Improving Methane Production from Waste Activated Sludge Assisted by Fe(II)-Activated Peroxydisulfate Pretreatment via Anaerobic Digestion: Role of Interspecific Syntrophism Mediated by Sulfate-Reducing Bacteria

Sulfate radical generated via peroxydisulfate (PDS) activation has been extensively applied to energy recovery from waste activated sludge (WAS) during anaerobic digestion (AD), while the residual sulfate is still a limitation for its further application. In this study, a novel coupling strategy of Fe(II)-activated PDS pretreatment [Fe(II)_PDS] with sulfate-reducing bacteria (SRB) mediated was explored to enhance methane yield from WAS during AD. Results showed that the similar methane yield was obtained in Fe(II)_PDS + SRB (16.6 mL/g VSS) and zero-valent iron (ZVI)_PDS + SRB (16.9 mL/g VSS), which enhanced by 19.9-56.5% than other groups, while the hydrolysis rate constant (0.0065 h^-1) in Fe(II)_PDS + SRB was much higher than that observed in ZVI_PDS + SRB (0.0046 h^-1). Meanwhile, the concentration of short-chain fatty acids (SCFAs) peaked at 360.9 mg COD/g VSS (5 d) in the Fe(II)_PDS + SRB group, with the acetic acid (HAc) and propionic acid (HPr) accumulative proportion of 73.5%, and the total concentration promoted by 28.5% than that of the ZVI_PDS + SRB group, which provided a more ideal substrate for methanogens. The ultimate utilization efficiency of SCFAs reached 62.4% in the Fe(II)_PDS + SRB group. Moreover, MiSeq sequencing, canonical correspondence analysis, and molecular ecological network analysis revealed the intrinsic interaction of the functional microbial consortia, that is, anaerobic fermentation bacteria, SRB, nitrate-reducing bacterium, and acetotrophic methanogen, with the abundances of 9.52, 2.4, 4.93, and 77.7% in the Fe(II)_PDS + SRB group. Considering the treatment performance and the difficulty of the subsequent disposal, Fe(II) may be the superior activator than ZVI for free radicals (i.e., SO₄^•- and HO^•) generation from PDS and further played the important role for interspecific syntrophism coupling with SRB during AD. Results of this study may provide a promising way for energy recovery from WAS.