Revealing the Mechanism of Biochar Enhancing the Production of Medium Chain Fatty Acids from Waste Activated Sludge Alkaline Fermentation Liquor

In this work, the effectiveness of biochar with distinctively different particle sizes (i.e., 2000–5000 and 75–150 μm) derived from anaerobically digested sludge was comprehensively evaluated in the production of enhancing MCFAs (medium chain fatty acids) using waste activated sludge alkaline fermentation liquor as feedstock with ethanol as the electron donor. The results showed that fine biochar (i.e., 75–150 μm) was more favorable for enhancing the production of MCFAs with an increased electron transfer efficiency reaching 92.6%. Metagenomic analysis revealed that the total microbial taxonomic composition shifted in favor of the production of MCFAs (e.g., Clostridium kluyveri sp., Clostridium magnum sp., and Clostridium botulinum sp.). The increased relative abundance of functional enzymes associated with the production of MCFAs confirmed that both the reverse β-oxidation pathway and the fatty acid biosynthesis pathway were enhanced by biochar. Mechanistic studies revealed that the biochar had high redox-active properties due to the functional groups (mainly quinones and hydroquinone) on its surface and an important ability to charge and/or discharge, which thus promoted the transfer of electrons for the production of MCFAs during the CE process.