1 Introduction

The need to transform cities into more resilient and sustainable urban environments led academics to develop several different urban typologies, such as "smart city", "low carbon city", and "resilient city", among others. Furthermore, it has increased with the effects of the COVID-19 pandemic since the measures taken to face this health emergency highlighted urban weaknesses related to environmental, economic, social, service, and resource provision [1, 2]

The circular city is one of the newest developed sustainable urban typologies and is highly present in China and European Union agendas. The first article identified in the Scopus and Web of Science databases on urban circular economy practices was "Material flow analysis in Guiyang", written by Xu Y and Zhang in 2004. However, the term "circular city", in the context of a circular economy appears for the first time in a scientific database between 2017 and 2018, with two articles: "Circular cities: mapping six cities in transition" [3] and "Circular Economy in the context of the alter-globalization"[4].

Implementing circular economy strategies in cities can disrepute if the scientific community does not collaborate with new paths and agendas for public policymakers to adopt [3]. Besides this warning, the following question arises: "does the circular city typology add value to urban sustainability?" In this context, there is a research gap regarding the circular city and its differences, similarities, and contributions compared to other sustainable urban typologies. To explore this question, this article aims to identify the characteristics of a circular city and compare them to other sustainable urban typologies.

A range of critical dimensions should be considered when evaluating sustainable urban typologies, such as energy efficiency, technological innovation, and sustainable land use. The power generation sector mainly contributes to global emissions and waste [5] Therefore, thinking about an efficient generation and energy consumption strategy is fundamental. Several commitments related to this concern have been adopted. EU is committed to achieving an energy transition and low-carbon economy by 2050, that is, a transition based on renewable energy sources [6].

However, the transition to new sustainable typologies based on low-carbon energy transition has led to issues such as energy poverty. In developing countries, it is due to high energy prices and the need for help accessing modern services. In developed countries, although modern services are more accessible than in developing countries, there is an increase in energy prices due to the penetration of renewable energy alternatives and global crises [7].

Another critical dimension is technology. Technological innovation is fundamental in implementing circular economy practices and other typologies like Smart City. An example of how technology can contribute to more sustainable economic development would be incorporating blockchain technology into a sustainable supply chain. This technology would eliminate waste from the entire supply chain using a cradle-to-cradle method [8].

Typologies such as Green City have also discussed the environmental dimension related to land use. Equitable management between planetary boundaries and human development needs is a significant challenge in sustainability-oriented initiatives [9]. An example of this trade-off is the land use of renewable energy technologies, such as photovoltaics and wind power, which compete with agriculturally arable land or biodiversity preservation [10]. In this context, the discussion presented in this paper regarding the relationship between the circular city and other sustainable urban typologies permeates strategies and characteristics of more sustainable urban environments that can contribute to aspects of energy efficiency, technological innovation, and sustainable land use, among other requirements of a more sustainable and resilient urban environment.

2 Frameworks and critiques

2.1 Definitions of circular cities

The practices that we now relabel as circular economy have existed for thousands of years since "all living things are open systems interacting with the environment through their mechanisms of metabolism” [11]. The word metabolism, in its original biological context, connotes the internal processes of living organisms that ingest rich energy—low entropy—and thereby perform essential functions of maintenance, growth, and reproduction. This process necessarily involves the output of waste that generates degradation and high entropy of materials [12]. Thus, before the studies on the circular economy, there was research on metabolism, which began in the fifteenth century and evolved until Karl Marx, in the nineteenth century, used the term metabolism to characterize the complex relationship between society and nature [13]. The studies about urban metabolism, also predecessors of the studies about circular cities, began with Abel Wolman, who in 1965 wrote the article "The metabolism of cities," where he states that "the metabolic need of a city can be defined from the set of all materials and commodities necessary to sustain its inhabitants in their homes, work and leisure activities" (p. 156); and that one should be aware that the planet is not able to assimilate unlimited garbage [14].

The origin of the circular economy occurs in works published since 1989 by a group of industrial ecology researchers who started to publish studies about the recycling of waste by the industry [15]. Concomitant to these studies, in 1994, the German Council of Experts on Environmental Problems established that environmental issues should be addressed through strategies that would contribute to the integration between production processes and natural cycles from the beginning of the supply chain. Thus, the "circular economy law" was instituted, which modified the German waste legislation [16]. In addition to Germany, Japan also applied circular economy strategies from the 1990s. Over the years, China has become a hotbed of circular economy cases and publications. Between 1990 and 2005, China developed national legislation and policies to implement reduction, reuse, and recycling strategies; supported circular economy research; and launched projects involving the establishment of recycling regions in ten cities and the implementation of forty-three pilot projects [17,18,19,20]

A city's metabolic flows involve all kinds of materials to sustain its inhabitants in all their activities, encompassing resources that enter, are in stock in the city, and leave it [14]. These flows require natural resource inputs and generate waste at the output.

