*Article* **The Potential of Industrial Symbiosis: Case Analysis and Main Drivers and Barriers to Its Implementation**

**Angela Neves 1,2,\*, Radu Godina 3,\*, Susana G. Azevedo 2,4, Carina Pimentel 3,5 and João C.O. Matias <sup>5</sup>**


Received: 8 October 2019; Accepted: 9 December 2019; Published: 11 December 2019

**Abstract:** Industrial symbiosis, which is characterised mainly by the reuse of waste from one company as raw material by another, has been applied worldwide with recognised environmental, economic, and social benefits. However, the potential for industrial symbiosis is not exhausted in existing cases, and there is still a wide range of opportunities for its application. Through a comprehensive literature review, this article aims to compile and analyse studies that focus on potential industrial symbiosis in real contexts, to highlight the margin of optimisation that is not being used. The cases reported in the publications identified here were characterised and analysed according to geographic location, type of economic activity, waste/by-products, main benefits, and the methods employed in the studies. From this analysis, we conclude that there is great potential for applications involving industrial symbiosis throughout the world, and especially in Europe, corresponding to 53% of the total cases analysed. Manufacturing stood out as the sector with the highest potential for establishing symbiosis relationships, and the most common types of waste streams in potential networks were organic, plastic and rubber, wood, and metallic materials. This article also discusses the main drivers and barriers to realising the potential of industrial symbiosis. The diversity of industries, geographical proximity, facilitating entities and legislation, plans, and policies are shown to be the main drivers.

**Keywords:** industrial symbiosis; potential industrial symbiosis; sustainability; eco-industrial parks; circular economy

#### **1. Introduction**

In recent years, resource-intensive use, rising industrialisation and urbanisation, modern lifestyles, energy-intensive use, and land use patterns have led to increased greenhouse gas emissions, with negative consequences for the environment and the population [1–3]. It is therefore urgent to find solutions that do not hinder economic development but can provide ways to reduce carbon dioxide emissions, which are largely responsible for greenhouse gases, and to use resources more efficiently. As a subfield of industrial ecology, industrial symbiosis, which is often defined as a collective approach in which one company's waste is used as raw material by another company [4], can help to address these problems without compromising economic development. This practice is similar to biological processes, in which different organisms associate in a "mutually beneficial relationship" [5], as it allows entities and companies that operate separately to come together in the physical exchange of materials, by-products, energy, and water, with competitive advantages for all participants [6]. In addition to

waste/by-product exchanges, this sharing of resources also encompasses infrastructure sharing and the joint provision of services [7,8].

Some of the problems faced by industries, such as increases in waste, waste treatment costs and high resource consumption, are common in urban areas [9–11]. Thus, if cooperation between companies is extended to urban regions, there is potential for both parties to achieve environmental and economic benefits and to mitigate certain common problems. This cooperation has been referred to in several publications as Industrial and Urban Symbiosis (also called Urban and Industrial Symbiosis) and takes place when waste generated in an urban area is used as a raw material or energy source in industry or when industries provide urban areas with waste heat resulting from their operation [12,13]. However, some authors have also used the term Urban Symbiosis to refer to the use of waste from urban areas as a raw material or source of energy [14–16].

The most well-known case of industrial symbiosis, and the one most often cited in the literature, is in Kalundborg, Denmark. This network arose spontaneously from a self-organising initiative between companies to address water scarcity [4,5,17] and over the years has increased not only in terms of the number of symbioses but also the number of participants and remains a successful case to this day.

However, industrial symbiosis is not confined to the Kalundborg case, and numerous examples of synergistic networks around the world have been reported in the literature, with a wide variety in terms of the numbers of participants, types of economic activities, and how they are organised. In Europe, numerous foci of industrial symbiosis [18] are spread across different countries, and most of the cases reported in the literature are in northern and north-western Europe, with the United Kingdom reporting the highest number of cases [19]. This is due to the voluntary programme that the government has launched to help companies find partners to use their waste as raw material, called the National Industrial Symbiosis Programme [20]. Finland also has several cases of industrial symbiosis, largely arising from the strong presence of the pulp and paper industry, which has driven the creation of synergy relations [21,22]. In Asia, a number of industrial symbiosis initiatives have also been reported, with the highest number of cases in China, largely due to constraints on carbon dioxide emissions and the numerous plans and policies that have been implemented to foster circular economy practices [23–25]. In Japan, there have been cases of industrial symbiosis and industrial and urban symbiosis across several cities, driven by the Japanese Eco-Town Programme that encourages the use of industrial, municipal, and commercial waste in industrial applications, with the aim of boosting the economy and reducing waste disposal [14,15]. In North America, and particularly in the US, there have been cases of industrial symbiosis that date back to the late 1970s. The case most frequently mentioned in publications is located in Barceloneta, Puerto Rico, where the strong presence of pharmaceutical companies has spurred the creation and development of industrial symbiosis between various companies [26,27]. Several cases of industrial symbiosis have also been reported in Australia [28,29], Brazil [30], Morocco [31], and Algeria [31].

Despite the recognised benefits that these synergy cases have provided to the environment, the companies involved and the local population, the potential for industrial symbiosis is not exhausted in these existing cases, and there is still great potential for application not only in developed countries but also in countries with developing economies. Of the various articles published on industrial symbiosis, a significant proportion have focused on the best ways to foster industrial symbiosis and the most effective ways of overcoming the various obstacles, including economic, technological, legal, and social obstacles, that are faced in the creation and development of industrial symbiosis. Although some of these publications have a predominantly theoretical content e.g., [23,32,33], most present in-depth studies conducted in real contexts, that is, in a given region, with a holistic analysis of those industries, wastes and products with potential for developing industrial symbiosis e.g., [34–37]. In some of these studies, the implications of these new synergies for the environment, the economic development of companies, and the surrounding population have also been studied e.g., [36,38,39]. This article focuses on the latter type of publication, and identifies these as cases of potential industrial symbiosis. All articles that study the possibility of implementing new industrial symbiosis relationships in a given

location and that indicate the types of economic activities and industrial symbiosis are included in this designation.

