Pathways for Cleaner, Greener, Healthier Cities: What Is the Role of Urban Agriculture in the Circular Economy of Two Nordic Cities?

Abstract

As major hubs for energy and resource consumption and carbon emissions, cities are at the forefront of the discussion on the impacts of megatrends, such as demographic changes, technological advancements, and the shift toward climate neutrality. Despite growing literature suggesting pathways for cities to cope with these challenges, the intersection between circular economy and urban agriculture for sustainable urban development has been little explored, especially concerning practical applications. To bridge this gap, this study aimed to explore the role of urban agriculture in promoting the circularity of resources at the city level. Aarhus, in Denmark, and Ås, in Norway, provide the empirical context for this discussion that uncovers the barriers that impact the successful implementation of C-E practices in the context of UA and delves into how these obstacles challenge cities in transitioning to circular and sustainable food production models. Using a case study approach and qualitative data sources, the findings suggest that while urban agriculture demonstrates potential in reducing resource consumption, it requires further evidence-based research and clear monitoring tools to assess its environmental impact and economic viability. Obstacles to urban agriculture implementation include regulatory challenges, social acceptance of waste, high investment costs, and limited recognition of its indirect impacts. Concerning recommendations, local governance and public policies were found to play a central role in fostering circular urban agriculture by promoting collaboration, fostering innovation, developing regulatory frameworks, and showcasing successful examples.

1. Introduction

The last decade was marked by high volatility and different shocks, from the aftermath of a global financial crisis and its political and socio-economic fallout to the more recent effects of the COVID-19 pandemic and the Russian-Ukraine war. The next 10 years are foreseen to be just as unpredictable, if not more, with several megatrends, such as demographic changes, technological developments, and the transition toward climate neutrality, expected to assume even greater importance.

In this context, cities are major hubs with a prominent role in the world’s energy and resource consumption and carbon emissions. In 2018, cities were already consuming around 70% of global resources and energy, producing over 70% of all greenhouse gases, and creating more than 70% of global waste [1]. Considering the projections pointing to 70% of the global population living in urban areas by 2050 [2], these numbers are expected to rise, emphasizing the urgency to transform urban dysfunctional ecosystems, in which natural resources are distributed, used, and disposed within the prevalent linear “take-make-dispose” economy, into more sustainable, resilient, and circular approaches [3].

Since linear, industrial agri-food systems are at the center of the socio-ecological crisis [4,5], and the role played by cities in the global transition toward more sustainable food production, consumption, and waste systems will only increase in forthcoming years, it is important to debate how to re-conceptualize urban food provision toward more closed loop paradigms and align it to sustainable development [6,7]. In this discussion, concepts like circular economy (C-E) and urban agriculture (UA) are worth addressing.

Given this background, the aim of this paper was to explore and discuss evidence concerning the potential impacts of UA activities in the transition to more resilient circular cities.

UA has been gaining importance as a strategy to tackle some of the social, economic, and environmental challenges cities face today [8,9]. Proponents of UA argue that it can promote agri-food sustainability and food security while delivering various ecosystem services with positive impacts, ranging from health benefits to the promotion of social inclusion [9,10,11], as well as supporting circular practices—such as regenerative techniques, pollution reduction, waste recycling, and efficient resource utilization [12,13]. Nevertheless, the literature also mentions drawbacks related to excessive resource consumption [14] and potential environmental contamination [15].

In reimagining cities’ role in the worldwide shift toward sustainable food production, consumption, and waste management systems, it seems therefore pertinent to discuss the role of a circular approach in cities. Considering the systemic, transformative nature of the C-E concept [16,17], change appears dependent on overcoming challenges of technological and economic nature, as well as socio-cultural factors. Nevertheless, the pathway is unclear, and different urban centers are facing different challenges when considering transitioning to a more circular UA paradigm.

Drawing on a case study analysis inspired by the recent literature [18,19,20], this study dives into two Nordic cases: Campus NMBU, in Ås Municipality in Norway, and two community gardens—“World Gardens” and “Brabrand Fællesgartneriet”—in Aarhus Municipality in Denmark. These cases demonstrate how different technologies can transform waste into resources for UA and thus are suitable to explore the following research questions:

• What are the barriers that impact the successful implementation of C-E practices in the context of UA?
• How are those barriers challenging cities in transitioning from linear, industrial agri-food systems to more circular and sustainable food production models? And how have innovative solutions been facilitating this transition?

The Nordic region has been highlighted for its strong commitment to cooperation and leadership in climate action, along with sharing similarities in planning systems and practices [21,22,23,24]. These similar commitments not only imply a potential for insightful findings, but also offer a chance for significant comparisons, given the similar systems in place. Despite employing different strategies and technologies, both examples have harnessed the potential of UA to contribute to closing the resource loop in cities. Therefore, the exploration of practical applications of C-E principles in UA, made available through the analysis of these cases, offered valuable insights into the different pathways in the pursuit of more regenerative and resource-efficient food systems in cities, as well as the main challenges linked to regulatory issues, social acceptance of waste, high investment costs, and limited recognition of its indirect impacts.

2. Literature Review

This section explores trends in UA and C-E research. It reviews the growing significance, controversies, and potential applications of the C-E approach in urban environments, with a focus on the role of UA in promoting circularity within cities.

2.1. Cities and the Role of Urban Agriculture

UA has been defined as the practice of food production within and around cities [25]. This umbrella concept is used to describe a range of multifunctional and multi-purpose practices that involve different actors and imply a variety of development options [26]. It can include commercial and non-commercial activities [25] and can be operated from intra-urban to peri-urban areas, on public or private land, employing a variety of more advanced (high-tech) to simpler (low-tech) technologies [27], including examples from community gardens to vertical farming [26].

Driven by global consumption and demographic trends, as well as increasing and rapid urbanization, UA is expected to play a crucial role in food production in the coming decades. It has been stressed as enabling shorter and more resilient food supply chains (e.g., with lower transportation costs and a lower food miles impact, as well as lower resource requirements concerning land, water, and fertilizers), supporting several ecosystems services (e.g., cooling, recreation, runoff mitigation) as well as reducing negative environmental impacts [28,29]. Additionally, in times of crisis and economic hardship, including rising energy costs and inflation uncertainty, UA can potentially alleviate poverty and reduce living costs while granting access to healthy food [30,31,32]. Overall, the benefits of UA have been underlined as supporting resilience, as it strengthens the capacity of urban systems to cope with shocks, ranging from climate change to other social, political, and economic challenges [9,29,33].