Strategies from the circular economy can contribute to mitigating this environmental liability. However, it is essential to point out that there is a long way to go. There are criticisms related to the need for the strategies and practices of circular economy that produce products and materials that are quality substitutes to the primary products and at competitive prices, maximizing the potential of displacement of end-of-life goods [21]. For this, circular economy strategies need to be functional, creating systemic change through innovative collaboration structures, generation of social results, technical efficiency, and introduction of sustainable technologies [22]

In a systemic approach, the circular economy's strategies are promoted by attending to ten principles that form the so-called "R structure." They are: (a) refuse, abandon the function of a product or have it otherwise; (b) rethink, make the use of the product more intensive (sharing) or give more functions to the product; (c) reduce, increase resource efficiency or decrease or use of them; (d) reuse, reuse of the discarded product running in the same function by a different user; (e) repair, repair defective products so that the original function can be preserved; (f) reform, recondition old products to bring-in updated them; (g) remanufacture, use the functional components of the product to make comparable products; (h) reuse, use the product or its components in a new product with different function; (i) recycle, use the materials of one product for application in another product; and (j) recover, incinerate materials with energy recovery [3, 23, 24]. Therefore, the circular economy proposes replacing the end of life of products and materials (linear economy) by employing the ten principles of the “10R circular economy structure”.

A circular city can incorporate the principles of the circular economy in all its functions, establishing an urban system that is regenerative, accessible, and abundant in its structure and planning (by design). Thus, it aims to eliminate waste and keep resources active and with high value for as long as possible [25]. Furthermore, the circular city conserves and reuses resources and products by sharing and increasing the use and utility of all assets to reduce resource consumption and waste [26]. It demands a structure to exchange and share products and raw materials [27]

Most definitions of a circular city focus on material and energy flow with strategic actions related to producing goods and services, augmenting the use value of resources, and waste management. This strategy involves a modular and flexible environment with renewable energy systems and efficient energy use, accessible and effective urban mobility systems, waste reduction or transformation into a resource, and production and trade systems that encourage local cycles. The transition to a circular city also depends on adaptive governance that generates environmental, social, and economic results [28].

Social actors' actions are one of the main elements for circular economy strategies to work well in cities. Besides, the transition to circular cities requires infrastructure, investments, and the performance of public policymakers [3]. Research carried out by Yalçin and Foxon [29] analyzed the implementation of circular economy practices in the cities of Brighton and Hove in the United Kingdom. They considered that a multilevel transition involves ecosystems of natural flows, technologies, institutions, business strategies, and strategies applied by all social agents, from citizens to companies and government entities.

The transition structure to a circular economy can be top-down (top-down change) or bottom-up change (bottom-up change). The "top-down" transition is an institution-driven change, with the local government as the main incentive of public–private partnership strategies, policies and projects. The "bottom-up" transition originates from social movements and innovation, such as entrepreneurial initiatives and activities from society, third-sector entities and companies. When government actions drive the transition, the public policy framework for cities can involve knowledge development, collaboration platforms, regulation, fiscal framework and infrastructure [3]. The urban circular transition involves many elements like resource looping and adaptation schemes with flexible infrastructure. It also contributes to ecological regeneration of the natural cycles [30].

Economic activity is interrelated with cyclical natural environmental and sociocultural processes [4]. The transition from a linear to a circular city can impact all these processes by contributing to job creation, a cleaner environment, new businesses and the rejuvenation of existing ones. Thus, it transforms cities into cradles and catalysts for developing circular economy strategies for more regenerative and resilient environments [31]. A circular city structure demands circular metabolism of urban flows and stocks with share and regeneration of resources [32].

A circular city must have the following elements: environmentally sustainable buildings, resilient and renewable energy systems with reduced costs and positive environmental impact, accessible and productive urban mobility systems, incentives for an urban bioeconomy and production systems that encourage the creation of local value loops. In addition, the urban transition to the circular economy can be optimized through stimulating instruments, such as implementing urban planning policies, recycling, sharing and reusing materials within or near the urban environment, and using digital technologies [25].

