Insight into magnetite effects on anaerobic digestion of waste activated sludge with thermal hydrolysis pretreatment: From reactor performance to metagenomics analysis

Although the promotion of magnetite in anaerobic (AD) has been widely reported, the role of magnetite plays in the AD of waste activated sludge (WAS) and the mechanism of the promotion effect are still needed to be explored. This study investigated the effects of magnetite in AD of WAS with thermal hydrolysis pretreatment (THP) from four aspects including reactor performances, sludge characteristics, microbial community structures and gene abundance. Results showed that magnetite is able to enhance the consumption of soluble protein (SPN) and soluble polysaccharide (SPS) by increasing the activities of alpha-glucosidase and protease. With the addition of 0.5-5 g/L magnetite, the cumulative CH4 production could be increased by 38.25 %-85.34 %. The sludge electrical conductivity (EC) grew with the increase of magnetite dosage, which were 3.02, 8.65, 13.00, 15.46 and 17.97 mu S/cm for G0-G4 respectively. In the presence of magnetite, the abundance of Proteobacteria increased which contained many genera of electroactive bacteria, such as Pseudomonas. By further metagenomics analysis, hydrogenotrophic methanogenesis pathway was shown to be enhanced with the addition of magnetite. The increasing abundance of pilA genes from Clostridium and Pseudomonas in bacteria and Methanospirillum in archaea confirmed that magnetite stimulated DIET in the AD of WAS.