Nitrogen Removal Capacity And Pathways Of The Simultaneous Nitrification And Denitrification Process Under Different Oxygen Conditions

To deal with the low nitrogen removal efficiency,unclear microbial distribution,coupling and interaction mechanisms and lack of stable operation parameters in wastewater treatment via simultaneous nitrification and denitrification(SND),the nitrogen removal efficiency and pathways of the aerobic SND reactor under the condition of 2 mg/L dissolved oxygen(DO)concentration were researched.The functions of heterotrophic nitrification and aerobic denitrification were explored;Based on the results,the nitrogen removal efficiency and pathways of the low-oxygen SND reactor were researched when the DO concentration was 0.2?0.3 mg/L.The possibility of the existence of SND under low oxygen conditions was explored,and the microbial community and microorganisms involved in nitrogen removal were deeply discussed;To make full use of the oxygen and organic carbons,the SND reactor with limited oxygen(the DO concentration was below the determination limit of 0.003 mg/L)was built.The influence of hydraulic retention time(HRT)and carbon to nitrogen ratio(C/N)on the nitrogen removal ability and the change patterns of the microbial community was investigated and the key control parameters were obtained.The main research results were as follows:(1)The nitrogen removal capacity and pathways of the aerobic SND were revealed and heterotrophic nitrification,aerobic denitrification,dissimilation nitrate reduction and nitrogen fixation were found to be important pathways except for the autotrophic nitrification and anoxic denitrification.When the influent NH₄⁺concentration was 100mg/L with a C/N ratio of 3.8,aerobic SND was achieved in the sequencing batch biofilm reactor(SBBR).The total nitrogen removal efficiency reached 73.2%with the effluent NO₂^-and NO₃^-concentration lower than 0.05 and 0.21 mg/L.The SND rate reached 99.8%;When NH₄⁺was the sole nitrogen source and the autotrophic nitrification and anoxic denitrification process were inhibited,the NH₄⁺removal efficiency reached 40.3%,and the concentration of NO_x^-was 2.02 mg/L.When inhibitors were not added,the NH₄⁺removal efficiency reached 72.4%,and the concentration of NO_x^-was 0.01 mg/L.Traditional autotrophic nitrification and anoxic denitrification removed 44.6%of the removed total nitrogen,meanwhile,heterotrophic nitrification and aerobic denitrification were also nitrogen removal pathways;The most abundant ammonia-oxidizing microorganism was aerobic complete ammonia-oxidizing microorganism(0.67%)and the abundance of the second abundant ammonia-oxidizing microorganism was 0.11%.The abundance of the complete ammonia oxidation module was 95.6%of the ammonia oxidation module.These results indicated that the nitrogen removal pathway in the aerobic area of the biofilm was complete ammonia oxidation.The abundance of enzymes Nir B and Nif involved in dissimilation nitrate reduction and nitrogen fixation was higher than other enzymes except for the nitrate reductase which indicated that the denitrification,nitrogen fixation and dissimilation nitrate reduction occurred in the anoxic part.(2)The nitrogen removal capacity and pathways of the low-oxygen SND were revealed and the main microorganisms involved in the nitrification were complete ammonia-oxidizing ones.A sequencing batch reactor(SBR)with continuous inflow and a continuous reactor(CR)with biofilm were successfully built to perform low-oxygen SND.When the inflow NH₄⁺concentration was 50 mg/L,the DO concentration,NH₄⁺removal efficiency and the SND rate were 0.2?0.3 mg/L,89%?90%and 50%,respectively;The nxr B genes in SBR and CR were similar.80%of them were derived from the complete ammonia-oxidizing microorganism Nitrospira nitrosa and 20%from the strict nitrite-oxidizing microorganism Candidatus Nitrospira defluvii.These results indicated the main nitrification pathway was the full process of nitrification;Anaerobic denitrifiers and aerobic denitrifiers were found in both reactors.But the abundance of the aerobic denitrifiers,Zoogloea(34.65%)and Burkholderiaceae(14.14%),were higher in the SBR while the anaerobic denitrifiers,OLB8(24.10%),Saprospiraceae(11.94%)and Denitratisoma(5.40%),were more abundant in the CR.These results indicated that the denitrification pathway was anoxic denitrification and aerobic denitrification.Anoxic denitrification was more important in the biofilm.(3)The nitrogen removal capacity and pathways of the SND under limited oxygen conditions were revealed and the nitrogen removal capacity and key running parameters of the process treating low C/N ratio wastewater were obtained.The SND under limited oxygen was achieved in SBR.When the influent NH₄⁺concentration was 50.0 mg/L and the air amount was 2.7 m³ per cubic meter wastewater,the DO concentration was at 0mg/L.The effluent NH₄⁺concentration was lower than 2.0 mg/L.The SND rate reached85%;When HRT decreased from 24 h to 20 h,the nitrification capacity increased by 5times and the denitrification capacity improved 1 time without NO₂^-accumulation.When C/N increased from 2.2 to 8,the nitrification capacity dropped 5.9 times and the denitrification capacity improved 1.1 times,however,the NO₂^-accumulated.The optimal HRT and C/N in the SND reactor with limited oxygen were recommended to be20 h and 2.2;When the HRT dropped from 24 h to 20 h,the abundance of the autotrophic nitrifiers in the reactor increased from 0.09%to 0.73%.The abundance of the heterotrophic nitrifiers increased from 5.42%to 2.95%.The abundance of anoxic denitrifiers increased from 41.46%to 62.01%.The abundance of aerobic denitrifiers increased from 4.63%to 8.57%.When the C/N ratio was increased from 2.2 to 8,the abundance of autotrophic nitrifiers dropped to 0.50%.The abundance of heterotrophic nitrifiers increased to 31.88%.The abundance of anoxic denitrifiers decreased to23.34%,The abundance of aerobic denitrifiers increased to 38.91%.These results indicated that the nitrogen removal pathways were heterotrophic nitrification,autotrophic nitrification,aerobic denitrification and anoxic denitrification.Short HRT was beneficial for autotrophic nitrification,anoxic and aerobic denitrification.High C/N was beneficial for heterotrophic nitrification and aerobic denitrification.The nitrogen removal efficiencies and pathways of the aerobic?low oxygen and limited oxygen SND reactors were revealed.The key control parameters were achieved.The microbial SND nitrogen removal theory in municipal wastewater treatment was enriched.The ways to save the energy consumption during the nitrogen removal process in the SND reactors were proposed.These results provided technical support for the future engineering application of this technology.