Alongside these benefits, however, some undesirable effects of UA have also been documented. These primarily relate to potential environmental impacts and risks, namely excessive water consumption [14], potential contamination of aquatic ecosystems, and water quality [15].

The impacts, as well as the challenges of UA, are nevertheless highly contextual and country-specific, dependent on the actors involved, purpose, land use, property, technology, and production system [25,31,34]. For example, in the Global South, UA has been identified as a means of survival and poverty alleviation, while in the Global North, its potential has been related to reducing negative urban development impact on the environment, improving the quality of life of urban communities, as well as improving physical and mental well-being and social inclusion [27,31,35,36]. Nevertheless, the current global challenges have emphasized the fragility of global food supply chains, blurring the South–North distinctions and calling for the re-evaluation of the concept, especially concerning its contribution to close resource loops in cities [31,35,37].

2.2. Circular Economy and Its Strategies at an Urban Level

At the same time that the conceptual debate on UA’s definition, purposes, functions, and overall impacts is still ongoing, UA as a sustainability practice is considered more viable when it becomes “circular (using regenerative practices, eliminating pollutants, recycling waste and maximizing exploitation of the inputs used)” [13].

C-E is an “economic system that replaces the ‘end-of-life’ concept with reducing, alternatively reusing, recycling, and recovering materials in production/distribution and consumption processes” [38]. Inspired by natural ecosystems, the concept conveys the possibility of moving away from the “linear” extraction, production, distribution, consumption, and disposal paradigm toward a permanently regenerative economy, focusing on circular flows of reuse, restoration, and renewability, encompassing the entire value chain [39].

While the debate on C-E’s “revolutionary” potential [40,41,42] is still on-going, its possible benefits to achieve SDGs [7,43] have permeated the global agenda. Overall, C-E has received immense attention in the global sustainability discourse: from the European Union Circular Economy Action Plan “(…) establishing an ambitious long-term path leading towards waste prevention and recycling” (EC, 2017a, p. 3), reinforced by the European New Green Deal [44,45], that influenced several Northern European countries C-E strategies and Action Plans [46,47,48]; to Asian legislative frameworks relating C-E to recycling initiatives, eco-parks, and eco-cities [49,50,51]; as well as recognition of the C-E’s potential benefits to the Global South [52,53,54].

As circularity strategies vary substantially depending on the geographies and contexts, the most common definitions encompass activities focused on the 3R principles—reduce; reuse; and recycle [17,39,55]—accepted in academia and C-E practices, and employed in global policy, namely on the European Union, United Nations, and Organisation for Economic Cooperation and Development. This recognition reinforces the potential of the C-E framework when addressing global challenges [49].

Although C-E objectives are straightforward—limit inputs, close loops/optimize resources, and avoid/transform waste—C-E implementation is a complex endeavor, especially at the urban level.

2.3. From a Linear to Circular Urban Agriculture

As cities are racing toward more sustainable, healthy, and resilient paradigms, C-E has emerged as a possible strategy [56,57], with metropolises such as London and Paris already deploying urban circular roadmaps [58]. At the same time that 80% of the global GDP was generated in cities in 2018, in a linear economy, cities are still “food deserts”, utterly dependent on production from rural areas [59]. The separation between “places of production” and “places of consumption” affects not only food supply but also waste management. When food and organic waste are the endpoints of a linear production-consumption-waste system, cities must contend with an expensive waste management problem [5]. Introducing the C-E concept at a city level can address these challenges, making urban environments more resilient, healthier, resource-efficient, and less dependent on external supply chains (Figure 1).

Cities are potentially ideal test beds for the implementation of C-E strategies. Urban environments concentrate and combine resources, knowledge, and economic activity in a limited geographical area. Cities have the capability to supply the necessary inputs (e.g., waste, byproducts) to develop circularity, while simultaneously implementing strategies that close the loop by recycling such materials and waste [13,59].

There is, therefore, a case to make concerning cities’ role in implementing C-E strategies [58,59,60] and UA part in fostering the transition from linear to circular systems [61].

UA systems can be designed considering regenerative cycles [5]. UA has been found to contribute to resource-efficient food production through the application of circularity strategies such as reduced transport of food products, reduced food waste, reuse of nutrients, use of underutilized spaces, and smart water use [62,63,64]. Transformative processes in UA systems can also enable circular resource flows, reincorporating resources that would otherwise be wasted [12,35,63]. However, despite the apparent interconnections and interdependencies between UA and the implementation of circularity strategies at the city level, a limited overall debate, along with conflicting perspectives, exists on the practical realization of these ideas [5,12,65,66]. The literature points out that, as market forces are the primary driver of land ownership in cities, UA is regarded as a low priority due to preference given to the “highest and best use” that rules land-use planning [67]. UA tends to be viewed by planners as “a placeholder or interim use”. Therefore, embedding agriculture within the fabric of the city on a long-term basis is likely to be limited without specific planning provisions that safeguard the land for its operation, consequently restricting its capacity to adopt circular practices.

Furthermore, the economic potential of UA still lacks robust evidence, jeopardizing its acceptance as a desirable function in cities [68] as well as its impact on the development of circular business models.

Also, the literature has seldom addressed UA’s social externalities, although some studies have pointed out problems such as vandalism [69] and green gentrification [70]. Horst et al. (2017) [68] pinpoint, for instance, that the associated benefits of UA for health, skill-building and jobs, contribution to community development, and food security should be considered with caution as UA may benefit privileged communities, as well as contribute to marginalization and even displacement of socioeconomically disadvantaged households. These shortcomings relate and contrast with the recent imperative for C-E to address the human dimension to achieve significant social objectives, such as enhanced health, improved working conditions, and reduced inequality [71]. Therefore, the conditions that hamper the implementation of CE strategies within UA approaches have been little explored [5,12,66], reinforcing the need to identify obstacles and propose strategies to overcome them to optimize the use of UA to achieve more circularity in cities. A roadmap that uses UA to achieve more circularity in cities needs to identify obstacles of transition.

