**1. Introduction**

Cities are expanding and increasing in number. In fact, c.a. 70% of the population will move to urban areas by 2050, leading to vast cities [1]. Such cities will require smart sustainable infrastructures to manage citizens' needs and to offer fundamental and more advanced services [2]. This urbanization process in cities of developing countries has led to an increase in the quantity and complexity in terms of composition of municipal solid waste (MSW) [3]. The worldwide MSW generation in 2016 was approximately 2.01 billion tonnes [4] and it is estimated that, by 2050, the production will rise to 3.40 billion tonnes with approximately 56% of organic content [5]. At a European level, the annual generation of MSW in EU-28 reached 483 kg per person in 2016, with the daily waste production per capita in the European countries ranging from 0.71 to 2.06 kg [6]. Therefore, it is expected that, in the coming years, both the increase of global population and the growth in developing countries will create

a boost in MSW production. In this context cities will have to face new challenges, linking technology to the improvement of the quality of citizen´s life, moving towards the smart city concept [7]. It is essential to be able to assess the environmental impact of cities by assessing their di fferent components, such as the waste managemen<sup>t</sup> infrastructure [8].

In response to the challenge posed by the generation and consequently managemen<sup>t</sup> of MSW, the European Union (EU) has established a legal framework targeting strategies to increase resource efficiency. The most relevant goals for MSW are the recycling rates proposed by the Packaging [9] and Packaging Waste Directive [10] and the Directive on Industrial Emissions (DIE) [11]. The Directive on Landfills [12], which aims to prevent or to reduce as far as possible the negative e ffects on the environment caused by waste landfilling, has set the amount of biodegradable municipal waste landfilled to be reduced to 50% in 2009 and to 35% in 2016 (compared to 1995 levels). In addition, the Waste Framework Directive [13] establishes a target of recycling and preparing for reuse of 60% by 2025 and 65 % by 2030. To reach these values, the EU fosters selective collection systems and recycling of the MSW fractions, ensuring the sustainability of smart cities moving towards a circular economy approach (Figure 1) [14]. This legal framework contributes to the EU Sustainable Development Goals (SDGs) indicator set. For instance, SDG 7: a ffordable and clean energy; SDG 11: sustainable cities and communities; SDG 12: responsible production and consumption and SDG 13: climate action [15].

**Figure 1.** Framework of the study.

At a national level, in most Spanish cities, glass, paper/cardboard and light packaging are currently collected separately, whereas the remaining fraction comprises the organic fraction because biodegradable waste collection system has not been still fully implemented [16]. However, to meet the goals of the European legislation [13], organic waste should be collected separately in a specific container for two main reasons: it represents around 40% of the MSW generated in Europe [17] and a suitable collection allows obtaining a higher quality compost [18]. In fact, one of the main components of the biodegradable fraction is food waste, which has been analysed in terms of prevention and recovery in many studies. Laso et al. [19] combined Life Cycle Assessment (LCA) and Data Envelopment Analysis (DEA) to assess the e fficiency of Spanish agri-food system. Garcia et al. [20] quantified the food losses at the distinct stages of the food supply chain in terms of mass, nutrients and economy. On the other hand, Thyberg and Tonjes [21] examined the impacts of food system modernisation on food waste generation. Finally, Gustavsson et al. reported two studies focused on the extent and e ffects as well as causes and prevention of food losses and food waste, one for medium/high income countries [22] and another for low income countries [23]. In Spain, both European and Spanish legislation have boosted the introduction of selective collection systems for biowaste [24]. In fact, some Spanish regions, such as Catalonia, Madrid and Navarre, have already introduced in their MSW collection systems the "fifth container" or "brown container", exclusive for compostable MSW (see Figure 2). These containers collect kitchen waste, garden rubbish, tree cuts and waste from food market and biodegradable bags [16]. In this way, the amount of MSW disposed in landfills is reduced by means of increasing the percentage of recycling through composting. Nevertheless, there are several disadvantages related to its storage and collection; for instance, the need of specific containers, additional bins and collection points, and the requirement of additional trucks and new routes [17]. All these factors depend on the collection method: street-side or underground containers, door-to-door or pneumatic collection.

**Figure 2.** Municipal solid waste (MSW) containers in (**a**) Navarre and (**b**) Madrid (Spain). Source: [25,26].

Waste collection accounts for 50–75% of the total MSW cost in developed countries, being one of hots spots from an environmental perspective, due to its energy consumption and related CO2 emissions [27]. The collection method varies from region to region. For instance, in Spain the conventional managemen<sup>t</sup> of MSW relies on the collection of the waste from containers placed in the street by means of a fleet of heavy trucks. In particular, in 2015, 92.6% of MSW was collected from street-side containers, whereas underground containers represented 5.2%, door-to-door collection covered 1.5% and pneumatic collection only 0.7% [28]. On the other hand, in Nordic countries such as Sweden, the collection from containers has been replaced by new technological and automated systems such as vacuum waste collection and underground container systems. The use of these systems has increased, particularly in cities and in newly built areas, moving towards more environmentally-friendly cities, but also in city´s historic areas where the conventional street-side collection systems is not feasible due to the difficult access for garbage trucks [29]. Therefore, society seems to be aware about the importance of the selective collection of different waste fractions, but there is more disagreement about the most environmentally suitable selective collection system [30].

