Pretreatment in Small-scale Biogas Systems: A Pathway to Mainstreaming Co-digestion and Agricultural Residue Utilization

Pretreatment can facilitate the use of agricultural residues in small-scale biogas systems by improving the digestibility of lignocellulosic feedstocks. In many developing countries, these plants rely primarily on cattle manure mono-digestion, resulting in feedstock shortages, underfeeding, and frequent system abandonment due to limited and variable livestock ownership. Crop residues offer strong potential as co-digestates because of their complementary physiochemical properties and their ability to stabilize feedstock supply. This study applied the Analytic Hierarchy Process (AHP) to prioritize suitable pretreatment methods for small-scale biogas systems using five evaluation criteria: technological simplicity, energy requirements, capital and operational costs, effectiveness, and environmental impact. Physical, chemical, and biological pretreatment methods were assessed for their suitability in improving access to lignocellulosic material for microbial degradation in small-scale settings. The results identified biological pretreatment methods, together with simple mechanical size reduction, as the most appropriate options due to their low energy demand, simplicity, and environmental compatibility. A multi-stage pretreatment strategy combining mechanical processing with biological pretreatment using digestate liquid fraction is proposed as a practical closed-loop approach for small-scale applications. The system has the potential to improve operational resilience, feedstock flexibility, and sustainability in small-scale biogas production.