Enhancement of biogas production from sludge anaerobic digestion via supplementing magnetic co-pyrolysis biochar: Dosage response and syntrophic metabolism

The addition of conductive materials to promote anaerobic digestion (AD) via direct interspecies electron transfer (DIET) has been attracted extensive attention, whereas seldom focused on the effect of co-pyrolysis biochar on sewage sludge AD. Here, a novel co-pyrolysis biochar derived from oil sludge and wheat straw was successfully applied in improving methane production. Experimental results suggested that the co-pyrolysis of wheat straw with oil sludge would increase the surface area of biochar, benefited for the methane production improvement. As high as 144.05 ​mL ​(g ​VS) ⁻¹ accumulative methane productivity and fast volatile fatty acids (VFAs) mainly acetic acids degradation rate was detected under the optimal operational condition with 1.6 ​g BC25 ​% (wheat straw: oil sludge ​= ​1:3) additive. Generally, the strong electron accepting capacity (71.8 μmol e⁻ g⁻¹) and donating capacity (27.5 μmol e⁻ g⁻¹) resulted from magnetic features and oxygen containing functional groups of co-pyrolysis biochar facilitated DIET process for boosting methane yield. Furthermore, co-pyrolysis biochar supplied sufficient trace elements (Ni, Cu and Zn) for activating the coenzyme F420, protease and electron transport system for accelerating methane yield. Microbial and Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis indicated acetoclastic and hydrogenotrophic pathways were both promoted due to the enrichment of archaea including Methanothrix, Methanobacterium, and Methanomassiliicoccus, as well as the typical bacteria of Chloroflexi. The fundamental understanding of underlying mechanisms is critical for the practical application of co-pyrolysis biochar in AD field.