Environmental Assessment of Resource and Energy Recovery in Waste Management Systems
Waste and resource management are complex, largely interlinked systems that are exposed to a number of influences. With the goal of optimizing the use of resources and energy, waste management increasingly aims for conserving resources through material and energy recovery, besides the more traditional goals of safe disposal and emission abatement. In view of the numerous possible utilization strategies and pathways, the political targets, and the technical feasibilities, indicators for steering and evaluating the systems performance are of great importance. While current targets focus mostly on collection and recycling rates, there is a need to also assess the environmental sustainability of waste utilization strategies. Several assessment tools, such as material flow analysis (MFA) and life cycle assessment (LCA), are used to evaluate waste management’s environmental performance. Generally, they are, however, applied in isolation. But only the combined implementation of MFA and LCA allows for the simultaneous consideration of resource availability and quality, capacity and market restrictions, as well as the environmental impacts of the system.
This dissertation aims at enhancing the understanding of the environmental assessment of national municipal solid waste management systems. All methodologies developed are tested on the Swiss waste management system, and provide support for political decisions. Five main tasks were addressed i) the assessment of the material availability in Switzerland using MFA and considering resource quality aspects; ii) the calculation and comparison of various resource efficiency measures, including an analysis of the measures meaning when targeting a circular economy and their ability to account for material quality; iii) the analysis of the effects of the separate collection on the quality of source separated material and the influence of material input-quality on the recycling process; iv) the development of a methodology to create future scenarios based on trends in Swiss waste management and consumption and in correspondence to energy scenarios; and v) the development of a modular LCA approach that takes material qualities into account.
This thesis starts with the application of a top-down perspective, and results in a comprehensive map of all waste and resource flows that breaks down the national waste stream into several material fractions and follows their treatment pathways. The resulting material flows were used to compare collection and recycling rates on a national level. Through assessment of the treatment pathways, including the secondary material production, differentiation between closed- and open-loop recycling could be achieved, providing novel insights with regard to the circularity of the Swiss waste management system. The results highlight the shortcomings of collection rates, namely the sole consideration of the collection behavior of consumers and collection logistics while neglecting the material purity and the recovery efficiencies in proceeding recycling processes. Comprehensive recycling rates, defined as the amount of secondary material produced after recycling divided by the amount of material consumed, are, therefore, recommended to be used as mass-based indicators. Complementing the MFA with an analysis of trends in waste occurrence and under the consideration of existing energy scenarios, a methodology to set up umbrella scenarios was developed. The participatory approach ensures a high level of assumption transparency for involved stakeholders, which has the potential to facilitate the implementation of the recommendations based on LCA results.
Additionally, two bottom-up case studies on resource quality and its influence on recycling processes, and vice versa, were carried out for ferrous scrap and PET bottles. Both studies highlighted that the quality of recycled material is not only determined by the use-phase, but also the collection and treatment schemes. Based on a decrease in quality of source-segregated PET bottles with increasing collection rates, a decreasing marginal environmental benefit from recycling with increasing collection rate was identified. The effect is expected to be even more prominent for materials where the benefits of secondary production are less pronounced. In addition, statistical analyses of a large industrial dataset have revealed an influence of the ferrous scrap quality on the recycling process. In particular, low-quality scrap was found to substantially increase the electricity demand of the recycling process.
Finally, the information from the comprehensive MFA and the case studies was combined and complemented with up-to-date life cycle inventories within a modular LCA approach. The flexibility of a modular approach allows for assimilating the acquired knowledge on the resource quality, on different recycling pathways and on material-quality dependent environmental impacts. The modularity allows for further integration of any newly occurring waste stream, for different quality classes of the same material or for innovative waste treatment processes in the future, as it can be extended as needed for the system under study. Also, modelling the LCA based on the detailed material flow distribution and the respective process models allows for ensuring the mass balance of the system.
The assessment of the current Swiss municipal solid waste management system highlighted substantial environmental benefits; the integrated waste management, i.e. the recovery of material and energy from waste and the related substitutions, compensated for 1% of the total climate change impacts and 4% of the cumulative energy demand of the Swiss consumption. As Swiss waste management is already rather advanced with high collection and recycling rates, a further increase of these rates has only a limited improvement potential. Therefore, the focus of political measures should be laid on (i) the utilization of secondary materials in applications where they replace high-impact primary production, and (ii) an increased recovery of energy in waste-to-energy plants.
By coupling MFA and LCA, a model was created to analyze large scale waste management systems. The modular LCA model enables for the consideration of actual mass flows, allows for evaluating different mass flow distributions, facilitates the integration of new modules, and allows for inclusion of material quality. This dissertation, therefore, provides a solid basis for developing new indicators for waste management systems. The modular LCA approach can further be used to analyze environmental consequences of Circular Economy strategies. Areas for further model development include the use of the modular LCA approach in mathematical optimization.
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