Modeling Anaerobic Digestion and Biogas-Fueled Engines for Sustainable Waste-to-Energy and Resource Recovery in Small-Scale Swine Farming

This work presents an integrated dynamic modeling framework to support sustainable waste-to-energy conversion, resource recovery, and decentralized renewable electricity generation in small-scale swine farms within the Brazilian rural context. Unmanaged swine manure poses severe environmental and socio-economic hazards, making anaerobic digestion a pivotal technology for circular agricultural systems. In this study, a control-oriented anaerobic digestion model for high-rate reactors is adapted to represent low-cost covered lagoon digesters operating under realistic hydraulic retention times, ambient temperatures, and organic loading rates. The adapted model predicts methane production and maps critical stability boundaries in relation to volatile fatty acid accumulation, enabling proactive process monitoring to prevent operational failure. To link organic waste treatment with sustainable energy access, a mean value model (MVM) of an Otto-cycle internal combustion engine is adjusted for raw biogas and scaled to low-power regimes (10–40 kW) compatible with family-run farms. The coupled framework estimates electricity output and biogas consumption, maintaining dynamic fidelity for control synthesis and facility sizing. Baseline consistency assessment against experimental data from a full-scale 5000-swine facility demonstrated close numerical agreement (2.28% relative error, predicting 685 m3/d versus 701 m3/d observed). Furthermore, the scaled engine model achieved electrical efficiency up to 22% with specific fuel consumption of 0.66 m3/kWh for a 10 kW genset. These results validate the framework as an accessible engineering and decision-support tool for monitoring and quantifying bioenergy production, optimizing decentralized waste treatment, and bolstering the socio-economic and environmental sustainability of rural communities.