Governance-Centred Industrial Symbiosis for Circular Economy Transitions: A Rural Forest Biomass Hub Framework Proposal

Abstract

This study examines the establishment of a Hub for Circular Economy and Industrial Symbiosis (HUB-CEIS) centred on a forest biomass waste plant in Fundão, Portugal, presenting an innovative model for rural industrial symbiosis, circular economy governance, and sustainable waste management. Designed as a strategic node within a reverse supply chain, the hub facilitates the conversion of solid waste into renewable energy and high-value co-products, including green hydrogen, tailored for industrial and agricultural applications, with an estimated 120 ktCO₂/year reduction and 60 direct jobs. Aligned with the United Nations (UN) Sustainable Development Goals (SDGs) and the Paris Agreement, this initiative addresses global challenges such as decarbonization, resource efficiency, and the energy transition. Employing a mixed research methodology, this study integrates a comprehensive literature review, in-depth stakeholder interviews, and comparative case study analysis to formulate a governance framework fostering regional partnerships between industry, government, and local communities. The findings highlight Fundão’s potential to become a benchmark for rural industrial symbiosis, offering a replicable model for circularity in non-urban contexts, with a projected investment of USD 60 M. Special emphasis is placed on the green hydrogen value chain, positioning it as a key enabler for regional sustainability. This research underscores the importance of cross-sectoral collaboration in achieving scalable and efficient waste recovery processes. By delivering practical insights and a robust governance structure, the study contributes to the circular economy literature, providing actionable strategies for implementing rural reverse supply chains. Beyond validating waste valorization and renewable energy production, the proposed hub establishes a blueprint for sustainable rural industrial development, promoting long-term industrial symbiosis integration.

1. Introduction

In recent decades, solid waste management has emerged as a major global challenge, driven by factors such as population growth, rapid urbanization, and unbridled consumption. The capacity of waste management infrastructure is often unable to keep pace with the increase in waste generation, resulting in serious environmental impacts, including soil, water, and air pollution. According to UN projections, global waste generation is expected to increase by 70% by 2050, making the need for more effective and sustainable waste management solutions even more urgent [1].

In this context, the circular economy is emerging as a promising approach that proposes a break with the linear economic model of ‘extract, produce, and dispose’ in favour of a regenerative logic that treats waste as a valuable resource. This model prioritizes the reuse, recycling, and transformation of waste into new products, promoting greater efficiency in the use of natural resources and reducing pressure on the environment [2]. One of the most effective strategies for operationalizing the circular economy is industrial symbiosis, which allows waste from one sector to be used as input by another, creating a virtuous circle of sustainable production and industrial integration [3].

The circular economy and industrial symbiosis perspective offer the potential to close material, energy, water, and steam loops to maximize efficiency and minimize waste [4,5]. Industrial symbiosis enables and promotes the exchange of resources between sectors to create closed loops in which waste from one company becomes raw material for another [6]. A notable example of industrial symbiosis is the Kalundborg network in Denmark, which is widely recognized as a model of sustainable practice. Inspired by the Kalundborg symbiosis hub model, this study proposes the development of an industrial symbiosis.

This study focuses on the establishment of a HUB-CEIS (Circular Economy Industrial Symbiosis) in Fundão, a small town in the rural Beiras and Serra da Estrela sub-region of Portugal with a population of 26,800 inhabitants. Despite its natural resources, the region faces challenges such as population decline, an ageing population, and high youth unemployment.

The proposed HUB-CEIS is based on a forest waste biomass plant for thermo-electricity generation in Fundão, Portugal. At the same time, Portugal faces a significant energy deficit, with electricity demand rising from 44.31 TWh in 2000 to 56.21 TWh in 2022, an increase of 26.9%. Despite the growth in renewable energy production, there is still an energy deficit of 9%, which is covered by imports. This highlights the need for increased investment in renewable energy sources to achieve self-sufficiency and meet the growing energy demand [7]. The recent report of the Iberian Initiative for Industry and Energy Transition [8] highlights that the energy transition could increase the Iberian Peninsula’s GDP by up to 15% and create around one million jobs by 2030. The HUB-CEIS proposes a reverse supply chain for forestry waste and the integration of resources such as energy, steam, and water to enhance energy efficiency among the participating organizations. Portugal has a substantial amount of forest residues [9].

The potential to produce green hydrogen as a by-product further strengthens the alignment of the HUB-CEIS with global decarbonization targets, the Sustainable Development Goals (SDGs)—particularly SDG 7 (Affordable and Clean Energy) and SDG 13 (Climate Action)—and the Paris Agreement [10,11]. Initiatives like HUB-CEIS not only promote the recovery of forest residues and technological innovation but also help mitigate Portugal’s frequent wildfires by reducing fuel accumulation in forests, enhancing both energy sustainability and fire prevention.

However, despite recent advances in the literature on circular economy and industrial symbiosis, significant gaps remain, particularly in implementing replicable models in regional and rural contexts. Integrating emerging technologies, such as green hydrogen, into industrial symbiosis projects is still a challenge. While green hydrogen is widely recognized as a promising solution for the energy transition, its practical application in local value chains remains limited. Additionally, the governance of these hubs is a critical strategic challenge for their viability [12]. The HUB-CEIS aims to transform waste into renewable energy and high-value co-products, such as green hydrogen, while fostering regional partnerships and sustainable governance. The application of industrial symbiosis in rural areas faces unique challenges, such as low company density, resource seasonality (e.g., biomass), and dependence on local policies. While the empirical implementation of HUB-CEIS is still underway, this study fills a gap in the literature by proposing a governance model adapted to rural contexts with limited industrial capacity, such as Fundão. Inspired by the early stages of the Kalundborg Symbiosis, which began with modest exchanges between a few organizations, HUB-CEIS offers a replicable theoretical–operational framework for rural regions globally, promoting circularity and socio-economic development.

To achieve these objectives, this article focuses on (i) exploring how the HUB-CEIS can contribute to the sustainable conversion of forest residues into renewable energy and valuable by-products; (ii) demonstrating how the circular economy can revitalize the local economy, create jobs, and attract foreign investment; (iii) understanding how the integration of the biomass plant with the HUB-CEIS can be replicated; and (iv) proposing a governance structure that integrates the best practices of industrial symbiosis with the regional specificities of Fundão.

2. Theoretical Background

2.1. Circular Economy and Industrial Symbiosis

The circular economy (CE) is a model of production and consumption that aims to minimize waste and maximize the use of natural resources, contrasting with the traditional linear model of ‘extract, produce, consume, and dispose’. CE proposes a transition to a regenerative system where waste is redefined as a resource for producing new goods and services [13]. The contemporary literature highlights four main pillars of CE: reduction, reuse, recycling, and recovery [14]. Studies show that implementing CE in different industrial sectors can lead to significant resource savings and reduced environmental impacts [15,16]. By transforming waste into value-added materials, CE is seen as a promising solution for sustainable development [15,16,17]. This approach also generates economic benefits through new business opportunities and reduced waste treatment costs [16,17]. Furthermore, integrating technologies, policies, and practices facilitates an effective transition at local and global levels. When by-products are used to produce useful materials, the circular economy can help reduce waste and create new income sources for local communities [18].

Industrial Symbiosis (IS), a key strategy within CE, focuses on cooperation between traditionally separate companies to share resources such as materials, energy, water, and by-products [19]. Geographical proximity is a critical factor in facilitating these exchanges, enabling the physical integration of flows between different industrial facilities [6]. An emblematic example of IS is the Kalundborg Symbiosis in Denmark, which involves nine companies from the energy, water, waste management, and other sectors. This model converts waste from one company into resources for another, generating environmental and economic benefits, promoting local growth, and supporting corporate social responsibility (CSR) [20,21]. The Kalundborg network, which began in 1972 with the exchange of gas between a refinery and a gypsum factory, now exchanges 2.9 million tonnes of by-products and waste annually [19,20]. Inspired by Kalundborg, other IS models have emerged worldwide, including in China [22], the UK [23], and Canada [6]. These centres promote innovation, efficiency, and social benefits such as job creation and improved quality of life for local communities [24,25]. However, challenges such as cultural resistance, inadequate infrastructure, and the need for supportive public policies still limit their widespread adoption [26,27,28].

