Mechanisms of biochar-mediated promotion of acidogenic fermentation in waste activated sludge and propionic 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 worldwide. However, the conversion of WAS to a certain acid among VFAs is difficult. Herein, for the first time, biochar was used to promote sludge solubilization, hydrolysis and acidogenic fermentation for the production of propionic acid. The results illustrated that biochar (prepared at 450 degrees C, containing environmental persistent free radicals (EPFRs) of 15.91 x 1015 spins/g) stimulated sludge solubilization, hydrolysis and VFAs production, enhancing sludge cell lysis rate, VFAs production, propionic acid ratio, protease, & alpha;-glucosidase and nicotinamide adenine dinucleotide hydrogen (NADH)/nicotinamide adenine dinucleotide (NAD+) ratio by 10.53 %, 107.42 %, 17.52 %, 68 %, 32 % and 76.10 % respectively, compared with those observed in the control group without biochar addition. And once the EPFRs were shielded, they decreased. The signal of EPFRs decreased from 15.91 x 1015 spins/g to 15.20 x 1015 spins/g on day 2 and 8.50 x 1015 spins/g on day 4. The EPFRs contained in the biochar played an important role in the intermediary function and interfered with the bacterial metabolic pathways. Metagenomics sequencing also revealed that biochar EPFRs promoted the sludge acidogenic fermentation to produce propionic acid ([EC:7.1.1.2]) and partly inhibited acetic acid production ([EC:1.2.1.3]). This study reveals that biochar could promote sludge solubilization, hydrolysis and promote propionic acid production, which could be used as a new strategy for acid production in the future.