Environmental optimization of Swiss municipal solid waste management to support the energy turn-around

Municipal solid waste (MSW) is widely used for energy and material recovery in Switzerland and contributes substantially to the domestic electricity and heat generation. In addition to energy recovery, material recovery from... [ view full abstract ]

Municipal solid waste (MSW) is widely used for energy and material recovery in Switzerland and contributes substantially to the domestic electricity and heat generation. In addition to energy recovery, material recovery from MSW can avoid energy use for the production of primary materials and therefore lower the energy and resource demand in industry. Therefore, a multi-objective environmental optimization is performed to support the prioritization of recycling and treatment strategies for MSW with respect to energy efficiency, resource quality and efficiency, carbon footprint and overall environmental impact. The optimization model is based on a material flow analysis (MFA) of the Swiss waste management system in 2012. The MFA includes recycling processes (e.g. paper, PET, glass etc.), co-processing in industrial plants (e.g. cement industry) and dedicated MSW treatments. Life cycle assessment (LCA) is applied to assess the environmental impacts of individual treatment options. A modular LCA approach, modeled with brightway2 and ecoinvent v3.3 data, is chosen in order to facilitate the optimization problem formulation. The LCA of the Swiss MSW management system in 2012 results in a net avoided impact over all environmental impact categories considered (e.g. for IPCC 2013 GWP 100a the annual climate benefit amounts to -1100 kilotons CO2-eq., the total cumulative exergy demand is -60 PJ-eq.) except for eco-toxicity. The benefits arise in the recycling of separate collected fractions and materials recovered from the fly and bottom ash but also through the recovery of energy in municipal solid waste incinerators. From a systems perspective, the largest burdens arise in dedicated treatments for organic waste (84% composting and 16% anaerobic digestion in 2012) and in the thermal treatment of MSW for some indicators like global warming potential. The results highlight the importance of the large recycling fractions glass, paper and cardboard for the overall LCA result. The overall contribution of metal recycling, which per kg results in the highest credits, is only small. The >120 LCA modules of treatment processes, collection schemes and material substitutions are then used to find the optimal mass flow distribution of each waste stream among the modeled treatments, accounting also for constraints such as capacity restrictions and treatment portfolio limitations due to the quality aspects (e.g. for plastic waste, aluminium and ferrous scraps). Optimal mass flow distributions are determined for the time horizons 2020, 2035 and 2050 under different energy scenarios available for Switzerland. These scenarios were extended to also cover waste management variables in collaboration with stakeholders by including additional system variables like future waste amounts and compositions and the availability of treatment technologies in and around Switzerland. Based on the optimization results, strategies for a more energy- and material-efficient use of waste streams are derived. The strategies can later be used to support decisions on national and cantonal level, e.g. to identify waste streams and technologies of high priority and to aim at a more energy-efficient and sustainable Swiss waste management system.