Abstract
This paper proposes a holistic vision of the urban metabolism (UM), viewing the city as a subsystem within an industrial ecosystem (IE) in which municipal-industrial symbiosis is essential to achieve sustainability goals. For this purpose, the metabolism of a large Spanish city, Zaragoza, was studied by analyzing the main fractions of its MSW. A methodology based on carbon footprint (CF) was developed to analyze the environmental impact—in terms of CO2—of the influence of households’ behavior, the City Council’s strategies, and the main MSW fractions. Zaragoza’s IE represents a footprint of 931,250 CO2 tons for the fractions studied, of which 438,000 CO2 tons are due to organic fraction, 180,371 to plastics and 154,607 to paper and cardboard, which are the three most significant contributors. If households selectively separated 100% of their waste, the footprint would drop to 648,660 tons of CO2. Furthermore, monetary savings were quantified through the CO2 emissions price. The proposed methodology accounts for the CF of the whole IE, not just the city. Moreover, it enables the creation of Sankey diagrams to visualize the distribution of emissions of each subsystem, highlighting the importance of cooperation between the city and its recycling industries to reduce its CF.
Highlights
It is possible to estimate all Municipal Solid Waste (MSW) generation by fractions, the different treatments that are applied to them and the amount of MSW that is delivered to recyclers, i.e., all processes that are the responsibility of local authorities and that are shown in Figure 1 in the so-called Stage 2
Most MSW management studies use Life Cycle Analyses (LCAs), but they start once a material or product becomes waste [22], so they do not consider the whole industrial ecosystem (IE), only the city. Some of these authors conclude that MSW management can be carbon neutral or even a carbon sink [12,16,17,40]. This can lead to confusion because it could be thought that an activity capable of absorbing carbon is better to recycle than to reduce MSW generation, which is false, as in no case is recycling capable of offsetting manufacturing footprints, as this paper demonstrates with the minimum footprint scenarios
This approach highlights the role of the recycling industry in transforming waste from the city’s households and businesses (H&B) into new products, closing cycles and moving one step closer to a circular economy and the sustainability of cities
Summary
The growth of urban populations—where already half of the world’s population lives—means that the annual waste generation is expected to increase worldwide by 70% from 2.01 billion tons in 2016 to 3.06 billion in 2050 [1]. This problem concerns the European Commission, as they have set a series of objectives to be met by the member states, such as a 10% reduction of MSW generation by 2020, with respect to 2010 and a recycling rate of 50% of MSW by 2020, rising to 55% of MSW by 2025 [2]
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