3. Methods

Inspired by recent studies [18,19,20,25,35], a systematic approach was employed to investigate the role of UA in cities’ circularity and identify the barriers that hamper the implementation of CE at a city level. As shown in Figure 2, this approach includes multiple case studies, different qualitative methods for data collection (further elaborated in Section 3.1), and a structured analytical framework (further elaborated in Section 3.2).

3.1. Case Study Identification and Data Collection

As a research strategy, case studies enable “an empirical enquiry that investigates a contemporary phenomenon in depth and within its real-life context, especially when the boundaries between phenomenon and context are not clearly evident” [72], allowing a greater understanding of the complexity of this phenomenon [72,73]. The use of case studies also opens the way for new findings that can foster new avenues of research when addressing topics that have not been explored sufficiently [74]. This strategy is, therefore, aligned with the exploratory nature of the research questions.

Globally, Nordic cities are frequently recognized as pioneers in climate action and circular economy frontrunners [21,22]. This makes Nordic examples a rich source of lessons and inspiration for other regions striving to address similar challenges. Also, notwithstanding different national contexts, the Nordic region’s unique governance and distinctive administrative structure are characterized by a balance between a robust central government and local government autonomy, as Nordic municipalities have a significant degree of autonomy in their governance [23,24]. This common ground offers an excellent opportunity for comparisons, allowing a deeper understanding of how these similar contexts approach sustainability challenges.

Within this context, and as in similar studies [18,19,20], the selection of case studies took into consideration their relevance concerning UA and C-E rather than their generalization possibilities. The two Nordic case studies, Ås municipality in Norway (specifically the University Campus Ås) and Aarhus municipality in Denmark (specifically the community gardens World Gardens and Brabrand Fællesgartneriet), can be juxtaposed for analytical purposes due to their numerous similarities in planning systems, governance practices, and their commitment to local sustainability [21,22,23,24].

The case study analysis also benefited from the data collected within the ‘Sino-European Innovative Green and Smart Cities’ (SiEUGreen) project, specifically the data used for the development of Action Plans and monitoring of activities for the implementation of circular systems in both case studies (see Sections S1–S5 for more information on the Action Plans). Both cases played a two-fold role in the implementation of the SiEUGreen project as they represented open labs that allowed testing both technical and social innovations in urban environments and different social, economic and regulatory challenges [75].

Different methods were used for the development and monitoring of the Action Plans. Semi-structured interviews were employed for different purposes. Firstly, they were used to identify the resources needed and the main challenges for the implementation, operation, maintenance, and usage of technologies (see Section S4). The outcomes of these interviews helped design the workshops carried out with the main stakeholders of the gardens in Aarhus and the site visits in both Aarhus and Ås (see

Table 1. Summary of data collection in the case studies.

Case Studies Data Collection
Aarhus (World Gardens and Brabrand Fællesgartneriet—both sites went through the same process) between December 2018 and 2021	Campus Ås between April 2021 and December 2022
Preliminary interviews before the start of the case studies (2018)	Semi-structured interviews on resource needs for the implementation (April 2021, see Section S4)
Semi-structured interviews on resource needs for the implementation (August 2019; see Section S4)	Site visit and interviews to understand main challenges and needs (October 2021)
In-person workshop to understand the main challenges and needs (September 2019)	Online follow-up meetings on the implementation and monitoring of the circular processes (three meetings between January 2022 and October 2022; see Section S5)
Site visits and interviews to understand the main challenges and needs (September 2019)	In-person follow-up meeting focused on evaluation and lessons learned (November 2022)
Online follow-up meetings and e-mail interviews on the implementation and monitoring of the circular processes (three meetings between June 2020 and April 2021, see Section S5)
In-person follow-up meeting focused on evaluation and lessons learned (September 2021)

) and were fundamental for the elaboration of the Action Plans for each showcase (see Sections S1–S3).

Secondly, semi-structured interviews were employed to monitor the implementation of the activities outlined in each Action Plan (see Section S5). This procedure included several meetings between the researchers and the main stakeholders of each case study, which followed the standard procedure of (i) discussing the advancements in the implementation of the technologies, (ii) the usefulness of the processes and activities proposed in the Action Plans, and (iii) the need to adjust the activities and/or the timeline for implementation if necessary.

In addition to the interviews, the site visits, workshops, and informal exchanges (e.g., e-mails and casual conversations) were essential for gaining further insights into the particularities of each case. A summary of the methods for data collection is presented in Table 1.

3.2. Analysis

This study is inspired by the most recent literature on C-E and UA [12,13,25], and the framework proposed by De Jesus and Mendonça [76] for the systematic exploration of C-E barriers. This framework, developed from an innovation studies perspective, identifies four main categories of challenges (see

Table 2. Barriers towards a circular economy.

Barriers
Technical	Related to technology availability and uptake, technological implementation, and technical know-how.
Market	Related to financial barriers to innovation, namely high initial costs, asymmetric information, and uncertain return and profit.
Institutional	Related to misaligned incentives, regulations mismatches, and inadequate institutional frameworks.
Cultural/Social	Related to limited social awareness and literacy concerning urban agriculture and rigidity of consumer behavior.

Source: Adapted of [76] to the urban agriculture setting.

) toward circularity that can be re-interpreted when examining specific UA examples and strategies.

This framework was helpful to organize the information from the case studies in the C-E–UA intersection, distinguishing between factors closely linked to techno-economic trajectories and others related to regulatory and socio-cultural issues. Following Auld et al.´s (2007) [77] decision tree, and considering the level of detail required to answer the research questions defined in Section 1, a qualitative manual analysis of the data was deemed the most suitable.

4. Results

The following section presents first some contextual background from both case studies. This is followed by the analysis of the main barriers using the evidence from both case studies.

4.1. Campus Ås

Ås is a municipality of Viken County located about 30 km south of Oslo (Norway). Within an area of around 101 km², the municipality includes several urban centers (Ås centrum, Kroer, Vinterbro and part of the urban center of Ski Municipality) that host a population of 21,350 inhabitants. Ås is one of the fastest-growing municipalities in the county, with approximately 1.5% annual population change between 2020–2023 [78]. Agriculture is the primary source of income, but one of the great assets of the municipality is the presence of the Norwegian University of Life Science (NMBU), which offers 64 educational programs to 6400 students while employing 1900 people.