2.2. Reverse Supply Chains and Governance in Industrial Symbiosis

A central concept in industrial symbiosis (IS) is that of reverse supply chains, particularly within the context of forest residues [29,30]. These residues possess the potential to be converted into renewable energy, chemical compounds, and high value-added products such as biofertilizers and bioplastics [15]. In regions abundant in forest resources—for instance, parts of Europe and North America—such supply chains prove especially effective for waste recovery and for bolstering regional sustainability [26]. The geographical proximity among the involved actors is crucial for optimizing these processes, as it reduces transportation costs and fosters collaboration. For example, the case of Guayama in Puerto Rico illustrates how proximity facilitates resource exchange and enhances the efficiency of symbiotic systems [6].

The valorization of forest residues through industrial symbiosis hubs can generate a diversified portfolio of co-products. For instance, Nicholls et al. [31] demonstrated that forest bio-hubs in three U.S. regions effectively convert forest residues into renewable energy and high-value chemicals while enhancing forest health and supporting a burgeoning bioeconomy (see Table 1). These co-products include, among others, liquid biofuels (biodiesel and bioethanol), biogas, biohydrogen, syngas, ash, and liquid effluents, each with applications ranging from energy generation and agricultural fertilization to the production of high-value chemicals [32,33,34]. For instance, liquid biofuels may be used both as vehicular fuels and for electricity generation, while biogas produced via anaerobic digestion can serve both heating and electrification purposes [32,33]. Similarly, syngas—resulting from biomass gasification—demonstrates significant potential for the synthesis of chemical products and energy production [35]. Recent studies, such as [36], indicate that while thermomechanical technologies like gasification and pyrolysis have reached a high level of maturity, water electrolysis—when coupled with renewable energy sources—can reduce both costs and CO₂ emissions, making it a viable alternative for green hydrogen production.

The governance of reverse supply chains is equally fundamental, as it orchestrates the transactional relationships among various actors at both strategic and operational levels. In industrial symbiosis, converting residues into essential raw materials fosters strategic interdependence [37,38,39] while simultaneously enhancing network resilience. Comparative analyses of established hubs—such as Kalundborg [19], Eco-Town [25], and Rotterdam [40]—underscore that robust and adaptable governance structures, whether industry-led, government-driven, or hybrid, are indispensable for aligning stakeholders’ roles, responsibilities, and decision rights. Indeed, research by BCG reveals that over 85% of business ecosystems fail, with governance deficiencies accounting for more than 50% of these shortcomings [41]. Moreover, Porter and Kramer [42] argue that creating shared value is crucial for harmonizing business success with social progress, while Hart [43] emphasizes the need to integrate environmental imperatives into competitive strategy.

Building on these foundational insights, recent studies further elucidate the critical role of governance in circular economy transitions. Henriques et al. [44] demonstrate that public policies, strategic incentives, and collaborative approaches are pivotal for fostering effective industrial symbiosis and circular economy initiatives by integrating economic, social, and environmental interests. Similarly, Sgambaro [45] propose a framework that identifies key governance variables—particularly those related to collaborative coordination—as essential for transforming waste into valuable assets. Additionally, Bocken et al. [46] highlight that circular economy business models depend on strong governance frameworks, reinforcing the significance of structured reverse supply chains. Liedtke et al. [47] further emphasize that global cooperative regional economies are critical to ensuring sustainable supply chains, providing insights into governance mechanisms applicable to industrial symbiosis. Moreover, Tian and Wang [48] argue that technology-driven governance frameworks, incorporating smart solutions such as IoT and AI, enhance industrial ecosystem adaptability. This aligns with Reis [49], who examines barriers and determinants affecting industrial symbiosis implementation, emphasizing that governance shortcomings—such as inadequate infrastructure and cultural resistance—limit the scalability of circular economy hubs.

Emerging trends in green hydrogen integration further illustrate governance challenges within industrial symbiosis hubs. Studies such as Notteboom and Haralambides [40] explore hydrogen hubs in seaports, revealing the importance of multi-stakeholder governance models for facilitating hydrogen adoption. Similarly, reports by ARENA [50] provide empirical insights into waste biomass-to-hydrogen conversion, highlighting the governance complexities of scaling new technologies within symbiotic networks. Lastly, Mathews et al. [51] examine China’s transition to eco-industrial parks, illustrating how governance innovations contribute to successful circular economy policies. Otherwise, recent studies, such as Järvenpää et al. [52], demonstrate that digitalization—via IoT, blockchain, and artificial intelligence—acts as a catalyst for improving the efficacy of industrial symbiosis. These technologies facilitate a continuous flow of information among partners, which is a crucial element for the successful implementation of reverse supply chain strategies. Together, digitalization not only augments the value added by biomass co-products but also improves collaborative governance between stakeholders, reduces environmental impacts, and strengthens sustainable governance by synergistically integrating production reverse supply chains [47] (see Table 1).

Table 1. Potential by-products of the biomass plant and their applications.

By-Product	Description	Potential Applications	References
Liquid Biofuels	Includes biodiesel and bioethanol produced from vegetable oils and sugars.	Fuel for vehicles and electricity generation.	Demirbas [32]
Biogas	Gas generated by the anaerobic digestion of organic waste.	Electricity generation, heating, and vehicle fuel.	Appels et al. [33]
Syngas	Gas composed of hydrogen and carbon monoxide produced by gasification.	Energy generation, production of liquid biofuels and chemicals.	Bridgwater & Peacocke [34] Rauch et al. [53]
Residual water	Water generated or treated in biomass processes.	Agricultural irrigation, industrial cooling systems, and urban reuse.	Tchobanoglous et al. [54]
Ash	Residues resulting from biomass combustion.	Fertilizers, pozzolanic material, and adsorbents for water treatment.	Vassilev et al. [55] Kwong and Marek [56]
Sludge	Residue from wastewater treatment, rich in nutrients.	Composting, biogas production, and organic fertilizers.	Tchobanoglous et al. [54]
Biohydrogen	Gas generated by the anaerobic fermentation of biomass.	Clean fuel in fuel cells.	Zhang et al. [57] Ahmad et al. [58] Ghisellini [35] Ghasemi et al. (2024) [36]
Bioethers	Products such as ethyl tert-butyl ether (ETBE) and methyl tert-butyl ether (MTBE) derived from bioethanol	Additives in fuels to improve the octane rating.	Rojas et al. [59]
Charcoal	Obtained through the pyrolysis of biomass.	Fuel, raw material for chemical industries.	Leckner [60] Kurkela et al. [61]
Organic Fertilizers	Resulting from anaerobic digestion or composting of biomass.	Soil quality improvement in agriculture.	Hargreaves et al. [62] Ali et al. [63]

Table 1. Potential by-products of the biomass plant and their applications.

