*Article* **Understanding Hazardous Waste Exports for Disposal in Europe: A Contribution to Sustainable Development**

**Carmen Callao 1,\*, M. Pilar Latorre <sup>2</sup> and Margarita Martinez-Núñez <sup>3</sup>**


**Abstract:** The concept of sustainable development was introduced in Europe by the Treaty of Amsterdam (1997) and was extended to waste management in the Waste Framework Directive. In order to achieve sustainable development, hazardous waste (HW) must be managed safely and in accordance with regulations. This also applies to worldwide HW transport, especially when HW is shipped for disposal. The United Nations, through the Basel Convention, aims to prevent the export of HW from developed countries to developing countries for disposal. In Europe, HW shipments are regulated by Regulation (EC) No. 1013/2006 of the European Parliament and by the Council of 14 June 2006 on shipments of waste. Additionally, all HW shipments must be in accordance with two principles contained in the Waste Framework Directive: proximity and self-sufficiency. Using data from 2014 and network analysis methodology, this paper fills the gaps in the scientific literature by looking at how shipments of HW travel for disposal in Europe, how the regulations affect these shipments and how GDP per capita influences the shipment of waste. The results show that countries with a high GDP per capita play an important role in the network (having the highest in-degree) and that the absence of landfill taxes for HW does not influence HW shipments for disposal. Therefore, countries in the EU act in accordance with the proximity and self-sufficiency principles.

**Keywords:** hazardous waste shipment; network analysis; gross domestic product per capita; disposal; proximity principle; self-sufficiency principle

#### **1. Introduction**

Sustainable development is a fundamental objective of the EU and was included in the 1997 Treaty of Amsterdam [1]. Since then, the Sustainable Development Strategy has gone through a great revolution.

Sustainable development includes waste management as the Waste Framework Directive (WFD) urges Member States to "promote and support sustainable production and consumption models" and introduces the United Nations Sustainable Development Goals in its objectives, showing the relation between waste management and sustainability.

Although sustainability is an aim of the European Union, the European waste management industry shows a weak model of sustainability [2].

The study of efficient hazardous waste (HW) management in relation to legislation can be a driving force towards the achievement of sustainable development [3]. It should be noted that, in order to achieve sustainability, certain regulations and directives must be met and fulfilled to ensure safe and environmentally sound practices are implemented. The implementation of the two environmental principles included in the WFD, proximity and self-sufficiency, affect HW exports in Europe. However, HW is not only a European concern but also a worldwide concern, especially regarding its disposal. HW disposal must

**Citation:** Callao, C.; Latorre, M.P.; Martinez-Núñez, M. Understanding Hazardous Waste Exports for Disposal in Europe: A Contribution to Sustainable Development. *Sustainability* **2021**, *13*, 8905. https:// doi.org/10.3390/su13168905

Academic Editor: Silvia Fiore

Received: 7 June 2021 Accepted: 31 July 2021 Published: 9 August 2021

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

be carried out safely and controlling HW shipments is essential to determine where and how such disposal takes place.

In the analysis of HW shipments transported for disposal, one must take into account how they affect sustainability. On the one hand, shipments of HW affect sustainability through carbon emissions, as road transport has a great impact on greenhouse gas (GHG) emissions [4–6]. However, an analysis of the regulations applying to the shipment of waste is also important, as transboundary waste shipments contribute to efficient waste management [7], and trade policies also affect environmental quality [8] and shipments.

Understanding these regulations and policy implications is key to achieving the Sustainable Development Goals [9].

Finally, disposal in landfill sites or in incineration facilities has a great impact on the environment and therefore on sustainability [10–14].

The main research problem is the lack of information on how HW is transported and disposed of in Europe, and the relationship between HW disposal shipments and compliance with European environmental laws. This article analyses different aspects of sustainability related to HW management and HW shipments and contributes by: (1) deepening the analysis of waste shipments in Europe and the way in which landfill taxes affect waste shipments; (2) analysing HW regulations and the adherence to the principles of proximity and self-sufficiency, and; (3) presenting a qualitative analysis of HW shipments for disposal using different variables – GDP, HW generation and the amount of HW shipped for disposal. To fulfil these aims, network analysis is used to display the importance of networks, giving us a picture of HW shipments, and showing the communities which arise in relation to HW exports for disposal.

Before the methodology is set out, there is a review of key aspects of HW shipments for disposal: a legislative review, including logistics, and a brief analysis of costs and capacity as they relate to the disposal of HW.

#### **2. Objectives and Scope to Present Legislative and Literature Review**

#### *2.1. Objectives*

Research literature shows how hazardous waste travels worldwide for disposal from rich countries to poor countries [15], making GDP an important element in HW shipments. European countries cannot export HW for disposal to countries outside the Organization for Economic Co-operation and Development. In order to achieve their objectives and fulfill regulations, countries can use different policies and landfill taxes.

In this research we answer the following questions, not yet analyzed by researchers:


With these questions we try to fill the gap in the research literature about how HW travels and the relationship between HW shipments and legal compliance with European environmental laws.

#### *2.2. Scope*

The geographical scope of the research is Europe.

Regarding waste management the scope is Hazardous Waste exports for disposal. The year analyzed is 2014.

It must be pointed out that the scope of this research has several limitations. Firstly, due the research time, only 2014 data of hazardous waste shipments for disposal have been used in the network analysis. Secondly, not all European countries are studied, as Eurostat only has data for the countries included in the table. Thirdly, no data on number of landfills in each country have been used. Finally, the network is made from legal and official data from Eurostat, even if data from illegal shipments are important to understand the impact on sustainability.

#### *2.3. Literature Review: The Origin of the Restriction of HW Exports*

The Basel Convention controls HW transport for disposal worldwide. The EU, as part of the Basel Convention, has incorporated its provisions through the European Waste Shipment Regulation (EWSR) [16].