The various studies that have been carried out on potential cases of industrial symbiosis have a wide scope, not only in terms of the characteristics of the synergy network but also in terms of the existence or otherwise of industrial symbioses in the location under study. In places where symbiosis networks already exist, relationships of trust are already established, and there is a knowledge of the benefits of this practice that can help to facilitate the process of mobilising other companies. Studies carried out of the industrial symbiosis networks in Weifang Binhai Economic-Technological Development Area, China [40], in the Västra Götaland region of Sweden [41], and in the Taranto provincial industrial district in Italy [42] represent some of these examples. In these cases, proposals have been put forward to extend the industrial symbiosis network to other companies, with the aim of repurposing some of the waste that was not yet being shared. However, several studies have been published that have proposed the creation of industrial symbiosis relationships in areas where this practice is not yet implemented but bring together a set of characteristics that reveal the potential for establishing synergy networks. Examples include studies of the development of industrial symbiosis carried out in Perth and Kinross, Scotland [43], Botos, ani and Piatra Neam¸t in Romania [44], and Konia, Liberia [45].

Although the potential for the application of industrial symbiosis is high, there are a number of constraints on its implementation. A lack of trust, uncertainty about the benefits, a lack of knowledge of the concept of industrial symbiosis, and a lack of information sharing [20,33,34,46–48] are the main factors that have been identified as restraining this process. However, there are also factors that are often referred to as drivers for the creation and development of industrial symbiosis networks, such as the need to reduce raw material and waste disposal costs and the potential generation of revenue [18,34,49–51]. In addition to these aspects, existing policies and legislation have also been identified as influencing industrial symbiosis practices. Regulatory pressure and landfill tax, which drive companies to find solutions for using resources more efficiently and reducing waste disposal [50,52], are examples of these. Policies and plans that aim to foster synergy networks, such as those in China [23,25] and the United Kingdom [20], have greatly contributed to the spread of these practices. However, existing legislation can also restrict these practices, for example if it is unclear or very restrictive on the use of waste, and can thus create difficulties for companies with regard to the application of industrial symbiosis [53].

A literature review reveals that although there are many studies of the compilation and analysis of case studies relating to industrial symbiosis [18,19,23,54–58], these focus on synergies that have already been implemented. Furthermore, although a study of the evolution of industrial symbiosis was carried out by Chertow and Park [59], with an analysis of the number of articles published and the countries that were the subject of the various publications, this work only included articles published until 2014. A comprehensive analysis of the cases of potential industrial symbiosis is still absent.

This article therefore aims to compile and analyse the various cases of potential industrial symbiosis in real contexts reported in the literature, and to highlight the margin of optimisation that is not being used. It also aims to identify and discuss the main drivers and barriers to the implementation of industrial symbiosis, as well as the various strategies for overcoming these barriers. To this end, the various cases are characterised and analysed by geographical location, type of economic activity, type of waste/by-product exchange, infrastructure sharing, and joint provision of services. Moreover, the methods employed in the different cases are analysed, as are the main environmental, economic, and social benefits that would be achieved if industrial symbiosis were to be realised. The cases of potential industrial symbiosis have been separated into those concerning the symbiosis between industries and those concerning the manufacture of new products and new uses for the different wastes enhanced by industrial symbiosis.

The rest of the article is organised as follows. Section 2 describes the methodology adopted for the research, selection, and analysis of publications, as well as the inclusion and exclusion criteria. Sections 3 and 4 contain the results and a discussion. Section 3 presents the results and an analysis of the cases of potential industrial symbiosis, in terms of the companies involved, the production of new products and new uses of waste. Section 4 discusses the main drivers, barriers, and strategies for overcoming these barriers to the creation of industrial symbiosis networks. Finally, Section 5 presents the main conclusions and discusses the limitations of this study and the scope for future research.

#### **2. Materials and Methods**

In order to fulfil the proposed objectives, a methodology was developed consisting of several stages, as illustrated in Figure 1. This methodology can be grouped into three main steps: a deep and systematic collection of the existing literature, the selection of publications and a content analysis.

**Figure 1.** Flow diagram of literature search and screening.

To enable a systematic and thorough review of the existing literature, several steps were followed. The first was the choice of keywords. To avoid limiting the search and thus obtain a more comprehensive set of publications, the terms "industrial symbiosis", "potential industrial symbiosis", and "eco-industrial park" were combined. These were used to search for articles in databases with more publications in this area, such as, Elsevier, Wiley Online Library, Springer, MDPI, Inderscience Online, IEEE Xplore, Taylor & Francis Online, ACS Publications, SAGE Journals, Nature Research, Emerald Insight, and Annual Reviews. In this research, no time interval was imposed, and the only exclusions were articles that were not written in English. The publications that resulted from this initial collection were submitted to a screening process in order to select the most relevant ones for the study. Titles, keywords, and abstracts were read, with the aim of selecting articles that focused mainly or significantly on industrial symbiosis. If there were any doubts about the inclusion of the articles, we analysed the frequency with which the keywords appeared throughout the publication and the context in which they were inserted in order to verify whether industrial symbiosis was the focus of the study or whether it only appeared as an example or to contextualise other concepts. This selection resulted in 591 publications, including research articles, review articles, conference articles, book chapters, and editorials. In our analysis, the references cited in these publications were used as secondary sources; however, this resulted in only a few articles, which may indicate the wide-ranging of the initial research.

In the next step, the 591 articles on industrial symbiosis were screened with the aim of finding only the publications whose object was the study of potential industrial symbiosis in a real context. This resulted in 103 articles, and these made up the final body of articles for which a more detailed content analysis of potential industrial symbiosis was carried out. Of these 103 articles, 89 concerned industries and 14 concerned potential uses of industrial symbiosis to manufacture new products or use different wastes. A content analysis of all these articles was then conducted to gather and analyse information on potential industrial symbiosis, such as location, types of economic activities involved in potential synergies, types of waste/by-products, and the existence or otherwise of infrastructure sharing. We also analysed the methods employed in the different analyses, the main environmental, economic and social benefits, and the factors underlying the potential for creating industrial symbiosis, which may condition or favour it.

#### **3. Potential Industrial Symbiosis**

The results of the study are presented below and are structured according to the main themes that emerged from the research and analysis of publications on potential industrial symbiosis, namely its scope, evolution over time, geographical distribution, and cases of potential industrial symbiosis relating either to companies or to the production of new products or new uses of waste.

#### *3.1. Evolution of the Number of Published Articles*

Of the 591 articles on industrial symbiosis selected and analysed according to the selection criteria defined in Section 2, 103 related to potential industrial symbiosis, accounting for approximately 17% of the total. Although these articles on industrial symbiosis were published from 1995 onward, it was only around 2001 that articles on potential industrial symbiosis began to appear, as illustrated in Figure 2.