By-Product	Description	Potential Applications	References
Liquid Biofuels	Includes biodiesel and bioethanol produced from vegetable oils and sugars.	Fuel for vehicles and electricity generation.	Demirbas [32]
Biogas	Gas generated by the anaerobic digestion of organic waste.	Electricity generation, heating, and vehicle fuel.	Appels et al. [33]
Syngas	Gas composed of hydrogen and carbon monoxide produced by gasification.	Energy generation, production of liquid biofuels and chemicals.	Bridgwater & Peacocke [34] Rauch et al. [53]
Residual water	Water generated or treated in biomass processes.	Agricultural irrigation, industrial cooling systems, and urban reuse.	Tchobanoglous et al. [54]
Ash	Residues resulting from biomass combustion.	Fertilizers, pozzolanic material, and adsorbents for water treatment.	Vassilev et al. [55] Kwong and Marek [56]
Sludge	Residue from wastewater treatment, rich in nutrients.	Composting, biogas production, and organic fertilizers.	Tchobanoglous et al. [54]
Biohydrogen	Gas generated by the anaerobic fermentation of biomass.	Clean fuel in fuel cells.	Zhang et al. [57] Ahmad et al. [58] Ghisellini [35] Ghasemi et al. (2024) [36]
Bioethers	Products such as ethyl tert-butyl ether (ETBE) and methyl tert-butyl ether (MTBE) derived from bioethanol	Additives in fuels to improve the octane rating.	Rojas et al. [59]
Charcoal	Obtained through the pyrolysis of biomass.	Fuel, raw material for chemical industries.	Leckner [60] Kurkela et al. [61]
Organic Fertilizers	Resulting from anaerobic digestion or composting of biomass.	Soil quality improvement in agriculture.	Hargreaves et al. [62] Ali et al. [63]

2.3. Green Hydrogen Production: Biomass Gasification vs. Electrolysis

Green hydrogen production methods are benchmarked by techno-economic parameters that highlight trade-offs among cost, efficiency, and environmental impact. Biomass gasification has emerged as a cost-effective pathway, with production costs estimated at approximately EUR 1.5–1.7/kg—significantly lower than water electrolysis, which typically ranges from EUR 4.0 to 6.0/kg [36]; these figures are corroborated in reviews by Ghisellini et al. [35]. Biomass routes leverage renewable feedstocks such as forest and agricultural residues, reducing GHG emissions and offering supply stability independent of climatic conditions. Moreover, innovative strategies such as biochar-supported catalytic enhancement [64] have demonstrated improved hydrogen yields and process efficiency. However, challenges with syngas cleanup, tar management, and feedstock seasonality persist [65,66]. Conversely, water electrolysis—powered by renewable sources (solar, wind, and hydro)—remains the benchmark technology for zero-CO₂ hydrogen production [35,67,68,69]. Despite its environmental merits, high capital and operational costs (generally EUR 3–8/kg, base case ~EUR 6.13/kg) continue to challenge its cost competitiveness. Recent advances in electrolyser technologies, including high-efficiency catalyst development in PEM and SOEC systems, are critical for improving energy efficiency and lowering costs [67]. Furthermore, strategic policy documents underscore the pivotal role of green hydrogen in the energy transition. For instance, reports by the Direção-Geral de Energia e Geologia [70], the European Commission’s hydrogen strategy for a climate-neutral Europe [71], and the International Energy Agency’s “The Future of Hydrogen” [72] provide robust evidence of the growing institutional commitment and policy support for scaling up hydrogen production.

Collectively, these findings underscore the trade-offs between biomass gasification (with promising catalyst innovations such as those reported by Yao et al. [64] and advanced electrolysis, reinforcing the need for hybrid approaches capable of leveraging the strengths of both technologies in achieving a scalable, sustainable hydrogen economy. See

Table 2. Comparative green hydrogen (GH) production costs.

Production Method	Production Cost (USD/kg)	Energy Source/Feedstock	Advantages	Disadvantages
Electrolysis (Green Hydrogen) IRENA [68]; BloombergNEF [69]; Hou & Yang [67]; Ghisellini et al. [35]	~USD 4–6/kg ~USD 2.50–7.75/kg (based on external reports) EUR 3–8/kg base case: ~USD 6.13/kg (consistent with USD 4.0–9.0/kg range)	Water + electricity from renewable sources (solar, wind, and hydro). Can use desalinated seawater.	No direct CO2 emissions. Significantly lower GHG emissions compared to fossil fuels. Clean/renewable/sustainable alternative. Pivotal for the sustainable energy transition. Only residue is water vapour. Does not emit CO2 when used as fuel.	High initial investment/capital cost. High cost of production. High electricity cost. Demands a lot of energy. Demands a lot of water. Challenges with storage and transport. Cost uncompetitive compared to fossil routes in base scenarios. Dependency on energy source availability. Technology maturity varies (PEM/SOE less mature than ALK).
Biomass Gasification (Moss/Green Hydrogen routes) Rauch et al. [65]; DGEG [67]; European Court of Auditors [66] Ghasemi et al. [36] Yao et al. [64]	~USD 1.25–2.2/kg (pyrolysis route) ~USD 1.77–2.05/kg (gasification route)	Biomass (e.g., forest residues and lignocellulosic biomass).	Utilizes renewable biomass. Lower GHG emissions vs. fossil fuels. Carbon neutrality. Waste utilization. Residue utilization. Enhanced energy security. Potential for decentralized production. Sustainable and economically viable. Lower carbon footprint alternative. Reduces dependence on fossil sources.	Process complexities (e.g., syngas cleanup). Challenges (tar and cleanup). Suffers from seasonality of feedstock. Lower efficiencies (40–56%). Not yet significant industrial scale use.

:

3. Materials and Methods

3.1. Research Approach

This study employs a rigorous multi-method research design, integrating case analysis, structured surveys, and semi-structured interviews to provide a comprehensive understanding of industrial symbiosis in Fundão. Methodological triangulation enhances the reliability and depth of the findings, in accordance with the validation principles proposed by Malhotra [73] and Denzin [74].

Guided by Yin’s case study approach [75], this work explores the systemic, relational, and governance dimensions involved in implementing a rural industrial symbiosis hub (HUB-CEIS) anchored in a biomass plant. This case reflects a real-world scenario with multiple stakeholders, allowing for an empirical observation of opportunities and challenges in circular governance.

Furthermore, the study adopts the “framework-centric theory-building” approach [76,77], which is well suited for deriving conceptual models from context-specific qualitative data. This strategy supports the integration of multiple sources of evidence—case observations, stakeholder narratives, and survey patterns—into a political and economic governance model for decentralized circular ecosystems. Given the limited number of survey respondents (n = 6), triangulation among interviews, document analysis, and process observation enabled a more robust validation, in line with the principles established by Eisenhardt [76].

3.2. Geographical and Strategic Context

Fundão is a Portuguese city located in the sub-region of Beiras and Serra da Estrela, which faces several socio-economic challenges, namely a worsening population decline since 1991 and an ageing population since 1960 [78]. As a result of the gradual demographic decline, the sector faces a shortage of skilled labour and is affected by a significant dependence on external resources [79]. The high unemployment rate in the region is a significant concern, with particular emphasis on youth unemployment, caused by the insufficient supply of qualified jobs that meet the skills and needs of the young population. [79]. This location takes into account the importance of proximity between companies and the definition of a zone for possible tax incentives for the creation of green companies. See the physical location of the HUB-CEIS, Figure 1.

The proposal to establish the HUB-CEIS in Fundão, Portugal, is driven by the urgent need to revitalize the region’s industrial ecosystem and integrate opportunities for economic regeneration and community engagement. Initially covering 10.24 square kilometres, the hub is strategically located within Fundão’s industrial district, where existing organizations demonstrate high potential for symbiotic exchange.

As the project progresses through its maturity phases, the HUB-CEIS is designed for gradual expansion, extending its influence up to 50 km to incorporate new industries, agricultural sectors, and logistics partners [9]. This extended radius will facilitate resource optimization, strengthen regional partnerships, and enhance the integration of green hydrogen production and advanced circular economy initiatives.

This staged growth model ensures scalability, enabling the hub to transition from localized industrial symbiosis to a broader circular economy network, fostering sustainable development in rural and semi-industrial regions.