The EWSR was modified in 2014 by Regulation (EU) No 660/2014 of the European Parliament and by the Council of 15 May 2014, amending Regulation (EC) No 1013/2006 on shipments of waste and aiming to strengthen inspections of waste shipments, in order to discourage illegal shipping. Even after this amendment, loopholes in the legislation have been found [17].

Recently, and after China's plastic waste ban, the EWSR has been modified by the Commission Delegated Regulation (EU) 2020/2174 of 19 October 2020 amending Annexes IC, III, IIIA, IV, V, VII and VIII of Regulation (EC) No 1013/2006 of the European Parliament and of the Council on shipments of waste. China's ban was caused by plastic pollution [18] and will affect the plastic waste trade networks which have been hereto established [19], as well as the global circular economy [20].

Besides the Basel Convention and the EWSR, Directive 2008/98/EC on waste management includes two principles connected with waste shipments, as described in Article 16 of the WFD: self-sufficiency and proximity. The self-sufficiency principle indicates that "Member States shall take appropriate measures, in cooperation with other Member States where this is necessary or advisable, to establish an integrated and adequate network of waste disposal installation. The network shall be designed to enable the Community as a whole to become self-sufficient in waste disposal." The proximity principle states that "the network shall enable waste to be disposed of or recovered in one of the nearest appropriate installations by means of the most appropriate methods and technologies to ensure a high level of protection for the environment and public health."

The proximity and self-sufficiency principles can increase the market power of local disposers [21], as Reggiani and Silvestri state, but these principles are also analyzed because of their legal importance [22,23].

Compliance with both sets of regulations, the EWSR and the WFD and its principles, should lead to fewer exports of HW for disposal, and better control the illegal traffic in waste to poor countries [17,18,24,25].

The application of the proximity principle decreases the dangers in the transport of HW [26–28] and the GHG emissions caused by the transport of waste by road, and the self-sufficiency principle can lead countries and companies to innovate in order to comply with the regulations [29–32].

#### *2.4. HW Management: Costs and GDP*

Waste management costs have been indicated as one of the reasons for illegal shipments [33,34] and a barrier to a circular economy in which recovery is prioritized over disposal [35,36].

GDP is an important variable in this analysis for two reasons: on the one hand, there is a link between GDP and waste generation [37,38], and, on the other, as HW travel worldwide from rich countries to poor countries, it is important to know how GDP affects the export of waste in Europe, and if HW is disposed in countries with a high or a low GDP.

#### *2.5. HW Shipment for Disposal and Disposal Taxes*

Disposal operations are classified by the WFD into 15 categories, identified with the codes D1 to D15. In research on disposal taxes and their effects, not all papers distinguish between different disposal operations [39–41]. Instead, they discuss disposal in general. However, Sigman's analysis of HW taxes [42] establishes a difference between landfill disposal and incineration. Dinan [39] proposed the taxation of disposal and the establishment of a reuse subsidy. Levinson [40] studied the effect of disposal taxes on HW shipments for disposal, finding that HW disposal taxes increase the decentralisation of HW disposal. The

literature on this topic has developed widely, studying not only the impact of landfill and disposal taxes [43–45] but also the impact of environmental taxes [46–48].

It is important to determine what kind of disposal operations should be taxed if the right effect is to be achieved and there is to be sustainable development. Incineration (D10) increases in countries with landfill taxes, which causes landfills (D1) to decrease [49–51]. Taxes and regulations that ban the landfill disposal of some types of waste have allowed the Netherlands to reduce its number of landfills [52]. Interestingly, Scharff (2014) points out that "underground storage" in Germany is in a grey area between disposal and recovery, while others recognise underground storage as a common disposal practice [53].

According to a study on landfill taxes in Europe [54], landfill prices vary among and within European countries according to waste classification (e.g. HW, non-HW and municipal solid waste). Bulgaria, Finland and Norway have no landfill taxes for HW, while in the Belgian region of Wallonia, HW is partially banned. In other countries HW is taxed, with rates ranging from less than 10 euros/tonne (Portugal) to more than 60 euros/tonne (UK, Ireland, Denmark, Czech Republic and Estonia), which may be one of the causes of the shipment of waste for disposal. Some of these countries have also established taxes on incineration to promote waste recycling (Austria, Denmark, France, The Netherlands and Norway), whereas in countries where only landfill disposal is taxed, incineration has increased.

The variation of landfill taxes is also verified by the information provided by the Confederation of European Waste-to-Energy Plants (CEWEP) [55], updated to December 2017.

#### **3. Materials and Methods**

#### *3.1. Network Analysis for HW Shipment for Disposal in Europe*

This research uses network analysis to determine the relationships among nodes (countries) and uses Gephi to show the relevance of these nodes in the network or the communities formed by the countries, in the framework of HW exports for disposal among EU Member States. Gephi is used not only to create a trade network to discover whether the self-sufficiency and proximity principles are being adhered to, but also to relate trade/shipments to GDP per capita and HW production. This paper analyzes how these variables affect HW shipments for disposal in Europe.

The stages of this research are presented in the following diagram (Figure 1):

#### **Figure 1.** Block diagram.

Gephi is a piece of software designed to explore networks and has previously been used in scientific studies related to waste management. Lepawsky [56,57] used Gephi to evaluate e-waste trade and to determine its evolution and patterns. In his research he used e-waste data import transactions reported by territories and available from United Nations Comtrade. Chen et al. [58] and Wang et al. [59] used Gephi in an analysis of the literature related to waste. While the former used data from the WoS Collection Database on the most cited publications on construction and demolition waste, the latter used data on the literature on incinerating waste to produce energy.

