Mechanisms of Biochar-Mediated Promotion of Acidogenic Fermentation in Waste Activated Sludge and Acetic Acid Production Pathways

The conversion of organic matter in waste activated sludge (WAS) into high value-added chemicals of volatile fatty acids (VFAs) has become a focus of attention. However, the direct conversion of WAS to a certain acid in VFAs is difficult. Herein, for the first time, biochar was used to promote sludge solubilization, hydrolysis and acidogenic fermentation directing the production of acetic acid. The results illustrated that biochar was able to promote the oxidation of NADH and maintain the appropriate level of NADH/NAD(+) ratio, thus increasing the acetic acid production. The promotion effect was directly proportional to the content of oxygen-containing functional groups (OFGs) in the biochar, and the maximum content of OFGs was found in the CT/Ni2O3@SBC, which had the lowest NADH/NAD(+) ratio in the anaerobic fermentation system, acetic acid accounted for the largest percentage (52.30%), which was 1.99 times higher compared to the blank. OFGs in the biochar were involved in the electron transfer between electroactive microorganisms in the fermentation system, which compete with the methanogenic microorganisms for the electrons and thereby inhibit the production of methane. This study reveals that biochar could promote sludge solubilization, hydrolysis and promote acetic acid production, providing a new strategy for directional acid production in the future.