The transition to a circular city demands strategies scaling. It is a multifaceted process [4, 33]. The transition path requires waste separation and reverse logistics for implementing closed-loop strategies. It also depends on the existence of industries for waste reprocessing in the city or its surroundings, intelligent systems for renewable resources management and the creation of symbiosis between all parts, activities and neighborhoods of the city, where one area can provide resources, waste and energy to others to benefit [34].

The transition to a circular city has the advantages of improving the quality of life, creating new jobs and business models, reusing and recycling routines that promote environmental cleaning, rejuvenating existing business sectors and creating new ones and improving competitiveness [35]. In addition, it can contribute to achieving SDG 11 by increasing inclusive and sustainable urbanization, reducing the per capita negative environmental impact of cities, and supporting positive economic, social and environmental relationships between urban, peri-urban and rural areas.

2.2 Analysis frameworks of circular cities and critiques

Four methodologies of transition to a circular city were identified in the literature review carried out in this article: ReSOLVE, circular cities hubs, circular city analysis framework and the methodology of the European Investment Bank.

2.3 ReSOLVE framework

The Ellen MacArthur Foundation developed the ReSOLVE framework. It organizes the transition strategies for a circular city into six types [3, 25]:

  1. (i)

    Regenerating: energy and materials regeneration.

  2. (ii)

    Sharing: application of reuse, rethink and reject principles.

  3. (iii)

    Optimizing: strategies related to life extension cycles (slowing loops) with remanufacturing, repairing and refurbishing principles.

  4. (iv)

    Cycling (loop): relates to closed-loop strategies by applying the principles of recycling, reusing and recovering.

  5. (v)

    Virtualizing: These strategies focus on using technology contributing to dematerialization.

  6. (vi)

    Exchanging: strategies for replacing old materials and products with renewable ones.

The ReSOLVE framework distinguishes between technical cycles (inorganic waste) and biological cycles (organic waste). Materials from the biological cycle can return to the system through composting and anaerobic digestion. This process contributes to the regeneration of living systems such as soil, providing renewable resources to the economy. On the other hand, technical cycles recover and restore products, components and materials through reusing, repairing, remanufacturing and recycling [36].

Critics of the ReSOLVE methodology consider that it is designed for companies rather than for cities, which are more complex systems, ignoring the land as a provider of natural resources and the need for infrastructure and adaptability of urban systems. However, if developed with comprehensive dimensions considering the city's consumption, scale and complexity, ReSOLVE can be applied in the urban environment for a circular city transition [37].

2.4 Circular cities hubs approach

The methodology of circular cites hubs emerged with a specific focus on application in urban areas. Its creation follows circular economy strategies applied in Paris, London, Amsterdam, Stockholm, Bristol and St. Petersburg, among other cities. Analyzing the strategies in these cities verified that the transition to a circular city could follow different paths.

For this reason, seven different subtypes of cities transitioning to the circular city were mapped. Each of these subtypes relates to a different set of transition strategies [38]:

  1. (i)

    The local city: it establishes local cycles of products and services consumption, organic food, renewable energy, local waste management and measurement of urban metabolism levels. This city subtype uses "location" strategies, flows of resources or activities within the city or region that can close the resource cycle. It focuses on the energy, food, waste recovery, recycling and reuse sectors.

  2. (ii)

    The looping city: it offers upcycling services and a structure that allows energy recovery and recycling and reuse of resources. This subtype of city uses "cycle" strategies, focused on closing the resource loop through recycling, renovation, recovery and reuse of resources (land, materials, infrastructure, energy and water) within the city or region.

  3. (iii)

    Substitution city: it promotes using renewable energy and resources, long-lived materials and products (slowing loops) and replacing structures and activities with more efficient ones through circular economy alternatives. This city subtype uses "replacing" non-renewable resources with renewable ones within the supply chain, resource-based activities with service-based ones, and non-durable with durable structures.

  4. (iv)

    Adaptable city: construction materials are recycled and reused, and there is resilience capacity to adapt spaces and infrastructures to different types of use. This subtype of city uses strategies to "adapt" urban spaces (infrastructure, buildings and spaces) to change conditions according to different needs, avoiding waste generation and closed technologies – which do not allow updating, for example.