#### *3.2. Network Model*

Different metrics, like degree centrality, eigenvector centrality and modularity, are used to analyze how European countries are linked to HW shipments for disposal. In the networks with origins in European countries in 2014, V represents the set of countries from Europe and E represents the shipments for disposal. Let (v\_i,v\_j )∈E, with v\_i,v\_j ∈V as an edge (i.e., export) in G, representing HW shipments among countries v\_i and v\_j. This analysis assumes that countries' relationships are unidirectional—that is, from exporter to importer—and, therefore, the graph is directed.

#### 3.2.1. Centrality Network Metrics

Centrality metrics measure how important a country is in the European network. In this analysis, centrality shows the importance and the role of a given country in HW exports for disposal. Centrality includes 'micro' measures that show how a given node relates to the overall network [60,61]. Knowing the importance of countries (i.e., nodes) in the generated network indicates the relationships between these countries in the shipment of HW for disposal.

#### Degree Centrality

Degree centrality [62] represents the number of links each country/node has in the network, using the following formula:

$$DC^{v\_i} = \frac{d(v\_i)}{|V| - 1} \tag{1}$$

where *d*(*v<sup>i</sup>* ) denotes the degree centrality (*DC*) of node *v<sup>i</sup>* in the network. This metric counts the direct links of each country in the network.

#### Eigenvector Centrality

Eigenvector centrality was proposed by Bonacich [63], as follows:

$$
\lambda \cdot EC^{v\_i}(G) = \sum\_{v\_j} g\_{ij} EC^{v\_j}(G) \tag{2}
$$

in which *gij* takes the value 1 if (*v<sup>i</sup>* , *v<sup>j</sup>* ) ∈ *E* and 0 otherwise (retrievable if *G* is represented using an adjacency matrix) and *λ* is a proportional factor (i.e., the eigenvalue).

Eigenvector centrality measures the influence of a node on a network. In other words, nodes that have high values of this measurement are well connected. Also, in this sense, they are good connectors as waste exporters and importers from a large number of countries and in large amounts. When the degree of centrality of the eigenvectors is greater, the cohesion of the group is greater.

#### 3.2.2. Structural Analysis of the Network through Modularity

Modularity is another technique used to observe the relationships of HW shipments among European countries. This notion of community partition using modularity was first proposed by Newman and Girvan in [64]. The vertices in networks create groups or communities, which means that some countries in the analyzed network have many edges (exports) while other countries have few edges. Countries in the same community are better connected, while those in different communities are less likely to be connected.

$$M(\Pi, \mathcal{G}) = \sum\_{\pi \in \Pi} e\_{\pi \pi}(\mathcal{G}) - \sum\_{\substack{\pi \in \Pi, \ \pi' \in \Pi, \pi'' \in \Pi}} e\_{\pi \pi \tau'}(\mathcal{G}) e\_{\pi' \pi''}(\mathcal{G})$$

where Π represents any community structure and e*ππ* (*G*) represents the fraction of all edges in the network that connect nodes in *π* to nodes in *π'*.

#### **4. Results**

The network analysis was performed with the disposal data obtained through Eurostat for the year 2014. As established in Regulation (EC) No. 2150/2002 of the European Parliament and the council of 25 November 2002 on waste statistics, Member States are obliged to provide data to Eurostat. The main reasons for analyzing the year 2014 are that in 2014 (1) the Circular Economy Package was presented and (2) the EWSR was modified. The Circular Economy Package was the starting point for legislative modifications in the Directives to regulate different waste streams and try to increase recovery and recycling. It is a key year to give a picture of HW shipments before the implementation of new recycling targets and new regulatory changes.

Table 1 shows tonnes exported for disposal from 2011 to 2015.


**Table 1.** Tonnes exported for disposal 2010–2015.

In 2015, a decrease in the quantities exported can be observed. The reason for this may be the change in landfill tax policies in some Member States, as CEWEP shows [55]. The Netherlands reintroduced its landfill tax, Norway repealed its landfill tax and Sweden established a fee in 2015. These changes in landfill taxes may have affected the exports of HW for disposal.

This study analyzes the shipments made in 2014 on the basis of the following scientific assumptions: countries are adhering to the proximity and self-sufficiency principles; as has been shown in research, there are difficulties in finding sites for HW facilities for disposal in the case of landfills and incinerators [65–67] because these must meet environmental, economic and social criteria; and countries must find the best routing model for their exports to minimize transportation costs and risks [26].

#### *4.1. From Data to Network Generation*

Taking the current network model (Section 4.1), let G = (V,E) be the graph representing the network for European waste disposal analysis, in which V is the set of operating countries and E is the set of existing shipments among them.

The figures show two different networks. Figure 2 shows the network based on the effective shipments of waste and the GDP per capita and Figure 3 shows the communities formed in the network.

In the export analysis, Italy (573,614 tonnes), Germany (237,777 tonnes) and the Netherlands (195,969 tonnes) were the countries with the greatest amounts of HW exported for disposal. The countries that generated the most HW were Germany (21,812,660 tonnes), Bulgaria (12,206,169 tonnes) and France (10,783,405 tonnes).

Table 2 shows that countries with the highest GDP per capita or with a GDP per capita above 40,000 euros in 2014, according to Eurostat, exported the most HW for disposal to other countries with a high GDP per capita.

**Figure 2.** Network of HW shipments for disposal in Europe.

**Figure 3.** Network for HW shipments for disposal in Europe (2014), displayed by modularity.


**Table 2.** HW exports from high-income countries.

The countries with the lowest GDP per capita or with GDP per capita lower than 15,000 euros did not receive HW for disposal, except Lithuania, which received HW for disposal from Latvia. Only two countries exported as much as 19% of the HW they produced (Malta and Slovenia), while the countries with the next highest exports exported under 10% of the waste they produced.

No data for HW landfill taxes were found from 2014. It is therefore not possible to assess whether these influenced HW shipments. However, in 2012, only three countries (Bulgaria, Finland and Norway) did not have landfill taxes for HW, and this did not appear to affect waste shipments—that is, European countries did not look to export to countries with no landfill taxes.

