Environmental performance of management scenarios for urban bio-waste: A German case study on the role of sources and pathways

Bio-waste treatment can cause substantial environmental impacts, but it also contributes to resource conservation through recovery of fertilizers, biogas, or other bio-based products. In this study, environmentally preferable management schemes for urban bio-wastes from different settlement structures and collection schemes are identified for the city of Kassel (Germany) as a case study. Life cycle assessment is used to assess management scenarios, taking flow-specific waste compositions and established bio-waste treatment processes (open and closed composting, tunnel and plug-flow digestion, municipal solid waste incineration (MSWI)) into consideration. Separate curbside collection rates for bio-waste ranged from 24% to 72%, depending on the settlement structure. Assuming average emissions of German treatment plants, biological treatment pathways are associated with higher impacts than MSWI for several impact categories such as climate change as well as terrestrial and marine eutrophication. Therefore, increased separate collection goes hand in hand with higher environmental impacts in these categories because of higher process emissions (e.g. CH₄, N₂O and NH₃) and impacts from compost utilization. On the other hand, substituting compost for mineral fertilizer results in substantial environmental savings with regard to freshwater ecotoxicity. Consequently, the contents of carbon, nitrogen and potassium were, among others, identified as sensitive parameters affecting environmental performance when composting bio-waste and digestate. The environmental performance of biological treatment pathways for urban bio-waste can be improved by decreasing process-related emissions through optimal operation of treatment plants and by utilizing compost as a substitute for mineral fertilizers.