(98b) Design of a Supply Chain for the Production of Biodegradable Plastics Based on the Revalorization of Sewage Sludge

The growth in the production of synthetic plastics since the 20^th century has promoted the development of the industry. The wide range of polymers and their properties make these materials promising alternatives for the manufacture of products used in the automotive, electronics and construction sectors, as well as for the production of containers and packaging used in the food industry ¹. Among all plastic applications, packaging represents the main market, accounting for 44% of global plastic demand ². The most commonly used polymers for packaging are polypropylene (PP), polystyrene (PS), polyvinyl chloride (PVC), polyethylene terephthalate (PET) and polyethylene (PE) ³, the later representing 63% of all packaging waste ⁴. A possible alternative to mitigate the environmental impact of this fossil – based polymers is the development of polymeric materials with renewable origin, biodegradables and with mechanical properties similar to polyethylene, as can be found in starch – based plastics ^5,6.

To evaluate the polyethylene substitution potential through the production of starch – based biodegradable blends, a multiscale analysis is performed including process scale up and supply chain conceptual designed. The supply chain is composed by the wastewater treatment plants (WWTP), CO₂ and sawdust sources and polymers productive centres. These facilities are integrated processes where the biopolymer is produced from renewable and waste sources such as sawdust, sludge, and CO₂, obtaining all the intermediates required (e.g., methanol, glycerol, starch) ⁵. The supply of sludge to the production centres has been considered through pipelines directly connected to the WWTP to facilitate the transport. In this way, the installation of these centres will be limited to the locations of the WWTP. For the case of sawdust, the supply to the production centres has been considered by land transport in trucks from locations with availability of shredded lignocellulosic material after forestry maintenance of pine populated regions. Finally, CO₂ is obtained from DAC and/or MEA – based CO₂ capture methods, being supplied to the production centre by pipeline. The integrated facility was optimized ⁵ and scaled up. Next, the supply chain model was formulated including linear algebraic constraints based on the availability of sawdust and CO₂ at the raw material locations together with capacity limits set by the availability of sludge at each location of the production centres. Additionally, economic aspects such as investment and operational costs of the production network and environmental impact have been included. In this way a mixed – integer linear model (MILP) is formulated to determine the optimal network which maximizes the fraction of demand for polyethylene replaced by starch-based biopolymer. The methodology presented has been applied to the case study of peninsular Spain to show the approach. Agricultural county has been considered as discretization level of the problem, accounting for 343 regions, solving a total of 1M equations, 1.4M continuous variables and 1400 binary variables solved in GAMS^® using CPLEX.

Considering a total budget of 2200M$, corresponding to the tax imposed by the European Union ⁷ over the use of non-renewable plastic manufacturing in Spain between 2024-2027, the optimization problem was solved focusing on the maximization of PE substitution by starch – based polymers. The results showed the ability to satisfy the 40% of the polyethylene demand, capturing a total of 1.5 Mt of CO₂ per year and promoting a decarbonization of 1% with respect to the year 2023. In addition, the total biodiesel production of the proposed network is 0.47 Mt/year, corresponding to a 1.47% of the diesel consumption in Spain.

Acknowledgments

This work was supported by funding to José Enrique Roldán San Antonio under the call for predoctoral contracts USAL 2021, co-funded by Banco Santander. This work was supported by projects PID2019-105434RB-C31 and TED2021-129201B-I00 of the Spanish Government

References

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