Selective Enrichment Of Complete Ammonia Oxidizing Bacteria Under Low Dissolved Oxygen

Energy saving and consumption reduction of sewage treatment system is of great significance to global carbon emission reduction.Nitrification is the core part of the biological nitrogen removal process.In order to ensure the smooth progress of nitrification,the sewage treatment process needs continuous aeration to provide oxygen,and the energy consumption of the aeration system can account for 45-75%of the total energy consumption of the sewage facility,which brings huge carbon emissions and high operating costs.Traditional biological nitrogen removal technology based on microbial nitrification and denitrification has always been the core link in the sewage treatment system.The traditional nitrification theory has always believed that the two steps of nitrification are assisted by traditional ammonia oxidizing microorganisms(AOM)and nitrite oxidizing bacteria(NOB).In 2015,a microorganism was successfully isolated and cultured.This kind of microorganism is called complete ammonia oxidizing microorganism(comammox or CAOB).The discovery of Comammox proved the existence of single-step nitrification theory,and became another revolutionary breakthrough in the history of nitrogen cycle development.Through phylogenetic analysis,it was found that the only isolated CAOB strain N.inopinata so far does not belong to the same branch as the main CAOB strain Candidatus Nitropsira nitrosa in the sewage treatment system.Its ecological undifferentiated mechanism and kinetic mechanism need further study,and the enrichment of the main strains in this sewage system is an important prerequisite for further research.In this study,in order to explore how to further selectively enrich the strain of N.nitrosa,we studied its niche under different oxygen concentrations and the possibility of using nitrification inhibitors to achieve selective enrichment,and discussed the reasons why it is difficult to achieve high abundance.The activated sludge in sewage treatment plant was cultured for 4 months under the condition of oxygen concentration of 0%,2%,5%,10%,20%,40%and 70%.Quantitative PCR analysis showed that CAOB dominated under hypoxic conditions,while the diversity and growth of ammonia-oxidizing archaea(AOA)and ammoniaoxidizing bacteria(AOB)were affected,indicating that CAOB played a major role under hypoxic conditions.In addition,phylogenetic and Pearson correlation analysis showed that Nitrospira nitrosa was the main advantage,while Nitrospira nitrifican had the possibility of growth under high oxygen concentration.1Octyne has been widely used as a specific inhibitor to distinguish the contribution of AOA and AOB by inhibiting the activity of AOB.1-Octyne can effectively inhibit the activity of AOB by inhibiting ammonia monooxygenase(AMO).Due to the unique structure of AMO,1-octyne has little inhibitory effect on AOA,which also reflects that AMO is a functional enzyme.Through 150 days of culture,the behavior and performance of AOB,AOA and CAOB at 1-octyne concentrations of 0 mg/L,20 mg/L,50 mg/L,100 mg/L,200 mg/L and 400 mg/L were comprehensively evaluated.Quantitative PCR analysis showed that when the concentration of 1-octyne was higher than 100 mg/L,CAOB became dominant after 3 months of operation(the highest relative abundance reached 98.90%).Phylogenetic analysis showed that N.nitrosa dominated CAOB(relative abundance>95%)at all 1-octyne concentrations,while the growth of AOB was inhibited.In addition,after stopping the addition of 1-octyne,reversible inhibition of AOA was observed under high 1-octyne conditions.After dilution transfer enrichment,the abundance of CAOB reached 70.27%,and the relative abundance of nitrosa reached 100%.Our study demonstrates the longterm effect of 1-octyne concentration on the niche differentiation of ammoniaoxidizing microorganisms and emphasizes the importance of selective enrichment of CAOB under high 1-octyne conditions.These findings pave the way for the use of 1-octyne as a nitrification inhibitor for selective enrichment,and for exploring the prospects of optimizing community structure and saving energy consumption in wastewater treatment.