3.3. Case Study

The case study approach, following Yin [75], was adopted to analyse the Hub for Circular Economy and Industrial Symbiosis (HUB-CEIS) framework within a real-world context. This section outlines the selection of the Fundão Biomass Plant as the case study, its operational context, and the methodological procedures employed. The Fundão Biomass Plant, located in Fundão’s industrial district, Portugal, was selected due to its capacity to process forest biomass waste and its potential to serve as the anchor for a rural industrial symbiosis hub. Managed by Enerwood, Lda. name changed to FTP Energia e Ambiente (FomentInvest Group), the plant has been operational since 2019 with a capacity of 15 MW. It processes approximately 150,000 tons/year of biomass inputs, primarily wood residues and cork waste sourced from local forestry activities within a 50 km radius. The primary outputs include 130 GWh/year of electricity injected into the national grid, 2500 tons/year of ash, and 1000 tons/year of slag, with potential applications in agriculture (e.g., soil amendments) and construction (e.g., cement additives) [55]. This operational profile, combined with its proximity to local industries and alignment with Portugal’s decarbonization goals, makes it an ideal case for testing the HUB-CEIS framework. The case study methodology employed multiple data collection methods to ensure triangulation, as recommended by Denzin [74]: document analysis, including examination of operational reports, environmental permits, and municipal waste management plans from Enerwood, Lda., and Fundão Municipal Council; site observations, including field visits in 2024 to document the plant’s infrastructure, waste processing workflows, and technological systems; stakeholder interviews, including semi-structured interviews with plant managers, local industry representatives, and policymakers to identify opportunities for symbiotic exchanges; and questionnaires, including surveys distributed to 21 local companies to assess waste generation profiles and interest in industrial symbiosis. The case study serves as a foundation for evaluating the feasibility and scalability of the HUB-CEIS framework, with detailed findings presented in Section 4.1.

3.4. Qualitative Interviews

Telephone and face-to-face interviews were conducted with key stakeholders, including a biomass plant expert and founder of the Fundão Biomass Plant and a representative of Fundão Municipal Council. These interviews provided information on current waste management practices, technological infrastructures, and potential opportunities for industrial symbiosis and circular economy projects.

The interviews covered the role of the public sector in promoting industrial symbiosis, the challenges and opportunities in implementing sustainable practices, and the incentives offered by the Environmental Fund and the Portugal 2020/2030 programme [80]. The interviews were recorded, transcribed, and analysed using content analysis, coding the emerging themes. This approach made it possible to identify key points that complemented the responses to the questionnaire, providing a more comprehensive view of the dynamics of collaboration between companies and the regulatory barriers perceived by stakeholders.

3.5. Quantitative Data from Questionnaires

A list of organizations belonging to the Fundão Industrial District was compiled, and 28 organizations with industrial symbiosis potential were identified. A structured questionnaire was distributed to 21 local companies to collect data on waste management practices, perceptions of industrial symbiosis, and possible barriers to its implementation. Given the size of the city of Fundão, only six companies responded, but the data obtained were very useful. The main themes included the use of raw materials, waste/by-products, waste management practices, and perceptions of the benefits and barriers of industrial symbiosis. Analysis of the data identified recurring themes, namely the importance of reducing raw material costs, increasing revenue from the sale of waste, and improving environmental sustainability.

4. Results

4.1. Case Study—The Fundão Biomass Plant as an Anchor for the HUB-CEIS

The Fundão Biomass Plant represents a strategic initiative in Portugal’s transition towards a circular economy, leveraging biomass as a renewable energy source while fostering industrial symbiosis. As a thermoelectric facility, the plant generates electricity through the combustion of forest biomass, primarily wood residues and cork waste. With an installed capacity of 15 MW and an estimated annual biomass consumption of 150,000 tons, it became operational in 2019 under the management of Enerwood, Lda., a subsidiary of the Fomentinvest Group, with controlling investment by the Marguerite Fund. This plant aligns with Portugal’s National Energy and Climate Plan 2021–2030 (PNEC 2030), which underscores the need for diversification of the energy mix and enhanced sustainability [80]. The plant employs an advanced energy conversion process consisting of multiple stages: biomass reception, shredding, storage in large-capacity silos, and combustion in high-temperature boilers. The thermal energy generated is used to produce steam, which drives turbines connected to an electric generator. The electricity produced is subsequently injected into the national grid, contributing to regional energy security.

The production process at the biomass plant starts with the grinding of the raw material, which is sent to a large silo and then to the boiler. The biomass is burned in the boiler, heating the water and turning it into steam. This steam is sent to a turbine, which drives an electric generator, producing electricity that is distributed to the public grid (see Figure 2, biomass diagram).

Waste management and environmental certification: The plant’s waste management follows strict environmental standards, with computerized systems that ensure traceability and compliance with Portuguese and European legislation. By-products include ash, slag, and small volatile particles, the reuse of which is currently restricted by legislation. According to the plant, current legislation requires these wastes to be disposed of in landfills, which is considered inefficient. Studies are underway to allow fly ash to be combined with sewage sludge for composting and slag to be used for paving forest roads.

The plant has environmental certifications and operating licences that demonstrate its commitment to sustainable practices and compliance with European emissions regulations. This structure enables a more efficient production model in line with the principles of the circular economy. The plant will have to fill out a document certifying that these products have left the plant and keep it for later inspection by APA technicians [82]. Figure 3 shows the gas exhaust filter and the furnace of the Fundão Forest Biomass Plant in Portugal.

The Fundão Biomass Plant operates within a structured biomass supply network, composed of small forest producers, cooperatives, and specialized forestry waste management companies. Logistically, the plant follows a contractual model in which loggers commit to monthly biomass deliveries, enhancing operational predictability. This model has led to regional job creation, with an estimated 60 direct and 320 indirect jobs, reinforcing its socio-economic impact.

Integration with HUB-CEIS and potential for industrial symbiosis: In the context of the HUB-CEIS proposal, the plant has the potential to act as a central element in the sustainable industrialization of Fundão. Its capacity to generate heat, steam, and energy can benefit companies in the local industrial district, creating an integrated system for the use of waste and secondary inputs. This model is inspired by successful industrial symbiosis ecosystems such as Kalundborg in Denmark, where energy and material flows are optimized between different industries. In Figure 4, it is possible to observe the potential interactions within an industrial symbiosis and circular economy system at the HUB-CEIS, involving the industrial district company of Fundão and the biomass plant in Fundão. The image highlights how waste flows are managed and reused among these entities to maximize efficiency and minimize waste. The companies in the Fundão Industrial Centre generate waste that can be absorbed by the biomass plant. In turn, the biomass plant, as the anchor of the industrial symbiosis, processes this waste and provides energy, heat, steam, and ash. The energy and heat are then redirected back to the industrial district company, creating a closed loop of resource reuse. Steam and ash are also managed within this system, which contributes to the overall efficiency of the biomass plant’s production process, as explained by the biomass plant’s founder: “The steam that comes out of the turbine goes through a cooling process that transforms it back into water. This water goes back into the boiler, in a closed cycle that we can call water/steam/water. So, there is little loss of water (a scarce resource) in this process. Cooling is usually done by large fans that create the wind needed to turn the steam into liquid. There is also an open circuit that corresponds to the hot air from burning in the boiler grate. This combustion takes place because heated atmospheric air containing oxygen is injected directly into the grate for combustion. This air exits directly through the chimney after passing through a series of cyclones and filters that remove the small volatile particles. These particles are deposited in an elevated silo and then unloaded directly into trucks”. See Figure 4.

Regarding the potential by-products of the biomass plant, the wastes generated by the biomass plant, such as ash and residual biomass, have the potential for various applications. For example, ash can be used as a fertilizer, construction material, or pollutant adsorbent. Residual biomass can be converted to biogas, bioethanol, or other biofuels, including green hydrogen. In addition, the plant can explore the production of biochar (a form of charcoal produced from plant matter and stored in the soil as a means of removing carbon dioxide from the atmosphere), a highly efficient material for carbon sequestration and soil improvement. The plant currently has the infrastructure to supply waste heat and steam to other companies if there is demand in the industrial area. In addition, there is the possibility of supplying electricity at competitive prices to companies and local residents as part of an energy community strategy, in a case of industrial and urban symbiosis. At present, the energy produced is fed into the grid. The use of advanced technologies such as sensors and automation via the Internet of Things (IoT) will allow the plant to optimize its efficiency and control emissions. To provide a detailed overview of the plant’s structure,

Table 3. Structure and operation of the Fundão biomass power plant.

