Exploring the environmental consequences of roadside grass as a biogas feedstock in Northwest Europe.

The Russo-Ukrainian war has highlighted concerns regarding the European Union's (EU) energy security, given its heavy dependence on Russian natural gas for electricity and heating. The RePowerEU initiative addresses this challenge by targeting a significant increase in biomethane production (up to 35 billion m3 by 2030) to replace natural gas, aligning with the EU methane strategy's emission reduction and air quality improvement goals. However, the use of energy crops as biogas feedstock has raised land-use concerns, necessitating a policy shift towards alternative sources such as agro-residues, livestock manure, and sewage sludge. This study investigates the environmental impacts of using roadside grass clippings (RG) as an alternative feedstock for biogas production, focusing on selected regions in Northwest Europe (Belgium, Netherlands). The aim is to evaluate the environmental performance of RG as a mono- or co-substrate for biogas production, comparing it to the current practice of composting. Additionally, the study assesses the environmental impacts associated with biogas end-use in these regions. The results indicate that co-digestion of RG with pig manure offers a more environmentally friendly alternative compared to mono-digestion of RG or the existing composting practice. This finding is primarily attributed to the avoided emissions resulting from conventional pig manure management. Furthermore, in terms of climate change impacts concerning biogas end-use, the study identifies that combined heat and power (CHP) systems are preferable to biomethane recovery in regions with a natural gas-based electricity mix. However, for reducing fossil resource use, biomethane recovery emerges as the preferred option. By providing insights into the environmental performance of RG as a biogas feedstock and evaluating the impacts of different biogas end-use options, this study offers insights to policymakers for the development of sustainable energy strategies in Northwest Europe.

Development of conversion factors to estimate the concentrations of heavy metals in manure-derived digestates.

During biogas production, a residual by-product rich in organic matter, nutrients, and trace elements - called digestate - is generated. Due to the nature of the anaerobic digestion process (i.e., conversion of organic matter into biogas) and the non-digestibility of trace elements, metal concentrations are higher in digestate than initially in the treated feedstock, resulting in a detrimental effect on the environment when directly applied as fertiliser on the soil. This study aims to predict the concentration of heavy metals in digestate through four different process parameters (Biogas yield - M1, Biodegradable fraction - M2, Dry matter - M3 and Power generation - M4) in full-scale biogas plants. For the validation of the process parameters, the predictions were compared against laboratory analyses of feedstocks and digestates samples from mono- and co-digestion processes. The convergence between the conversion factors based on laboratory data and process parameters (CLD and CFA, respectively) ranged in the following order: M3 > M2 > M1 > M4. Based on laboratory analyses, better predictions were obtained for Al, Cr, Cu, Fe, Mn, and Zn employing M3. Moreover, a robust convergence was achieved between the CLD and CFA conversion factors for the mono-digestion process. Further assessment of a diverse range of feedstocks is needed to increase the convergence between the conversion factors based on process parameters and laboratory data, specifically for the co-digestion process M3. The concentrations of Cd, Co, Ni, and Pb elements were below the detection limits, whereas Cr, Cu, and Zn did not exceed the legal threshold limits of the legislations.