A lab-scale methanolic wastewater-fed (3000 mg COD L−1) UASB reactor was operated for 235 days to evaluate the influence of the sulfidogenesis process on metabolic routes, the re-granulation of dispersed granules and long-term process performance. Various sulfidogenesis scenarios were created by stepwise decreasing the influent COD/SO42− ratio from 20 to 0.5 at a fixed organic loading rate (OLR) of 12 g COD L−1 d−1. It was shown that the conversion of methanol to methane was stable at a wide COD/SO42− range of ≥2, attaining high biogas production rate of 3.78 ± 0.32 L L−1 d−1 with efficient concurrent removal of the total COD (96.5 ± 4.4%) and sulfate (56.3 ± 13.0%). The methane content in biogas remained relatively stable at 81.5 ± 1.6% for all COD/SO42− ratios tested. The particle size of the granules was shown to clearly increase as the COD/SO42− ratios decreased. A slight linear decline was noted in the number of electrons utilized by methane producing archaea (MPA) (from 98.5 ± 0.5% to 80.0 ± 2.4%), whereas consumption by sulfate reducing bacteria (SRB) increased (from 1.5 ± 0.5% to 20.0 ± 2.4%) with the decreasing COD/SO42− ratio. According to the results of activity tests and microbial community analysis, the conversion of methanol to methane at a low COD/SO42− ratio, except from Methanomethylovorans sp., depends not only on low levels of acetoclastic and hydrogenotrophic methanogens, but also on incomplete oxidizer SRB species (e.g. Desulfovibrio sp.) that utilize H2-CO2 with acetate to mineralize the methanol. This serves to diversify the metabolic pathway of methanol. Further analysis through scanning electron microscopy (SEM) revealed that a lower COD/SO42− ratio favored the sulfidogenesis process and diversified the microbial community inside the reactor. The benefical sulfidogenesis process subsequently invoked the formation of a sufficient, rigid [-Fe-EPS-]n network (EPS: extracellular polymeric substances), binding and immobilizing the sludge, and resulting in the re-granulation of the dispersed granules.
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