For degree centrality, three main nodes were considered (Germany with 26 relationships, France with 18 relationships and Belgium with 15 relationships). These countries are in central Europe, and, following the proximity principle, the logistics connectivity for these countries may have been greater. Furthermore, these three countries correspond to the highest in-degree values.

The results are shown in Tables 3 and 4 and Figures 2 and 3. Table 3 shows the amount of HW produced, the GDP per capita, the amount of HW exported, the in-degree (from how many countries waste is received or imported), the out-degree (to how many countries HW is exported), the degree (in-degree + out-degree) and the ratio of exports to generation.

Figure 2 highlights the waste tonnage generated by each node (i.e., the node size corresponds to the tonnage generated). The nodes are green, with their shades varying according to GDP per capita (a darker color corresponds to a higher GDP per capita). Finally, the thickness of the line corresponds to the amount of export flow between the countries.

Figure 3 shows the network displayed by modularity; each color represents a different community.


**Table 3.** Results from the Network Analysis for HW Shipments for Disposal in Europe in 2014.

**Table 4.** Communities in the European Disposal Network.


The node size is proportional to the waste tonnage that the country exports. The thickness of the line corresponds to the size of the export flow between the countries.

The modularity shows groups/communities in the network. These groups account for GDP per capita and show how the European countries apply the proximity and selfsufficiency principles.

Table 4 shows the communities formed in the network. The reasons for these communities are discussed in Section 6.

The largest community is the third (purple), which is composed of 10 countries: Bulgaria, Germany, Greece, Italy, Latvia, Lithuania, Luxembourg, Malta, Poland, Portugal and Romania. Germany has the highest eigencentrality score but the other members of this community have a score of nearly 0. Germany is also the country with the highest in-degree, which indicates that it receives the highest volume of HW. The second most important community is the first (green), which is composed of five countries: Belgium, Ireland, France, Cyprus and the Netherlands. Community six (blue) consists of Croatia, Hungary, Austria and Slovenia. The other communities are small.

#### **5. Discussion**

This paper analyses HW shipments for disposal, the effect of the regulations on the shipment of waste, the application of the two principles contained in the Waste Framework Directive—proximity and self-sufficiency—and the way in which GDP affects these shipments.

The adherence to these principles shows a low density network, while HW for recovery in the same year shows a high density network [68]. The density of the networks represents the links between the nodes, showing there are many fewer shipments for disposal than shipments for recovery.

The communities formed by some of the countries show that there is one country with a higher eigencentrality value [63], that is, a country that has a bigger relevance to the network.

The centrality shown by Germany can also be seen in the literature, as there has been a thorough analysis of waste treatment facilities in this country [69].

The countries with the highest in-degree (Germany 20, France 12, Belgium 15 and the Netherlands 12) are, except Belgium, countries with a high incineration capacity. These countries also have a GDP per capita above 30,000 euros.

In contrast with the "Pollution Havens" described in the research literature, in which waste travels from rich to poor countries [70,71], in Europe, HW is sent to be disposed of in countries with a high GDP.

This shows that high GDP makes these countries more able to use the best available techniques for wastes incineration [13].

The countries with high incineration capacity (France, Germany, Sweden, Denmark, the Netherlands, Austria and Finland) have a GDP per capita above 30,000 euros. These countries also have an important value for in-degree. It must be taken into account that, Sora [72] states that the opening of the incineration market threatens the application of the proximity principle.

#### **6. Conclusions**

One of the novelties of this study was the use of network analysis to fill in the gaps in the research literature about how HW travels for disposal in Europe and the relationship between HW shipments and legal compliance with environmental laws in relation to sustainability and sustainable development in Europe.

Network analysis is a useful tool to answer these research questions and to find out if HW travels for disposal to European countries with a low or high GDP and how countries interact to fulfil the principles of self-sufficiency and proximity.

HW is shipped for disposal to countries with a high GDP and high incineration capacities, which means that when countries must apply proximity and self-sufficiency principles, waste is shipped to countries with a high GDP, because these countries have better treatment facilities. This demonstrates how GDP is a determining factor in the export of waste.

Countries with a high GDP per capita have more incineration facilities; they are better prepared for the disposal of HW.

Good practices for the environment and for sustainable development are demonstrated by networks, showing coherence in the fulfilment of the principles of self-sufficiency and proximity, and the adherence to legal regulations.

The absence of a landfill tax does not affect the export of waste; countries with no landfill tax did not have higher in-degrees than countries that applied a landfill tax.

The network analysis demonstrated the relationships between countries when HW is shipped for disposal, and the association between countries generated from the adherence to the proximity and self-sufficiency principles.

Degree centrality demonstrated that countries in central Europe (Germany, France and Belgium) were the main nodes. Following the proximity principle, this may be because of better logistics connectivity. The application of these principles helps to improve efficiency in HW management systems, since it minimizes emissions from HW transport and indicates that countries have sufficient capacity for the disposal of the HW they generate.

Further research should be undertaken to establish the quantities of HW exported and imported for landfill and incineration in each country. Additionally, two circumstances may affect HW shipments: (1) the exit of the UK from the EU may affect waste shipments to and from this country and (2) the plastic waste ban imposed by China. New data may show how these circumstances affect waste trade and the stability of the communities.

The control of compliance with the analyzed regulations will be fundamental to avoid illegal waste trafficking and to protect the environment and citizens' health.

The capacities for waste management in Europe (i.e., landfill and incineration capacities) should also be determined. In future, Europe should establish appropriate regulations that take into account all these circumstances, in order to make a better contribution to sustainable development.

