Effects of Fe3O4 nanoparticles on anaerobic digestion enzymes and microbial community of sludge

Fe3O4 nanoparticles (NPs) have potential effects on the anaerobic digestion of excess sludge. The effects of different concentrations of Fe3O4 NPs (0, 100, 200, 400 and 600 mg/L) on enzymes and microorganisms in anaerobic digestion were studied to explore the mechanism of the effect of Fe3O4 NPs on anaerobic digestion. The results showed that 100, 200 and 400 mg/L Fe3O4 NPs could promote anaerobic digestion, and 200 mg/L Fe3O4 NPs had the most obvious promoting effect. The activities of protease, cellulase, dehydrogenase, acetic kinase and coenzyme F420 in the 200 mg/L Fe3O4 NPs group reached 120 U/mg VS, 71.75 U/g VS, 135 U/mL, 94 mol/L, 1.37 umol/g VSS, respectively, which were 3.8, 1.5, 1.2, 1.2 and 1.6 times of the blank group, respectively. However, when the concentration of Fe3O4 NPs reached 600 mg/L, the activities of cellulase, dehydrogenase and acetic kinase were lower than those of the blank group, and anaerobic digestion was inhibited. The above conclusions can also be confirmed by high-throughput sequencing. The abundance of longilinea and ornatilina in the 200 mg/L Fe3O4 NPs group was 8.8% and 4.1% respectively, and methanthrix abundance was 69%, which was more conducive to decomposition acetic acid into CH4.