**Figure 2.** Number of publications on industrial symbiosis and potential industrial symbiosis (PIS) per year.

Figure 2 shows the growing evolution of the number of publications on industrial symbiosis. Although low numbers of studies were published during the early years, from 2007 onward, there was a growth in the number of publications, which continued over the following years. Despite these small numbers, publications on potential industrial symbiosis have also shown growth. The evolution of these publications essentially comprises two distinct periods. In the first, between 2001 and 2014, there was a greater oscillation between the number of publications and in which the growth of articles was not very significant. In the second, between 2015 and the current year, more pronounced growth

could be seen, and whilst there was a drop in 2016, this was insignificant in light of the increase over the following years. Although this second period was shorter in terms of years, the proportion of publications was clearly higher than those in the first and accounted for 63% of the total number of publications. This increase coincided with the increase in publications on industrial symbiosis, and although it began to grow more significantly from 2007 onward, it was from 2014 onward that it began to obtain greater expression, revealing the growing scientific interest in this issue and the recognition of its potential to achieve sustainability in terms of its environmental, economic, and social aspects.

The proportion of the articles on potential industrial symbiosis in relation to the total number of publications on industrial symbiosis did not show significant variation over the years, except for 2001 and 2005, in which these articles made up 50% and 60% of the total number of publications, respectively. Three periods are of note, due to the fact that the values were very close in consecutive years—the period between 2006 and 2008, with an average of 20% of the total number of publications on potential industrial symbiosis; the period between 2009 and 2014, with an average of 12%; and the period between 2015 and 2018, with an average of 21%. Most of these 103 publications (about 84%) relate to research articles, while 12% are conference papers, and the remaining 4% are book chapters.

#### *3.2. Geographic Distribution*

The study of potential industrial symbiosis has shown great diversity with regard to geographical location; it has been studied in 31 different countries, revealing the great potential of this practice both in developed countries and in countries with emergent economies, as illustrated in Figure 3. Europe leads in terms of the number of publications on potential industrial symbiosis, with 48 articles, corresponding to approximately 53% of the total published. It is followed by Asia, with 26 publications, corresponding to 29% of the total, and North America, South America, and Oceania, with six, four, and four publications, respectively, corresponding to 7%, 4%, and 4% of the total. With three publications, equivalent to 3% of the total, Africa has the fewest studies of potential industrial symbiosis. This distribution is very different from that presented for industrial symbiosis publications [59]; here, China very significantly predominates in terms of publications, followed by the US and Australia.

**Figure 3.** Distribution of the number of published studies on potential industrial symbiosis by country.

The European countries in which most case studies of potential industrial symbiosis were carried out were Italy, Sweden, and Finland, with 11, seven, and six articles, respectively. All cases in Sweden were related to the study of potential industrial symbiosis between industries, while those in Italy and Finland focused both on synergies between industries and on the investigation of waste and new products fostered by industrial symbiosis.

Of the eight countries in Asia with studies of potential industrial symbiosis, most related to China, with approximately 46% of the total for Asia.

The US occupies the fifth place on the list of countries with articles published on potential industrial symbiosis, with three articles focusing on the synergy between industries and two more on the study of new uses for waste produced by the automobile industry. This figure is still low compared to the countries with the most publications, which is also true for case studies of industrial symbiosis—although the USA occupies second place on this list, the difference between the number of articles published compared to China, which ranks first, is very significant [19]. While some incentive measures have been applied to create synergies, such as an Environmental Protection Agency–funded project to identify possible synergies in six counties in North Carolina [60], they have not had a major impact on the increase in cases of symbiosis, and even existing studies of potential industrial symbiosis between industries date back many years.

In recent years, studies have been carried out to analyse the implementation of industrial symbiosis practices in developing countries such as the Philippines, Liberia, India, Bangladesh, Colombia, Mauritius, and Egypt. While these countries have little in the way of a tradition of synergy practices between companies and the number of published case studies is still small [19], the important role of industrial symbiosis is recognised as a means to enhance the development of these regions [34,45,61].

#### *3.3. Cases of Potential Industrial Symbiosis*

Although industrial symbiosis is well established in many countries, there are still many possibilities for creating and developing new industrial symbioses. The numerous studies that have proposed and evaluated new synergy networks are examples of this. In places where there is already industrial symbiosis, the process of creating new relationships is facilitated, since there are already relationships of trust between the actors and the benefits offered by this practice are well known; however, this is not an essential factor. In places where there is no synergy, factors such as the location, the waste generated, or the nature of the industries can drive the creation of symbiosis relationships. Moreover, this potential is not bound to a particular region, country, type of activity, or number of entities (NE) involved, as proven in the many studies that have been performed to evaluate the creation of new industrial symbioses (shown in Table 1). This table summarises the main characteristics of the various cases of potential industrial symbiosis that have been published (Table A1 in Appendix A provides more details of these cases). By analysing Table A1, it is possible to verify the huge untapped potential and the huge diversity of opportunities for the development of industrial symbiosis. These cases are characterised by the location, number of entities involved (NE), type of economic activity, waste/by-product exchange, sharing of infrastructure, joint provision of services, methods used in the study, and assessment of the potential of industrial symbiosis. The various economic activities are grouped into sections, which are defined according to the International Standard Industrial Classification of All Economic Activities, Revision 4 (ISIC, Rev.4). Of the 21 sections defined in the ISIC, 16 are relevant to the various cases of potential industrial symbiosis. In Table 1, the section designations are more concisely defined. The different streams of waste exchanged in these cases are grouped into types, such as organic (e.g. food and food processing wastes, biomass, livestock and fisheries wastes); plastics and rubber; wood; metallic; non-metallic (e.g. glass, waste from construction and demolition, lime-based waste), paper, waste heat and steam; ash, water, and wastewater; chemicals; sludge; waste oil; and others (e.g. textile waste).








administrative and support service




**Table 1.** *Cont.*







The cases compiled here are organised by region, and these are arranged into descending order based on the number of cases studied, i.e., Europe, Asia, North America, South America, Africa, and Oceania. Within each region, the various countries are also listed in descending order based on the number of cases studied, and within each country, the same process was carried out in ascending order based on the date of publication of the article.

In the following sections, the main characteristics of the cases of potential industrial symbiosis, the methods used in the analyses and the main potential benefits of these synergies are analysed and discussed.

