Enhanced Nitrogen Removal From Black-Odorous Water Via Aerobic Denitrifiers And Iron-Carbon Micro-Electrolysis Carriers:Performance And Microbial Mechanism

Serious nitrogen pollution is usually present in black-odorous water bodies.And excess nitrogen can not only deteriorate water quality,but also pose a potential threat to the ecological environment of the water body.Currently,aeration is commonly used for nitrogen removal in black-odorous water bodies.Although aeration can effectively convert ammonium nitrogen（NH₄⁺-N）,it tends to create an oxygen-rich environment,where the heterotrophic microorganisms in the water body will also consume a large amount of organic matter.Under such conditions,the denitrification can often become a challenge,mainly constrained by both excess oxygen and limited carbon sources.Therefore,there is a certain contradiction between ammonium removal and total nitrogen（TN）removal from black-odorous water bodies,which is one of the key issues in the remediation of nitrogen-polluted water bodies.Based on the analysis of nitrogen removal bottlenecks,a study was conducted to enhance the TN removal from black-odorous water via iron-carbon micro-electrolysis（IC-ME）carriers and aerobic denitrifiers.According to the characteristics of the investigated nitrogen-rich black-odorous water bodies and the current status of nitrogen removal,aerobic denitrifying bacteria Pseudomonas ATCC 9027 and IC-ME carriers were used to establish single and coupled types of remediation systems for black-odorous water.The nitrogen removal performance among these systems were investigated and compared,and the potential mechanism of enhanced nitrogen removal assisted by Pseudomonas ATCC 9027 and IC-ME carriers was revealed.The main contents were as follows:（1）The characteristics of black-odorous water bodies and the status of nitrogen removal were analyzed.The result showed that these water bodies were in a state of anoxic conditions,with low redox potential and high nitrogen pollution.Proteobacteria andδ-Proteobacteria were the main sediment bacterial phylum and class,respectively.Environmental factors significantly influenced sediment community composition.Sediment microbial communities had the potential to metabolize pollutants.Denitrifying enzymes were also detected in these sediments.Further investigation was conducted for aerated black-odorous water,and NH₄⁺-N was found to be effectively converted.However,aerated black-odorous water bodies were characterized by high dissolved oxygen concentrations and low carbon to nitrogen ratios,and nitrate（NO₃^--N）and TN removal in these water bodies needed to be improved.This phenomenon suggests that aerated black-odorous water bodies indeed suffer from oxygen suppression and insufficient electron donors in terms of denitrification.（2）The effect of IC-ME carriers on denitrification was explored.Results showed that IC-ME carriers can provide electrons for denitrification.At the same time,IC-ME carriers increased the electron transfer activity of the sediment microbial system（ETSA）,enriched denitrification genes and enhanced nitrate reductase（NAR）,nitrite reductase（NIR）,nitric oxide reductase（NOR）and nitrous oxide reductase（NOS）activities.After the denitrification process was strengthened,the TN removal performance of black-odorous water was improved.These results show that IC-ME carriers can improve the denitrification ability of microorganisms in terms of electron production,transport and consumption,thereby enhancing the TN removal of black odorous water bodies.（3）The addition of the exogenous aerobic denitrifier Pseudomonas ATCC 9027also facilitated the process of TN removal by not only allowing NO₃^--N to be reduced under aerobic conditions,but also by driving the succession of indigenous microbial communities.It was found that adding Pseudomonas ATCC 9027 enriched indigenous anoxic and aerobic denitrifiers and increased the cumulative abundance of denitrifiers.Besides,the addition of Pseudomonas ATCC 9027 changed the bacterial co-occurrence network,not only by enhancing the numbers of core genera with denitrifying properties,but also by changing the composition of core genera.Thus,exogenous aerobic denitrifying bacteria can promote the succession of indigenous microbial communities in a direction favorable to denitrification.（4）Pseudomonas ATCC 9027 and IC-ME carriers were found to have no significant effect on the nitrification process.While NO₃^--N and TN concentrations in the overlying water were reduced by 6.34 and 7.78 mg/L,respectively,with the synergistic effect of Pseudomonas ATCC 9027 and IC-ME carriers.Therefore,the promotion of TN removal in the overlying water was mainly attributed to the enhancement of denitrification processes.Moreover,the coupled system achieved the best nitrogen removal performance,whose TN concentrations in the overlying water and sediment were lower than all other groups.For nitrification,aeration can completely oxidize NH₄⁺-N to NO3^--N.For the denitrification process,Pseudomonas ATCC 9027 and IC-ME carriers together improved the reduction of NO₃^--N.Briefly,adding aerobic denitrifying bacteria Pseudomonas ATCC 9027 can allow NO₃^--N to be reduced under aerobic conditions and improve the performance of indigenous denitrifiers（including anoxic and aerobic denitrifying members）,solving denitrification inhibition caused by oxygen.Adding IC-ME carriers can provide electrons for denitrification,which addressed the shortage problem of electron donors.Simultaneously,the electron transport activity of the sediment microbial system was boosted（0.07μg O₂/（g protein·min））.With the synergistic effect of Pseudomonas ATCC 9027 and IC-ME carriers,enzymes and genes associated with denitrification were both upregulated.Specifically,the improvement for activities of nitrate reductase（NAR）,nitrite reductase（NIR）,nitric oxide reductase（NOR）,nitrous oxide reductase（NOS）was 0.93,3.74,0.18 and 0.09μmol N/（g·d）,respectively,and the cumulative abundance of denitrification genes showed an increase of 1.48%.These findings reveal the microbial mechanism of aerobic denitrifiers and IC-ME carriers in improving nitrogen removal from the aerated black-odorous water bodies,providing a theoretical basis for in situ remediation of nitrogen-polluted water bodies.