Advances challenges and future directions of pretreatment strategies for enhancing biogas production from agricultural waste

The efficacy of biogas production from agricultural waste is largely determined by the pretreatment methods applied, which are pivotal in enhancing the conversion of organic matter into biofuel. Biogas production from agricultural waste offers a sustainable pathway for renewable energy. However, the recalcitrant lignocellulosic structure of many residues limits microbial accessibility and methane yield. This review synthesizes findings from 119 recent studies published between 2020 and 2026, providing quantitative benchmarking of physical, chemical, biological, and combined pretreatment techniques for agricultural residues. Each pretreatment method possesses distinct advantages and limitations. Physical pretreatments, like mechanical milling and steam explosion (TRL 7–9) achieve methane increases of 10–50% and suit large-scale, low-lignin feedstocks but demand high energy. For chemical methods, alkaline and organosolv (TRL 8–9 and 4–6) deliver 20–100% increases for lignin-rich biomass, yet risk digestate contamination through salt accumulation and inhibitor formation. Among biological pretreatments, ensiling and fungal treatment (TRL 7–9 and 5–7) provide 15–70% increases with low operating costs and positive digestate quality, though treatment times extend to days or weeks. Combined strategies generate synergistic effects: digestate recirculation achieves 86% operating expenditure savings, while ensiling reduces greenhouse gases by 250 Mg CO₂eq/1000 hectares, albeit with trade-offs including increased nitrogen leaching and soil carbon loss. Environmental hotspots are context dependent. Electricity driven physical methods burden grid carbon intensity, steam explosion risks methane slip (− 0,134 kg CO₂-eq/kWhₑₗ); and chemical pretreatments introduce salt accumulation unless lime recirculation is employed. Nanotechnology (TRL 3–5) is an emerging enhancement layer capable of improving direct interspecies electron transfer, but barriers in cost, toxicity, and regulation confine it to research applications. A decision framework integrating feedstock lignin content, plant scale, digestate quality, economic feasibility, and pretreatment limits is proposed. Advancing pretreatment methods is a cornerstone of the biogas cycle, essential for a sustainable circular energy future. Future research must prioritize region-specific, multi-feedstock studies, life cycle assessments, predictive modeling, and scalable hybrid systems that balance methane yield against sustainability. A significant need for integrated, standardised studies that are economically and environmentally sustainable and take into account the diversity of waste and the underlying microbial mechanisms.