A conceptual model for a circular city: a case study of Maribor, Slovenia

By 2019 over 55% of the global population resided in urban areas (United Nations 2019). With their hindsight areas, cities consume over 60% of global energy and over 75% of the planet’s material resources (Dodman et al. 2017; World Economic Forum 2022). Such consumption rates generate an increased impact on the environment and society. Thus, cities are responsible for producing up to 50% of global solid waste and 70% of greenhouse emissions yearly (OECD 2021). Projections by Gao and O’Neill (2020) indicate an increase in urban areas by 1.8 to 3.6 by 2100 in comparison with the year 2000, with the residing population expected to rise by 15 per cent, according to United Nations (2019), while World Economic Forum (2022) projecting 80% of global population residing in urban areas by the year 2050. The increased growth of cities and their population presents a unique challenge for policymakers, researchers, experts, and society in achieving sustainable development in cities.

The importance of cities in combating rising economic, social, and environmental issues has not gone unnoticed. The United Nations emphasised the cities as an important area to be considered, thus including them under the 11th goal of Sustainable cities and communities (General Assembly UN 2015). European Union (EU) delivered a renewed Urban Agenda for the EU (Urban Agenda for the EU 2023), further supporting countries' dedication towards sustainable development in urban areas. Such directives have fostered the development of various city models (smart, green, sustainable cities), including circular cities, encompassing circular economy principles for urban areas. The circular city model was supported by the EU's "Circular cities and regions initiative” to reduce pressure on natural resource consumption and create sustainable and circular jobs (European Commission 2022). The initiative adopted the European Green Deal (European Commission 2021) and the Urban Agenda for the EU (Urban Agenda for the EU 2023).

The emergence of various city models for urban areas poses a challenge for decision-makers, experts, and researchers. However, there is a distinction in tackling acceleration towards a sustainable city for various city models. According to the existing literature review, we can perceive the prevalent features of the mentioned city's models. For example, smart cities employ smart and digital technologies (Moradi 2020; Tiware et al. 2021), circular cities introduce principles of circular economy from the strategic and practical perspectives (Crippa et al. 2022), while green cities focus on greening public spaces (Jia et al. 2021; Zhang et al. 2022). City models’ reviews are beneficial for furthering the city's development. However, the rapid rise of publicised review papers presents an obstacle between discerning potential breakthroughs or rewriting published findings (Nature Sustainability 2021). Understanding city models' underlying fields and potential overlaps can forward implementing beneficial ideas in practice.

In recent years, there has been a tendency towards the circular economy and subsequent implementation in urban areas (European Commission 2015; European Commission 2019). The circular economy implementation within cities faced several barriers, as the circular economy was primarily developed for industry and services (Williams 2019), including product development (Boeri et al. 2019). The transition towards redefining the circular city model as a built or urban environment has begun with increased research and support by organisations such as the EU, Ellen MacArthur Foundation, and the UN (Murray et al. 2017). A transformation of the established circular economy approach to better accommodate the intricate systems presented by cities is needed (Prendeville et al. 2018). The increased interest in circular cities spurred the development of several research areas, such as building on resource efficiency (Ness and Xing 2017), circularity in port cities (Gravaguolo et al. 2019; Cerreta et al. 2020), establishing circular tools (Girard & Nocca 2019), nexus for energy, food and transport (Paiho et al. 2021). Cities are considered complex built systems, and defining circularity models should consider a systems approach (Wijkman et al. 2019). Systems approach in cities were perceived in the "ReSOLVE” approach (Prendeville et al. 2018), complex urban system structure (Rios et al. 2022), and inclusion of resilience hubs across urban systems (Boeri et al. 2019). However, these models focused more on various areas (e.g. energy, waste, food sector), not comprehending the entire system. Also, statistical validation and a transition measurement system are needed to provide adequate information for decision-makers. Inspired by the existing studies and research the main challenge is not a circular approach implementation but measuring acceleration towards circularity and providing continuous improvements backed with statistical and reliable data (Birgovan et al. 2022; Paoli et al. 2022). A solution can emphasise a circular city model based on the Define-Measure-Analyse-Implement-Control (DMAIC) tool and problem-solving system, represented within this paper. The DMAIC tool, as part of Six Sigma, is a data-driven technique that enables a project-based approach, providing an established five-phase structure, enabling the identification of possible solutions, their development and implementation (Jamil et al. 2020). Such an approach allows for semi- and final loops throughout the process (Sokovic et al. 2010) and allows continuous improvement to already implemented circular approaches while enabling a system perspective. As this approach is data and statistically-driven (Monday 2022), it enables management through measurement techniques. It allows the promotion of “state-of-the-art” maps before and after the implementation (de Mast and Lokkerbol 2012). Furthermore, it coincides with the need for measuring systems for transitioning from a linear to a circular economy (Birgovan et al. 2022; Paoli et al. 2022). The problem-solving structure enables one to focus on the roots of issues (Mingers and Brocklesby 1997). Engaging DMAIC and problem-solving into one model assures a systemic structure supported by measurement techniques and supports developing strategies for the transition towards circularity.

