Exploration Of Nitrate Metabolism Pathway And Influencing Factors

Nitrate is one of the common pollutants in sewage treatment,which is often treated by biological denitrification method in sewage treatment plants.In the biological denitrification process,the nitrate conversion pathways are mainly divided into denitrification(DNF),dissimilatory nitrate reduction to ammonium(DNRA)and assimilatory nitrate reduction(AN).Among them,denitrification is often the most ideal nitrate removal method,but intermediate products such as NO and N2O released during this process can cause pollution to water,soil,and atmospheric environment.The increasingly environmental issues require us to explore more environmentally friendly nitrate removal pathways.Due to their advantages in reducing nitrogen pollution and reducing nitrogen loss by recovering and recycling nitrogen while removing nitrogen from wastewater,DNRA and AN have attracted widespread attention.According to research reports,the nitrate conversion pathways in the aquatic ecosystem are affected by many factors,such as carbon nitrogen ratio,salinity,and nitrogen source type.However,this study focuses on the individual and systematic regulatory effects of carbon nitrogen ratio and salinity on the nitrate conversion pathways and microbial community in activated sludge.In view of the above situation,this study has constructed four sequencing batch reactor(SBR)systems with stable nitrogen removal function using denitrification activated sludge.The influent COD/N values were set to 3,5,7.5,and 10,respectively(denoted as R1,R2,R3,and R4),and the influent salinity was gradually increased through salinity domestication.In other words,Four systems simultaneously increase salinity from 0%to 3%with a gradient of 0.5%(w/w),named stages Ⅰ-Ⅶ.By monitoring reactor performance and analyzing microbial community composition,the denitrification performance,nitrogen conversion mechanism,and changes in microbial community structure of the four systems under salinity control were explored.The main achievements of this study are as follows:(1)Durin’g the increase in salinity(0-3%),the denitrification performance of the four SBR systems underwent significant changes.After more than two hundred days of continuous operation,the nitrate removal rates in the four systems decreased from 67.64%,100%,100%,and 100%in Phase Ⅰ to 9.44%,9.79%,15.31%,and 8.96%in Phase Ⅶ.There was nitrite accumulation in the whole process of R1 system,and it was speculated that heterotrophic denitrification process is dominant under the condition of low organic carbon.When the salinity in the R2,R3,and R4 systems is below 1.5%,nitrate nitrogen was completely removed and there was no accumulation of nitrite nitrogen.When the salinity rises to 1.5%or above,the nitrate removal rate significantly decreased,but nitrite has not yet been accumulated.The reduction of nitrate in the system may mainly rely on the process of assimilation of nitrate.Throughout the salinity increase phase,extremely low ammonia nitrogen concentrations were maintained in each system,indicating no significant DNRA process.(2)The research results of nitrogen transformation mechanism in each stage of salinity showed that the nitrate removal rate decreased with the increase of salinity,and nitrite nitrogen accumulated in different degrees in seven stages when COD/N is 3,which indicated that denitrification process was more obvious in the environment of low COD/N.Under sufficient COD/N conditions(COD/N≥5),the accumulation of nitrite nitrogen disappeared within the salinity range of 1.5-2.5%.At a salinity of 3%,nitrite nitrogen accumulated in small amounts at a concentration of 0.2mg/L.But ammonia nitrogen maintained a low concentration throughout the entire process.The overall results were consistent with the long-term data of the reactor.The proportion of nitrate reduction pathways in four systems under different salinity was estimated through batch experiments.The results showed that compared to other influent environments,in the stage of carbon nitrogen ratio of 10 and salinity of 1.5-2.5%,assimilation nitrate reduction accounted for a relatively large proportion,which were 71.13%,56.02%,and 68.20%,respectively.Therefore,changes in salinity have a certain impact on the selection of nitrate reduction pathways.The comprehensive consideration of the synergistic regulation of carbon nitrogen ratio and salinity indicated that high carbon and high salt conditions are more conducive to the selection of nitrate assimilation pathways.(3)The increase in salinity caused significant differences in the bacterial community structure of activated sludge.High salinity enriched various marine halophilic bacteria with poor nitrogen removal efficiency,such as Hypomonas,Oceanicaulis,and Phenonobacterium,while Lentimicrobium,Thauera,Terrimonas,and other bacterial genera with denitrification function under initial conditions were all affected in high salt environment.OLB13 was mainly responsible for the accumulation of nitrite and was greatly affected by changes in COD/N.Pseudomonas with nitrogen assimilation function was enriched when salinity is higher than 2%,which may be responsible for driving nitrate assimilation pathway under high salinity.(4)The increase in salinity affects the abundance of nitrate metabolism functional genes.High salinity had a negative impact on denitrification functional genes narG/nxr A,napA,nirK,norB,norC and assimilation nitrate functional genes nasB,nirA,while higher organic carbon concentration was conducive to the enrichment of denitrification functional genes nar A,nirK,norB,norC and dissimilatory nitrate reduction functional genes nirB,nirD.That is,high organic carbon is conducive to driving the DNRA process.The relative abundance of the assimilated nitrate functional gene nasA was relatively high in the range of 1-3%salinity,while the relative abundance of the gene nirA reached its highest value at a COD/N of 10 and a salinity of 1.5%.This once again confirmed that the assimilated nitrate reduction process may be more likely to occur under high salt and organic carbon conditions.To sum up,this study explored the impact of different influent conditions on the nitrogen removal performance of denitrification activated sludge,clarified the impact of COD/N and salinity on nitrate conversion paths,and also provided a theoretical basis for the regulation direction of nitrate conversion path.It may have broad application value in future innovation of sewage treatment processes and nitrogen circulation.