  5. (v)

    Sharing city: resources have an extended lifecycle and are exchanged, rented, and shared. This subtype of city uses strategies of "sharing" resources within the city through strategies of exchange, sharing, collaborative consumption, rent, and loan, among others.

  6. (vi)

    Regenerative city: the natural resources of the city and its region are regenerative, with local production and consumption, reduced pollution, regulated hydrological and climatic processes, mitigation of contaminated areas, carbon sequestration and protected biodiversity. This subtype of city uses strategies to "regenerate" natural capital through support (nutrient cycling and soil production), production (e.g. energy and food) and regulatory services (carbon sequestration, climate regulation, hydrological cycles, etc.).

  7. (vii)

    Optimizing city: it promotes efficiency in the use of resources. This subtype of city uses strategies to "optimize" the use of resources through the efficient use of technologies and the general reduction of resource redundancy in the urban system.

Depending on the characterization of each city for size, location, socio-economic status and level of industrial and service activity, it can fall into one or more of the subtypes of cities presented in the model and thus fulfill one or more of the parts of the model. The greater the transition level, the more model parts the city will apply, generating a spiral of transition to a circular city.

2.5 Circular city analysis framework methodology (CCAF)

The CCAF structure aims to analyze the circularity in cities and demonstrate its multisectoral perspective. It is composed of the Circular City Diagram (CCD). This drawing shows the relationships established within the city and, in the sequence, the motivating agents (drivers) and the synergies or strategies involved. The first step of applying the CCAF involves the development of a structural basis that requires elements such as demographic data analysis, technological support, education and public policies [39].

For the CCAF, three tables are necessary. The first describes the motivating agents involved: sectors, agents, technologies, behaviors, indicators, objectives and current situation. The second table of synergies brings the strategies employed in each sector involved. Civil construction, transport, energy, food, waste and water management are examples of sectors. For each sector, sets of strategies are developed, such as generating of energy with waste, vertical agriculture and renewable energies, among others. The third shows policies, coverage levels (regional, national or international), affected sectors and alternatives to the identified gaps [39].

2.6 Methodology of the European investment bank

This transition methodology to a circular city considers that the R-structure principles of the circular economy require strategies that allow resource' sharing and renting. Some of them are cradle-to-cradle for products, components and materials, urban bioeconomy with urban gardens and composting, reverse logistics, digital technologies, shared urban mobility system, clean energy, local production within value cycles and industrial symbiosis, local renewable energy production, and modular constructions, shared and designed to be dismantled.

To implement these strategies, there is a planning, action and control structure based on fifteen circular steps for cities (Table 1):

Table 1 The 15 circular steps for cities. Source: [26]
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These transition models for circular cities are based on technical criteria and have a preponderant focus on meeting the circular economy's economic and environment dimensions. However, to add sustainable value, the transition to a circular city needs to evolve to parameters that also address the social dimension.

Implementing circular economy strategies and transitioning to a circular city should contribute to political and governance structures that contribute to democratic participation, social justice, and the strengthening of a local economy [40, 41]. (The social participation of citizens and civil society in the transition process of the city should engage the various segments of society in such a way that the roles of each social agent are discussed, as well as their social needs [42]. Another point is that implementing its strategies can contribute to reducing consumption and not only to reintegrating waste and materials back into the economy [43]).

Another point to raise is that the implementation of circular economy strategies in cities has advanced in recent years, both in developed and developing countries. However, it needs to move further in the latter. In Latin America, for example, urban waste management, health, and hygiene issues could be better worked with strategies, business models, and techniques derived from circular economy strategies implemented in partnership between public and private sectors [44].

Issues related to urban metabolism and circular economy have advanced in Asia. Singapore is an example of a city with several experiences implementing circular economy strategies and whose transition process results in experiences for other cities with similar characteristics [45].

The transition to a circular city must also be a path towards urban sustainability in developing countries. In this sense, examples of peripheral urban areas and slums where circular economy strategies occur (such as Kibera, Kenya; Dharavi, India; and Rocinha, Brazil; among others) must receive support so that strategies may become the starting point for think about the transition to a circular city in developing countries [46].

