Effects of COD/TN ratio on nitrogen removal efficiency, microbial community for high saline wastewater treatment based on heterotrophic nitrification-aerobic denitrification process

[Display omitted] •The optimal treatment performance was achieved in high saline HN-AD system with COD/TN ratio of 25.•The dominant nitrogen removal functional bacteria and pathways shifted under different COD/TN ratio.•HN-AD bacteria Pseudomonas, Bacillus and Paracoccus played a key role in the system.•Other functional bacteria Halomonas, Nitrincola and Oceanimonas participated in denitrification. High salinity and high organic concentration impose negative impacts on autotrophic nitrification, which hinders efficient nitrogen removal. To achieve efficient nitrogen removal, high saline wastewater nitrogen removal systems with different COD/TN ratios based on heterotrophic nitrification-aerobic denitrification (HN-AD) process were established in this study. Results demonstrated that the COD/TN ratio had significant effects on nitrogen removal efficiency, microbial community structures and metabolic pathways. The optimal COD/TN ratio was 25 for nitrogen removal, with NH4+-N and TN removal rates of 11.86 mg·L−1·h−1 and 11.50 mg·L−1·h−1, respectively (3.65 and 3.31 times higher than those COD/TN ratio of 10). 16S rRNA high-throughput sequencing revealed that HN-AD functional bacteria (Pseudomonas, Bacillus, Paracoccus) with highest abundance (7.61%) played a key role in high saline wastewater treatment. And Halomonas, Nitrincola and Oceanimonas participated in the denitrification process. Moreover, the abundance of genes related to nitrogen removal was the highest (1.90%) revealed by functional genes prediction.