System performance and functional analysis for the methanogenic bioreactor of a two-phase anaerobic digestion system: The effect of influent sulfate

In this study, a lab-scale bioreactor was built to investigate the influence of sulfate on the overall performance, microbial community, and metabolic pathways in the methanogenic bioreactor of a two-phase anaerobic digestion system. The results demonstrated that by gradually cultivating the sludge of the methanogenic bioreactor, a chemical oxygen demand (COD) degradation efficiency of over 65% was achieved for sulfate contents from 10,000 mg/L to 30,000 mg/L. The sulfate reduction performance could only be maintained at a high level for influent COD/sulfate ratios no less than 1.88. In addition, the methane production rate (MPR) decreased from 0.92 L/L/d to 0.28 L/L/d, and the proportion of methane in biogas dropped from 51% to 33% when the influent sulfate exceeded 30,000 mg/L. High-throughput sequencing coupled with PICRUSt2 revealed that the dissimilatory sulfate process could be enhanced by sulfate within 30,000 mg/L. While an amount of greater than 5000 mg/L sulfate remarkably inhibited the assimilatory sulfate reduction process, and this inhi-bition was induced by the down-regulation of the cysNC, cysC, cysI and cysJ genes. For the methanogenic process, sulfate contents within 30,000 mg/L had no obvious impact on all of the acetoclastic, hydrogenotrophic, and methylotrophic methanogenesis processes, but serious suppression was observed for sulfate amounts greater than 30,000 mg/L. This inhibition primarily originated from the decrease in mcrA, mcrB, and mcrG genes that regulate the conversion of methylcoenzyme M to methane, the last step of all three methanogenesis pathways.

Automated summary

This study investigates the influence of sulfate on the performance, microbial community, and metabolic pathways of a methanogenic bioreactor in a two-phase anaerobic digestion system. The results show that sulfate content affects the degradation efficiency of chemical oxygen demand (COD), methane production rate (MPR), and the proportion of methane in biogas. Different concentrations of sulfate also have different effects on the different metabolic processes.