**Author Contributions:** Conceptualization, C.C., M.P.L.; methodology, M.P.L.; software, M.P.L.; validation, M.M.-N. and M.P.L.; formal analysis, M.P.L., M.M.-N., C.C.; investigation, C.C.; resources, C.C.; data curation, C.C.; writing—original draft preparation, C.C.; writing—review and editing, M.P.L., M.M.-N. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data presented in this study are openly available in https://ec. europa.eu/eurostat/cache/metadata/en/env\_wasship\_esms.htm (accessed on 25 June 2019).

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

#### **References**


**Ana Isabel Abellán García 1,\* , Noelia Cruz Pérez <sup>2</sup> and Juan C. Santamarta <sup>2</sup>**


**\*** Correspondence: ana.abellan.garcia@alumnos.upm.es

**Abstract:** Sustainable urban drainage systems (SUDS), or urban green infrastructure for stormwater control, emerged for more sustainable management of runoff in cities and provide other benefits such as urban mitigation and adaptation to climate change. Research in Spain began a little over twenty years ago, which was later than in other European countries, and it began in a heterogeneous way, both in the SUDS typology and spatially within the peninsular geography. The main objective of this work has been to know through bibliographic review the state of the art of scientific research of these systems and their relationship with the different types of climates in the country. These structures have a complex and sensitive dependence on the climate, which in the Iberian Peninsula is mostly type B and C (according to the Köppen classification). This means little water availability for the vegetation of some SUDS, which can affect the performance of the technique. To date, for this work, research has focused mainly on green roofs, their capabilities as a sustainable construction tool, and the performance of different plant species used in these systems in arid climates. The next technique with the most real cases analyzed is permeable pavements in temperate climates, proving to be effective in reducing flows and runoff volumes. Other specific investigations have focused on the economic feasibility of installing rainwater harvesting systems for the laundry and the hydraulic performance of retention systems located specifically in the northeast of the Iberian Peninsula. On the contrary, few scientific articles have appeared that describe other SUDS with vegetation such as bioretention systems or green ditches, which are characteristic of sustainable cities, on which the weather can be a very limiting factor for their development.

**Keywords:** sustainable urban drainage systems; green infrastructures; stormwater green infrastructure; Mediterranean climate; arid climate; template climate; Spain

#### **1. Introduction**

A new approach to urban stormwater management emerged in the 1980s and 1990s, introducing a holistic and environmentalist approach to urban hydrology, and which is increasingly spreading around all cities of the world [1]. This methodology reproduces, as faithfully as possible, the natural hydrological cycle to minimize the impact of urban development. It aims to reduce the negative impacts in terms of quantity and quality of runoff, as well as maximize the landscape integration and the social and environmental value of the elements involved in urban stormwater management [2]. This new way of treating urban stormwater took different names around the world. A very widespread one is Sustainable Urban Drainage Systems (SUDS), those elements of the infrastructure (urban–hydraulic–landscaping) whose mission is capture, filter, retain, transport, store, and infiltrate the urban runoff, trying to reproduce as close as possible the natural water cycle [3]. This definition is similar to green infrastructures in the United States: an approach to hydrological cycle that uses soils and vegetation to enhance and/or mimic the natural hydrologic cycle processes of infiltration, evapotranspiration, and reuse [4]. On the other

**Citation:** Abellán García, A.I.; Cruz Pérez, N.; Santamarta, J.C. Sustainable Urban Drainage Systems in Spain: Analysis of the Research on SUDS Based on Climatology. *Sustainability* **2021**, *13*, 7258. https:// doi.org/10.3390/su13137258

Academic Editors: Margarita Martinez-Nuñez and Mª Pilar Latorre-Martínez

Received: 29 April 2021 Accepted: 25 June 2021 Published: 29 June 2021

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

hand, in Europe, the concept of green infrastructure is broader, encompassing all those elements that provide connectivity to ecosystems, provide ecosystem services, and contribute to the mitigation and adaptation to climate change; these are classified in different scales: the local level, municipal level, and regional or state level [5]. Therefore, the SUDS would be urban green infrastructures to be implemented at the local–municipal level. In addition, since some SUDS are characterized by the use of vegetation (for example: green roofs, bioswale, artificial wetlands, bioretention areas . . . ), they can also be included within the so-called Nature-based Solutions (NbS), which include those elements in Nature that inspire facing new social challenges efficiently and responsibly with the environment [6].

SUDS cover a wide variety of elements or techniques such as [7] rainwater harvesting systems, green roofs, permeable surfaces, bioretention systems, vegetated swales, filter strips, infiltration systems, and detention–retention systems.

In Spain, SUDS appeared later than in other countries such as the UK or USA and were not as widely distributed or studied [8]. Thus, the objective of this article is to determine the state of the art in Spain and identify possible deficiencies in the research and/or experiences (if there are any of them); more specifically, it is to identify which techniques are the most analyzed and if they depend on the climate of the area or not.

#### *Study Area*

According to the Spanish State Agency of Meteorology (AEMET), in the Iberian Peninsula, there are mainly three types of climates in agreement with the Köppen classification [9]: (i) Dry climates (type B): BWh (warm desert) and BWk (cold desert), corresponding to the provinces of Almería, Murcia, and Alicante, where minimal rainfall occurs, and BSh (warm steppe) and BSk (cold steppe) for Extremadura and the Balearic Islands; (ii) Temperate climates (type C): Csa (temperate with dry and hot summer, known as Mediterranean climate) is found in approximately 40% of the surface of the Iberian Peninsula and the Balearic Islands, being the most common climate, it extends over almost all the southern half and much of the Mediterranean shoreline, Csb (temperate with hot and dry summer) in most of the northwest of the Peninsula and inland mountainous areas, Cfa (temperate without dry season with hot summer) in the northeast of the peninsula and in a strip of medium altitude in the Pyrenees, Cfb (temperate without dry season with mild summer) in the Cantabrian region; (iii) Cold climates (type D): Dsb (cold with dry and temperate summer) and Dsc (cold with dry and cool summer), Dfb (cold without dry season and mild summer) and Dfc (cold with dry summer and cool summer) in high mountain areas of the Pyrenees, the Cantabrian Mountains, and the Iberian System. Figure 1 shows the spatial distribution of the different climatic classes in Spain.