Understanding complex phenomena such as cities and their interrelationships with the environment can be an arduous task. In this sense, classification efforts are often undertaken to provide a richer understanding of these phenomena. The task of constructing a typology is to classify all observations into meaningful groups of mutually exclusive and jointly exhaustive categories. The goal is to minimize variability within the group while maximizing variability between groups. That is, each group member should be as similar as possible to the others but as distinct as possible from the members of other groups [47]

However, due to the diversity of typologies aimed at cities and their ideal prototypes, this classification is not always homogeneous and unanimous. An example of this is the typology of smart city. Many studies focused on this ideal prototype in developed countries. However, more research needs to be conducted in a developing country context, specific to urban typologies with socio-economic and culturally diverse backgrounds. In other words, a site-based approach would be more attractive to identify areas emphasizing smart cities that consider local advantages, disparities, and unique cultural characteristics [48].

Despite the conceptual limitations related to typologies, they have the potential for diagnostic and prognostic analysis. From the diagnostic point of view, defining the similar characteristics of a particular group allows a more accurate diagnosis of its possible members. From the prognostic point of view, typologies can be applied in creating prototype cities for future large-scale simulation, as they provide a framework for analyzing relevant future scenarios in different types of cities [49] That is, typologies can be used as a reference for diagnostic analysis in which the city already has the aspects common to the typology or to evaluate aspects that can be improved. Additionally, they can be helpful in prescriptive analyses to evaluate the transition potential for that typology in a given context and the path to follow.

Science results from heterogeneous groups of actors' work, ranging from researchers of diverse areas to professionals who implement actions on the research object. For this reason, the construction of knowledge generates tension centers that result in a boundary object, a theme, or a research object with a weak structure in the joint center to all actors who study it and a firm structure at the individual level. That is, it can have different characteristics, subtypes, results, and even concepts depending on the conditions in which it is employed or researched [50]. The circular city is a boundary object because of its multiple and differentiated translations in the literature that result in contradictions and contestations about this sustainable urban typology [51].

Not only the circular city but also all other urban typologies presented in this study can be considered as boundary objects due to the complexity and multiplicity of variables involved in its studies, such as dimensions (technology, energy, land use), social reality (developed or underdeveloped country, for example) and environmental and economic characteristics. In this context, this paper does not seek to deepen the diversity of concepts, subtpologies, and characteristics of each sustainable urban typology presented here. Moreover, it is also emphasized that these typologies can not even be compared methodologically, and each is strong at a specific point of the complex path to urban sustainability. The contributions of a smart city focus on technological tooling, those of the circular city focus on the life cycle of products and materials, those of the resilient city focus on adaptation and mitigation of the effects of climate change.

This paper focuses on verifying, from the main characteristics (weak structure in the common center) of each typology, how they interact with the concepts and characteristics of the circular city. The motivation for the research is the premise that all these sustainable urban typologies, including the circular city, are still in a border region, which, implemented alone, cannot meet the complex needs of a transition to a more sustainable and resilient urban environment. The smart city, for example, is the sustainable urban typology with the most significant number of publications currently. However, when analyzing the lists of the smartest cities in the world [52] the technological progress implemented contributes weakly or tangentially to improving sustainability indicators. Other typologies that can be put here as an example are the city with solutions based on nature and low carbon city whose strategies of implementation can contribute to the improvement of environmental indicators but need to cover the other dimensions of sustainability.

Thus, it is evident that the simple transition or implementation of a set of strategies of a single sustainable urban typology, whatever it may be, needs to offer more solutions for a more sustainable and resilient urban environment. In this sense, when this study presents the relationship between the characteristics of a circular city in interaction (and integration) with other sustainable urban typologies, it contributes to future discussions and research on the joint application of various sustainable urban typologies.

3 Methods

This article carries out a narrative review of articles in the Scopus database and reports from international entities through a qualitative approach. The Scopus scientific database was chosen because it contains most publications on the topics under study. A narrative review does not follow a defined protocol since it does not exhaust the sources and the search for publications follows the researcher's choices [53].

In addition to the central theme of this study, the circular city concept, other sustainable urban typologies appear in the research: sustainable city, green city, smart city, resilient city, eco-city, low carbon city and city with nature-based solutions. These typologies were chosen because they are the most cited ones in the Scopus database related to urban sustainability. Therefore, within the scope of this study, the term "sustainable urban typology" refers to a set of methodologies, characteristics, and strategies to improve the sustainability indicators of a city.

Table 2 shows the number of publications in Scopus, presenting at least one of the typologies studied here as a keyword. Only articles and reviews formed the database of this work. Besides, the sustainable urban typology had to appear in the publication's title, abstract or keywords. The circular city is the newest concept among other typologies and has the lowest number of publications, thus reinforcing the relevance of this study, as demonstrated below.