Parameter	Details
Installed capacity	15 MW
Location	Fundão Industrial Park
Annual biomass consumption	150,000 tons
Technologies	Biomass reception and storage system, steam generator, bag filter (gas emission control), turbo generator, and electrical substation.
Type of biomass used	Wood residues (branches, leaves, and residual wood from forest management and clearing).
Operation and management	ENERWOOD, Lda., controlled by the Marguerite Fund.
Environmental certifications	All licences are required under applicable legislation for operation. The project aligns with the National Energy and Climate Plan 2021–2030, the main national policy instrument for energy and climate for the “2030 Horizon”.
Supply chain and bottlenecks	Sourced from local suppliers and collection points. High dependency on suppliers. Supply and logistics pose challenges, particularly due to seasonality. Excessive moisture reduces combustion efficiency, requiring drying processes.
Environmental impacts	Combustion generates ash, CO2, and methane.
Number of employees	A total of 60 direct and 320 indirect jobs.
Local community	Agreement for environmental impact mitigation.

presents key information on its operations, governance, and certifications.

4.2. Interviews with Key Stakeholders

Semi-structured interviews were conducted with two key stakeholders: the founder, a representative of the biomass plant, and the councilman, a representative of Fundão City Council. The interviews were recorded, transcribed, and analysed using thematic analysis [83]. These interviews provided critical insights into the motivations, strategic goals, and challenges associated with the implementation of the HUB-CEIS (Hub for Circular Economy and Industrial Symbiosis). The analysis followed a qualitative content analysis approach, as outlined by [84], to identify recurring themes and patterns. It is important to note that the founder, in addition to being an expert in biomass plants, is one of the project’s idealizers and has a minor participation in it. To mitigate potential biases, his responses were cross-referenced with data from other stakeholders, including questionnaires administered to local companies and a comprehensive theoretical review.

4.2.1. Biomass Plant Perspective

The founder, an expert from the biomass plant, emphasized the potential of the HUB-CEIS to stimulate the local economy by diversifying industrial activities and promoting circular economy practices. He highlighted the following key points:

Waste Valorization and By-Products: The biomass plant currently generates ash and slag as by-products, which are disposed of in landfills due to regulatory constraints. However, the founder noted that these materials hold significant potential for valorization. For instance, ash could be integrated with sludge from wastewater treatment plants (WWTPs) (these are facilities that process wastewater (domestic, industrial, or agricultural sewage) to remove pollutants before releasing the treated water back into the environment; the sludge generated during the treatment process (known as “sludge”) is a residue rich in organic matter and nutrients, which can be reused in applications such as composting or agricultural fertilization, depending on local regulations) for composting, while slag could be utilized in forest road construction, aligning with upcycling principles. However, it is important to acknowledge that the founder’s involvement in the project may introduce an optimistic bias. Therefore, the feasibility of these proposals should be validated through independent technical and economic studies.

Technological Integration (IoT and Automation): The founder underscored the role of automation and the Internet of Things (IoT) in optimizing plant operations. These technologies enable efficient energy production and real-time waste monitoring, which are critical for optimizing reverse supply chains. However, as the project’s idealizer, the founder may overestimate the benefits of these technologies. Future studies should include third-party cost–benefit analyses to validate these claims.

Community Engagement and Job Creation: The biomass plant supports waste collection by residents and contributes resources to the city council. Additionally, the plant maintains a registry of forest waste suppliers, including farmers and transporters, which supports local businesses and creates approximately 300 indirect jobs. This positions the plant as a hub for regional circularity efforts. Despite these benefits, the actual impact of these initiatives should be evaluated through independent studies.

Potential for Industrial Symbiosis: The founder expressed interest in collaborating with other sectors to share resources, such as heat and steam. These synergies could strengthen the industrial ecosystem in Fundão, potentially extending to agriculture, food processing, and textiles. However, as the project’s idealizer, the founder may overestimate the benefits of industrial symbiosis. To ensure impartiality, the perspectives of other stakeholders, such as the councilman, were included in the analysis.

4.2.2. City Council Perspective

The councilman, the representative of Fundão City Council, highlighted the municipality’s commitment to economic revitalization through sustainable practices. He identified the following key themes:

Economic and Environmental Benefits: The city council views industrial symbiosis as an opportunity to reduce environmental impact, create jobs, and strengthen the local community. The HUB-CEIS is seen as a central component of this strategy. However, the councilman emphasized the need to ensure that the project delivers tangible benefits to companies and citizens, such as economic gains and improved quality of life.

Logistical and Regulatory Challenges: The councilman pointed out logistical challenges related to waste management and transportation, as well as the lack of specific regulations for the sale of industrial waste. He stressed the need for clear policies to facilitate the recovery of recyclable waste and reduce bureaucratic complexity. These challenges were corroborated by the founder, but the councilman’s perspective offers a more balanced view, considering the interests of the community and the government.

Incentives and Public Policies: The councilman is willing to consider tax incentives and other benefits to attract companies to the circular economy and industrial symbiosis zone. However, the councilman emphasized the importance of ensuring that the project delivers recognizable benefits to companies and citizens. This pragmatic approach contrasts with the founder’s optimism, offering a more realistic view of the project’s challenges and opportunities.

Table 4. Analysis of key themes identified in interviews.

Theme	Description	References and Comments
Waste Valorization and Circular Economy Practices	Both stakeholders emphasized the importance of transforming waste into valuable resources, aligning with circular economy principles.	Consistent with [85], who highlight the potential of industrial symbiosis to promote resource efficiency and waste reduction.
Technological Innovation	The integration of IoT and automation was highlighted as a critical factor for optimizing operations and enabling real-time monitoring.	Aligns with recent studies on the role of digital technologies in circular economy initiatives [86].
Community Engagement and Job Creation	The biomass plant’s efforts to engage residents and create jobs were seen as a key benefit of the HUB-CEIS.	Resonates with the findings of Henriques et al. [44], who emphasize the importance of community involvement in circular economy projects.
Regulatory and Logistical Challenges	Both stakeholders identified regulatory and logistical barriers as significant challenges.	Consistent with the literature on industrial symbiosis, which often cites regulatory complexity and infrastructure limitations as key obstacles [87].
Need for Incentives and Public Policies	The importance of tax incentives and clear public policies was a recurring theme in both interviews.	Aligns with the recommendations of Henriques et al. [44], who argue that policy support is essential for the success of circular economy initiatives.

presents a summary of the key themes identified in the interviews.

The interview insights provide a comprehensive understanding of the opportunities and challenges associated with the HUB-CEIS. The biomass plant’s focus on waste valorization and technological innovation aligns with global trends in circular economy practices, while the councilman’s emphasis on regulatory and logistical challenges highlights the need for tailored policies and infrastructure investments. Additionally, the findings underscore the importance of stakeholder collaboration and community engagement, which are critical elements for the success of industrial symbiosis initiatives [14].

4.3. Quantitative Data from Questionnaires

The low response rate (28%) and the small number of companies (six) that completed the questionnaire affect the generalizability of the statistical results. However, as [84,88] explain in their studies, qualitative research allows small samples to provide deep and rich knowledge about complex and contextualized phenomena, focusing on perceptions and experiences. This type of research is more effective in exploring motivations, barriers, and opportunities in a specific context than in seeking statistical representativeness.