At the same time that the municipality is expected to grow, triggering increased demand for infrastructure, natural resources, energy, and food in the future, the presence of one of the most significant interdisciplinary academic environment in life sciences in Norway allows for the demonstration of how different types of technology could connect UA and the urban waste resources via circular flows. In Campus Ås, a closed system was implemented, using two wastewater streams as input for growing food in a bubble green house. In this closed system, black water collected from 24 student apartments equipped with vacuum toilets is treated in an anaerobic digestion reactor. After this process, post-treatment methods allow the recovery of nutrients from the sludge (e.g., biochar and struvite), which are used to grow food in the greenhouse.

Grey water from one of the university’s office buildings is also treated in filters/tanks that use activated carbon and nanofiltration. After the treatment, the recycled water is used to produce soap bubbles that enable the greenhouse to reach and maintain higher temperatures than standard greenhouses and increase the possibility of growing food in a cold climate for a more extended period of the year. Figure 3 shows the bubble greenhouse and the reactor used to purify the blackwater and provides further information on the technologies. Figure 4 shows the schematic representation of the closed system in Campus Ås.

4.2. Taste Aarhus

Aarhus is one of 19 municipalities which make up the East Jutland Province (Østjyalland) in the Central Denmark Region (Midtjylland). It is the second largest city in Denmark, with a population of approximately 340,421, with the majority living within the urban core (around 93.7%) and the remainder dispersed throughout the municipality. The population distribution contrasts with the occupation of the territory as approximately 35% of the municipality’s area is urbanized, and 65% is dedicated to other land uses, including green open spaces. The pattern of occupation mirrors the significant population density of around 700 people per km² [80].

In the local economy, agriculture, forestry, and fishery activities contribute only 2% of total business. Nevertheless, Aarhus has been regarded as a ‘lab’ for UA, with more than 200 UA activities implemented across the city with the support of the Taste Aarhus program. Since 2015, this program, managed by Aarhus Municipality, has used UA to bring people together, activate underutilized spaces around the city and engage people in growing their food. This strategy has resulted in a variety of UA activities that take place in public and private spaces and restricted access areas (e.g., hospitals, schools, associations) [81].

Among these initiatives, Brabrand Fællesgartneriet and World Gardens were test beds for SIEUGreen technologies. A broad set of innovative strategies have been implemented in these gardens, from promoting the construction of polytunnels made of recycled materials to the construction of a solar dry toilet and further nutrient recycling from the waste which was tested as a soil additive for growing vegetables. Figure 5 illustrates and provides further information on the technologies tested in both gardens. Figure 6 shows the schematic representation of the closed system in both community gardens from Aarhus.

4.3. Analysing Barriers That Influence the Successful Implementation of Circular Economy Practices in the Context of Urban Agriculture

4.3.1. Technic and Technological Barriers

Technical solutions play a crucial role in harnessing the full potential of UA in closing the resource loop in cities. Many non-renewable resources, such as phosphorous, are becoming scarce, which is critical in the production of soil additives to optimize agricultural production [82]. Additionally, water scarcity affects 40% of the world’s population [83], highlighting the need to adopt sustainable water management systems [84]. However, at the same time that several recent technology developments—from light-emitting diodes (LED) to the use of 5G for automation and smart urban farming—have been drivers in the implementation of UA activities, the implementation and scalability of technical solutions are often hampered by different types of challenges [28,85]. When discussing the case study examples, it is important to recognize that both account for small-scale projects. Technical barriers can hinder the implementation of closed-loop systems on a bigger scale. For instance, the limited availability of large-scale waste segregation infrastructure can pose obstacles to the broader implementation of closed-loop systems. Additionally, transitioning from the current centralized to a decentralized sanitation system raises additional considerations regarding technical security. Security is already a significant concern in centralized systems, as the biogas produced in anaerobic digestion plants is composed of several substances (methane (50–75%), carbon dioxide (25–50%), water, nitrogen, oxygen, hydrogen sulfide, and ammonia), and some of them are extremely harmful to human health. For example, methane is highly combustible, and hydrogen sulfide and ammonia are poisonous. Transitioning to decentralized sanitation systems would thus multiply these risks as the increased number of treatment plants would put more places in danger [63,86,87]. From the waste management perspective, several lock-ins remain when reordering existing sanitation systems. As waste management methods require the engagement of a multitude of stakeholders and processes—not least of which the municipality—a change in the collection and treatment of residential waste would require major transformations of many urban systems [88]. On the other hand, the importance of skilled manual labor was also identified as a potential barrier. For instance, installing vacuum toilets and treating the blackwater requires expertise. Despite not being a limitation in Campus Ås, as researchers and experts monitored the system, this would probably become a shortcoming with the upscaling of the technology at the city level. This aspect points to the importance of addressing challenges to current management structures, new technical skills and tools, and even reshaping the roles of civil servants and consultants to enable them to cope with technical requirements to manage these systems safely and effectively. The members of Brabrand Fallaesgartneriet in Aarhus experienced a similar barrier with the operation of the solar dry toilet. Due to the waterless system, this toilet required regular and careful maintenance. In addition, the manager of the garden and the researchers involved in the experiment had to respond to unexpected issues, such as complications with the excess of liquid waste that infiltrated the soil and posed contamination challenges and caused undesirable odors.

Also, from the case studies, it seems that social acceptance and awareness toward these technologies require effective engagement strategies that foster the reshaping of social behaviors and address the negative perceptions of waste. In Brabrand Fællesgartneriet, composting the waste from the solar dry toilet successfully demonstrated the possibility of recovering nutrients from human feces, but its use as fertilizers for horticultural production was perceived as a health risk as many users were concerned with food safety. The lack of social acceptance of new technologies is a barrier to closing the loop (further addressed in Section 4.3.3). In Campus Ås, the technical capabilities of the vacuum toilets were also an issue. According to Bhatti (2021), who conducted several interviews with the users, the noise from flushing, unpleasant odors when emptying the waste tank, blockages, and vacuum pump failures were the main pushbacks to the technology. The interviewees also found the requirements to use only expensive toilet paper and non-chemical cleaning products inconvenient to ensure the functionality and safety of the system. Despite these shortcomings, many interviewees supported the technology as it enabled a significant reduction in water usage compared to its gravity-based counterpart and allowed for the recycling of nutrients from waste.