#### 3.3.1. Level of Implementation

Similar to industrial ecology and the circular economy, industrial symbiosis can be implemented at three levels: the micro, meso and macro levels [40,125–128]. These are related to the boundaries at which industrial symbiosis relationships develop, i.e., at company (micro) level; between businesses with geographic proximity, such as eco-industrial parks (meso level); and activities that are carried out at regional or national level (macro level). Table 2 illustrates the diversity of potential industrial symbiosis cases, shown in Table 1, for each level of implementation.


#### 3.3.2. Industries Potentially Involved in Industrial Symbiosis

The diversity of economic activities with the potential to become part of an industrial symbiosis network is very wide, as illustrated in Tables 1 and A1. Manufacturing, which comprises activities involving the transformation of materials into new products, is the predominant sector in these cases of potential industrial symbiosis, as shown in Figure 4. This figure represents the distribution by country of all the economic activities involved in the various cases, grouped into sections according to the format established in the International Standard Industrial Classification of All Economic Activities, Revision 4 (ISIC, Rev.4). Of the 21 sections defined in ISIC, 16 are relevant to the various cases studied, with manufacturing accounting for 63% of the total occurrences of all sections. Sections such as agriculture, forestry and fishing, electricity and water, and waste management and recycling are also among the most frequently seen.

Within the manufacturing sector, the most frequent economic activities in the cases of potential synergy involve chemical, iron and steel, pulp and paper, construction materials, and wood and wood products. While there are cases in which manufacturing is the only sector in the synergy network, such as the study conducted in the Val di Sangro Industrial Area in Italy [64] or in Achaia in Greece [82], most of the cases involve several different industries within the manufacturing sector and other entities in the network that carry out other types of activity. This diversity of sectors has been highlighted by some authors [18,129] as being very important for the establishment of industrial symbiosis networks, as it widens the opportunities available.

**Figure 4.** Distribution of the categories of economic activities existing in cases of potential industrial symbiosis by country.

#### 3.3.3. Types of Waste/By-Product Exchange, Infrastructure Sharing, and Joint Provision of Services

The types of waste/by-product that have the potential to be used in the various cases of industrial symbiosis are very diverse, and are directly related to the nature of the economic activities that are carried out in the various networks of potential synergy. The most frequently reported wastes are organic (food and food processing wastes, biomass, livestock and fisheries wastes), plastics and rubber, wood, metallic, waste heat and steam, non-metallic (e.g. glass, waste from construction and demolition, lime-based waste), ash, water and wastewater, chemicals, sludge, and paper.

The sharing of infrastructure and the joint provision of services between companies has also been highlighted as a kind of symbiosis between entities, with potential to be developed and to provide benefits to the participants, albeit in lower numbers than for waste flow. The most frequently mentioned cases are facilities associated with the management and treatment of waste, water and recycling [34,62,64,65,86]. This type of scheme can assist companies in their waste management by freeing them of some of the costs associated with the storage and treatment of waste and by facilitating the direction of waste to various other companies. In addition to these aspects, the sharing of expertise, consultancy, equipment, logistics and transport, energy, and water supply infrastructure have also been identified as having potential to be established. Economic, environmental, and social benefits are identified as being likely to be attained if these types of sharing are established, such as cost savings in the construction of facilities [111], saving of resources used in waste treatment [62], reduction of inefficiencies [64], and job creation [65], and these can serve as drivers for the creation of new symbiosis relationships. Furthermore, the existence of shared and efficient infrastructures can foster new synergies between existing companies in the area and can also be an incentive for new companies to establish themselves in the region with the intention of being part of this symbiosis network. The sharing of services such as transport can also be an important factor in promoting symbiosis networks. Since most of the waste sold does not have a very high commercial value, any reduction in costs such as via

the sharing of services can increase the potential economic benefits to the companies involved in these synergies and can foster the creation of new symbiosis relationships.

#### 3.3.4. Methods Used in the Analysis and Assessment of Potential Industrial Symbiosis

In order to identify potential synergy relationships and assess the feasibility of implementing various cases of industrial symbiosis, several methods were used, as listed in Tables 1 and A1. The main objectives of this work were to study the best ways to establish synergy networks with regard to the most potential waste streams and the companies with the highest potential for integration and to assess the potential impacts that industrial symbiosis can have on the environment, the companies involved and the local population.

One of the first steps in the design and analysis of a potential synergy network is the collection of information and quantitative data. Various methods have been used for this, such as interviews [44,65,78,82,106], questionnaires [9,37,63,85,111], site visits [64,84,86,112], and focus groups [53,64,68,85].

To enable the realisation of industrial symbiosis networks, it is also essential to obtain knowledge not only of the possible participants but also of the numbers and types of waste/by-product available. Thus, in addition to meetings with local businesses that help foster possible relationships, as achieved in Emilia-Romagna, Italy [53], the Catania and Siracusa districts in Italy [66], and Colombia [34], the use of digital programs and platforms can also facilitate this interaction by building a common base with potential participants and waste and by optimising possible relationships. Examples of these are the development of a digital web platform for the electrical and electronic equipment sector [38], the Looplocal tool, which is useful in countries with geographically dispersed industries [72], the eSymbiosis multilingual service, implemented as an accessible web service [80], the SymbioSyS tool [87], and the ESOTA®platform, which is based on relationship mimicking [112].

One of the factors that can enhance the realisation of industrial symbiosis relationships is realisation of the potential benefits to businesses, not only the economic benefits, which are essential in encouraging companies to create synergies, but also the environmental and social benefits. While some of the studies carried out only a qualitative analysis, a significant proportion assessed at least one of these benefits. Of the three dimensions of sustainability, the environmental aspect was the most frequently discussed, and the most commonly applied method was the use of environmental indicators [10,13,39,88,105]. The reduction of carbon dioxide emissions was the most frequently addressed in the various analyses [35,36,38,63,70,73,98], followed by quantification of savings in the consumption of resources such as energy, water, raw materials and fossil fuels [9,39,63,88,95], and quantification of the reduction of waste sent to landfills [9,34,39] resulting from the potential application of industrial symbiosis. Life cycle assessment [9,38,63,84,98] was the method most commonly used to assess the environmental impact. The economic aspect, which was the second most frequently assessed, was measured using the life cycle costing method [104] and several metrics that primarily reflected reductions in raw material [39,60], fuel [13,106], energy consumption [35,36], waste disposal costs [38,39], and increased revenue [34,35,38,45,109]. Job creation [45,80,120,121] was the indicator most commonly used to assess the social benefits of realising potential industrial symbiosis relationships. The material flow analysis method [9,10,39,88,111] has also been used several times to quantitatively assess potential industrial symbiosis.

