From waste to circularity: the potential of different treatments of poultry manure and forestry residues in a hot-spot production region in Portugal

The management of biogenic residues in a way that extends the lifetime of carbon within the systems is a focus of the present work. Different waste management systems are assessed in terms of GHG emissions and removals, aiming to identify that with the lowest impact on climate change. A life cycle comparative analysis is performed, assessing the mitigation of the impact on climate change of the current waste management system (A) of the top two residues of a Portuguese region – composting of poultry manure and open air burning of forestry residues – considering two bioenergy solutions: biochemical conversion of poultry manure and thermochemical conversion of wood residues (B); biochemical conversion for both residues together (C), implying fungal pre-treatment of the wood residues, which adds a food product to the supply chain. Data were retrieved from literature and from authors’ experimental work. Assumptions underlying the methodological framework (e.g., cut offs inherent to comparative assertions) are discussed. A sensitivity analysis covers uncertainties underpinning bioenergy systems assessments (e.g., unperfect substitution and rebound effect phenomena). The highest impact on climate change is observed with system A, greenhouse gas (GHG) emissions 2 to 3 times higher than those of the other two systems. System A also performs poorly in conserving the biomass, e.g., forestry residues burning results in carbon being directly released to the atmosphere with no valuable intermediate use. In the base scenario, system B results in the lowest GHG emissions of the three systems, producing biochar, which contributes to carbon sequestering, and biodiesel. Nevertheless, system C can result in the lowest GHG emissions, in a scenario considering unperfect substitution with renewable sources being selected to match the biodiesel supply of system B. System A is least preferable among the three options. Between systems B and C, ranking depends on whether unperfect substitution is considered. System B performs better in all scenarios, except with unperfect substitution, where system C outranks B. Moreover, system C achieves a higher yield of digestate, a product providing other ecosystem services. Thus, in future research, the inclusion of other impact categories in LCA can reveal a better overall environmental performance for system C.