Full length articleLife cycle assessment as a decision-making tool for the design of urban solid waste pre-collection and collection/transport systems
Introduction
Municipal solid waste (MSW) management includes different stages (pre-collection, collection/transport and treatment). The pre-collection stage consists of the elements needed to deposit citizen's waste (surface and underground containers in conventional systems; pre-collection boxes in pneumatic systems). The collection/transport is the intermediate stage between the pre-collection and treatment stages, in which the waste is collected and transported to recovery and/or disposal centres. Each of these stages generates a growing impact: for instance, discarded materials embody lost energy and non-renewable resources; waste disposal generates air, soil and water pollutants; their transport to treatment plants consumes energy, produces air emissions and imposes economic and public health costs in terms of roadway-related congestion, damages and accidents (Miller et al., 2014).
The growth in MSW generation is determined by the increase in the world's population and the greater consumption of products and services, which compromise a sustainable future (Severo et al., 2018). According to World Bank 2020, the world's total urban population is increasing at a rate of 2% yearly and the MSW generated by urban residents is expected to get almost doubled from 3.5 million metric tons/day in 2002 to 6.1 million metric tons/day in 2025 (Khandelwal et al. 2018). Then, one of the most challenging issues for building sustainable cities is the improvement of MSW management (Wilson, 2015), which requires a substantial effort to reduce its production and improve its collection, transport and treatment (United Nations (UN) 2013).
MSW management contributes to many environmental problems: global warming, human health hazards, photochemical ozone formation, stratospheric ozone depletion, ecosystem damages, or mineral and renewable resource depletion (Laurent et al., 2014). In order to reduce these impacts, the decision-making requires an assessment to minimize the associated hazards. The main methodology used to evaluate the environmental impact of the different stages of any system in general, and MSW management in particular, is the life cycle assessment (LCA), based on the standards ISO 14040 and 14044 (ISO 2006a; 2006b). This methodology has been widely applied to MSW treatments in cities all over the world (i.e., Al-Salem et al., 2014; Buratti et al., 2015; Erses Yay, 2015; Fernández-Nava et al., 2014; Habib et al., 2013; Herva et al., 2014; Laurent et al., 2014; Margallo et al., 2014; Montejo et al., 2013; Parkes et al., 2015; Vergara et al., 2011; Yildiz-Geyhan et al., 2016). It has also been applied to evaluate the environmental loads from the collection/transport stage (i.e., Chàfer et al., 2019; Hidalgo et al., 2018; Maimoun et al., 2013; Peri et al., 2018; Punkkinen et al., 2012; Rose et al., 2013). The contribution of this stage varies by the type of collection and transport system in place: pneumatic systems or conventional systems that rely on trucks. In the last case, the fuel used has a significant influence on the environmental loads, especially in carbon footprint (Jayaratne et al., 2010; López et al., 2009; Maimoun et al., 2013; Rose et al., 2013; Quiros et al., 2017) and urban air quality (Fontaras et al., 2012; Giechaskiel et al., 2019; Grigoratos et al., 2019; Lozhkina and Lozhkin, 2016; Sandhu et al., 2014; Suthawaree et al., 2012; Vermeulen et al., 2018).
Nevertheless, there are few researches applying the LCA methodology to the pre-collection stage, either as part of the MSW management analysis (Bovea et al., 2010) or only for this stage (Rives et al., 2010). Its contribution to the total impact caused by MSW management is lower than collection/transport and treatment stages (Bovea et al., 2010; Mühle et al., 2010; Vergara et al., 2011). However, an inadequate pre-collection capacity, a bad distribution, or an improper use at the urban level can needlessly worsen the environmental impact. In addition, the associated inefficiency also affects the transport stage through unnecessary routes, higher number of vehicles, lower average driving speeds, or increased number of stops, leading to higher environmental impact. Then, in Pérez et al. (2017a), the research team carried out a deep analysis of the pre-collection stage, considering aspects involving collection capacity, total MSW collected or collection effectiveness, which determine the environmental impact. That work presents a methodology that covers the previous gap of estimating the environmental impact for this stage, applying LCA methodology. The methodology was applied to the City of Madrid for 2013, considering only conventional systems with surface containers, not for underground containers or pneumatic systems. Subsequently, the research team published similar methodologies for calculating the carbon footprint of the collection/transport (Pérez et al., 2017b) and treatment stages (Pérez et al., 2018).
In the case of the pneumatic systems, the pre-collection and collection/transport stage are completely linked. Therefore, a comparison with conventional systems must take into account the pre-collection and the collection/transport stages. According to Miller et al. (2014), these systems have been attracting a lot of attention and some cities have started to use it, replacing conventional systems based on containers and collection trucks. Those authors stated that the costs and environmental impact of the installations will vary depending on the design of pneumatic (amount of MSW managed, number of MSW fractions, length of pipeline network and number of pre-collection boxes) and the conventional system characteristics (distances travelled, routes, truck type, MSW generation density).
