Nitrification,N₂O Emission And Comammox In Weakly Acidic And Low Dissolved Oxygen Activated Sludge

In the nitrification reactor,it is necessary to maintain sufficient dissolved oxygen（DO）and suitable alkalinity conditions with the aim of efficient nitrification perfoemance.Therefore,a large amount of aeration energy consumption is consumed,and additional alkalinity is needed to buffer the proton releases by nitrification for wastewaters with a low alkalinity.If the DO in the nitrification reactor is reduced from the traditional≥2 mg/L to≤0.5 mg/L,the aeration energy consumption can be saved by about 30%.However,while low DO or low p H affects the nitrification reaction kinetics,it also affects the emission of the strong greenhouse gas N₂O.The comammox can oxidize ammonia nitrogen and nitrite.It has strong ammonia affinity and can adapt to low-oxygen environments.It also has the potential to reduce N₂O emissions,so it helps to improve the nitrification efficiency under adverse factors and regulated the production of greenhouse gases.In this study,a high DO acidic（DO>2 mg/L;p H=6.3-6.8）and a low DO acidic（DO<0.5 mg/L;p H=6.3-6.8）wastewater nitrification bioreactors were operated to treat simulating domestic wastewater,studying the nitrification performance and N₂O emission charact-eristics under low DO concentration and low p H conditions;using molecular biology methods and high-throughput sequencing to analyze comammox and other nitrifying bacteria under different working conditions.The abundance and community structure were explored and the effects of comammox on the nitrification and N₂O release of the nitrification reactor under weak acid and low oxygen were also studied.The main conclusions of this research are as follows:（1）With the influent NH₄⁺-N concentration of 48 mg/L,the effluent NH₄⁺-N,NO₂^--N,and NO₃^--N of the high DO acidic reactor are 0.06±0.09 mg/L,0.04±0.02 mg/L and 44.2±2.3 mg/L,respectively;those of the low DO acidic reactor are 0.64±0.86 mg/L,0.44±0.46 mg/L and 42.8±2.8 mg/L,respectively,indicating that the nitration reaction can be carried out efficiently under acidic or even low-oxygen acidic conditions.（2）The N₂O emission factor of the high DO acidic reactor and the low DO acidic reactor are 0.15±0.08%and 0.12±0.08%,respectively,which were lower than those in the general aerobic nitrification process（0.27%）,indicating that long-term weak acidic operation will not lead to a large amount of emissions of the strong greenhouse gas N₂O.The emission factors of N₂O are not significantly related to the concentration of DO and NO2--N in the reactor,indicating that the production of N₂O is probably not produced by nitrifier denitrification pathway.By measuring the SP value of N₂O,we also found that the contributions of nitrifier denitrification to N₂O in the two reactors were not more than 30%,which were lower than the reference values of the same type of process.This may be one of the reasons for the low N₂O emissions in the long-term weakly acidic nitrification system Compared with the high-oxygen condition,the N₂O emission factor of long-term low DO operation still has a slight drop（p=0.171）,indicating that the low-oxygen weak acid nitration reaction has the advantage of energy saving and emission reduction.（3）Using molecular biology methods and high-throughput methods,we found that comammox can adapt to acidic environments.Based on real-time fluorescent quantitative PCR,we found that after long-term operation,comammox has achieved a significant advantage in the competition with canonical AOB.The ratio of comammox to canonical AOB under high oxygen conditions is 6.6,while low oxygen conditions were conducive to the competition between comammox and traditional AOB,resulting in the ratio of the comammox to canonical AOB at 17.1.Based on amplicon sequencing and metagenomic sequencing,we also found that comammox related to Candidatus Nitrospira nitrosa was more suitable for low DO conditions,while those related to Nitrospira sp.SG-bin1comammox was more suitable for high DO conditions.（4）Using PTIO as a selective inhibitor of comammox,we found that comammox has strong acid resistance and can achieve efficient ammonia oxidation in an environment of p H=4.3.Through high-throughput sequencing of c DNA in a stable operating acidic nitration reactor,we further confirmed the activity of comammox under acidic conditions.