Biochemical Methane Yield and Process Performance in Thermophilic Anaerobic Digestion of Abandoned Organic Solid Wastes

In the context of the global energy transition, renewable and sustainable resources are increasingly being explored as an alternative to fossil fuels. Lignocellulosic and organic waste biomass is abundant, low-cost, and represents a promising feedstock for bioenergy production. However, the valorization of abandoned or underutilized residues remains largely unexplored. This study evaluated the bioenergy potential of eight solid organic waste materials collected from abandoned sites, including: (1) landfilled woodwaste, (2) softwood and (3) hardwood sawdust, (4) fresh pine bark (5) decomposed pine bark, (6) decomposed leaf and yard waste, (7) decomposed organic food waste (8) and aged barn dust. Physicochemical characterization revealed their high organic matter content across all substrates, with volatile solids (VS) ranging from 40% to 95%, whereas the C/N ratio varied widely from 10 to 1297. To optimize conditions, co-digesting was performed at a fixed substrate-to-inoculum ratio (SIR) at 1, which effectively balanced the high carbon content and enhanced process stability. Under thermophilic anaerobic digestion, organic matter degradation ranged from 16% to 71%. The highest specific methane potential reached 89.9 ± 7.7 L CH4·kg VS added−1 for fresh pine bark, while the lowest was 25.2 ± 6.8 L CH4·kg VS added−1 for decomposed organic food waste. The resulting digestates were rich in nutrients, demonstrating high agronomic value. Anaerobic digestion of abandoned lignocellulosic and organic residues presents a dual benefit: it reduces pollution while producing renewable energy in the form of methane and valuable by-products that can be used as fertilizers, thereby ensuring a circular economy. This study demonstrates the significant potential of utilizing overlooked waste streams as valuable resources in sustainable bioenergy generation.