| Nitrogen pollution has been one of the most serious environmental challenges that could affect the water quality and ecological health.Although the widely-adopted biological nitrogen removal processes are cost-effective and environmentally benign,the process efficacy of these technologies is frequently contained under low-temperature conditions（e.g.,below 25℃）.The winter season lasts 4-6 months in the vast northern regions of China,during which the biological nitrogen removal process is ineffective in meeting the wastewater discharging quality standards.Maintaining the denitrification activity of the microorganisms in the sludge is an important measure to ensure the effectiveness of the treatment process.Previous studies have highlighted the existence of diverse nitrogen-removal bacteria,nevertheless,the vast majority of these microorganisms are mesophilic,and there is still a lack of microbial resources that can remove nitrogen in low temperature environment.It is thus of great theoretical and practical significance to strengthen the exploration of psychrotolerant microbial strains and to gain a deeper understanding of the nitrogen removal performance in cold regions.In this study,psychrotolerant bacteria with nitrogen-removal potential were screened,by spread plate method,from activated sludge collected from a wastewater treatment plant in northern China.Meanwhile,the nitrogen-removal capabilities of isolated strains at various low temperatures（0°C～25°C）were evaluated.In addition,microbial blends were constructed and tested.The main results were:（1）in total of 114 bacterial strains were obtained from the initial screening,and 12 isolates exhibited efficient heterotrophic nitrification and denitrification capabilities while degrading COD at 15°C.The alignment of 16 S r RNA gene sequences showed that strains H6,H8,W4,W6,L1,L3,L5 and L6 share the highest sequence similarity（>99%）with Pseudomonas species.Strains H3 and H4 are closely related to Rahnella aquatilis（100% similarity）,and strains H12 and H13 are 100% similar to Variovorax paradoxus and Delftia acidovorans,respectively.（2）At 5°C,15°C and 25°C,all strains of H6,W4,L1,L3,and L6 could efficiently remove ammonium,nitrate,nitrite and COD,with the highest removal rates of 84.3%-97.3%,66.3%-80.2%,60.0%-73.6% and 93.6%-100.0%,respectively.（3）The above five strains all exhibited simultaneous heterotrophic nitrification-aerobic denitrification denitrification（HN-AD）activity at C/N of 34,and under 5°C,15°C and 25°C conditions.HN-AD activity at 0°C was observed only by strain L1 and 92.1% of ammonium,51.8% of nitrate and 93.7% of COD was removed within 11 days.The HN-AD process was incomplete for all strains when the C/N was 23 and 11.（4）Bacterial blends,consisting aliquots of cell cultures of strain L1,L3 and W4,showed higher HN-AD activity at 5°C,and C/N of 34.This blends removed 96.3% of ammonium,95.5% of nitrate and 87.1% of COD within 144 hours.The psychrotolerant nitrogen-removal bacteria obtained in this study enrich the microbial resources in China.The performance of these strains or microbial blends at 0～25°C contribute to the subsequent potential applications,which in together will help improve the efficacy of biological wastewater treatment in cold regions in China. |
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