The environmental and economic components of sustainability have also been used to optimise and select the best potential symbiosis relationships. In order to compare industrial symbiosis with an equivalent system in which companies remain separate, several forms of synergy network integration have been evaluated and optimised based on their total costs [69] and carbon dioxide emissions [70] using the MIND method, an optimisation method based on mixed integer linear programming. Three models were developed using mixed integer linear programming to model and optimise waste streams in an industrial park in France with regard to cost and environmental impact [75].

SWOT analysis has also been used several times [67,76,111,114] to study the internal and external factors in the potential application of industrial symbiosis networks.

#### 3.3.5. Potential Environmental, Economic, and Social Benefits

An analysis of the various cases of potential industrial symbiosis leads to the conclusion that most of them intend to achieve environmental, economic and social benefits through these practices. Table 3 gives some examples of this. The environmental component was most frequently measured, largely due to international constraints on reducing greenhouse gas emissions, as well as national constraints on emission reductions and the amount of waste sent to landfills and incinerators. It is therefore important to ascertain whether these practices can be effective in meeting these limitations.

The economic component, which is often cited as a determinant factor in decisions by companies to establish symbiosis relations [34,130], was the second most frequently quantified component of sustainability. However, not all cases of industrial symbiosis have the potential to provide economic benefits to all participants, such as an example of symbiosis in a chemical industrial park in the west of Urumqi City, China. In this case, one of the companies did not receive economic benefits, largely due to the fact that the price of the raw material was lower than some types of industrial solid waste that were used for the production of bricks [130]. The environmental benefits, such as reduced consumption of natural resources and greenhouse gas emissions, justify the implementation of these networks of synergy, even without economic benefits, although, in these cases, it is important that local or national governments provide economic incentives that encourage companies to create these synergies.

It can therefore be concluded that the implementation of these cases of industrial symbiosis can provide a number of environmental, economic, and social benefits that translate into an efficient use of waste and resources.





#### *3.4. Cases of Potential Industrial Symbiosis Applied to New Products and New Uses of Waste*

Research into new uses for waste and the manufacture of new products based on industrial symbiosis is essential in order to reduce the consumption of raw materials and reduce waste sent to landfills and incineration plants. However, despite the environmental and economic benefits of this reduction, the process of moving from research to practice is not always rapid or, indeed, possible. In addition to the barriers to the creation and development of industrial symbiosis, which are often referred to in the literature, such as a lack of trust among potential collaborators [33,116], the risks and uncertainties associated with the costs and benefits of such synergies [18,131], and a lack of knowledge [34,41], there are other more specific obstacles that impede these new uses. Current legislation restricting the integration of new waste materials into productive processes [132,133] and the toxicity of some of these waste materials [134] are examples of barriers that can hinder the flow of waste materials and thus condition the development of future synergies. Thus, several studies have investigated future relationships of industrial symbiosis with a focus on the use of new waste materials and their reutilisation in the manufacture of existing or new products. Table 4 presents a summary of these studies, and in addition to the location and main characteristics of the industrial symbiosis, describes the methods used in the various studies. The different case studies are grouped into regions, i.e., Europe, Oceania, North America, and South America, which are sorted into descending order in terms of the frequency with which they appear in the studies. In cases with an equal number of articles, the ordering was carried out based on the date of publication, in ascending order.

It can be seen that, although these cases are few in number, a great variety of industries is involved in potential industrial symbiosis, and there is a wide range of different waste materials and potential uses of these in the manufacture of new or existing products. The geographical distribution is also relatively varied, with studies carried out in Europe, North America, South America, and Oceania. Although not all of these studies were contextualised with regard to a specific location, the vast majority studied the potential for using new waste materials within a given geographical context.



**Table 4.** *Cont.*


**Table**

**4.**

*Cont.*


**Table 4.** *Cont.*

In some cases, geographical location was not constraining, and the use of certain waste materials could be transferred to several locations. For example, waste materials from common industries and those available in most countries can be used to extend the range of application of industrial symbiosis. One example of this is a case study of the production of potential symbiosis products such as soil amelioration pellets, low-competence concrete, and mine filler from a mixture of waste materials from multiple industries, such as pulp and paper mills, carbon steel plants, mines, and power plants [133]. Although this was studied in the context of Finland, this symbiosis could be replicated in numerous different locations due to the nature of the industries available. Another example was a study of the use of wine grape pulp to produce a bio-adsorbent for the removal of copper sulphate from water [145]; this symbiosis between the winery and environmental industries could also be reproduced in several distinct locations.

In other cases, however, geographical location can condition or incentivise the use of certain waste materials in the symbiotic process. For example, the strong presence of a particular type of industry can be an enhancer for industrial symbiosis and the search for new solutions to the waste generated by the production process. One example is the pulp and paper industry, which has a long tradition in Finland and is responsible for large volumes of production [21] and consequently high levels of waste generation. One of the studies focused on the potential uses of sludge resulting from the processes of wastewater treatment in the forestry industry and of fly ash resulting from the production of bioenergy for the production of a forest fertiliser [135]. In Brazil, the steel, mining, and construction sectors have a strong presence and are responsible for large-scale generation of waste and greenhouse gas emissions, and this has boosted the search for sustainable solutions. Thus, new solutions for the use of iron and steel mining waste were studied as an example of potential symbiosis in the production of solid brick/construction blocks in the Quadrilátero Ferrífero zone [143].

Current progress and the consequent emergence of new products have created new streams of waste, and with them the need to provide solutions which promote a more sustainable end of life. If these wastes result from or are integrated into a sector or product that has a significant environmental impact over its life cycle or a certain part of it, industrial symbiosis makes it possible to reduce the environmental impact of this sector or product. One of the examples found in the literature was the potential use of end-of-life electric vehicle lithium-ion batteries as storage systems for the renewable energy produced from photovoltaic systems in the generation of electricity for buildings [138,141]. In addition to using a waste material that is expected to increase over the next few years, this potential synergy also contributes to the reduction of carbon dioxide emissions from two sectors that are responsible for high greenhouse gas emissions—buildings and the automotive sector.