**Figure 1.** Köppen Climate Classification. The province's written names are the places where studies of real cases have been carried out. Source: Adaptation of Mapas climáticos de España (1981–2010) Y ETo (1996–2016) [10].

The weather in most of the country is characterized by temperate temperatures and rainfall regimes divided into two periods: one maximum in autumn and the other secondary in spring (except for the west and south of the peninsula, where the rainiest periods are autumn. and winter) [11]. This irregularity in the distribution can affect the development of plants [12] (many SUDS, such as bioretention areas or green roofs, have vegetation) and the performance of drainage elements as well [13]. So, the climate is a key element to consider in SUDS operation. Another characteristic of the rainfall in the Mediterranean and dry areas is the high intensity of rainfall events that are expected to increase in the future because of climate change [14].

SUDS are solutions for climate change adaptation and mitigation [15], and for this, they appeared as recommendations in publications for urban sustainability [16]. However, as we know the importance of the weather, doubts arise about the performance of these solutions in regions with different weather conditions, and therefore, there are concerns as to whether they are translatable. For this reason, this analysis intends not only to elucidate the state of the art of research on these techniques in Spain (if it is homogeneous throughout the national territory or not, if all the techniques arouse equal interest, what are the parameters, characteristics, or functionalities most analyzed) but also to find out the operation of the different sustainable drainage technologies under different climatic conditions.

So, although the main question to answer in this article was about the state of the art in scientific research on sustainable drainage systems in Spain, there have also been attempts to answer other questions in this regard, such as: Is there any relationship between climatology and the techniques studied? Since they are multifaceted structures, does the research focus on hydrological–hydraulic performance, or are other potential benefits evaluated?

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

The scientific publications compiled in SCOPUS on the sustainable treatment of urban runoff in Spain were the starting point for this analysis. SCOPUS was selected because it was a search engine that includes a greater number of journals compared to others such as Web of Science, and its citation analysis was faster [17]. Previously, we verified that the journals where researchers of specialized university centers (such as GITECO (https://www.giteco.unican.es/SUDSlab/inicio.shtml (accessed on 22 February 2021)) or IIAMA (https://www.iiama.upv.es/iiama/es/ (accessed on 22 February 2021)) published could be found indexed to SCOPUS.

In this bibliographic review, those publications made in scientific journals with DOI (and indexed in JCR or SJR) have been considered, looking for scientific evidence that shows the performance of these techniques in their different facets under the climatological characteristics of Spain [9].

The development of the methodology followed has had the corresponding steps: (i) Bibliographic search in SCOPUS, to find any paper about SUDS in Spain; (ii) Selection of the bibliography found, which was focused on obtaining the necessary data to answer the research question in this article (What is the state of scientific research on SUDS in Spain?); and (iii) Obtaining information from the selected documents focused mainly on knowing the temporal evolution of research in this field in Spain, if there were more theoretical studies than empirical cases, what techniques were most analyzed according to the different climates of the country or if there was some type of stormwater green infrastructure that has not been studied or monitored. Each of these points is detailed below.

#### *2.1. Bibliographic Search in Scopus*

The search in the SCOOPUS database included publications of any nature, without a time limit and geographically affiliated with Spain that contain the following key concepts (Table 1), since one of the main objectives of this article was to know the status of the scientific research on sustainable urban drainage, the years of experience in this area, and the amount of research carried out.


**Table 1.** Keywords used in the search for articles related to SUDS in Spain. Source: Prepared by the authors.


#### *2.2. Selection of Bibliography*

The initial search provided a total of 424 records, including articles, book chapters, lectures, and reviews. However, several articles with different keywords appeared with different search criteria, so there were some duplicate items. The screening process consisted in the elimination of duplicates, exclusion of publications without DOI, and we reviewed and read the papers to ensure that the practical cases were located within the Spanish geography (without include the work of Spanish researchers or Spanish entities in foreign locations). Thus, the number of documents to be analyzed became 137, of which 116 were articles, 5 were reviews, 9 were books or book chapters, and 7 were conference papers. This analysis considered the information contained in articles reviews and conferences. Most of the papers consulted belong to journals indexed in prestigious scientific databases, such as Journal Report Citation (JCR), SJR, and SCOPUS.

#### *2.3. Extraction of Information*

To answer the questions related to the state of SUDS research in Spain, which is one of the objectives of this study (Which techniques are the most analyzed? What are the most studied parameters and the main characteristics or functionalities? Is the study distributed evenly throughout the country? Does the study of these techniques arouse interest over time?), the minimum information collected included the following:


urban thermal comfort), economic (cost–benefit studies and life cycle analysis of systems), social (citizens' perceptions about urban drainage and related urban policies), and planning (proposals for the inclusion of SUDS at the urban level, urban drainage design and management methodologies).

The information to delve into the analysis of the SUDS based on the weather included the following:


#### **3. Results**

A total of 128 publications met the selection requirements. Figure 2 shows the total number of articles according to the technique analyzed and the evolution in the number of publications since the first explicit reference to the SUDS in the 1990s [18]. Under the title of several techniques are the articles that deal with projects that contemplated the operation (hydrological and quality of runoff) of several techniques simultaneously. With SUDS, we refer to those articles that deal with the sustainable management of urban runoff in a general way and not always using that term, also, green infrastructures.

Table 2 shows in a more concrete way the classification of the articles according to type of technique, type of study, and main subject studied in each case. The numbers indicate the number of publications found in this regard and in the last column of the table, the references of the classified articles.

Figure 3 schematically shows the articles grouped and counted according to the analyzed study parameters and also to the type of study (see Section 2.3).

