Network design optimization of waste management systems: the case of plastics

Failure in appropriately managing complex municipal solid waste systems in general and waste plastics in particular negatively affects public health and environment. A decision-making tool incorporating concepts of logistics and supply chain, considering different types of waste was developed. A multi-objective, multi-period mixed-integer linear programming (MILP) formulation, focusing on the plastic recycling network, incorporating features of multimodal transportation was implemented. The model aims at minimizing net present cost (NPC) and environmental impacts, illustrated by a case study of a Swiss waste treatment cluster. A reference situation was used for benchmarking and 7 new scenarios considered - corresponding to different plastics collection configurations. From the solutions space, designs arise that lead to a 2-fold decrease of NPC at the expense of increasing by 20 % current emissions, as well as solutions that provide small net gains in environmental aspects - close to 3 %, albeit a 4-fold increase in NPC due to large investments in recycling facilities and increased transportation costs. This work sheds light on the impact of supply chain and logistics aspects on the separate collection and treatment of waste plastics, in the context of a real municipal waste treatment policy, where a thermodynamic model to deal with the conversion processes is integrated into the general supply chain decision. As a result, important insights to guide decision-makers were obtained.