Eco-Efficiency of Rural Biodigesters: Mono- and Co-Digestion of Agricultural Waste

The increasing generation of agricultural waste poses both environmental and economic challenges, particularly in rural areas with limited infrastructure. Anaerobic digestion has emerged as a sustainable alternative, enabling the valorization of waste and the production of biogas and biofertilizer. This study evaluated the economic and environmental gains of mono- and co-digestion of equine manure and vegetable waste using biodigesters of different capacities across four simulated projects—Project 1 (15 m2 biodigester with monodigestion), Project 2 (15 m2 biodigester with co-digestion), Project 3 (20 m2 biodigester with monodigestion), and Project 4 (20 m2 biodigester with co-digestion). Economic feasibility was assessed through indicators such as Net Present Value (NPV), Internal Rate of Return (IRR), Modified IRR (MIRR), Profitability Index (PI), Benefit-Cost Ratio (B/C), Discounted Payback Period, sensitivity analysis, and Monte Carlo simulation, adopting a Minimum Attractiveness Rate (MAR) of 6.43% per year. Environmental benefits were estimated based on the annual reduction of CO2 equivalent emissions. The results showed that all projects were economically viable and had the potential to mitigate up to 36 tons of CO2eq per year. Additionally, an eco-efficiency indicator (NPV per CO2eq avoided) was calculated to enable an integrated assessment of economic performance and environmental impact. Projects using 20 m3 biodigesters achieved the best results, with Project 3 being the most eco-efficient (USD256.05/tCO2eq), while Project 4 yielded the highest absolute return in all economic analysis tools: NPV (USD 9063.81), IRR (25.10%), MIRR (10.95%), PI (USD 1.65), B/C (USD 1.65) and DPP (4.56 years). The integrated analysis underscores the significance of co-digestion and economies of scale in encouraging the adoption of this technology by small rural producers.