**Figure 2.** Number of articles according to the analyzed technique and temporal evolution of the number of publications. Source: Prepared by the authors.

**Table 2.** Types of studies and subjects analyzed according to the exposed drainage technique or techniques in the papers. Source: Prepared by the authors.



**Figure 3.** The graph on the left shows the percentage of study of the main subjects covered in the articles, and the graph on the right shows the proportion of papers according to type of study. Source: Prepared by the authors.

#### **Table 2.** *Cont*.

#### *3.1. Sustainable Urban Drainage*

This classification includes those articles that talk about sustainable drainage and green infrastructures in a global way, how they should be implemented, projects executed, and the benefits that these systems provide.

It also includes a recent bibliographic review of the SUDS in Spain from 2019: "The potential of sustainable urban drainage systems (SUDS) as an adaptive strategy to climate change in the Spanish Mediterranean" [136]. This paper is a compilation of some of the techniques implemented in Spain, particularly in Alicante.

Since these were not real case studies that could be affected by the weather, we did not delve further into the content of this classification.

#### *3.2. Projects With Several SUDS*

In addition to real cases of independent SUDS, publications that contemplated the simultaneous operation of different SUDS also appeared in the search: a pair referring to the AQUAVAL project [129–131] in Valencia province (Csa climate) and out in the north of the country (Cfb climate), whose conclusions appear in Table 3.

**Table 3.** Main conclusions of the studies on several SUDS together. Abbreviations used in Kind of Study: C, Comparative; DC, Data collection. Abbreviations used in Subject Studied: HHP, Hydrological–hydraulic performance; RQ, Runoff quality. Source: Prepared by the authors.



In addition to the real cases, there were also investigations that propose models to evaluate the suitability of the use of these techniques in flood control (giving positive results) [124–128], improving adaptation to change climate [122,125], and providing another environmental benefits [123].

#### *3.3. Green Roofing*

Green roofs, with 41 publications, were the most analyzed techniques, and 24 of these articles showed the results of monitored roofs. Table 4 summarizes the main conclusions obtained in investigations of real cases in Spain in a semi-arid climate (type B) and in a Mediterranean climate (type Csa).

**Table 4.** Main conclusions of the studies on green roofs. Abbreviations used in Kind of Study, TNT, Test new technologies and/or materials; S: Survey; C: Comparative; DC: Data collection; MA, Model application. Abbreviations used in Subject Studied: HP, Hydrological performance; EP, Energy performance; V, Vegetation; C, Component, or system layer; LCA, Life Cycle Analysis; ES, Ecosystem services; UP, Urban policies; SP, Social perception. Source: Prepared by the authors.






Given the importance of climate in the development and maintenance of vegetation and that the climate of much of the national territory is characterized by long periods of drought, an important part of the research carried out has focused on the functioning of species such as *Brachypodium phoenicoides*, *Crithmum maritimum*, *Limonium virgatum*, *Sedum sediforme*, *Sporobolus pungens*, [46,47] and *Asteriscus maritimus* [47], which were studied in the Balearic Islands; Sedums such as *lbum, sexangulare,* and *spurium* [48], which were observed in Lleida; and *Silene vulgaris, Silene secundiflora, Crithmum maritimum, Lagurus ovatus, Asteriscus maritimus*, and *Lotus creticus* in Murcia [42].

#### *3.4. Permeable Pavements*

Permeable pavements were the second technique with the most publications, with a total of 35 papers. Nine of them reflected the results of experimental installations, and 20 were laboratory tests. Table 5 shows the most representative conclusions of the study cases in locations with a temperate mesothermal climate (type Cfb) and in places with a Mediterranean climate (Csa).

**Table 5.** Main conclusions of the studies on permeable pavements. Abbreviations used in Kind of Study: C, Comparative; DC, Data collection. Abbreviations in Subject Studied: HP, Hydrological performance; EP, Energy performance; LCA, Life Cycle Analysis; S, Survey; RQ, Runoff quality; C, Component. Source: Prepared by the authors.



Regarding the use of this technique for adaptation to climate change, the Life CER-SUDS project [91] has investigated the capacity of these forms of permeable surfaces made from ceramic elements systems to mitigate the expected effects.

#### *3.5. Rainwater Harvesting*

There were also several studies of rainwater harvesting and potential uses (Table 6), which were all located in places with Mediterranean climatology (Barcelona and Girona).

**Table 6.** Main conclusions of the studies on rainwater harvesting. Abbreviations used in Type of Study: S, Survey; C, Comparative; DC, Data collection; MA, Model application. Abbreviations used in Subject Studied: UP, Urban policies; RQ, Runoff quality; EE, Economic study; SP, Social perception.



A large portion of the articles are economic analyses on the use of rainwater among this type of analysis, including the creation of a software, Plugrisost [105], to evaluate the profitability and environmental impact of rainwater tanks, which has been used to estimate water prices (for different uses) from which it is economically profitable to install a rainwater harvesting system [106] and to carry out an environmental and economic analysis of rainwater storage systems that supply water for laundry [109].

#### *3.6. Green Channel*

There was hardly exhaustive research on green channels, although it is necessary to mention a laboratory investigation [26] focused on the temperature variations in the different layers of a green channel.

The hydrological behavior of green channels was effective from the hydrological point of view [129–131] and improving the runoff quality [131,132].

#### *3.7. Detention Systems*

The studies of detention systems observed were mainly hydrological–hydraulic models applied in Barcelona [24], Granada [19,20], and Valencia [21,22,25], cities with a Csa climate, and Cantabria [21] with a Cfb climate.

#### *3.8. Research by Climate*

The studies found from the end of the 1990s to the end of the year 2020, although they were not homogeneously distributed throughout the Spanish geography, they cover a large part of the national territory based on the climate. In more arid climates (type B), it seems that research (there are 16 articles of empirical studies carried out in arid climate) has focused more on energy performance (eight papers of 16) and optimizing vegetation selection (six papers of 16), while in temperate climates (type C), it has focused more on

hydrology (eight papers of 29 based on C climate). Figure 4 graphically shows the amounts of articles dedicated, on the one hand, to each subject of study and on the other to each type of specific technique.