Thus, the research focus has been given to the three main areas. Firstly, to examine the extent of correlation in developing different city models, exploring their shared areas of interest and the advantages of adopting an integrated approach. This analysis has been conducted via a research papers' content analysis on circular, green, and smart cities. Then, to identify key research fields, crucial for the progression of circular cities and their transition from the current linear strategies. Understanding the existing research focus is essential in establishing a foundation for devising strategies and projects that can guide policymakers when incorporating circular activities. Third, to design and test a conceptual model based on the DMAIC (Define, Measure, Analyse, Improve, Control) tool and a problem-solving structure. This model incorporates the identified research fields that promote circular growth. Through a case study, strengths and potential shortcomings of the model are identified and addressed to enhance its effectiveness. This research is essential as it can guide policymakers in formulating strategies that facilitate continuous improvement of their cities and lay the groundwork for possible unified standards in the transformation towards a circular city.

This section represents a methodological approach and methods, comprehending details on literature review and content analysis, implementing and testing DMAIC process. A visual presentation of our research is seen in Fig. 1.

This section represents content analysis results, establishment of circular research fields and comparative analysis with current standing of Maribor. The content analysis is followed up by a case study analysis of Maribor based on the DMAIC model phases.

The research provided information and cooperation on developing and designing smart, circular, and green city models from research paper content analysis. Results revealed an increased publication number for city models. An increase coincided for models with the introduction of SDG17 goals by the United Nations (General Assembly UN 2015), the Circular Economy Action Plan for circular cities (European Commission 2015; European Commission 2019), and the smart City Expo World Congress in Barcelona for smart cities (European Commission 2019). According to the city model orientation, differences between models were identified in publications and WoS categories.

The findings derived from the content analysis have revealed distinct research trends concerning the conceptual core of each city model. Smart cities, for instance, prioritise the development of innovative technologies and services that facilitate enhanced control over city activities and their ease of use by residents (Moradi 2020). Additionally, there is a growing interest in establishing smart networks and digital databases to enable seamless communication between technology and users, with the ultimate aim of reducing environmental impacts and integrating different sectors of the city into a unified system (Tiwari et al. 2021; Hajek et al. 2022). Consequently, implementing sustainable development in smart cities entails the establishment of smart environments, IoT solutions, and networks (Moradi 2020).

The analysis of green cities has revealed a distinct focus on integrating a green mindset and technologies. The research indicates that green cities need help attempting to incorporate green urban planning, particularly concerning the inclusion of green spaces and the promotion of natural environment regeneration, which can be hindered by existing city layouts (Jia et al. 2021). However, researchers are addressing these issues by investigating the potential benefits and advantages of promoting a healthy, sustainable lifestyle for residents, such as increased biodiversity and economic opportunities (Jai et al. 2021; Zhang et al. 2022; Javidroozi et al. 2023).

Through a cross-analysis of the three city models, a total of twenty-seven research fields were identified. While some overlap between the models, the results surprisingly demonstrated a high degree of interaction. The overlap indicates that city models are actively evolving and seeking potential opportunities that can accelerate their transition towards sustainable development. Comparisons with the findings of Javidroozi et al. (2023) and Ahvenniemi et al. (2017) suggest that cities progressively integrate proven and beneficial solutions from competing city models. By adopting this approach, cities can expedite the development of solutions, offer established ideas to policymakers, and ultimately gain an advantage in achieving their sustainable goals (Javidroozi et al. 2023). The high level of interaction between the city models indicates a positive trend towards adopting successful strategies and solutions from different models, accelerating the progress towards sustainable development.

