Bioaugmentation Of Immobilized Heterotrophic Nitrifying-aerobic Denitrifying Bacteria To Enhance Nitrogen Removal Efficacy Under Psychrophilic Conditions

Biological treatment technology is widely used in China’s wastewater treatment plants due to the advantages of low cost,high efficiency and Environmental friendliness,however,low temperatures can significantly constrain the activity of the biological nitrogen-removal process.The cold season in China’s northeast and other vast areas lasts 4-6 months long annually,during which the effectiveness of nitrogen removal and COD_Cr degradation in wastewater treatment plants is reduced,affecting the effluent quality.Most of the existing microorganisms are mesophilic,and their nitrogen-removal activity is inhibited under low temperature conditions.In this study,the psychrotolerant heterotrophic nitrifying and aerobic denitrifying（HN-AD）strains e1（Pseudomonas jessenii）,f3（Pseudomonas psychrophila）and f6（Pseudomonas sp.）were used to evaluate bioaugmentation（free cells dosing or immobilized cells）for improving nitrogen-removal efficacy in sequencing batch reactor.The main results and conclusions of the study are:（1）The filler made of modified synthetic fibres for low temperature resistant HN-AD strain immobilisation was more effective than the Moving Bed Biofilm Reactor（MBBR）filler and ceramic pellet filler,with the maximum COD_Cr and NH₄⁺-N removal rates of 88.1%and 70.2%,respectively.By 30 d,the mixed liquid suspended solids（MLSS）value of 665 mg/L was observed.（2）The SBR reactor was operated in anaerobic-aeration-sedimentation（1h-6h-1h）mode with a hydraulic residence time of 1 d under different low temperature conditions.At 15°C,the average removal efficiencies of NH₄⁺-N and COD_Cr of the control reactor inoculated with activated sludge only（（1）CK）were 58.2%and 55.1%,respectively,which were significantly lower than those of the reactor that were inoculated with free HN-AD bacteria（F-SBR,81.6%and 81.1%）,and the reactor with immobilized HN-AD bacteria（I-SBR）had the highest removal rates,reaching 85.9%and 83.9%;when the SBR reactor was operated at 10°C,the average COD_Cr,NH₄⁺-N and NO₃^--N removal efficiencies of the（1）CK reactor were 74.6%,15.5%and 45.3%,respectively,which were significantly lower than those of the F-SBR reactor（88.5%,67.8%and85.7%）,and those of the I-SBR reactor（90.9%,75.0%and 84.8%）;when the SBR reactor was operated at 5°C,the average COD_Cr,NH₄⁺-N and NO₃^--N removal efficiencies of（1）CK were 74.2%,33.0%and 44.3%,respectively,which were still significantly lower than those of the F-SBR reactor（84.3%,71.0%,75.1%）and the I-SBR reactor（86.5%,76.0%,78.7%）.（3）The reactor with immobilized Pseudomonas jessenii e1（（5）e1 M）and the enhanced reactor with immobilized Pseudomonas sp.f6（（9）f6 M）were the most effective in improving the nitrogen-removal efficiencies at 5°C.The average removal rates of COD_Cr,NH₄⁺-N and NO₃^--N were 88.3%,77.6%,81.2%and 89.2%,79.8%and 81.7%,respectively.（4）High-throughput sequencing results showed that the bioaugmentation technology could enhance the bacterial diversity in the SBR reactor;the the psychrotolerant HN-AD bacteria,at 5°C,were able to gradually dominate within the sludge,with Pseudomonas genus up to 12.47%（second dominant genus）and 35.75%（first dominant genus）in the sludge of F-SBR reactor and the I-SBR reactor,respectively.In summary,this study clarifies that bioaugmentation with HN-AD bacteria significantly improves biological nitrification and denitrification,and COD_Crdegradation,under low-temperature conditions.The findings here can provide useful references for enhancing the efficacy of wastewater treatment processes in cold seasons in northeast China and other regions.