The study of new solutions based on industrial symbiosis is not only due to the inherent characteristics of certain waste materials, such as their toxicity and associated value, but also to the fact that the recycling process is often expensive and a large consumer of energy, meaning that it is not a viable solution. In the case of retrieval of valuable metals such as platinum, the recovered value sometimes does not cover the costs inherent in retrieval [137] if these are present in low concentrations, and industrial symbiosis can address this limitation. One example is a case study of Baglan, South Wales, in which, due to the local proximity between the stakeholders of potential synergy, the recovery of platinum for the production of catalytic electrodes for dye-sensitised solar cells could be translated into environmental, economic, and social benefits [137]. The potential direct use of sheet metal scrap from the automobile industry in the manufacture of new facade systems for the exterior of buildings could also lead to a reduction in costs of approximately 40% and a reduction in energy consumption of approximately 67% compared to a conventional recycling process [142].

All these studies were supported by several methods with different objectives. Since the main aim of these publications was to promote the use of new waste materials or the production of new products empowered by industrial symbiosis, it is not surprising that the predominant methods were those associated with laboratory-scale experiments. These tests were carried out to study not only the characteristics of waste materials [137,138] but also the final products [135,143] in order to guarantee their functionality and suitability for these purposes. A knowledge of the potential environmental, economic and social benefits that these new uses of waste can provide is also very relevant, as these can drive realisation. In the same way as for studies of potential industrial symbiosis between companies, the environmental component was the most frequently analysed aspect [135,138–140], followed by economic factors [137,140,142,144], and finally social components [140]. The potential benefits from the use of new waste materials and the manufacture of new products based on industrial symbiosis are extremely diverse. Table 5 lists the main environmental, economic, and social benefits that could be achieved if some of these potential symbioses were put into practice.


environmental,andsocialbenefitsoftheofand



#### **4. Drivers and Barriers to the Realisation of Potential Industrial Symbiosis and Strategies to Overcome These Barriers**

A knowledge of the drivers and barriers to the implementation of industrial symbiosis is essential in order to develop measures that enhance the application of this practice. Based on the studies of potential industrial symbiosis analysed above, this section compiles the various drivers, i.e., factors that promote and facilitate the development of industrial symbiosis, and barriers, i.e., the factors that hinder the implementation of this practice. Selected strategies for overcoming the various barriers are also highlighted, as these can create conditions for the various cases of potential industrial symbiosis to materialise.

#### *4.1. Drivers and Enablers of the Realisation of Potential Industrial Symbiosis*

An analysis of the articles on potential industrial symbiosis leads to the conclusion that there are a number of factors that play important roles in the realisation of industrial symbiosis relationships. Knowing the environmental, economic and social benefits that this practice provides is important in promoting the creation of synergy networks [146]; however, these are not always the main drivers of this practice, and many other drivers have been identified in studies of potential industrial symbiosis as being conducive for companies to participate in symbiosis networks. In most cases, it is not one factor but a set of factors that create favourable conditions for the development of symbiosis.

The economic, environmental, political, and social context of a country can be decisive in the way sustainability issues are addressed and consequently in how they can favour or condition the development of industrial symbiosis. The distribution of a number of potential industrial symbiosis articles by country, as illustrated in Figure 3, reflects the characteristic context of each country.

Existing legislation, plans and policies in each country are also repeatedly referred to as drivers of industrial symbiosis [82,101,115,118,120]. Companies are encouraged to set up synergy networks through imposing limits on emission or waste disposal through regulations and taxation, facilitating the use of waste, and allocating funds.

The higher numbers of studies of potential industrial symbiosis in Europe cannot be dissociated from the efforts that have been undertaken by European countries to reduce greenhouse gas emissions and to promote the more efficient use of resources. These efforts have been driven by the European Commission, which has established a number of directives, communications and programs with the provision of funds. One example is the "Roadmap to a Resource Efficient Europe" communication, which proposed a framework for action to ensure the sustainable management of all resources without sacrificing economic growth [147]. Another example is the communication "Closing the loop—An EU action plan for the Circular Economy". This communication underlined the importance of industrial symbiosis and proposed to facilitate this practice through cooperation with the Member States, guaranteed funding through cohesion policy funds, and the research and innovation framework program Horizon 2020 [148]. Another initiative launched by the European Commission was Directive 2018/851 on waste; in addition to highlighting the great advantages of improving the efficiency of waste management and recognising waste as a resource, this acknowledges the importance of industrial symbiosis and encourages Member States to take steps to facilitate it [149].

The European countries for which the highest numbers of cases of potential industrial symbiosis have been published are Italy, Sweden, and Finland. All of these countries have common factors that may have contributed to fostering the study of new industrial symbiosis relationships and their implementation, such as (i) a greater concern with environmental issues and the search for sustainable solutions, (ii) the established existence of cases of industrial symbiosis over several years, (iii) a considerable number of cases of self-organised symbiosis networks, (iv) the existence of facilitators through national agencies or local governments, and (v) more stringent environmental regulations [18,21,41,150,151].

The realisation of two cases of industrial symbiosis, involving the automotive [64] and agri-food [65] industries in the Italian Region of Abruzzo, is another example of the combination of several factors in realising the potential of symbiosis. In this case, good communication routes, favourable geographical positions, stakeholder involvement, and the facilitating role of the president of Consorzio Italiano Subfornitura Impresa (CISI) in the case of the automotive industry were viewed as driving factors in the development of industrial symbiosis [152].

China has the highest number of published cases of industrial symbiosis [19] and potential such cases. This may be associated with a set of measures that China has implemented over recent years, such as the implementation of policies and plans, financial incentives, and research incentives. These measures have attempted to contain the negative effects of increasing industrialisation and urbanisation in recent years [153,154], such as increased carbon dioxide emissions [153,155,156], increased amounts of industrial solid waste [154,157], and increasing resource consumption [39,97]. The National Pilot Circular Economy Zone Programme, launched by the State Environmental Protection Administration in 2001, and the laws that have been applied since 2003 to promote the circular economy, are examples of these measures. While not primarily aimed at promoting industrial symbiosis, they contribute to the spread of this practice by providing increased awareness of the importance of resource reuse and recognition of the fundamental role of the circular economy in China's development [23,24]. The China National Eco-Industrial Demonstration Programme launched in 2000 by the State Environmental Protection Administration [25] has also contributed to the increase in the number of potential industrial symbiosis cases and China's leadership in the publication of case studies [19]. This programme has enabled the development of the largest national network of eco-industrial parks, in which industrial symbiosis practices have been promoted [158,159].

