Dry anaerobic digestion of organic fraction of municipal solid waste in a vertical plug digester: Start-up procedures, digestate recirculation, organic loading rates, and energy balance analysis

This study addresses the challenges of efficiently processing the organic fraction of municipal solid waste (OFMSW) through dry anaerobic digestion in a vertical plug digester under mesophilic conditions. The focus is on optimizing hydraulic retention time (HRT) and digestate recirculation (REC) to enhance system performance. During the start-up phase, a mixture of digestate from a wet biogas plant and organic municipal solid waste (OFMSW) was employed, with a gradual introduction of 15 % garden waste to improve the carbon-to-nitrogen ratio and increase the total solids content. Various HRT levels (25, 30, and 40 days) and REC rates (17 % and 35 %) were assessed during steady-state operation. The start-up stage results suggest that increasing the feedstock C/N ratio, gradually removing excess water from the digestate, and achieving a 60 % REC are effective strategies to expedite the start-up process. Optimal performance was observed at HRT of 40 days and REC of 35 %, yielding a specific methane production (SMP) of 311.6 NL/kg VS. Maximum volumetric methane production (VMP) of 2.55 L/L/day and methane content of 62.2 % were recorded at HRT of 25 days with the same REC rate. Additionally, energy balance analysis revealed that digester heating constituted the highest energy consumption, with a maximum surplus energy of 44.0 % of gross biogas energy achieved under optimal volumetric methane production treatment. The study highlights the potential for implementing this system like Dranco process in heterogeneous waste treatment plants, emphasizing the advantages of moderate digestate recirculation. Future research should explore energy balance and scalability.