Impact of Biochar Amendment on Pilot-Scale Anaerobic Digestion of Agro-Industrial Waste with Digestate Recirculation.

This study evaluated the impact of biochar amendment on the performance of a pilot-scale anaerobic digester treating a mixture of olive pomace mill wastewater, cheese whey, and chicken manure under digestate recirculation. The 180 L mesophilic reactor operated for 160 days at a hydraulic retention time of 20 days, with biochar added to the feedstock at 10 g L⁻¹. Process monitoring included measurements of biogas production rate, methane content, and key physicochemical parameters (pH, TS, VS, sCOD), while microbial community dynamics were assessed via 16S rRNA amplicon sequencing and real-time PCR. Biochar addition led to a modest reduction in biogas production rate (from 290.7 ± 17.5 L d⁻¹ to 273.8 ± 14.3 L d⁻¹), but enhanced methane content from 70 % to 73 %. Notably, the relative abundances of Methanothrix and Methanospirillum increased substantially (from 47.1 % to 74.1 % and 2.2 % to 5.6 %, respectively), indicating strengthened direct interspecies electron transfer pathways. These findings demonstrate that biochar can selectively enrich syntrophic methanogenic consortia and improve methane yield, offering a promising strategy for optimizing pilot-scale anaerobic digestion of agro-industrial residues.