The predominance of certain types of industry within countries can also be a driving factor for the creation of industrial symbiosis networks. This is particularly true if they are large consumers of resources and emitters of greenhouse gases, such as the steel and iron industry in China [97,160], and if they play a key role in economic development, such as the agri-food industry in Italy [65] and the iron, steel, and cement industries in China [160,161]. Moreover, these industries have a longstanding tradition in these countries and are located in industrially mature areas, which Jensen et al. [92] have shown to facilitate industrial symbiosis.

A diversity of industries has also frequently been highlighted as conducive to the establishment of industrial symbiosis relationships [76,114], since this opens up a range of opportunities due to the variety of wastes and the numbers of companies that produce them and have the potential to incorporate them into their processes. If there are several companies carrying out the same type of economic activity, this can be an added advantage, since it ensures a more constant flow of waste [77], while if there are no other companies nearby to ensure the incorporation of these wastes into their processes, the viability of industrial symbiosis is compromised. The fact that there are several industries carrying out the same type of economic activity may also enhance other synergies, such as infrastructure sharing and the joint provision of services.

If a company can function as an anchor tenant, this can be an important factor in driving the realisation of industrial symbiosis relationships [76,82,91,93,114]. These companies are able to attract and anchor a network of companies, not only in terms of the supply of materials but also the reuse of waste. There are some examples of such cases reported in the literature, such as a power plant in Honolulu in the US [162], mining firms in Gujiao, China [163], and a pulp and paper mill in Kouvola, Finland [164].

Although not an indispensable requirement for establishing the synergy network, geographical proximity between the potential participants in industrial symbiosis is often referred to as a facilitator [11, 36,82,91]. Establishing symbiosis networks with nearby companies can increase trust in the relationship. In addition, the fact that waste is mostly of low economic value, transportation and environmental costs may no longer compensate for the symbiosis connection over long distances.

The existence of industrial symbiosis networks that have already been established in a given place can be a driving force for creating new synergy linkages and extending the network to new companies, since the internal structures [114] and trust relationships that facilitate this development [40] are

already established. In addition, there is evidence that these networks can be of benefit to the parties involved, not only in terms of the reduction of waste treatment and landfill costs, but also in terms of the savings made in the acquisition of raw materials, and profits from the sales of waste. If there are entities that can support and facilitate existing on-site cases of industrial symbiosis, these can also act as enablers for the creation of new connections. This role can be played either by public entities such as local governments or by private entities such as business associations [18,41]. These entities, which are aware of the reality of the site, can more easily identify new partners for infrastructure sharing and joint provision of services, as well as new companies that may be able to use waste that is not yet in use, or that can provide waste to companies already involved in industrial symbiosis. However, in places where no synergy networks have been established, the role of these facilitators can be highly relevant, as mentioned in some of the cases analysed here [60,76,82,107]. They can provide training for companies, facilitate the exchange of information [53], foster cooperation and trust between companies [76], and coordinate and help identify possible symbiosis relationships [107].

#### *4.2. Barriers to the Realisation of Potential Industrial Symbiosis*

Despite the recognised environmental, economic and social benefits that industrial symbiosis can provide, there are a number of barriers that hinder its development. The literature review shows that these barriers can be of various types, such as economic, technical, regulatory/legal, organisational, social, and cultural [18,33,47,131,165]. By analysing selected publications on potential industrial symbiosis, it was possible to identify several of these barriers.

Several studies of potential industrial symbiosis have pointed out the lack of appropriate policies as a barrier to the application of this practice [53,65,116,122]. Low taxes on landfill disposal [122], a lack of policies that encourage and regulate industrial symbiosis [116], a lack of funds to promote this practice [116], and deficient regulatory frameworks [122] are some of these barriers. In addition, existing legislation may limit the implementation of synergy relationships, especially if it is too rigid, unclear, or inconsistent. One example is Italy's regulatory system, which is referred to in studies of potential industrial symbiosis [53,65] as constraining companies with regard to the use of waste.

There are also others barriers to the creation of industrial symbiosis networks. The first is associated with the reluctance of companies to establish synergistic relationships, not only due to a lack of knowledge of the industrial symbiosis mechanism [116,122] but also due to a lack of knowledge of other companies with the potential to receive or provide waste [9,68]. In addition, a lack of trust [82,116], resistance to providing data on processes and generated waste [122], and uncertainty related to the profitability of the symbiosis network [75] and the associated costs and risks [82] were also identified as barriers to the development of symbiosis relationships.

The fact that companies are implementing measures to reduce waste generation has also been identified as a barrier to the development of industrial symbiosis [68], as there is a concern that the stream of waste involved cannot be guaranteed.

The economic component has been referred to by various authors [18,166,167] as being essential in inducing companies to take the initiative to establish an industrial symbiosis relationship. If the economic value of raw materials is very close to that of waste, there is no incentive for companies to use waste in their production processes [37]. Moreover, the price that companies are willing to pay for waste may not be economically advantageous for the company producing such waste. In this case, there is no incentive for companies to divert waste from landfills and start a symbiosis relationship [68]. In addition to these factors, the role of stakeholders in deciding whether or not to initiate symbiosis relations should be highlighted, as there is often a lack of openness [34] and willingness [116] to initiate this kind of collaboration.

For the establishment of some symbiosis relationships, such as the sharing of waste heat, the initial costs associated with infrastructure are very high, and this makes companies reluctant to establish such symbioses [75,106,107]. A lack of availability of technologies required [9,116] and the high costs of equipment [85] for the realisation of industrial symbiosis have also been identified as inhibiting the realisation of this practice.

The social and economic instability of a country can also condition the establishment of synergies, since although the issue of sustainability is recognised as being important; there may be social issues which take precedence [168]. In addition, the issue of survival is imperative in some countries with developing economies, and since the time between setting up projects and achieving results may often be long in these countries, this may constitute a barrier to the implementation of symbiosis [45].

#### *4.3. Strategies for Overcoming the Barriers to the Realisation of Potential Industrial Symbiosis*

Regulations and policies were most often referred to as being important for encouraging or limiting the establishment of industrial symbiosis relationships [85,111,122,169]. While the decision to establish a symbiosis relationship is made by a given entity, the role of policies is critical in encouraging this practice. Thus, in order for these to not function as barriers, it is necessary to provide legislation and policies that are clear, consistent, and less bureaucratic and can facilitate the process of waste use [170].

Economic incentives have also been highlighted as being important in the realisation of industrial symbiosis [9,122]. To create more efficient legislation to facilitate this practice, programmes can be coupled with the provision of funds to promote industrial symbiosis and offer monetary support for companies in terms of the construction of infrastructures or the acquisition of the equipment necessary for the realisation of these relationships [122].

