Overcoming methanogenesis barrier to acid inhibition and enhancing PAHs removal by granular biochar during anaerobic digestion

Alleviating acid inhibition is the most important for optimizing anaerobic digestion (AD), but the recalcitrant organic pollutants in digestates have long been ignored. In this study, we aimed to simultaneously enhance both organics bio-methanation and recalcitrant polycyclic aromatic hydrocarbons (PAHs) removal in AD system by using granule biochar and short-term ethanol stimulation. The results showed that adding biochar effectively alleviated volatile fatty acids (VFAs) accumulation and achieved 313.6 mL CH4/d•g COD average yield. Meanwhile, the efficiency of phenanthrene (PHE) biodegradation, as the model PAHs pollutant, was significantly promoted by 37.1%. Additionally, more and compact extracellular polymeric substances improved electron transfer and mass transfer among microbes, and higher enzymes (ATPase, NADH and α-Amylase) activities met the physiological demanding of efficient microbial metabolism in biochar-added reactor. Microbiological analysis revealed that biochar-mediated direct interspecies electron transfer (DIET) selectively enriched exoelectrogens (Bacteroides, Proteiniphilum, f_Christensenellaceae, etc.) and acetoclastic Methanosaeta, which contributed to increasing biomethane production. The promoted PAHs biodegradation was mainly dependent on the cooperation between fermenting f_Paludibacteraceae and hydrogenotrophic Methanolinea, and potentially related to biochar-mediated DIET. Metabolic analysis indicated that the metabolic pathways of acetogenesis and methanogenesis were significantly strengthened by biochar, thus alleviating acid inhibition. Also, biochar stimulated functional genes expression in PAHs anaerobic biodegradation. These findings were further verified by a series of short-term metabolism tests.