In this sense, in the last years, Life Cycle Assessment (LCA) methodology [31] has been applied to evaluate and to compare several MSW collection systems developing ad hoc methodology [32]. Some authors have assessed the most common collection systems, door-to-door and street-side containers under environmental perspectives. Mora et al. [33] used LCA to assess the environmental impact of a waste managemen<sup>t</sup> system based on kerbside collection. On the other hand, Gilardino et al. [34] combined operational research techniques and LCA to create an effective collection-route system for garbage compactor trucks to attain a reduction in environmental impacts in the city of Lima. Other authors, such as Pérez et al. [35] described a methodology to evaluate the environmental impact of the urban containerization systems by using LCA, whereas Pires et al. [36] also evaluated, apart from the environmental, the economic aspect of a kerbside system and an exclusive bring system. Also, the social perspective of the most common collection systems was assessed under a social perspective [37]. The introduction of new technologies and automated systems boosted the comparison of conventional collection with novel systems such as vacuum collection. In this case, Teerioja et al. [27] compared a hypothetical stationary pneumatic waste collection system with a traditional vehicle-operated door-to-door collection system in an existing, densely populated urban area, from an economic point of view, while Punkkinen et al. [38] and Aranda-Usón et al. [39] applied LCA to compare the environmental sustainability of both collection systems. On the other hand, Iriarte et al. [30] used LCA to quantify and to compare the potential environmental impacts of three selective collection systems: mobile pneumatic, multi-container and door-to-door. The main conclusions previously obtained state that a pneumatic system generates more air emissions due to the consumption of electricity and installation materials. Only when the loads are close to 100%, the vacuum system had the best environmental performance compared to the conventional systems. In addition, under an economic perspective, the pneumatic collection is estimated to be six times more expensive than traditional systems [27].

The mentioned studies consider the managemen<sup>t</sup> of MSW taking into account glass, paper/cardboard, light packaging and bulky fraction. However, the introduction of the "brown container" makes necessary assessing the energy efficiency of the conventional and new collection systems, in order to determine the influence of the biodegradable fraction and its subsequently treatment by means of anaerobic digestion. The organic fraction of MSW is a substrate of interest due to its availability and characteristics [40]. Therefore, this study compares the energy efficiency of door-to-door system vs pneumatic waste collection, considering two alternatives: (i) the bulky fraction includes the organic fraction and (ii) the organic fraction is collected separately for further anaerobic digestion. The collection using street-side containers was excluded from the assessment since the study is placed in historic areas of Spanish cities.

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

The LCA is a tool to assess the potential environmental impacts and resources consumption throughout a product and service life-cycle [41]. In this regard, LCA has become one of the most relevant methodologies to help organisations perform their activities in the most environmentally friendly way along the whole value chain. In this work, LCA is conducted following the recommendations of the ISO 14040 [31] and 14044 [42] international standards in which LCA methodology is divided into four phases: (i) goal and scope, (ii) life cycle inventory (LCI), (iii) life cycle impact assessment (LCIA) and (iv) interpretation of the results.

#### *2.1. Goal and Scope Definition*

In this phase an accurate specification of the product or products to be investigated is done, as well as a clear description of the intended application of the study and its scope, in terms of system boundaries and functional unit (FU) [43]. Moreover, allocation procedures, cut-off rules and assumptions are also defined in this phase [44].

The purpose of this study is to assess the primary energy demand (PED) and the environmental efficiency of both conventional door-to-door and the alternative pneumatic waste collection systems considering only the managemen<sup>t</sup> of the organic fraction. The results are expected to provide an interesting discussion on the suitability of using different collection systems depending on the waste fraction managed.

#### 2.1.1. Function, Functional Unit and System Boundaries

The function of the compared systems is the managemen<sup>t</sup> of MSW and, in particular, of the organic fraction generated in historic areas of Spanish cities. These areas are characterised by narrow and tortuous streets where, in most cases, the transit of people and vehicles is not feasible, causing traffic jams and the disturbance of the daily routine of citizens. To handle this problem, many cities have pedestrianised and widened the streets making difficult the waste collection by means of street-side containers and garbage trucks. In addition, in the last years, these areas have turned into the shopping and leisure centres of the city, combining banks, company headquarters and public institutions with shops, hotels and restaurants which generate high amounts of MSW that have to be managed properly. To quantify this function, it is necessary to define a FU, to which all the inputs and outputs will be referred. In this case, the FU is described as the collection of one t of MSW with the composition showed in Table 1, in which biowaste is the major fraction (42%), followed by paper/cardboard (15%).


**Table 1.** Average composition of the Spanish MSW (year 2016) [45].

The system boundaries comprise the stages of the supply chain from cradle to gate, that is to say, the waste collection system, the transport of the collected waste to the sorting plant, the waste classification in the sorting plant and its treatment by means of anaerobic digestion. The waste collection system includes the manufacturing of components (i.e., bins and/or pipes) and the use stage (i.e., operation and maintenance) (see Figure 3).

**Figure 3.** Diagram of selective waste collection boundaries.

#### 2.1.2. Description of Selective Collection Systems

As previously mentioned, door-to-door and pneumatic systems are the waste collection alternatives analysed in this study, that, together with multi-container collection are the waste collection systems most implemented in urban areas. The combination of several collection systems depends on budget limitations, public participation, urbanisation age and municipal and regional planning, among others. In particular, the pneumatic system has been implemented for more than a decade and its future development will depend, just like the other systems, on economic, social and environmental aspects [30].

