Comparison of local and centralized biowaste management strategies – A spatially-sensitive approach for the region of Porto

The increasing concern with greenhouse gas emissions and nutrients cycling creates a need for cost-effective, practical and environmentally sensible biowaste management strategies. Centralized systems have struggled to comply with those needs. Decentralized systems, treating waste at source, promise local nutrient circularity and increased resource sovereignty. The large-scale performance of decentralized systems remains unclear, especially concerning the local sink capacity to assimilate the treatment products. This study aimed to compare centralized and decentralized systems for the region of Porto and assess whether creating additional urban farms could reduce costs and environmental impacts. Spatial analysis was used to assess waste generation, potential compost bin locations, peri-urban and potential urban farmland available, and collection and transport requirements. The carbon footprint of different scenarios was determined using life-cycle assessment. The results show that local composting led to cost savings over centralized systems. However, this system encompassed positive carbon emissions and most districts evidenced limited sink capacity for compost application. Additional urban farms added significant sink capacity, however, their impact on cost and carbon footprint was insignificant. The carbon footprint of centralized systems was heavily dependent on factors influencing collection such as population density, and affected by the renewable content of the electricity grid. Anaerobic digestion was the most climate-friendly option in the urban center and local composting in remote and less dense districts. Municipalities may benefit from tailoring the treatment systems to specific districts, creating additional jobs while reducing cost and climate impacts overall