Potential mechanisms of granular activated carbon in reducing inhibition of sulfate reduction on methanogenesis

The competitive inhibition of sulfate reduction on methanogenesis seriously hinders anaerobic digestion performances of sulfate-contaminated organic wastewater. The application of granular activated carbon (GAC) to boost anaerobic digestion has recently attracted many researchers' attention. However, the effect of GAC in reducing inhibition of sulfate reduction on methanogenesis has yet been well studied. In this study the role and mechanisms of GAC in enhancing the anaerobic digestion performances under different chemical oxygen demand (COD)/SO 4 2- ratios were explored. As COD/SO 4 2- ratio decreased, methane yield decreased. However, under the same COD/SO 4 2- ratio, methane yield increased with GAC addition, with exhibiting 5.06 %-18.38 % higher ETS activity compared to control, demonstrating that GAC alleviated competitive inhibition of sulfate reduction on methanogenesis. The internal driving mechanism was mainly due to increase of pH and reduction of oxidation-reduction potential, which provided favorable environment for methanogens. Notably, with the rise in pH value sulfides mainly presented as sulfide ions, which posed less toxicity to microorganisms than free H 2 S. Besides, with GAC addition interspecies electron transfer was enhanced, protein synthesis and repair, and adenosine triphosphate synthesis were promoted, creating favorable thermodynamic condition for methanogenic pathway, maintaining microbial physiological activity, and providing more energy for growth and metabolism of methanogens. The other data, such as volatile fatty acid (VFA), alkalinity, free ammonia, and gas-phase H 2 S may be important for quantitative interpretation of process. These findings elucidate potential mechanisms of GAC addition driving efficient methanogenesis under sulfate reduction condition, expanding the ecological role of GAC in sulfate organic wastewater anaerobic digestion system.