Different strategies, such as co-creation with users, advance research and information campaigns, concerning waste as a resource, and additional technical adjustments in the closed-loop systems, are primordial for improving users’ acceptability and supporting the new technologies’ long-term success [79]. In Ås and Aarhus, comprehensive communication campaigns were launched to enhance social acceptance and awareness of these technologies. These campaigns aimed to increase public awareness, highlight successful examples, and demystify the use of byproducts as fertilizers (see for example, videos about the transformation of human waste into resources for growing food in Campus Ås [89] and Brabrand Fallaesgartneriet [90]). Finally, further research on the economic potential of these technical innovations is essential to build a business case for up scaling these solutions in urban settings, as both cases had limited private funding.

4.3.2. Market and Financial Barriers

The economic benefits of UA vary significantly based on the type and process of implementation, presenting different challenges and opportunities related to spatial location, functional focus, market orientation, and even farmer income level [27,32]. Recent market research shows promising prospects for the global urban farming market, which was already worth USD 137.5 billion in 2021 and is expected to reach USD 281.9 billion by 2030 [91].

The technologies tested in Campus Ås resulted in the potential marketable option named GREENergy [92]. GREENergy is an integrated waste and wastewater management system, resulting in bio-based products in cities and districts. This circular decentralized treatment system demonstrated near-zero-emissions to water and air, converting resources into fertilizers and soil amendments to enhance cities’ resilience and promote climate-neutral urban development. However, the viability of such initiatives in a private setting and the limited success of previous examples have raised questions about feasibility (see Figure 7).

Public instruments, when designated as climate investments, confer legitimacy upon urban agriculture, potentially promoting their adoption. However, research indicates that these instruments may fall short in offering adequate incentives to achieve diverse sustainable objectives [93]. As for private financing, it remains a barrier as banks are cautious about granting loans in this new and uncertain market [94]. The initial capital investment necessary for establishing UA initiatives, such as vertical farming, is another issue that may hamper its markets [30,95]. Capital costs vary depending on the size and level of automation, and the reliance on highly skilled manual labor increases operational expenses, making it less competitive than food production in rural areas.

Less commercially driven types of UA, like those implemented in Aarhus, play a crucial role in the economy despite focusing on self-supply or small-scale production for local markets. Community gardens, for instance, offer social services and the potential to generate employment related to garden maintenance and governance, thereby benefiting the local economy and property values—an advantage to property owners and municipalities collecting property taxes [13,63,96].

Nevertheless, several barriers hinder the economic viability and competitiveness of UA products. Linear economy lock-ins, where chemical fertilizers are cheaper than bio-fertilizers, limit commercial motivations [94]. The limited crop variety grown through UA techniques also affects marketability [30,35]. For instance, interviews with farmers from different UA initiatives from the Taste Aarhus program revealed that none of them are motivated to grow food commercially.

Moreover, challenges related to land costs and access hinder the expansion of UA, especially when competing with residential and commercial interests [30,35]. For instance, in 2020, Aarhus Municipality indicated the peri-urban area where Brabrand Fællesgartneriet is a priority zone for urban development. This strategy has increased the land’s value and attractiveness, expanding the prospects of displacement of the community garden association from the area. During an interview, the association manager mentioned how this barrier impacted further activities.

“Fællesgartneriet [is] fighting right now against the municipality and the new plans for our land. (…) The possibilities in large-scale urban farming and peri-urban farming are sadly unknown to the planners in Aarhus; they want us [the gardens] to move out soon and stop what we are doing [in the gardens]”

(E-mail interview in December 2020, see Table 1 in Section 3)

Addressing the knowledge gap concerning circular business models for valorizing agro-waste and byproducts is another crucial aspect to discuss when debating the economic prospects of UA [97,98]. In Aarhus, UA appears to account for a small portion of food consumption. Although some small farmers’ markets operate semi-regularly, and farms where it is possible to buy directly from the producers exist, the majority of food consumed in the municipality is sold in large supermarkets that mostly offer food from imported sources [81].

While UA products may face challenges in mainstream commodity markets and compete against global prices, they can thrive in market niches related to the demand for circular, fresh, ethnic, and tasteful products with short food supply chains and strong producer-consumer relationships. These conditions can be enablers for marketing both agricultural products and services associated with UA [37]

Finally, the passive economic impacts of UA are still poorly recognized and accessed. In this regard, research can further enhance the ongoing discussion, particularly by emphasizing the importance of implementing transparent monitoring tools and conducting thorough evaluations of various UA typologies and their effects on the environment and the economy. Public authorities often overlook the passive financial gains of UA (e.g., ecosystem services) as well as other social impacts, namely on health benefits, alleviating poverty, and improving lifestyles [13].

4.3.3. Social and Cultural Barriers

Several studies have highlighted the diverse opportunities that UA can potentially foster in terms of promoting social inclusion [66,99,100], reducing poverty and crime [9,35,101], dismantling social barriers [10,101,102], empowering women [9,102,103], preserving and maintaining traditional knowledge and cultural connections [104,105,106], supporting human health and well-being [105,107,108], and improving environmental education [10,103,104].

In the Aarhus case study, UA proved to be a valuable tool in enhancing social capital and democratic participation, with different gardens fostering varied social bonds. Some gardens strengthened connections between acquaintances, while others facilitated new bonds among previously unrelated individuals. For example, a group of neighbors adopted an underutilized public area, which became a place for leisure and social interaction. Inspired by a documentary about the value of UA for sustainability, a woman mobilized a group of people to develop a vibrant social garden at the back of the churchyard [81].

Overall, the Taste Aarhus program promoted knowledge about sustainable and circular food production processes, fostered citizen participation and changes in consumption behaviors, increased environmental awareness, and disseminated C-E practices, emphasizing the potential of UA to provide fresh and healthy food. A survey with the participation of 49 out of 100 members of Brabrand Fællesgartneriet revealed that over 80% of respondents were motivated to engage in gardening activities to access fresh organic food and reduce the environmental impact of their food consumption [109]. Despite being less empirically assessed, changes in consumption behaviors, improved environmental awareness, and the dissemination of CE practices are some potential benefits of UA.