Table 2 Sustainable urban typologies publications.
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The term "circular city" from the Economic Perspective came up with studies related to transportation costs. The city is considered a central market to which all products must be transported for sale or export at free market prices. The main criteria of these studies were freight transport costs, size, and road structure of the city [54, 55]. Subsequently, the studies extended to other types of costs and arrived at the analysis of strategies of competition and cooperation between firms in an oligopoly and circular market (Central Business District). Those strategies to reduce costs motivated firms to form networks and partnerships with other firms more distant [56,57,58]. This circular city approach was not the focus of this paper. Thus, it was necessary to find studies concerning circular cities in the context of the circular economy, it was necessary to combine the keywords "circular economy" and "circular city". After performing this filter, all articles in the Scopus database were read for the circular city typology until June 2023.

For the other sustainable urban typologies presented in this study, the initial idea was to conduct a systematic review of the literature following the protocol of Chandler However, the publications selected by the research protocol did not meet the objective of this study because the empirical publications generally do not cover contents related to concepts, characteristics, and strategies of urban typologies, leaving many gaps or blind spots. For this reason, the research was restructured as a narrative review. It was used reports from entities such as the Ellen MacArthur Foundation and the European Investment Bank too. Table 3 presents the methodological protocol followed even though it is a narrative review.

Table 3 Methodological protocol Source: Own elaboration
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After defining the materials to be analyzed, we proceeded with the technique of global data analysis [59]. As a preparatory stage, a floating reading of the texts referring to the circular city was carried out, noting key terms. In the next stage, the structure was improved, highlighting the central characteristics and strategies of the circular city. The sequence was followed with the production stage of a spreadsheet in Excel, each row of the spreadsheet, the source of the material collected, and each column, the concepts, structures, and central strategies of the circular city and the model cities studied. The result of the analysis is detailed in Table 4.

Table 4 Published articles with “circular city” as a keyword.
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Once the central characteristics of the circular city were defined, the structured analysis of the textual material referring to the other sustainable urban typologies began. In this qualitative content analysis technique, we seek types or formal structures in the material. [59]. The material was extracted and condensed to certain content domains. That is, at the same time that we searched for unique and exclusive aspects of each typology, we also sought similarities between each typology and the circular city. Finally, it was possible to classify the material according to the interrelations in the form of scale (Fig. 2), which resulted in the four dimensions: (i) strong relationship—has many strategies that also exist in the circular city typology, (ii) medium relation—has an average number of strategies that are also present in the circular city, (iii) low relation—has few strategies of the circular city and (iv) without relation—has no strategy or characteristic that may be related to the circular city.

This formalization of the procedure generated a scheme of categories, which facilitated the comparison of the different typologies and the typology of the circular city. From the categorization, it was possible to synthesize the typologies in generic concepts (Table 5) and to deepen the interrelationships between the studied typologies (Table 6).

Table 5 Sustainable urban typologies.
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Table 6 Sustainable urban typologies and interrelationships with the circular city.
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4 Results and discussion: relationships

There are prospects of transition to a circular city as a simple sum of circular economy initiatives through circular businesses in these locations, while others consider the transition only when there is integration of the entire urban set focusing on a system. The transition to a circular city covers all the city's activities, sectors, services, resources, lifestyles, and practices. It integrates them into a contextualized and socially inclusive circular system [60]. In consonance with this assertion, it is considered in this paper that the term "transition to the circular city" is different from "city with circular economy projects." That is because, in the process of transition to a circular city, as will be demonstrated throughout this section, there is always a documented transition project/program that has as main features: (a) strategic planning involving several sectors; (b) development of strategies involving different types of social actors within each sector; (c) establishment of strategies covering as many principles as possible of the R structure of the circular economy; (d, e) action protagonist of public power through legislation, regulation and motivation of other social actors involved in transition strategies. Thus, if there are not all these elements, it is considered that it is not a case of transition to a circular city, but only application of projects or programs of circular economy in a city. Thus, the cities in Table 4 are those whose authors related their projects to transition to circular cities.

Based on the concepts in the table, a circular city adopts the 10R circular economy structure by implementing a structured and formalized set of strategies to regenerate, share, optimize, cycle and replace resources. Its higher level of transition involves, in addition, strategies of adaptation and dematerialization. Dematerialization situations are related to the "reject" principle of the circular economy, which usually uses virtualization strategies. Some examples are property rental services or replacing physical products with digital ones, such as books.

