Influencing Factors And Accumulation Characteristics Of N₂O And NO In Denitrification Process Of Nitrite

Intermediate products such as nitric oxide（NO）and nitrous oxide（N₂O）are usually produced in biological nitrogen and phosphorus removal of wastewater.NO is an important trace gas in the atmosphere,which directly participates in the oxidant balance.NO is easily oxidized to N₂O.N₂O is also a major greenhouse gas,which will not only destroy the stability of the atmospheric structure,but also endanger the balance of the earth’s ecosystem.In the process of wastewater treatment,nitrite denitrification may also discharge NO and N₂O to the atmosphere,so it has attracted great attention in recent years.In order to understand the influencing factors of nitrite denitrification process and the accumulation characteristics of N₂O and NO,anaerobic/anoxic/aerobic（An/A/O）sequencing batch reactor（SBR）was used to study the accumulation and release characteristics of NO and N₂O in the process of nitrite denitrification and phosphorus accumulation,as well as the effect of different p H values（6.61 and 7.37）on the production of NO and N₂O;The effects of different electron acceptors（NO₃^--N、NO₂^--N or O₂）,different dosing methods（primary dosing,secondary dosing,tertiary dosing）,different influent concentrations（COD90,180mg/L）on denitrifying phosphorus removal and the production of NO and N₂O were studied.By increasing the concentration of ammonia nitrogen（30mg/L）,the cause of ammonia nitrogen loss in the early stage of hypoxia was explored.16Sr RNA high-throughput sequencing was used to analyze the colony structure.The following conclusions are drawn:（1）NO and N₂O accumulated in the process of denitrifying and phosphorus accumulating in the liquid phase,and the accumulated NO inhibited the reduction process of nitrite and NO obviously.The accumulation of NO leads to the increase of DO,and there is a significant correlation between NO and DO.The release factors of NO were 0.86%and0.58%,respectively,and the total N₂O production was 30.65mg/L and 21.24mg/L.NO accumulation may be related to electron accumulation in anaerobic stage,inhibition of Nor activity by free nitrous acid（FNA）and inhibition of NO toxicity.N₂O accumulation is mainly related to inhibition of Nos activity by FNA.High p H value can effectively reduce the production and release of no and N₂O.（2）In the denitrification phosphorus removal system,the absorption of phosphorus is largely affected by the type of electron acceptor.NO₃^--N（phosphorus absorption16.3mg/L）>NO₂^--N（phosphorus absorption 7.3mg/L）>O₂（phosphorus absorption6.1mg/L）.When NO₂^--N was used as electron acceptor,the liquid phase accumulations of NO and N₂O were 0.47mg/L and 38.85mg/L,which were much higher than those of NO₃^--N as electron acceptor.The results showed that the bacteria acclimated with nitrite could also adapt to the accumulation of nitrate,but could not adapt to the accumulation of aerobic phosphorus.（3）The denitrification rate（0.94,0.75,0.5mg/（L·min））and phosphorus absorption rate（0.261,0.192,0.19mg/（L·min））decreased with the increase of the times of adding NO₂^-,which was related to the decrease of intracellular PHA.With the increase of NO₂^-dosing times,the efficiency of denitrifying phosphorus removal also increased,indicating that the toxicity of high concentration of nitrous acid inhibited the denitrifying phosphorus accumulation.In the process of adding NO₂^-by stages,the change of DO concentration decreases with the increase of the times of adding NO₂^-.The change of DO concentration is 0.07mg/l when NO₂^-is added for one time,0.01 and 0.02mg/L when NO₂^-is added for two times,and0.03mg/L when NO₂^-is added for three times.The amount of NO production is closely related to the amount of DO change.When the total amount of NO is the same,adding NO₂^-in sections can effectively reduce the amount of NO production.（4）The concentration of ammonia had NO effect on the removal of COD,NO₂^--N and PO₄^3--P.After running for a period of time under the condition of ammonia nitrogen influent concentration of 30mg/L,the accumulation of NO will decrease and the decrease of ammonia nitrogen concentration will increase during denitrifying phosphorus removal.It is speculated that the anammox bacteria will use NO and ammonia nitrogen to react to generate N₂,and then reduce the production and release of NO and N₂O.（5）Based on 16Sr RNA high-throughput sequencing technology,Proteobacteria and Pseudomonas are the dominant bacteria in the activated sludge microbial community with influent NH₄⁺concentration of 1.88mg/L,which can improve the efficiency of nitrogen and phosphorus removal.The dominant microflora of the activated sludge microbial community with the influent NH₄⁺concentration of 30mg/L is Proteobacteria,and the dominant microflora is Thauera,which is not only conducive to denitrification and phosphorus removal,but also to the transformation of NO₂^--N into N₂.After increasing the concentration of ammonia nitrogen in the influent,the proportion of planctomycates gate increased,and the bacteria with the function of anaerobic ammonia oxidation were the branches under the gate.It was speculated that NO and NH₄⁺could be used as the sole substrate for the growth and metabolism of anaerobic ammonia oxidation bacteria.