Waste collection systems (WCS) are implemented worldwide for public health reasons, firstly, and ultimately to recover materials to be reused, recycled, or recovered. In developing countries, waste is collected by manual labor, deposited in bins, and then transported by vehicle for disposal (Amponsah and Salhi, 2004) and informal waste recycling (Wilson et al., 2006). In developed countries, WCS have evolved to collect waste via several waste flows, such as in Sweden, where in 10 different materials are separated (Dahlén et al., 2007). Optimization models are also commonly used to reduce WCS costs (Faccio et al., 2011), to use environmentally friendly fuels to reduce environmental impacts (Yang et al., 2014), and to use filling-level sensors and radio-frequency identification in containers and trucks with global positioning systems (Anghinolfi, 2013; Faccio et al., 2011; Johansson, 2006). Requirements related to locally specific situations, such as economic or aesthetic criteria, have also been applied to customize WCS (Kogler, 2007; Rives et al., 2010). Real-world WCS applications and descriptions are diverse and not well documented, and when they do exist are often inappropriate, complicating comparison and evaluation (Dahlén and Lagerkvist, 2010; Kogler, 2007) . WCS planning requires information about environmental impacts, economic indicators, and efficiency; however, environmental studies on WCS only characterize aspects such as material collected and container type by bags and containers in terms of location, material, and format. For example, "common bin" does not specify type of container or whether collection type is by mobile pneumatic, multi-container, or door-to-door (Iriarte et al., 2009); and "street-side containers" (Usón et al., 2013) is not sufficiently defined to compare results with other studies. Both economic and environmental assessment of WCS by Larsen et al. (2010) characterized WCS as the "bring" scheme with drop-off containers, curbside collection, recycling centers, and underground containers system. In addition to environmental and economic aspects, De Feo and Malvano (2012) also analyzed technical aspects of municipal solid waste (MSW) curbside collection and some information concerning vehicles and containers, but their findings were still insufficient to characterize WCS, specifically vehicles.
Regarding efficiency of waste amounts collected, Gallardo et al. (2010, 2012) compared WCS only as "curbside bins" or "drop-off points" but provided no detailed information on bins, preventing comparison with other WCS. In benchmark studies such as Karagiannidis et al. (2004), WCS were identified only as back-loaded trucks, with no information on container type. An analysis by Teixeira et al. (2014) only identified containers as street-side and drop-off. In WCS modeling, the features addressed are container volume and number, and trucks are characterized by their capacity and work team size (Mora et al., 2014). In these comparisons, lack of specifics on containers, vehicles, and emptying processes further complicates benchmarking and modeling and prevents analysis of time factors.
The fundamental problem of characterizing and describing WCS must be addressed before analysis and modelling or technical development can advance. Without a common understanding of capabilities and features of WCS (i.e., equipment, technologies, and functionality), there is a risk of slowing down not only the assessment of operational aspects but also the development of novel and sustainable WCS. A standardized taxonomy could provide a common language for systematic storage, efficient and effective teaching/learning, recall for usage of knowledge, and promote knowledge expansion and growth (Eksioglu et al., 2009).
The objectives of this study were to (1) briefly review existing classifications of WCS; (2) develop a useful, comprehensive, and parsimonious taxonomy to classify WCS that provides key elements concerning technologies and applications; and (3) test the taxonomy through the application of a case study in the Greater Lisbon area, Portugal. The taxonomy proposed in this work is devoted to MSW collection only and does not include transportation. Pneumatic or central vacuum collection systems were also excluded because these are high-technology solutions with a low dissemination level. Considering waste sources, the taxonomic focus is on residential and household-like commercial waste.
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