Iriarte et al. (2009) used the LCA methodology to compare the environmental impact of three MSW collection/transport systems in urban areas: mobile pneumatic versus two conventional systems, multi-container and door-to-door systems. Those authors concluded that the mobile pneumatic system has a greater environmental impact in the categories of global warming, fresh water aquatic ecotoxicity, terrestrial ecotoxicity, acidification and eutrophication, with values between 30% and 122% higher than those of the other two systems. The door-to-door system shows the greatest environmental impact in the categories of depletion of abiotic resources, ozone depletion and human toxicity, with higher values (26-61%). The impacts of door-to-door collection are mainly the effect of the diesel emissions produced by the waste transport associated with the longer collection routes typical of this system.
Eisted et al. (2009) concluded that a pneumatic collection/ transport system in Denmark generates a carbon footprint that can vary between 18 and 77 kg CO2 eq/tMSW.
Punkkinen et al. (2012) calculated the environmental loads for a hypothetical pneumatic waste collection/transport system modelled on an existing dense urban area in Helsinki (Finland) and the results are compared to those of the prevailing, container-based, door-to-door waste collection system. The results indicate that replacing the prevailing system with stationary pneumatic collection/transport in an existing urban infrastructure would increase total air emissions: 3 times for CO2 emissions. They also concluded that: 1) in the waste collection area, emissions would nonetheless diminish, as collection traffic decreases; 2) electricity consumption of the pneumatic system and the origin of electricity have a significant bearing on the results; 3) emissions due to manufacturing the components of the pneumatic system prove decisive.
Aranda Usón et al. (2013) compared a stationary pneumatic system with a conventional system based on multi-container and collection trucks, in a neighbourhood of Zaragoza, Spain. In that case, carbon footprint of the pneumatic system varies between 33 and 147 kg CO2 eq/tMSW, depending on the system load factor.
Hidalgo et al. (2018) also studied the impact of the MSW collection/transport system in two Spanish cities (Barcelona and León) and concluded that the pneumatic system needs more electricity but fewer fossil fuels when compared with the conventional collection/transport system using trucks. Nonetheless, this work does not consider the whole life cycle.
Cháfer et al. (2019) remarked the relevance of the electricity consumption when evaluating different waste collection/transport systems in terms of LCA. The environmental impact associated with the electricity consumption depends on the method of electricity generation.
The existing references do not consider underground containers and their environmental impact. In addition, other deficiencies have been identified: 1) in collection/transport stage for conventional systems, the vehicle life cycle is not considered in a large number of studies, which only consider the fuel life cycle (Erses Yay, 2015; Iriarte et al., 2009; Maimoun et al., 2013; Punkkinen et al.,2012); 2) those studies that take the fuel life cycle into account primarily deal with the tank-to-wheel stage (Al-Salem et al., 2014; Aranda Usón et al., 2013; Bovea et al., 2010; Buratti et al., 2015; Fernández-Nava et al., 2014; Habib et al., 2013; Hidalgo et al., 2018; Parkes et al., 2015; Pastorello et al., 2011); 3) not all possible pre-collection systems are being considered; 4) comparisons are not being made under the same conditions.
Then, this paper focuses on a deep evaluation of the environmental impact of the pre-collection and collection/transport stages, applying the LCA methodology, under different scenarios that consider the available options. As it was mentioned above, the contribution of these two stages to the total impact of the MSW management is lower than that of the treatment stage. However, local entities have to decide on their design and subsequent implementation (Gallardo et al., 2015; López Álvarez et al., 2009) by adhering to environmental criteria that minimise the impact (Khandelwal et al., 2019). In that sense, a deep evaluation of the environmental impact associated with the entire life cycle of the different pre-collection and collection/transport systems is needed. Even more as there are few exhaustive researches that compare all the existing options. Thus, the results obtained in the environmental comparison of all the existing conventional systems against underground or pneumatic systems would help, together with other economic and social aspects, the decision making process for a more sustainable urban planning.
Section snippets
Pre-collection and collection/transport systems
Based on a literature review (Bovea et al., 2010; Gallardo et al. 2015; 2012; 2010; Iriarte et al. 2009; Rodrigues et al. 2016a; 2016b; Universidad Politécnica de Madrid (UPM) 2014; Yildiz-Geyhan et al., 2016), there are different pre-collection and collection/transport systems (Fig. 1). Beyond special fixed and mobile drop-off points (located at specific areas of the cities), which are places provided to collect every MSW fraction, including those materials that exhibit special characteristics
Results and discussion
The LCI data collected in previous section, and the SimaPro 8.5.2 software (PRé, 2017) and the Ecoinvent 3.4 database (Ecoinvent, 2018), were used to determine, first, the environmental emissions and then the environmental impact. The results for each impact category are shown in Table 6 in the specific units to each impact category, per tonne of waste collected/transported.
In order to facilitate the analysis, the results obtained are evaluated based on the relative differences of each scenario
Conclusions
The obtained results show that, in general, the environmental impact from pneumatic systems is higher than from conventional systems. Furthermore, within the conventional systems, underground installations have a higher impact than surface containerisation systems.
For example, regarding the impact on climate change, the pneumatic scenario exhibits a carbon footprint of 68 kg CO2 eq/tMSW, which 63% corresponds to the manufacture of the pipes and all their sub-processes while the contribution of
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was done as part of the research project entitled OPTIMIZATION OF THE MUNICIPAL WASTE MANAGEMENT (reference number CTQ2013-48280-C3-2-R) granted by the Ministry of Economics and Competitiveness.
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