"An urban management strategy adapted to the needs of the future modifies the practices of the present urban system and projects its expansion, promoting well-being in line with the reduction of the inflow of resources"[66]. A study carried out by Jong et al. [67] in a review of 12 sustainable urban typologies considered that each one presents a different view of the form of governance and functioning of the city concerning infrastructure, the economy and its interactions with the environment.

The circular city differs from other typologies of sustainable cities – green city, for example—because these others focus on environmental indicators but do not focus on the economic results their strategies can produce [4]. The circular city is regenerative concerning the flow of resources, which contributes to the fact that, unlike the others, its strategies add economic value. These strategies contribute to maximizing the use of materials, product parts and products that have their value reestablished and continue to be part of an abundant economic cycle, rather than being discarded and failing to generate economic value.

The highest level of interaction between circular city and other sustainable urban typologies found in the reading of materials for this article was with the smart city. This interaction aims to implement higher-level strategies in the transition, such as those linked to reject and rethink, which generate virtualization and resource sharing. An example is apps that allow quick access to sharing services or renting resources ranging from cars to appliances and tools. Circular businesses promote these services and result in consumer cost savings [28, 36, 65].

Digital technologies applied to the circular city can positively impact environmental, cultural, social and economic dimensions. Moreover, this process strengthens the relationship between the circular and smart city as it contributes to a more sustainable urban structure [1].

The relationship between the circular city and other sustainable urban typologies, besides the smart city, requires further analysis. For example, the relationship between the circular and low-carbon city, since circular economy practices in the city and methodologies applied in low-carbon cities can come together in industrial symbiosis strategies, where one type of industry uses the waste of another [68]).

Research conducted by Anttiroiko [41] shows the process of structuring the transition of the urban region of Tampere (Finland) to an intelligent circular city. The governance structure is one of the main points for this transition, and it happens through the protagonism of the public sector in several coordinated instances of action.

Ref. [69] relates the definition of a circular city with nature-based solutions (NBS). Implementing NBS strategies can recover resources through water, nutrients and materials cycling. This connects the circular city to the Nexus structure, whose implementation of strategies contributes to food, water and energy cycling.

Langergraber et al. [69] demonstrated that there is a relationship between urban circularity challenges (UCCs) and urban nature-based solutions (UNBS). Among the UCCs for the shift to circular management of resources are the restoration and maintenance of the water cycle, treatment, recovery and reuse of water and waste, recovery and reuse of materials, energy efficiency and recovery, and building system recovery. On the other hand, urban NBS strategies, such as rainwater for urban design to improve infiltration and runoff, vertical garden systems and green roofs or urban gardens, can contribute to circularity.

Table 5 summarizes the information collected concerning each sustainable urban typology studied in this article regarding the concept and main characteristics.

Table 6 presents inferences from the literature review regarding the relationship between circular cities and other sustainable urban typologies.

The ReSOLVE and circular cities hubs approaches should be united for a transition to a circular city. Each principle of the R-structure of the circular economy relates to a set of transition strategies for a circular city that, in the union of the two methodologies, are as follows: localise, loop, regenerate, optimise, adapt, share, virtualise and exchange (Fig. 1).

Fig. 1
figure 1

Source: Own elaboration based on the ReSOLVE framework (EMF, 2018) and Circular Cities Hubs (Williams (2017)

Relationship between circular city principles and strategies.

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The diagram presented in Fig. 2 highlights the main characteristics identified in the literature review for each sustainable urban typology studied in this article. It is verified, through the figure, that the circular city, in the center of the figure, shares characteristics with all the other typologies studied.

Fig. 2
figure 2

Source: Own elaboration based on literature review

Characteristics of other sustainable urban typologies in the circular city.

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The diagram (Fig. 2) may represent a sustainable and circular urban system. Contributing to this is the perception that urban problems present in both small and large cities, such as pollution, low resilience to the effects of the climate change crisis, and waste management difficulties, among others, are examples that there is still a long way to go to make cities more sustainable and resilient environments. Furthermore, more than implementing strategies isolated from sustainable urban typologies is required. A set of steps involving strategies of different types can contribute better to urban sustainability.