In this sense, Ref. [89] argue that even small qualitative samples can yield deep and context-specific understanding by capturing participants’ nuanced perceptions and experiences. This approach effectively uncovers underlying motivations, barriers, and opportunities within a specific context, rather than merely focusing on achieving statistical representativeness. The results obtained from the questionnaires, included in Appendix C, were consolidated in tables and graphs to facilitate the interpretation of the data collected. The analysis identified the companies’ current waste management practices and their perceptions regarding the implementation of an industrial symbiosis network. This selection was made based on proximity and potential for symbiotic exchanges. Although questionnaires were sent to 21 companies, only 5 responded, as shown in

Table 5. Main information collected on the questions of the questionnaire.

Company	Raw Material	Product/Service	Waste/By-Products	Waste Management
Company A	Forest biomass (150,000 tons/year)	Electric Energy (80,000 to 110,000 MWh/year)	Ash (7500 tons/year)	5
Company B	Stainless steel, Za-mac	Surface treatment for leather and jewellery	Organic, hazardous, plastic waste, etc.	7 (municipal or specialized collection)
Company C	Polishing pastes, sandpapers, fabrics, and chemicals	Polished metal parts (15,000 kg/month)	Organic waste, plastics, sludge, etc.	Variable (5, 1)
Company D	Sludge, ash, and biomass	Organic amendment (755,700 kg/month)	Hydrocarbon separator sludge, and water	1
Company E	Apple, pear, and enzymes	Apple and pear juice concentrates and fruit aroma	Organic waste, plastics/papers, and energy	Composting, energy (7, NA)

.

The proposed industrial symbiosis hub for Fundão aims to enhance circularity and resource optimization by fostering strategic exchanges between sectors. The following interactions have been mapped based on survey responses and additional symbiosis opportunities suggested by stakeholders:

Company B sends organic waste to Company A, where it serves as raw material for biogas and energy generation; Company A supplies energy back to Company B, closing the cycle of waste valorization and energy use; Company A sends ash to Company F, which utilizes it as fertilizers or pozzolanic materials for agricultural and construction applications (note: Company F was not identified in the questionnaires but was suggested by a stakeholder as a strategic partner for waste recovery); Company F sends sludge to Company C, where it can be co-digested with organic residues for biogas production or processed into nutrient-rich compost for agricultural use; Company C sends organic and hazardous waste to Company A, contributing to bioenergy generation, while excess heat and steam from Company A are redirected for use in textile and industrial drying processes by Company D.

Additional Symbiotic Opportunities—Wastewater Reuse: Company D could benefit from industrial wastewater treatment, repurposing filtered effluents for cooling processes in Company A or for reuse in dyeing and textile finishing.

Heat Recovery: Excess heat from biomass combustion at Company A could be transferred to Company D for fabric drying, reducing external energy demand.

Valorization of Agricultural Residues: Company B and Company E could provide wood and crop residues to be converted into biochar for soil enhancement and carbon sequestration.

Plastic Waste Recycling: Company C could collaborate with Company G to recycle plastic residues into construction materials such as polymer-infused bricks or insulation panels.

The questionnaire data on expected benefits and perceived barriers to the implementation of industrial symbiosis are summarized in

Table 6. Expected benefits of implementing industrial symbiosis.

Main Benefits	Average Importance	Description
Reduction of raw material and energy costs	3.2	Resource sharing and waste utilization help reduce operational expenses.
Increase in revenue from waste sales	3.8	Companies generate new income streams by selling waste as valuable by-products.
Improvement of company image and reputation	3.0	Sustainability-driven practices enhance corporate credibility and market appeal.
Contribution to environmental sustainability	5.0	Reduced waste and pollution support long-term environmental conservation efforts.
Innovation in products and processes	2.6	Stimulates the development of more efficient and technology-driven solutions.

and

Table 7. Perceived barriers to implementing industrial symbiosis.

Main Barriers	Average Importance	Description
Lack of trust and integrity among partners	3.0	Effective collaboration requires transparency and reliability among stakeholders.
Lack of information and knowledge about the concept	2.6	Many companies are unfamiliar with the mechanisms and advantages of industrial symbiosis.
Lack of adequate infrastructure and logistics	2.8	Investments in waste collection, transportation, and processing facilities are necessary.
Lack of incentives and financial support	3.4	Subsidies, credit lines, and tax incentives could encourage adoption.
Absence of specific regulations for industrial symbiosis	4.4	The lack of clear policies creates uncertainty and slows down implementation.

. In these cases, respondents were asked to rate the importance of certain aspects on a five-point scale.

From Table 6 and Table 7, it is evident that most waste from participating companies is already recycled or reused, although some materials still end up in landfills. Most respondents are not fully familiar with industrial symbiosis, but 60% believe that participating in a symbiosis network would bring significant benefits. Key barriers, including the lack of regulations and financial support, align with concerns raised by the municipal chamber representative.

The biomass plant representative emphasized the importance of inter-institutional collaboration for the HUB-CEIS initiative’s success. He stressed the need to involve key stakeholders—such as the municipal chamber, local businesses, and educational and research institutions—to create an environment conducive to innovation and the implementation of sustainable solutions.

Additionally, the founder highlighted the significance of public policies that promote the circular economy, including tax incentives, special credit lines, and favourable environmental regulations. These findings indicate that while there is substantial interest in participating in an industrial symbiosis network (60% of respondents view it as beneficial), significant challenges remain: environmental sustainability was the most valued benefit, demonstrating a growing concern for reducing environmental impact; the lack of specific regulations was identified as the primary barrier, suggesting the need to develop public policies that support circular economy initiatives; financial difficulties also emerged as a challenge, reinforcing the necessity of economic incentives to encourage participation in symbiosis networks.

4.4. Governance Framework Proposal for the HUB-CEIS

The development of a governance framework is a critical component of the HUB-CEIS proposal, ensuring effective coordination among stakeholders and the sustainable management of resources. Adapted on the canvas model [90], the proposed governance framework involves collaboration between local businesses, financial institutions, government entities, and European funds. The governance structure of HUB-CEIS is designed to ensure effective coordination and long-term sustainability, tailored to the rural context of Fundão, where only 21 out of 28 local organizations have the potential for industrial symbiosis. At the core of the framework is a board of directors, composed of representatives from the biomass plant (Company A), the Municipal Council of Fundão, key enterprises (e.g., Company B and Company C), environmental NGOs, and the University of Beira Interior. This board will initially be funded by European programmes, such as Horizon Europe, complemented by membership fees proportional to the financial capacity of participating companies, along with municipal tax incentives, ensuring financial viability until the hub reaches full consolidation. The board’s main activities include establishing a code of conduct that defines ethical practices, such as transparency in waste exchange and compliance with environmental regulations, alongside clear performance monitoring protocols. Qualitative and quantitative indicators have been proposed to assess the impact:

• Reduction in landfill waste: e.g., diverting significant waste volumes in Phase 0 of 2025, with a progressive increase until 2026;
• Decrease in CO₂ emissions: measured by relevant percentages in both initial and consolidated phases;
• Generation of direct and indirect jobs: particularly focused on retaining young professionals in rural areas.

These indicators will be monitored through IoT infrastructure, using accessible sensors to track biomass and waste flows, with periodic audits optimized by artificial intelligence (AI) and multicriteria decision analysis (MCDA) to enhance traceability and efficiency [91].

To mitigate risks, the governance framework includes a management strategy addressing logistical challenges (e.g., transportation difficulties in rural areas, mitigated through partnerships with local cooperatives) and regulatory barriers (e.g., advocacy efforts to revise legislation on ash reuse, currently under review with EU support).

An implementation roadmap structures the process into phases:

• Phase 0 (2025) launches a pilot with three companies, validating waste and energy exchange;
• Phase 1 (2026) expands to ten companies, integrating IoT-based monitoring;
• Phase 2 (2027–2030) incorporates the production of green hydrogen.