However, even if there are a number of policies and programs that facilitate and encourage companies to establish symbiotic relationships, the companies themselves are often reluctant to make such connections. Thus, to drive the implementation of industrial symbiosis, it is necessary to disseminate information to companies. This can be realised through workshops, working group discussions, and other actions [116,122] that provide the necessary information to companies on industrial symbiosis and its potential benefits. A knowledge of this practice can create the willingness to cooperate, which is fundamental for the realisation of symbiosis networks [68].

The role of facilitators such as local governments and industry associations has also been highlighted as a way to overcome various barriers [34,111], including a lack of confidence, a reluctance of companies to share their data and a fear of relying on other companies. These entities can provide training to various employees on the concept of industrial symbiosis, assist in the creation of trust and cooperation relationships, and help to identify new symbiosis relationships [34,165].

Some barriers such as a lack of knowledge of companies with potential to start symbiosis relationships and lack of trust can be overcome using digital platforms and programs [87]. These tools can enable social interaction between companies and facilitate a search for companies that can provide or receive waste. In addition, where appropriate, they can facilitate the choice of the best option based on prices, distances and potential environmental and economic benefits [38,80,87].

In order to overcome the barriers associated with a lack of available technology, there is a need for increased investment by governments in research and development into technological innovations and greater involvement with research teams from university or business associations [9,37].

In the case of poorer countries with social problems such as high unemployment rates, low incomes, illiteracy, or low life expectancy, such as the cases studied in the Philippines, Liberia, India, and Bangladesh, the implementation of industrial symbiosis practices is more difficult. However, when properly supported, the implementation of this practice can make a positive contribution to the long-term sustainable development of these countries, as it makes it possible to combine environmental and economic issues with social aspects. These advantages translate into job creation, long-term links between companies and the possibility of small businesses entering the synergy network [45,61,171]. It is therefore essential to support these countries in the development of industrial symbiosis. Several authors have proposed measures for overcoming the characteristic barriers in these countries, such as (i) extending the symbiosis network to other stakeholders such as

community leaders and government organisations [45], (ii) the establishment of policies that encourage symbiotic relationships between small businesses, such as small farms [45], (iii) the provision of subsidies [122], and (iv) helping to obtain international support created specifically for sustainable projects in developing countries [122].

The main drivers and barriers to achieving potential industrial symbiosis are very diverse, as illustrated in Figure 5, and overcoming the various barriers and achieving further dissemination of industrial symbiosis requires concerted action at various levels. It is therefore essential to coordinate the various entities and resources and to restructure existing regulatory systems. It is also necessary to support companies in paradigm shifting and raising awareness of the advantages of more sustainable practices, and in particular industrial symbiosis.

**Figure 5.** Key drivers, enablers, and barriers to the realisation of potential industrial symbiosis, and strategies to overcoming these barriers.

#### **5. Conclusions**

Despite the large number of existing cases of industrial symbiosis, there is still huge scope for growth, as evidenced by the various studies that have been carried out to assess the potential application of this practice. A comprehensive review of the literature reveals that there is potential for the development of new industrial symbiosis relationships around the world, with a wide diversity in terms of network size, the types of economic activities involved and the types of waste stream. Most existing studies focus on countries where symbiosis is already widely applied, such as China, Sweden, Finland, and the US. However, the potential for industrial symbiosis has also been studied in countries where this practice has few or no existing cases, such as Egypt, the Philippines, and Colombia. Although most of these cases reproduce existing symbiosis relationships with regard to the activities and types of waste involved, there have been studies that have looked into the potential use of new waste and the manufacture of new products based on industrial symbiosis relationships. Furthermore, the potential for applying industrial symbiosis is not limited to replacing resources with waste; there are also many opportunities for other types of synergies, such as infrastructure sharing and joint provision of services.

Despite this great potential, it was only possible to verify the realisation of two of the cases of potential industrial symbiosis in the literature review. It can be inferred that there was either no interest from industry in the implementation of industrial symbiosis or, if the potential was realised, there was no follow-up that resulted in a publication. However, it is important to understand that there is interest from industry in implementing these cases. Some of the studies have a more theoretical character, and many of them resorted to interviews and site visits, which implies that industry is aware of the potential for industrial symbiosis. Thus, it is important to monitor the implementation of symbiosis in order to better understand the dynamics of implementation of this practice and the main factors enhancing its development. If potential cases of industrial symbiosis are not realised, it is relevant to analyse with companies the main barriers to this implementation.

The work carried out in this paper regarding knowledge of the potential for industrial symbiosis and the main barriers and drivers to its implementation may have theoretical implications. The characterisation of the various cases can contribute positively to the research efforts that have been developed to increase the application and diffusion of industrial symbiosis. Knowledge of the main drivers and barriers may also have implications for the development of theory, in terms of an understanding of industrial symbiosis and the main mechanisms that can drive or hinder it.

While an effort was made to ensure that the review of the bibliography was as comprehensive as possible, we limited our search to articles written in English and using only research articles, conference articles, book chapters and editorials, and this may have overlooked some cases of potential industrial symbiosis that could provide a greater understanding of this topic.

Of the various types of resource sharing available, most studies address the potential use of waste and the advantages that arise from its integration as a raw material in the production process. However, in future research, it is important to examine more case studies assessing the potential of infrastructure sharing, the joint provision of services, and the potential benefits to the companies, environment, and society. It will also be important to focus on the most favourable conditions and the factors determining implementation of symbiosis.

Future research could also assess whether various cases of potential industrial symbiosis have been implemented in order to increase our understanding of the mechanisms that drive or condition the creation of synergies and thereby promote the growth of industrial symbiosis initiatives.

**Author Contributions:** A.N. conducted the study, and wrote and prepared original draft. R.G. handled the conceptualisation, and the writing and editing of the manuscript. S.G.A. and J.C.O.M. supervised, revised and corrected the manuscript. C.P. helped with bibliography review.

**Funding:** Radu Godina would like to acknowledge financial support from Fundação para a Ciência e Tecnologia (UID/EMS/00667/2019). This work was also financially supported by the research unit on Governance, Competitiveness and Public Policy (UID/CPO/04058/2019), funded by national funds through FCT—Fundacão para a Ciência e a Tecnologia.

**Conflicts of Interest:** The authors declare no conflict of interest.