Impacts on mental health are also attributed to UA as it creates opportunities for people to socialize, fosters life satisfaction, and reduces stress and loneliness [13,94,110]. At Brabrand Fællesgartneriet, a survey showed that participants experienced relaxation, stress relief, and overall health improvements due to their engagement in UA [109].

However, social and cultural barriers can hinder the implementation of circular strategies in UA. Perceptions and misgivings about waste can limit the adoption of closed-loop approaches [13]. As Antić et al. (2020) [111] highlighted, the use of human waste as a fertilizer for growing food needs further research not only to clarify the implications it could have on human health but also to explain the benefits it can bring to the environment and food security. This aspect was noticed both in Ås [112] and especially in Aarhus. For instance, laboratory analysis showed that nutrients recovered from the human waste collected in the solar dry toilet in Brabrand Fællesgartneriet could be safely used as an additive for food production [113]. Nevertheless, informal interviews with the gardeners revealed their significant concerns about using human excreta in food production, primarily due to apprehensions related to safety and hygiene, fears of chemical contaminants passing from the fertilizer to food production and its possible impacts on human health and soil quality [79]. To alleviate these worries, information campaigns including newsletters, videos [90] and the involvement of key stakeholders through participatory mechanisms (e.g., engagement of the “urban gardeners” in the process, collecting their inputs and addressing their concerns) have guaranteed the successful acceptance and usage of the solar-driven toilet while demonstrating that is feasible and safe to use human excreta as fertilizer to grow food. These strategies point out the importance of addressing C-E and UA at the local level in close proximity to the community to enhance the likelihood of their effectiveness [39,114].

Alongside these social hesitancies toward UA, inadequate incentives to reuse bio-based materials [5] and the possibility of exacerbating gentrification processes [68,70,115] have also been mentioned in the literature as adverse effects of UA. In connection to gentrification, it is worth mentioning the process of revitalization of the Gellerup district where World Gardens is located. In Danish law, the Gellerup district is defined as a ghetto area, as the residents have a low income and high unemployment rate. The law enforces that such areas should be refurbished to counteract negative socio-economic trends. In Gellerup, this process included the construction of a new administrative building to host the municipality’s planning department, the embellishment of open public spaces, the rehabilitation of some residential buildings, and the construction of a new building for the community association. These measures have improved the spatial qualities of the neighborhood; nevertheless, they were not as positive for the social fabric of the area—the renovations provoked the eviction of some residents due to the demolition of several apartments during that process. Furthermore, some gardens that were located between buildings and acted as a source of food production and social interaction were razed for the sake of improving the open public spaces. While these changes aimed to enhance the area, they also caused unrest, resulting in acts of vandalism, including the damage of a polytunnel located in a public square [79].

This serves as a cautionary tale pointing to the important role of local governance and public policies in defining a strategy and supporting and promoting UA as a method for overcoming social barriers and anchoring circular practices that use waste streams to grow food in cities. Since local governments are responsible for a wide range of local infrastructure (e.g., water supply, waste management, sewerage treatment), they have a pivotal role in establishing a strategic view enabling practices and mechanisms (e.g., financial incentives, regulations) that encourage closing loops in cities and optimizing food supply chains, and at the same time, limiting societal inequities.

4.3.4. Regulatory and Institutional Barriers: Governance for a More Circular Urban Agriculture

The policy landscape and various political strategies, ranging from recognizing food, energy, and water as human rights and incorporating them into the UN SDGs to initiatives like the European Green Deal Farm to Fork Strategy, underscore the significance of these sectors in addressing global challenges such as climate change and economic, environmental, and social security. Public policy coordination and enabling framework conditions, such as legal regulations, taxes, incentives, and infrastructure development, play a pivotal role in addressing market failures, technological limitations, and socio-cultural difficulties to improve the circularity of resources in cities. However, despite the potential gains from linking food, energy, and water policy areas, certain limitations still hinder this development.

One of the primary hurdles is the regulatory frameworks, which can limit the utilization of waste streams due to varying and restrictive national regulations. For instance, despite the successful demonstration of recovering nutrients from wastewater in Campus Ås and the potential savings with fertilizers, the Norwegian legislation is restrictive concerning the use of wastewater for agriculture [116,117]. This control relies upon fears that wastewater may contain harmful quantities of organic pollutants or heavy metals, which can enter the food chain directly through irrigation. Further research on the hygienic risk of local use of waste resources and thorough routines for waste treatment, as well as coherent regulations, are still necessary [113]. At the same time, as mentioned in Section 4.3.1, upscaling the use of waste as a resource for food production in cities challenges current management structures. It requires transformative governance models addressing social/cultural challenges and fostering the diffusion of UA and C-E information for both UA enterprises and civil society. For instance, one of the success factors attributed to the operation and management of the solar dry toilet in Brabrand Fællesgartneriet was the employment of a janitor who took care of the waste [79]. These learnings underline the need to identify new professional roles and necessary skills to enable local administrations to drive the transition from linear to circular systems within UA.

Planning-related hurdles also exist, with many local authorities failing to integrate UA activities into urban plans [35]. In fact, territorial planning instruments and urban land use regulations have an essential role in officially acknowledging UA as a viable and worthwhile use of urban space [13]. In Aarhus, even if the municipality strategy is to become “a good city for all, where there is room for unfolding and diversity, and where we [the Municipality] together help those who need it” [118]), UA is seldom mentioned in the municipal plan. During an interview, a collaborator of Aarhus’s planning department mentioned that UA is attractive as a temporary measure to activate unused spaces in the city and bring people together, but it has little to do with planning. In his words: ‘When you plan, you plan for the future, not for the temporary’ (interview, 2018, see Table 1). The disconnection between planning and UA mirrored in his statement points to a narrow perspective on UA that overlooks the myriad of co-benefits it can bring for cities and, consequently, synergies with strategic planning. It also hints at the importance of strategic governance towards UA implementation within a circular approach: the need to develop a coherent roadmap to align policies at several levels and areas.