The content analysis results pertaining to circular cities are centred around the core concept of the circular economy. The circular cities' focus is to achieve sustainable development in line with the goals set by the United Nations for establishing sustainable cities (United Nations 2022). The findings reveal key areas requiring attention to implement circular economy principles. One important aspect is the establishment of urban flow maps and the principles of urban metabolism for future urban planning (Lucertini and Musco 2022). These approaches can progressively increase resource circularity and self-sufficiency within cities (Kalmykova and Rosado 2015; Mazzarella and Amenta 2022). Another significant area identified is the integration of the Water-Energy-Food nexus with the 9R strategies. Cities are major consumers of resources (Mulier et al. 2022), which are often quickly disposed of. The challenge lies in retaining these resources, and while recycling and reuse activities are being implemented, the need for upper 9R strategies suggests slow development in this regard. Research by Potting et al. (2017) indicates that the full realisation of a circular economy requires implementing upper strategies, which are currently underrepresented in the literature.

Furthermore, circular cities face the challenge of improving their inhabitants' economic and social aspects while mitigating potential environmental impacts associated with urban development (General Assembly UN 2015; Circular City Funding Guide 2023; United Nations 2022). Since cities are built environments, most changes occur in infrastructure and its utilisation for commerce, residential sectors, transportation, and services. Limiting resource losses through construction and demolition activities is crucial by employing upper 9R strategies such as repurposing and reusing existing buildings (Cimen 2021). These solutions aim to adapt buildings to meet the proposals of the inhabitants, thereby increasing the cultural and economic value for a more socially sustainable urban lifestyle (Cimen 2021; EMF 2023).

The case study conducted on Maribor revealed that the city has a well-established circular strategy. Furthermore, by integrating this model with the research focus areas of circular cities, a systematic analysis of the city's progress towards achieving circular goals was facilitated. The results indicate that Maribor is aligning its activities with its projected plans while considering all three dimensions of sustainability as defined by the United Nations (United Nations 2022) and the Circular City Funding Guide (2023). However, it is important to note that the final phase of the circular strategy, namely the control phase, has yet to be fully established. This phase could be crucial in advancing future planning efforts to improve circularity. The challenge of monitoring progress is not unique to circular cities but is also a concern for smart cities (Lai and Cole 2023) and individual sectors (Zope et al. 2019; Batalhao and Texeira 2020). As the authors point out, the main issue lies in establishing effective monitoring management systems that assess the current situation and provide data for future development.

The results gained from our case study and content analysis, suggest that cities are implementing project oriented approach in establishing circular activities in cities. The main issue is not implementing circular approach, but establishing monitoring management, that would enable assessing current standings, benefits and provide groundwork for future strategies and policies to be employed (Marin et al. 2020; Henrysson et al. 2022). Future projects should provide monitoring frameworks with circular city indicators (Vangelsten et al. 2020; Lindgreen et al. 2020; Paoli et al. 2022), establishing a consequential “state-of-the-art” map of the progress towards circularity. Continuous improvement and promotion of more permanent loops are dependable on the progress we achieved and needed to attain sustainable development, as our established city model constituted in the control phase.

Gaining insights into current research trends and the core conceptual evolution of city models is crucial for achieving sustainable development. While each city model pursues its approach to sustainability, their specialisation presents an opportunity for improvement through cross-pollination. The content analysis identified similarities and differences among the smart, circular, and green city models and twenty-seven research trends that correspond to all three city models. Such identification is necessary to properly manage cites according to the identified models.

An additional content analysis focused on circular cities only enabled the identification of specific trends within the circular city model. Combining the findings with the previously established twenty-seven research trends led to identifying twenty-four fields contributing to the development of an optimal circular city model. To structure these fields, we incorporated them into a conceptual model based on the DMAIC framework and problem-solving tools. The results showed that DMAIC-based tool allows cities to establish a continuous improvement cycle in their transition towards circularity. The model emphasises the establishment of semi-loops and loops and, when combined with a problem-solving systematic approach, provides a systematic and constructive means for potential policymakers to develop circular strategy maps.

The case study on the city of Maribor validated the proposed model, and also revealed the current weakness of inadequate monitoring of the progress towards a circular economy. Additional case studies focusing on the conceptual model are necessary to validate further and improve its application. Future research will enhance the conceptualised model, particularly by establishing measurement frameworks and indicators for validation during the control phase. Understanding the effectiveness of implemented circular activities is vital for future development and avoiding the unnecessary expenditure of time, funds, and resources in achieving a sustainable society.