This perception of a sustainable and circular urban system was based on inferences raised by Marjanović et al. [60] that consider that the transition to circular cities should go beyond implementing circular economy strategies. It should be more holistic in covering social issues, governance, the various social actors, and sectors of activity existing in the urban ecosystem, evolving thus into circular urban systems. In this sense, the path of transformation of current cities into more resilient and sustainable urban environments based on environmental and social aspects will not be possible with the implementation of strategies of a single sustainable urban typology but a set of them, according to the needs and reality of each urban environment (Fig. 2).

The literature review showed that the circular city has some relationship (strong, medium or weak) with all the other sustainable urban typologies studied in this article, with none where this relationship is non-existent. Based on the literature review, the green city, eco-city, and sustainable city are "conceptual" terms that integrate the idea that cities need to be sustainable. However, these concepts need a strategy or practice for achieving sustainability standards and indicators. These three typologies are, therefore, "ideals to be achieved" and have many similarities, although they are not synonymous. Therefore, the typologies of a smart city, resilient city, city with NBS and circular city are considered in this study as operational typologies, as they present strategies that can focus on improving sustainability indicators. Figure 3 shows the relationship between the circular city and the other typologies studied in this article and organizes these typologies into two nomenclatures: conceptual and operational.

Fig. 3
figure 3

Source: Own elaboration

Interrelationship of the circular city with other sustainable urban typologies.

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5 Final considerations

The main characteristics of a circular city are related to the fact that it is a city that: (i) locates flows that can become more efficient; (ii) cycles, shares, replaces and optimizes the use of resources; (iii) regenerates natural resources or environmental services as a result of applying its strategies; (iv) adapts products, services and activities to make resource use more efficient; (v) focuses on the financial result but also in positive results in the social and environmental dimensions of sustainability; and (vi) at its highest transition level, it may involve virtualization strategies for activities and services focusing on dematerialization.

The four methodologies presented in this article should be used together in the planning and management of the transition process to a circular city. Today, the RESOLVE structure is the most applied and the most researched. One of the least applied is the CCAF. The use of CCAF Methodology is indicated only for the beginning of the process, in the initial data collection phase. Besides, as demonstrated in this article, the fusion between ReSOLVE and circular cities hubs methodology serves to organize transition principles and strategies. The structure presented in the European Investment Bank methodology, applied in a strategic planning format, contains all the programs/projects that will be applied in the city’s transition.

This work highlights that the circular city shares characteristics with all other typologies by comparing the circular city and other sustainable urban typologies. Thus, its transition contributes to urban sustainability and the implementation path for all the other typologies presented here. Research conducted by Winslow and Coenenv [51] considered that the circular city strategies need to be employed in a vitalist worldview and that it is an object boundary, as its concepts and strategies present contradictions and contestations in the literature. Bortolotti et al. conducted a study on circular economy and divided publications on this theme into two types of approaches: conceptual (such as urban metabolism, industrial ecology, and environmental economics) and hands-on approach (such as urban planning and product design). Each type of approach promotes different visions, solutions, and results, as well as limitations. The work carried out in this research contributes to this discussion by agreeing with the following considerations: the study of circular economy in cities is multidisciplinary and dynamic; it is a boundary object; it is divided into conceptual and practical approaches. The present paper contributed to the urban typologies discussion by subdividing the urban typologies between those that are conceptual and those that are practical, by considering them as boundary objects, and by reinforcing that their integration is necessary to achieve the goals related to the transition to more sustainable and resilient urban environments.

The transition to a circular city is still being prepared. It is a path that is being built. The literature reviewed in this paper focused mainly on the environmental benefits of urban sustainability. However, there is a need for research that deals with the social results arising from implementing circular economy strategies since there is a lack of discussion about its development potential concerning the social dimension of sustainability.

It is impossible to deal with sustainability if only two dimensions are studied. The economic benefit motivates and drives agents—companies, government, and citizens—but there is a need to research the social effects related to issues such as the potential of circular businesses for small entrepreneurs, the impact and potential quality of the jobs that circular economy strategies can generate and the situation of current circular economy workers, such as people who work in the recycling chain.

In developing countries, where 'recycling' occurs through informal work that helps people live on the margins of society and below the poverty line, circular economy strategies can promote economic and social development. Therefore, policies for the transition to a circular city should be concerned with the training and development of people involved in issues related to waste management.

Future studies' suggestions indicate researching the transition to a circular city from the social dimension, such as the quality of the jobs that are generated, the energy poverty, and the potential and incentives to create circular businesses, since sustainability is not an end in itself, with a final result to be achieved, but a path to be followed with a focus on continuous improvement and development.