Nitrate reduction pathways and microbial community to different COD/NO3−-N ratios in mesophilic and thermophilic denitrification systems

Hydrolytic acidification integrated with denitrification (DEN) presents a promising treatment strategy for high-temperature industrial wastewater, such as oil, leather and ethanol wastewater. This study aimed to investigate the synergistic influence of temperature and the COD/NO3−-N ratio on the nitrogen reduction pathway using a series of semi-continuous flow reactors. The results revealed that the optimal total nitrogen (TN) removal performance was achieved under thermophilic (T = 60 °C) and COD/NO3−-N ratio of 37, with the highest TN reduction amount of 594.3 ± 41.8 mg/L. However, under other conditions, the TN removal efficiency reduced due to NH4+-N accumulation. Higher temperature facilitated the nitrogen reduction rate and butyrate yield rate, while inhibiting the dissimilatory reduction to ammonium (DNRA) process to 20.7 %. Furthermore, the higher temperature increased the bacterial diversity and the relative abundances of Clostridium, Acidaminococcus and Ruminococcus, thus, enhancing organic matter hydrolysis and nitrogen removal efficiency. In contrast, Carnobacterium, with percentage account of 34.4 %, was only detected under thermophilic conditions with a low COD/NO3−-N ratio. The COD/NO3−-N ratio only generated a slight effect on the production of volatile fatty acids (VFAs) and nitrogen reduction pathways, but had significant influence on the microbial community. These findings provide novel insight into the implementation of hydrolysis acidification coupled with anaerobic denitrification process in thermophilic wastewater treatment. [Display omitted] •Hydrolytic acidification integrated with DEN was simultaneously performed.•Thermophilic conditions facilitated the DEN process.•Providing novel ideas for CS wastewater treatment.