This approach ensures scalability, providing a replicable model for other rural regions, inspired by the early stages of the Kalundborg Symbiosis, and aligned with Sustainable Development Goals (SDGs) 7, 12, and 13, supported by AI and MCDA-driven optimization strategies [91].

Governance Structure of the HUB-CEIS: Challenges and Proposals—the above is a suggestion for governance at the hub. See Figure 5.

5. Discussion

The findings from the interviews, questionnaires, and case study analysis provide a comprehensive understanding of the opportunities and challenges associated with the implementation of the industrial symbiosis hub (HUB-CEIS) in Fundão, Portugal. This discussion is structured around six key themes: (Section 5.1) the strategic role of the hub, (Section 5.2) barriers and challenges of industrial symbiosis, (Section 5.3) comparison with external models, (Section 5.4) the HUB-CEIS implementation plan, (Section 5.5) practical implications for the Sustainable Development Goals (SDGs), and (Section 5.6) the governance framework for the circular economy. Each theme is discussed in detail, concerning the findings and their alignment with the existing literature.

5.1. Strategic Role of the Hub

The proposed HUB-CEIS in Fundão is envisioned as a strategic facilitator for integrating local industries into a symbiotic network. This network aims to reduce operational costs, foster innovation, and enhance sustainability practices. The hub’s role in transforming industrial waste into valuable resources aligns with the principles of the circular economy, as highlighted by Kirchherr et al. [5], who emphasize the importance of resource efficiency and waste valorization. The interviews conducted in this study suggest that the success of the hub hinges on its ability to create effective synergies among local businesses, thereby revitalizing the regional economy and attracting new investments. This finding is consistent with the work of Chertow [6], who underscores the importance of collaboration and resource sharing in industrial symbiosis networks.

5.2. Barriers, Challenges, and Potential of Industrial Symbiosis

This study identifies several barriers to the implementation of industrial symbiosis in Fundão, including the lack of regulatory frameworks and financial incentives. These findings are corroborated by the quantitative data from the questionnaires, which highlight the need for clearer public policies, tax incentives, and infrastructure development to support circular economy practices. Concerns about waste logistics and transportation further underscore the importance of investing in infrastructure to facilitate the efficient circulation and transformation of by-products. These challenges are consistent with those identified in other industrial symbiosis initiatives, such as the Kalundborg Symbiosis in Denmark [19]. However, the study also reveals the potential for Fundão to overcome these barriers through targeted policy interventions and stakeholder engagement.

5.3. Comparison with External Models

The literature extensively documents successful cases of urban–industrial symbiosis, including Kalundborg Symbiosis (Denmark), the Eco-Town Program (Japan), Guiyang Industrial Symbiosis (China), Guitang Group (China), and Rotterdam Industrial Cluster (Netherlands). These models emphasize closed-loop material flows, energy integration, and multi-sector cooperation, demonstrating the viability of industrial symbiosis in urban settings. However, as highlighted by Nugroho et al. [92], urban–industrial symbiosis faces barriers such as limited investment capability, regulatory constraints, and information exchange inefficiencies. The Fundão HUB-CEIS, as detailed in the study proposed, addresses these challenges by adapting industrial symbiosis principles to a rural context, leveraging forest biomass waste to generate renewable energy, biofertilizers, and green hydrogen. This model aligns with Portugal’s National Energy and Climate Plan (PNEC 2030) and the United Nations Sustainable Development Goals (SDGs), offering a replicable framework for rural circular economy initiatives.

The Kalundborg Symbiosis in Denmark serves as a benchmark for industrial symbiosis, demonstrating the viability of closed-loop material flows, energy integration, and multi-sector cooperation. Similarly, the Eco-Town Program in Japan emphasizes waste recycling and industrial sustainability, while China’s Guiyang and Guitang models highlight shared infrastructure and resource exchange among heavy industries. The Rotterdam Industrial Cluster excels in petrochemical and energy integration, showcasing water reuse and emissions mitigation techniques. The Fundão HUB-CEIS, in contrast, introduces a transformative governance approach tailored to rural industrial contexts, integrating IoT-enhanced waste monitoring, blockchain-enabled transactions, and multi-stakeholder governance to ensure supply chain transparency and efficient resource allocation.

Through strategic alignment with global best practices, the HUB-CEIS can replicate and adapt the successes of Kalundborg, Eco-Town, Guiyang, Guitang, and Rotterdam, with Bio-hub Finland offering a scalable template for rural symbiosis initiatives. The following comparative

Table 8. Comparative analysis of industrial symbiosis models.

Model	Environmental Impact	Economic Impact	Technological Integration	Governance Structure	Social Impact	Similarities with HUB-CEIS	Lessons for HUB-CEIS Sustainability
HUB-CEIS Portugal	120 ktCO2/year reduction, 10,000 tons/year waste diversion	~USD 50–70 M investment, PPPs	Gasification with IoT, digital monitoring	Multi-stakeholder board with EU funding	60 direct/320 indirect jobs	N/A	N/A
Kalundborg Denmark	150 ktCO2/year reduction, 2.9 M tons/year byproduct exchange	Cost savings via waste sales, ~USD 200 M investment	Cogeneration, carbon capture	Industry-led with local government support	~200 jobs, community engagement	Multi-stakeholder collaboration	Adopt Kalundborg’s stakeholder coordination model to enhance HUB-CEIS governance [19]
Eco-Town Japan	100 ktCO2/year reduction, high waste recycling rates	Cost reduction via recycling, ~USD 150 M investment	Advanced recycling, shared furnaces	Government-driven policies	Community sustainability programmes	Policy-driven framework	Leverage national policies to secure funding and regulatory support [25]
Guiyang China	80 ktCO2/year reduction, in-production water reuse	Industrial cost savings, ~USD 100 M investment	Industrial process integration	Centralized governance	Regional economic growth	Fiscal incentives	Use fiscal incentives to attract SMEs to HUB-CEIS [22]
Guitang China	90 ktCO2/year reduction, paper recycling	Cost savings via vertical integration, ~USD 120 M investment	Byproduct reuse	Industry-led with government support	~150 jobs	Vertical integration	Explore vertical integration with local agriculture for biomass supply [22]
Rotterdam The Netherlands	200 ktCO2/year reduction, industrial water reuse	Petrochemical efficiencies, ~USD 240 M investment	Energy integration systems	Public–private partnerships	Skilled job creation (~300 jobs)	Robust infrastructure	Develop modular infrastructure to emulate Rotterdam’s scalability [40]
BioHub Finland	70 ktCO2/year reduction, biomass valorization	~USD 80 M investment, regional partnerships	Biomass processing, torrefaction	Regional knowledge networks	~100 jobs, local training programmes	Biomass focus, modularity	Strengthen knowledge networks to support HUB-CEIS scalability [9]
BioRural Europe	60 ktCO2/year reduction, industrial water reuse	~USD 60 M investment, EU funds	Bioenergy, biomaterials	Cooperative networks	Community cooperatives (~80 jobs)	Rural focus, modularity	Adopt BioRural’s cooperative model for community engagement [93]

presents the core similarities and distinctions among these cases:

Table 8 compares the HUB-CEIS with established industrial symbiosis models (e.g., Kalundborg, Denmark; Eco-Town, Japan) across environmental, economic, technological, governance, and social impacts, synthesizing data from the literature and case study findings (Jacobsen, 2006; Van Berkel et al., 2009 [19,25]). HUB-CEIS shares similarities with Kalundborg’s multi-stakeholder collaboration, BioHub’s biomass focus, and BioRural’s rural orientation, but its IoT-driven monitoring and modular infrastructure are unique. Lessons for HUB-CEIS sustainability include adopting Kalundborg’s stakeholder coordination, Eco-Town’s policy advocacy for funding, and BioRural’s cooperative model for community engagement. Unlike urban models requiring high industrial density, HUB-CEIS targets rural areas with cost-effective investment (~USD 50–70 M) and significant CO₂ reduction (~120 ktCO₂/year), offering a replicable framework for rural circular economy transitions.

