Deciphering the synergistic effects and mechanisms of biochar and magnetite contained in magnetic biochar for enhancing methane production in anaerobic digestion of waste activated sludge

Adding conductive materials is one of the most extensive enhancement strategies while treating waste activated sludge via anaerobic digestion. Magnetic biochar (MBC), as one composite conductive material, is capable of enhancing methane yield and production rate because of its favorable characteristics. However, whether the synergistic effects formed or not between biochar and magnetite contained in MBC on anaerobic digestion is still unclear. This study investigated the synergistic effects and corresponded mechanisms of biochar and magnetite contained in MBC with semi-continuous anaerobic digestion mode. Results showed that the co-addition of biochar and magnetite performed non-synergistic effects on methane production potential, with decrease ratios of 2.2% and 7.4% respectively compared to that in biochar and magnetite groups. Interestingly, the biochar and magnetite contained in MBC formed synergistic effects, with an extra improvement of 5.5% compared to the sum of those obtained in biochar and magnetite groups. The synergistic effects came from efficient hydrolysis and acidogenesis stages, including the thorough degradation of soluble organic matters and the rapid conversion of acetic acids. MBC also produced synergistic effects on the hydrophilia and redox properties of extracellular polymeric substances, the activities of enzymes involved in interspecies electron transfer like cytochrome c, and the contents of adenosine triphosphate (ATP). Specifically, the enhancement potentials contributed by MBC exceeded the total enhancement potential contributed by biochar and magnetite, with the extra enhancement ratios of 13.1% and 19.4% for cytochrome c and ATP, thus, the biochar and magnetite contained in MBC formed synergistic effects for promoting electron transmembrane and transfer from kinetic aspects. The correlation coefficient between methane production performance and the microbial electron transfer activity reached 0.96, correspondingly, the highest electron transfer activity of microorganism was presented in MBC. As for microbial communities, the functional and electro-active microorganisms were enriched with the addition of MBC, such as Peptoclostridium, Anaerolineaceae, Methanosarcina, and Methanosaeta, which facilitated the conversion of organic matters and established direct interspecies electron transfer methanogenesis. The findings of this study revealed the synergistic effects and mechanisms between biochar and magnetite contained in magnetic biochar in enhancing sludge anaerobic digestion, and provided an effective strategy to recover bioenergy from waste activated sludge, potentially boosting carbon neutrality in wastewater treatment.