Characteristics and dominant microbial community structure of granular sludge under the simultaneous denitrification and methanogenesis process

•Characteristics of the SDMGS were investigated.•Methanosaeta and Methanobacterium were the dominant archaeal communities.•Proteobacteria, Bacteroidetes and Firmicutes were the three dominant bacteria.•Enterobacteriaceae could be the core bacterium accounted for the SDM.

Batch experiments under different COD/NO3−-N ratios were carried out to investigate physicochemical characteristics and microbial community structure of granular sludge under the simultaneous denitrification and methanogenesis (SDM) process. COD/NO3−-N ratio of 8.0 was proved to be a critical point of the SDM process and sludge at this ratio was selected for analysis. BET, SEM, FTIR and zeta potential measurement were used to characterize the micro-structure, functional groups and surface charge of the granular sludge related to nitrate addition. SEM observation showed that rod-shaped bacteria were predominant at the surface of granules and FTIR spectrum (1745 cm−1) presented an evidence for the carboxyl group protonation upon reduction of the cytochrome c oxidase. Furthermore, high-throughput sequencing technology was used to analyze the microbial structure and diversity. Archaea was found to be accounted for 3.33% of the total microbial communities and Methanosaeta and Methanobacterium were the dominant archaeas. Otherwise, Proteobacteria (63.00%), Bacteroidetes (21.79%) and Firmicutes (9.73%) phyla were identified to be the three dominant bacterial communities. Enterobacteriaceae was detected with a content of 50.24% of the total bacterial sequences and might be the core bacterium contributed to the SDM process. The results would provide vital guidances for the design and stable operation of nitrate-containing wastewater treatment.