N₂O Emission Characteristics And Mechanisms During Nitrogen Removal In A Sequencing Batch Biofilm Reactor

The sequencing batch biofilm reactor?SBBR?,expecially the anaerobic/oxic/anoxic SBBR has emerged as a promising process,due to its high nutrient removal efficiency and low energy consumption.However,the nitrous oxide?N₂O?may be produced during biological nitrogen removal via this process.N₂ O is an important greenhouse gas.Therefore,it is very important to study the mechanisms of biological nitrogen removal and characteristics of N₂ O emission from the SBBR process.In this study,a laboratory scale SBBR was used to investigate the emission of N₂ O during nitrification under different DO concentrations.An activated sludge sequencing batch reactor?ASSBR?and SBBR were carried out to study N₂ O emissions during nitritation from ammonium-rich wastewater at two different influent ammonium concentrations.And the dominant microbial community structures related to N₂ O emission were successfully tracked.The characteristics of N₂ O emissions were investigated in an anaerobic/ aerobic/ anoxic SBBR and an anaerobic/aerobic SBBR.And the characteristics of pollutants removal and N₂ O emissions from an anaerobic/aerobic/anoxic SBBR were compared under different influent COD/nitrogen?C/N?ratios?from 1-4?.The main conclusions are as follows:?1?Results showed that a higher DO led to a smaller N₂ O emission?the N₂ O production amount per cycle account for 5.47% of the removed ammonia nitrogen for high DO?2.70±0.11 mg/L?and 4.77% for low DO?1.92 ± 0.14 mg/L??.Meanwhile,under the different DO conditions?1.92±0.14 mg/L,2.34±0.11 mg/L and 2.70±0.11 mg/L?,the low DO operation conditions are favorable for the process of acclimating the simultaneous nitrification and denitrification via nitrite in SBBR.?2?During the stable operation in ASSBR and SBBR,there was no significant difference of NH4+-N removal efficiencies between the two reactors.At the end of nitrification,both nitrite accumulation rates were above 85%.However,SBBR had a better total nitrogen?TN?removal efficiency?81.7 ± 1.5%?than that of ASSBR?66.5 ± 5.7%?.According to typical cycles,when influent NH4+-N concentrations were 120 mg/L and 240 mg/L,the N₂ O emission amount during nitritation in SBBR account for 2.3 ± 0.4% and 6.5 ± 0.9% of the total nitrogen removed,respectively,which were 1/3-1/2 of that in ASSBR?6.6 ± 0.6%,14.1 ± 1.6%?.In addition,the N₂ O production increased with the increase of the feed ammonia concentration in both reactors.High-throughput pyrosequencing analysis revealed that N₂ O was primarily produced by betaproteobacterial microorganisms,mainly,the genera Thaurea,Denitratisoma and Nitrosomonas.?3?An anaerobic/oxic/anoxic SBBR was developed to treat low C/N wastewater without external carbon addition.By controlling the aeration time,simultaneous nitritation and denitritation via nitrite was achieved during oxic period.Under extending anoxic condition,the denitritation using internal carbon source was carried out to improve the removal efficiency of TN.The results showed that the significant high TN removal efficiency?95.97 ± 2.85%?was obtained in the anaerobic/oxic/anoxic SBBR.Compared with the anaerobic/oxic SBBR,the higher TN removal efficiency and less N₂ O emission were achieved in the anaerobic/oxic/anoxic SBBR.?4?The C/N ratios affected the quantity of polyhydroxybutyrate?PHB?and residual organic substances after the anaerobic period.When the C/N ratios increased from 1 to 4,the quantity of PHB and residual organic substances after the anaerobic period were increased with the increased C/N ratios,leading to the quantity of N₂ O production from PHB-driven denitrification and the N₂ O reduction based on residual organic substances-driven denitrification were increased with the increased C/N ratios.Hence,the largest N₂ O emission during aerobic period occurred at a C/N of 2.Moreover,during the anoxic PHB-driven denitrification period,the rapid decline in the dissolved N₂ O concentration indicated that the nitrite inhibition threshold for N₂ O reduction increased with the increased C/N ratios,which means the higher influent C/N ratios could lower the inhibition of nitrite on N₂ O reduction.Finally,more PHB and residual organic substances were provided to denitrification at a high C/N ratio,resulting in less total N₂ O emission was achieved at a high C/N ratio in the anaerobic/oxic/anoxic SBBR.