**Figure 4.** The graph on the left shows the number of studies of the main subjects covered in the articles about real cases according to climate, and the graph on the right shows the techniques studied in the papers. The authors have separately accounted for each of the techniques reflected in the articles that contemplated several simultaneously. Source: Prepared by the authors.

#### **4. Discussion**

A difference between the recent bibliographic review [136] (that compares the implementation of SUDS in other countries with respect to Spain) and the recent publication Sustainable Urban Drainage Systems in Spain: A Diagnosis [145] (an exhaustive compilation of implemented techniques) is that this paper only considers those cases in which a scientific investigation process had been carried out.

The usefulness of SUDS as effective and sustainable management of urban runoff in different climatic regions of Spain is widely demonstrated in several papers [64,96,129–131] analyzed in this review, as well as their potential in other fields such as mitigation of climate change in cities [136,139], but the success rates of local–regional SUDS in Spanish different climatology are still not validated.

By far, the most studied techniques are green roofs and permeable surfaces (Figure 2), followed by rainwater harvesting and detention systems. In contrast, typical green street techniques [146] such as bioswales, bioretention areas, or filtering strips providers of several ecosystem services [147] have hardly been analyzed.

The study of SUDS is unequally distributed throughout the Spanish geography; Catalonia and Cantabria are the regions with the greatest number of studies of these techniques, their components, and their operation. In Cantabria, the GITECO research group has carried out a large number of investigations [8], but these have almost entirely focused on permeable surfaces and their hydraulic–hydrological performance. In Catalonia, research centers with different objects of study, such as ICTA (https://www.uab.cat/web/ icta-1345819904158.html (accessed on 8 February 2021)) or CREAF (http://www.creaf. cat/es (accessed on 9 February 2021)) have investigated mainly green roofs and rainwater harvesting systems from different points of view (not only dealing with the hydrological and hydraulic performance, but energy, biological, and economic).

The establishment of vegetation is essential for the correct long-term operation of a green infrastructure [148], and it depends directly on the weather. Since some areas of Spain are predominantly dry [9], one of the main concerns could be the selection of species that can withstand water scarcity. Perhaps this is the reason why the regions where vegetative growth and development have been most investigated are the Balearic Islands, Lleida, and Murcia, which are characterized by their low rainfall [9]. The deductions that can be drawn after observing Table 4 is that for the prevailing dry climate in the country, it is advisable

to use a mixture of perennial and annual plants with porous and light substrates [42]; the presence of vegetation is decisive for the functions of thermal insulation [56] and water retention with the characteristic rainfall regime of dry areas [45]. However, not all vegetation is equally effective [48]; species such as Sedum sediforme [60] give better results than others [47,48,60]. Regarding the hydrological operation, the effectiveness of green roofs has been demonstrated, but the results of improving the quality of runoff are not satisfactory [130], so further tests in real facilities are recommended, since the results differ from those obtained in the laboratory.

However, more interest seems to focus on the condition of insulation against the heat of these techniques due to the number of publications in this regard (see Table 2).

Permeable pavements work well hydrologically regardless of the climate in which they have been analyzed (see Table 5); although their performance in quality management depends largely on the composition [77–79], there are no records of its operation in arid climates.

Although bioretention systems and green channels or bioswales are some of the green infrastructure solutions recommended at the urban level due to their multifunctionality [149], there are not plentiful investigations, as occurs with other techniques. Its multifunctional performance depends largely on the biota [150], which derives from factors such as location and climate (predominantly dry and with little precipitation in Spain [9]); plant selection and plant conditioning factors can be a limiting factor [151]. Therefore, it would be advisable to investigate further which plant elements and components are the ones that would work best under long-term peninsular climatic conditions, since ecosystem services will depend on plants, such as urban biodiversity or CO<sup>2</sup> reduction, and maintenance costs, among others [152].

#### **5. Conclusions**

The SUDS study includes different disciplines, hydrology, edaphology, ecology, economics, etc. [7]. However, in Spain, the study is highly polarized; that is, the papers with various techniques and those about permeable surfaces deal with the hydrology, while green roofs papers are focused on the improvement of the energy efficiency of buildings, and rainwater harvesting investigations show their economic performance. This can be associated with the fact that the studies are carried out by specialists who tend to prioritize their own fields without considering the important impacts of other fields [153].

There are many more types of SUDS than those found in this research, such as filter strings, trenches or infiltration wells, artificial wetlands, etc. However, although there is evidence that they have been implemented in the Spanish geography, there are no studies that evaluate its operation: neither the hydrological–hydraulic performance nor its potential components or possible secondary benefits.

It is interesting to mention that the most analyzed techniques in Spain are "in situ" control. That means there is too much to investigate about local and regional control SUDS—in other words, techniques that manage runoff from streets, municipalities, or large areas. This may be because it is easy to install a green roof or a permeable pavement in a university building or research centers, but it is more complicated to follow and monitor techniques located in the public space. In this case, it is necessary to have a collaboration between the researchers, the public administration, and citizens.

It would be advisable to carry out more interdisciplinary studies or a holistic analysis of these techniques in their operation in urban areas. Especially SUDS such as bioswales or bioretention systems that develop populations of living beings are limited in their growth by the rainfall regimes of the country.

**Author Contributions:** Conceptualization, J.C.S.; methodology, A.I.A.G.; formal analysis, N.C.P. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Some or all data, models, or code that support the findings of this study are available from the corresponding author upon reasonable request.

**Acknowledgments:** To all the scientists and researchers whose contribution to research into a new approach to runoff management has made this article possible.

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

#### **References**