Looping resources through UA seems to require the development of urban transformative capacities, which encompass the “collective ability of all actors in an urban innovation ecosystem to conceive of, prepare for, initiate, and perform transformative change at social, organisational and ecosystem levels, thus enabling sustainable future development” [119]. An interesting example of an action fostering transformative capacity is the headquarters of the Taste Aarhus program, the “Green Embassy”, which is located at the main public square of Aarhus. This space not only provides information related to UA (e.g., where to find edible resources in the city) but also invites the public to co-create public spaces, as anyone can get support to initiate an urban garden. With a small but qualified civil service capacity (manager, communication, and garden expert), the Taste Aarhus program has been successful in enhancing cooperation between different sectors of the municipality—healthcare schemes, educational, and social programs [81].

5. Discussion

Cities play a crucial role in spearheading the global transition toward more sustainable food production, consumption, and waste management systems, highlighting the interconnectedness of urban development, sustainable lifestyles, and responsible production and consumption within the sustainable development discussion [6,7,37]. While the definition, purposes, functions, and impacts of UA are still subject of discussion in the literature, its successful realization seems contingent on adopting circular practices encompassing resource limitation, regenerative approaches, and waste recycling [5,29,33]. In this context, the Nordic case studies provided the empirical context for identifying, reviewing, and discussing the barriers that hinder the effective implementation of C-E practices at the city level within the context of UA.

Table 3. Summary of the recommendations to overcome/minimize the main barriers identified in the case studies.

Barriers in UA	Campus Ås	Aarhus
Technical	Address technologic limitations—adjust and improve existent sanitation methods as well as develop new, more efficient ones. Ensure supply of skilled manual labor. Need to address security issues regarding treatment of waste streams.	Ensure supply of skilled manual labor. Address security concerns related to the treatment of waste streams and its impact on food safety and health.
Market	Expand research assessing urban agriculture’s economic impacts (active and passive). Address limitations concerning the economic potential, financing, and business case for large-scale implementation of technical solutions.	Address the insufficient economic recognition of urban agriculture’s social and environmental benefits and the still competitive linear economy product prices. Address the potential of urban agriculture to generate employment and improve the local economy. Address the incompatibility of land costs versus financial returns of urban agriculture. Improve market niches linked with the demand for “circular”, “fresh”, “ethnic”, and “tasteful” products.
Cultural/ Social	Address cultural and behavior limitations to use waste as a resource. Foster environmental education and make available accurate information and research on health impacts.	Address cultural and behavior limitations to use waste as a resource. Foster environmental education and make available accurate information and research on health impacts. Find pathways to overcome weak incentives to reuse bio-based materials.
Institutional/ Regulatory	Address several regulatory limitations and mismatches concerning waste and waste management. Expand research on the hygienic risk of local use of waste resources. Rethink governance models addressing social/cultural challenges and foster the diffusion of urban agriculture and circular economy information for both urban agriculture enterprises and civil society.	Integrate urban agriculture and circular activities into urban plans and limit siloed development of planning policies.

summarizes the main barriers the case studies faced in implementing circular systems through UA.

As the case studies feature some of the most recent research on the intersection between C-E and UA, the findings and recommendations to address the different barriers can be seen as a springboard for a broader discussion on the impact of UA activities within a C-E approach. In that context,

Table 4. Examples of the impact of urban agriculture on case studies’ circularity.

C-E Strategy	Impact of Urban Agriculture on Case Studies’ Circularity
REDUCE: Input minimization and efficient use of regenerative resources. This strategy focuses on the prevention and reduction of raw materials and energy consumption.	Reduced use of artificial fertilizers by recycling nutrients in wastewater and food waste by making liquid and solid organic fertilizers in Campus Ås. Reduced use of tap water for irrigation by use of treated grey water in Campus Ås. Reduced use of peat soil as growth media by making compost from food waste, bio residue in black water and garden waste in Campus Ås. Reduced use of water with the installation of the solar dry toilet.
REUSE: Life-cycle extension and systems reconceptualization. This strategy is related to expanding/optimizing lifespans, re-conceptualizing products to greater lifecycles from the outset, facilitating maintenance, repair, reconditioning, and re-manufacturing options, and creating new business models.	Extending the lifespan/function of a product/resource through sharing, repair or refurbishing with the elaboration of polytunnels with recycled materials in World Gardens in Aarhus. Reconfiguring unused public spaces for urban gardening in Aarhus Municipality. Sharing of tools between the members of Brabrand Fællesgartneriet in Aarhus.
RECYCLE/RECOVER: Waste reduction, valorization, and minimization. This strategy relates to waste management and recycling of waste that cannot be reused or re-manufactured. It also involves using waste/byproducts from one process as raw materials for another, thereby ascribing a higher value to waste materials as potential resources that can feed production.	Polytunnels made of recycled materials in World Gardens in Aarhus. Black water turns into fertilizers, and grey water is used to make the bubbles in the greenhouse at Campus Ås. On-site composting of food waste and garden waste at Campus Ås Liquid and solid fertilizers (e.g., struvite) from biogas production based on black water at Campus Ås. Recovery of nutrients from human waste in Brabrand Fællesgartneriet.

Note: inspired and adapted from [17,39,120], and examples from the case studies.

adds to that, stressing how different UA activities that took place in the case studies actively fostered C-E strategies.

Overall, when considering UA, the context and the specific circumstances of each location matter. Factors such as social and historical elements of the local environment play a crucial role in determining the diverse forms of UA and circular practices used [22,121]. In the Nordic examples analyzed, there appears to exist a noteworthy emphasis on sustainability, as well as an inclination toward fostering resilient urban ecosystems [22]. Aarhus, in particular, is renowned as a ‘lab’ for UA [81]. However, these cases also face significant challenges due to the region’s climatic conditions, particularly long and harsh winters. These challenges can explain the focus on experimentation with technologies and activities, such as the use of bubble greenhouses in Ås and polytunnels in Aarhus, aimed at extending the UA season throughout the year.

Another important aspect of embracing UA is the need to re-conceptualize cities’ role in food provision under the inspiration of circular and regenerative principles and practices. By designing city food provision and UA systems with C-E regenerative cycles in mind, cities can experience numerous benefits from closing the urban resource loop [57]. However, achieving this transformation faces challenges due to existing components of the linear, take-make-disposal economy that reinforce barriers. These barriers encompass technological, economic, cultural/social, and institutional limitations, as demonstrated in both case studies.

