Influence of inoculum-to-substrate ratio in the pilot-scale anaerobic co-digestion of organic fraction of municipal waste and sewage sludge

This study evaluated the effect of the inoculum-to-substrate ratio (ISR) on anaerobic co-digestion (ACoD) at pilot scale using a 70:30 mixture of the organic fraction of municipal solid waste (OFMSW) and sewage sludge (SS), with stabilized anaerobic sludge as inoculum. Experiments were performed in 1 m³ CSTRs under three ISR conditions (4:1, 2:1, 1:1) to determine their influence on methane production, pH, alkalinity, organic matter degradation, pathogen inactivation, and heavy metal behavior. The 2:1 ISR showed the best performance, achieving process stability and the highest methane yield (906 mL CH4/g VS) due to improved microbial adaptation and resilience. Conversely, extreme ISRs (4:1 and 1:1) reduced stability and methane output (662 and 608 mL CH4/g VS, respectively). Pathogen inhibition was also observed, with >99% removal of E. coli, Salmonella spp., and helminth eggs. These findings highlight the importance of selecting an appropriate ISR to enhance biogas production, optimize waste treatment, and minimize operational issues. While this research advances understanding under real operating conditions, further studies should test long-term continuous operation to validate and refine system performance.