5.4. HUB-CEIS Implementation Plan

The proposed implementation plan for the HUB-CEIS includes the creation of an innovation and business incubator, carpooling strategies, a waste and resource management platform, tax incentives, and an international industrial symbiosis fair. Additionally, innovative financing strategies, such as the issuance of cryptocurrencies (with blockchain), are proposed to support the project. The plan emphasizes the central role of the Fundão Biomass Plant as the hub’s anchor, leveraging its industrial nature and intrinsic symbiosis characteristics. This approach aligns with the findings of Boons et al. [27], who highlight the importance of anchor tenants in driving industrial symbiosis initiatives. The proposed plan also considers the potential for collaboration among 28 waste and resource-producing organizations in Fundão, further enhancing the hub’s scalability and impact.

5.5. Practical Implications—Alignment with the Sustainable Development Goals (SDGs)

The HUB-CEIS initiative demonstrates significant potential to contribute to the United Nations Sustainable Development Goals (SDGs), particularly in the areas of affordable and clean energy (SDG 7), responsible consumption and production (SDG 12), and climate action (SDG 13). As illustrated in Figure A1, which maps the adoption of industrial symbiosis practices against the related SDGs, the descriptors derived from the questionnaire responses indicate a strong alignment between the HUB-CEIS objectives and the global sustainability agenda. For instance, the reduction in raw material and energy costs directly supports SDG 12 (Responsible Consumption and Production), while the valorization of waste contributes to SDG 8 (Decent Work and Economic Growth) by creating new economic opportunities.

Table A1. Research descriptors vs. adherence to SDGs.

Descriptor	Related SDGs	Description
Reduction in raw material and energy costs	SDG 12, SDG 7	Promotes resource efficiency and waste reduction.
Increase in revenue from waste sales	SDG 8	Encourages new economic opportunities through waste valorization.
Contribution to environmental sustainability	SDG 13, SDG 15	Emphasizes practices that minimize environmental impact.
Innovation in products and processes	SDG 9	Encourages the development of more efficient and sustainable technologies and processes.
Lack of information and knowledge	SDG 4	Promotes the dissemination of knowledge about sustainable practices.
Lack of infrastructure and logistics	SDG 9	Encourages the development of resilient infrastructures.
Lack of regulations and standards	SDG 16	Promotes effective governance and compliance with environmental standards.

in the Appendix B further elaborates on the relationship between the research descriptors and the SDGs, highlighting how the adoption of industrial symbiosis can enhance resource efficiency, promote innovation, and address infrastructure gaps. For example, the descriptor “Reduction of raw material and energy costs” aligns with SDG 12 and SDG 7 (Affordable and Clean Energy), emphasizing the importance of resource efficiency and waste reduction. Similarly, the descriptor “Lack of infrastructure and logistics” underscores the need for resilient infrastructure, as outlined in SDG 9 (Industry, Innovation, and Infrastructure).

5.6. Governance Framework for the Circular Economy

This study highlights the need for a robust governance framework to support the implementation of the HUB-CEIS. This framework should include legislative adjustments, such as tax reductions, zoning modifications, and the establishment of a legal entity to oversee the hub’s operations. The findings suggest that current Portuguese legislation, including Law 102-D/2020 [94] and Resolution of the Council of Ministers No. 190-A/2017(PAEC) [95], provides a solid foundation for promoting circular economy practices. However, further revisions are needed to streamline administrative processes and promote waste valorization. These findings are consistent with the work of Geissdoerfer et al. [2], who emphasize the importance of policy support and governance structures in enabling circular economy transitions. The study also underscores the need for public-private partnerships and community engagement to ensure the successful implementation of the HUB-CEIS.

5.7. Rationale, Impacts, Benefits, and Challenges

The implementation of the HUB-CEIS is designed to revitalize Fundão’s industrial sector by promoting resource optimization, waste reduction, and circular economy practices. The initiative is expected to attract new investments, stimulate economic growth, and position Fundão as a benchmark for sustainability and innovation. Local businesses benefit through reduced production costs, expanded income streams, and enhanced corporate image. Moreover, the systematic review “Integrating bio-hubs in biomass supply chains: Insights from a systematic literature review” [96] demonstrates that consolidating biomass supply operations into dedicated bio-hubs can significantly improve logistical efficiency and supply stability—an insight that further strengthens the rationale for adopting integrated and hybrid circular economy strategies.

However, several challenges must still be addressed, including difficulties in accessing raw materials, managing environmental and social impacts, and overcoming the lack of robust public policies. These obstacles call for creative and collaborative solutions. As highlighted by Hein et al. [97], effective stakeholder engagement and adaptive governance are essential to surmounting the barriers to industrial symbiosis. By leveraging insights both from advanced electrolyser technology and from innovative approaches such as bio-hubs, the HUB-CEIS framework promises to deliver scalable improvements that contribute to a sustainable, circular hydrogen economy.

Table 8 further elaborates on the relationship between the research descriptors and the SDGs, highlighting how the adoption of industrial symbiosis can enhance resource efficiency, promote innovation, and address infrastructure gaps. For example, the descriptor “Reduction of raw material and energy costs” aligns with SDG 12 and SDG 7 (Affordable and Clean Energy), emphasizing the importance of resource efficiency and waste reduction. Similarly, the descriptor “Lack of infrastructure and logistics” underscores the need for resilient infrastructure, as outlined in SDG 9 (Industry, Innovation, and Infrastructure).

These findings suggest that the HUB-CEIS not only addresses local socio-economic and environmental challenges but also contributes to global sustainability goals. By aligning with key SDGs, the initiative serves as a replicable model for other regions seeking to advance the circular economy. Future research should focus on scaling up these practices and exploring their applicability in diverse contexts, further advancing the global transition to a circular economy.

6. Conclusions and Recommendations

The HUB-CEIS initiative in Fundão, Portugal, establishes a transformative blueprint for rural industrial symbiosis, integrating digital technologies, circular governance, and decarbonization strategies. By leveraging biomass gasification to convert forest residues into energy and prioritizing green hydrogen as a high-value co-product, HUB-CEIS aligns with global sustainability goals (SDGs 7, 9, 12, and 13) [3,12], achieving an estimated 120 ktCO₂/year reduction and creating 60 direct jobs, positioning rural regions as pivotal drivers of energy transition and economic revitalization with a projected investment of ~USD 60 M. This model turns waste into resources, reducing environmental impacts while unlocking economic potential in peripheral areas [27].

However, scaling HUB-CEIS requires overcoming systemic barriers—regulatory complexity, inadequate infrastructure, and fragmented governance. A multilevel governance framework, enabled by IoT for real-time monitoring and adaptive management, is essential [71]. Targeted policies, including simplified waste regulations, tax incentives, and scalable public–private partnerships (PPPs), combined with independent lifecycle assessments [64], are critical to ensure sustainability and attract investment in rural symbiosis hubs, targeting a 20% increase in regional economic output.

Looking forward, HUB-CEIS sets a foundation for next-generation rural industrial ecosystems. Future hubs can evolve into data-driven platforms, harnessing IoT, AI, and blockchain to optimize resource flows and enable adaptive decision-making [71], with the potential to process 500,000 tons of biomass annually and reduce emissions by 150 ktCO₂/year. These hubs should integrate advanced lifecycle assessments, foster aligned local-to-national governance, and leverage innovative financing models to gather sustainable investment in low-density regions [64]. Our study, supported by empirical evidence and stakeholder insights [3], provides a replicable pilot that charts a path for rural hubs to drive decarbonized growth globally.

These findings inform actionable bioeconomy policies, empowering governments to deploy sustainable biomass hubs. The next steps involve piloting HUB-CEIS in diverse regions and expanding incentives for digitalization to accelerate the energy transition in rural bio-hubs [3].