Multiple Goals for Biomass Residues in Circular Bioeconomies? Assessing Circularities and Carbon Footprints of Residue-Based Products

Biomass residues are often considered key in a reorientation towards circular bioeconomies, both by returning organic matter and nutrients to soils and by expanding the feedstock base for fossil-free products. Different indicators are available to assess progress towards circularity, but many available indicators and assessments seem to focus on product or material circularity, and lack in coverage of ecological or nutrient circularity. This study therefore applies both material and nutrient circularity indicators to two cases of residual biomass’ valorisation: plastics production from wheat straw, and jet fuel production from animal by-products, in order to better understand the potential of the different types of indicators to assess the circularities of bio-based products. Both the studied production systems achieve approximately 50% material circularity in the base case, but the scores are significantly lower when upstream processes such as cultivation and animal husbandry are included. In the plastics case, the nutrient circularity scores are consistently lower than material circularity scores. The contribution to circularity from composting and recycling of different streams can be interpreted differently following the different types of circularities and, in addition, considering the potential climate impact of different strategies. This study shows that a combination of methods and indicators can shed light on different types of circularities and goals, but also that a wider discussion on what circularity may entail for biomass and biomass residues, and how it can be measured, is needed to develop useful indicators for bio-based circularity and circular bioeconomies.