Despite the existence of innovative technological developments that support and promote various UA activities across all aspects of the C-E strategies (i.e., 3R: reduce, reuse, recycle/recover), implementing these solutions requires further experimentation, namely large-scale testing [63,122]. When examining the case studies, it becomes apparent that their success can be attributed to their relatively small and localized scale. From a waste management perspective, there are significant challenges in reorganizing existing sanitation systems, as this requires substantial transformations in various urban systems and involves multiple stakeholders, including municipalities. Additionally, the transition from centralized to decentralized sanitation systems raises technical security concerns, particularly regarding the safety handling of biogas produced by anaerobic digestion (as seen in the Ås example). The encouragement of local research and case studies becomes crucial to identifying context-specific barriers and developing policy solutions to overcome these challenges.

Implementing technologic solutions also requires validation regarding their efficiency, positive environmental impacts, and economic viability [123,124]. On the economic front, it is of note that while the technologies tested in Campus Ås led to the development of GREENergy, it remains a potential marketable solution with uncertain prospects once the SIEUGreen project concludes, as the novelty of transforming waste into resources requires systemic changes and poses risks to investors. This aspect highlights how investment costs for some UA technologies and difficulties accessing financial support limit implementation. Additionally, competition from food produced in larger supply chains and the lack of robust business cases for circular resource models pose obstacles [98], as evidenced by the lack of commercial motivation in Aarhus. Moreover, the indirect economic impacts of UA, such as the beneficial effects on health, local economy, social inclusion, and poverty reduction [125,126], are often overlooked. This aspect was noticeable in non-commercially driven UA examples of Aarhus, in which UA directly competed with residential and commercial interests.

Even when the direct economic potential is limited, UA can yield valuable social and health benefits [9,10,108]. These advantages are also evident in the case studies, particularly in Aarhus, where urban gardens play a significant role. These gardens foster diverse social connections; promote knowledge about sustainable production and consumption, especially among vulnerable communities; and contribute to improving mental health. While technological progress, innovation, and the development of a business case for UA are all important aspects, the profound societal impact these activities can offer should not be neglected, as well as their impact on C-E implementation. Positive health impacts of UA, such as increased access to fresh and locally grown food, are often underestimated, hindering efforts to align UA with C-E principles of sustainability, reduced external resource dependency, and the development of C-E business models. Furthermore, UA’s social inclusion opportunities are not adequately acknowledged, undermining the establishment of C-E practices that emphasize inclusivity and community participation. The example of Aarhus underscores this challenge, as it illustrates the tension between UA and other urban functions. This aspect jeopardizes C-E practices in UA and emphasizes the crucial need to balance conflicting interests to implement sustainable initiatives successfully.

However, it is also important to note that behavior and cultural barriers have a profound impact when fostering a closed-loop approach to UA and C-E, particularly in overcoming prejudices against waste as a resource. This aspect was observed in Ås [112] and especially in Aarhus. Further studies, such as those conduced in Brabrand Fællesgartneriet on the safe use of biowaste [113], along with communication of these findings to the community and the involvement of various key stakeholders to ensure the success of activities through participation, need to be further explored as a means to dispel these misconceptions [39,114].

Existing research and the case studies presented in this study emphasize how government interventions shape UA and the central role of local governance in fostering closed loops in cities. One of the critical challenges is overcoming the siloed development of planning policies and encouraging collaborations within the urban innovation ecosystem. The Taste Aarhus Program provides a valuable example of minimizing this challenge. Supporting UA initiatives in schools, as well as rehabilitation and community centers, the program enhances cooperation across different sectors of the municipality [81]. The transition to a circular UA seems, therefore, to benefit from structural changes and the establishment of supportive conditions by local and national governments [127]. This includes monitoring environmental impacts, establishing sound regulatory frameworks, promoting knowledge exchange and demystifying C-E processes, fostering collaboration among urban and regional actors, and showcasing successful examples to develop a compelling business case for UA.

6. Conclusions

This study aimed to explore the role of UA in promoting the circularity of resources at the city level. By integrating UA and C-E, it explored practical applications of C-E principles in UA, offering novel insights when creating more regenerative and resource-efficient food systems in cities. Grounding the investigation in real-world case studies enabled the uncovering of different challenges cities face in transitioning to sustainable and resilient food production and waste systems. The Aarhus and Campus Ås case studies offered a Nordic perspective that could serve as a model for similar cities. The research unveiled distinct barriers within each case study: Campus Ås faced several technological challenges, while Aarhus grappled with social acceptance and urban planning obstacles. Nevertheless, both cases also shared common and comparable barriers. Despite the presence of innovative technologies capable of supporting various facets of circular UA, further testing and validation seem required to evaluate their efficiency and environmental impacts. Financial barriers, such as high investment costs and the absence of a robust business case, pose significant hurdles to full-scale implementation. Regulatory challenges and the need to foster social acceptance and awareness of the value of waste were also underscored.

The findings suggest that achieving a transition to circular UA hinges on the necessity for structural changes and favorable conditions. These conditions can be cultivated through government intervention, encompassing actions such as environmental impact monitoring, regulatory development, knowledge sharing, collaboration among stakeholders, and showcasing successful examples.

Several challenges and complexities should also be highlighted when conducting case studies on UA and C-E initiatives. Data availability and comparability limited the initial choice of case studies and implied extensive efforts to gather relevant information from various sources. Employing a restricted set of case studies also advises caution regarding the potential for generalizing the main findings and conclusions. However, this perceived limitation underscores the importance of conducting additional research in similar contexts and possible applicability to other geographies [22] and paves the way for further investigations. A more extensive analysis could encompass a broader range of cities to serve as inspiration and provide valuable insights on the role of UA in the transition to cleaner, greener, healthier, and overall circular cities.

Overall, the study adopted a multi-dimensional approach, combining circular economy principles with UA through practical case studies, providing essential lessons for cities aiming to enhance sustainability and resilience in their food systems. Therefore, from an analytical standpoint, the findings offer insights for policymakers, facilitating the understanding of urban C-E pathways when viewed through the lens of UA. This understanding can lead to better alignment of policy interventions. For other stakeholders, this research provides inspirational stories and signals regarding market interest in various technologies and circular activities. Finally, the findings are relevant for academia as this work contributes to the ongoing discussion surrounding the advantages and limitations of UA and the contributions of the C-E approach to achieving several SDGs.