriculture can provide the benefit of stormwater retention by reducing runoff during peak flows [73
]. Second, water harvesting in appropriate containers during rain or peak flows can be used to irrigate urban agriculture and for other consumptive uses.
Finally, other noticeable benefits of urban agriculture include purifying reclaimed wastewater and revitalization of brownfields [34
]. Treating wastewater using a hydroponic system allows nutrient recovery and reuse while producing food [74
]. Another study shows that wastewater supplemented with artificial lighting improved nitrogen removal [76
3.5. Enhancement of Ecosystem Services
Ecosystem services are a crucial component of urban sustainability and resilience. They are the processes that directly or indirectly contribute to sustainable human wellbeing, and thus they are considered to be “natural capital” [77
]. Table 4
illustrates examples of ecosystem services provided by integrated systems of UA and stormwater GIs. The quality and quantity of ecosystem services depend on biodiversity, which is a key component of the environmental benefits of urban agriculture. As indicated in Table 4
and Figure 4
, integrated stormwater GIs and urban agriculture management help cities to overcome the existing barriers related to urban agriculture and focus on the synergies of the ecological benefits of the two sectors. Integration of stormwater management and urban agriculture reduces, if not eliminates, possible sectoral disservices. For example, urban agriculture may result in ecosystem disservices, including nutrient runoff and competition for water from other services [78
]. Table 4
depicts possible enhancements of ecosystem services provided by integrating urban agriculture and stormwater management. This indicates, in addition to providing and regulating services, the integrated system can have a significant contribution to the creation of cultural, economic, and educational opportunities for a city.
Nevertheless, the selection of an appropriate type of stormwater GIs is imperative to efficiently treat stormwater quality. Table 5
illustrates typical examples of bioretention cells with treatment efficiency for different water contaminants. Based on this example, the removal rate of total nitrogen (TN) and total phosphorous (TP) by the bioretention cell can range from 30% to 90%, and 67% to 95%, respectively. According to Jay et al. [88
], bioretention cells can remove more than 80% of polycyclic aromatic hydrocarbons (PAHs). There is limited data available for the PAHs removal. Overall, one should note that the removal rate of bioretention depends on various factors including soil mix, design, depth of saturated zone [79
Another typical example of integrated stormwater GIs and UA is rooftop farming, also known as a productive green roof. It is often referred to as the food-water-energy nexus [62
]. This type of integrated system provides multiple ecosystem services, including stormwater management, food production, energy conservation, education, aesthetics, recreation, economic and social benefits. Brooklyn Grange of urban rooftop farm in the New York City [92
], and the Farm at Mill Creek in the city of Mill Creek, Philadelphia [93
] are among many such integrated rooftop farms in the USA. Furthermore, rooftop farming is popular in developing countries [94