Mechanisms Of The Enhanced Ammonia Nitrogen-sulfate Wastewater Treatment By Microbial Electrochemical Regulation

Ammonia nitrogen（NH₄⁺-N）and sulfate(SO₄^2-)wastewater,as important by-products in the industrial process,will do serious harm to the natural environment and human health if they enter the river directly without effective treatment.When traditional physical/chemical methods are used to treat NH₄⁺-N and SO₄^2-wastewater,a large number of chemical agents are often added,and the treatment energy consumption is high.As a mainstream technology for treating wastewater containing NH₄⁺-N and SO₄^2-,biological treatment requires introducing H₂ or organic carbon sources as electron donors for Sulfate-reducing bacteria（SRB）in the process of heterotrophic sulfate reduction desulfurization.The addition of carbon sources also leads to the competition between Methanogens and SRB,which affects sulfur reduction metabolism.Similarly,traditional biological nitrogen removal often requires aeration and adding carbon sources,leading to the emission of greenhouse gases such as N₂O and CO₂.In contrast,Anaerobic ammonium oxidation（Anammox）technology is widely used for NH₄⁺-N wastewater treatment because of its low cost,high nitrogen removal efficiency,and no need to add carbon source.However,the Anammox process is limited by the ratio of NH₄⁺-N and NO₂^--N in wastewater.This ideal condition is quite different from the actual NH₄⁺-N wastewater,which seriously restricts its engineering application.Anaerobic ammonia oxidation bacteria（AnAOB）and SRB can accumulate biofilms on the surface of the electrode and have the function of extracellular electron transfer.This provides a new idea to solve the problem that Anammox is limited by NH₄⁺-N/NO₂^--N ratio and sulfur reduction adds carbon source resulting in competition between MA and SRB,and realizing synchronous biological nitrogen and sulfur removal.The purpose of this paper is to explore the feasibility of electrocatalytic induction of electroactive AnAOB and SRB biofilm enrichment,regulation of extracellular electron transfer in the process of Anammox and sulfate reduction,elucidate the succession rule and response mechanism of the microbial community at the electrode interface under electrical induction,and achieve efficient nitrogen and sulfur removal of wastewater containing NH₄⁺-N and SO₄^2-.（1）Explore the impact of external electrical potential on the enrichment and biofilm formation of electroactive AnAOB and SRB,and evaluate the feasibility of enhancing Anammox coupling sulfate reduction through bio-electrocatalysis.The characteristics of Anammox anode denitrification and SRB cathode denitrification under electrocatalytic control were investigated by constructing a microbial electrolytic cell reactor.The results showed that the removal effect of NH₄⁺-N was improved with the stimulation of 0.2V low potential.With the increase of potential to 0.4V,abundant biofilms were enriched on the surface of carbon felt electrode.Tryptophan,humus,and other EPS with enhanced electron transfer function increased fluorescence intensity;Microbial analysis showed an abundance of Ca.Brocadia increased to 27.4%at 0.2 V,and SBR1031 increased from 13.0%to 35.6%.While at 0.4 V,the abundance Ca.Jettenia and Ca.Kuenenia decreased,and Anammox metabolism was affected.The abundance of Ca.Brocadia and f＿A4b reached the highest level,and abundance increased to 27.4%and 5.4%from 1.1%and 0.5%under the initial condition.These results indicate that appropriate electrical stimulation can enrich electroactive AnAOB.When the voltage was further increase to 0.6 V,the removal efficiency of SO₄^2-reached77.0%,and biofilms were enriched by filamentous bacteria on the carbon-felt electrode surface.With the increase of external electric potential,MA growth and metabolism are inhibited,which is conducive to the growth of electroactive SRB.When the electrical potential exceeded 0.6 V,the electron transfer was performed on the carbon-felt electrode by nano-filamentous bacteria,resulting in stable SO₄^2-removal efficiency.The results show that electrical stimulation can promote the enrichment of AnAOB and SRB,and enhance Anammox and sulfate reduction.（2）Develop an upflow microbial electrochemical simultaneous denitrification and sulfur removal reactor（UMER）,and the strategies of electroactive AnAOB and SRB enrichment and electron transfer enhancement at the biofilm interface were proposed to reveal the mechanism of bio-electrochemical regulation to enhance the simultaneous denitrification and sulfur removal of ammonia nitrogen and sulfate wastewater.The results showed that the removal rate of NH₄⁺-N at the anode was maintained at 99.9%when the hydraulic retention time（HRT）of the cathode was controlled at 12 h and the HRT of the anode water inlet was 4 h under continuous stimulation with an external potential of 0.2 V.The formation of a dense biofilm on the surface of the carbon-felt electrode enhanced the EET and H⁺transfer across the PEM to the cathode.The analysis of microbial community structure showed that SRBs such as Desulfobacterota,Desulfomonile,Syntrophobacter,and Desulfatiglans gradually became dominant populations.After stabilization,the effluent concentration of SO₄^2-was 33.4 mg/L,and the removal efficiency reached 77.8%.The median particle size of the sludge increased from 38μm to 60μm.In addition,at the anode,the abundance of Ca.Kuenenia and Ca.Jettenia decreased due to high influent nitrogen load,while Ca.Brocadia increased to 8.4%on the surface of carbon-felt electrodes due to its ability to use insoluble carbon-based materials as electron transfer mediators and gradually became the dominant population of the community.Therefore,bio-electrocatalysis induction can enrich the reactive AnAOB and SRB,enhance the efficiency of EET and H⁺transport,and support UMER engineering applications.（3）Explore the biofilm formation mechanisms of electrochemically enriched AnAOB and SRB in UMER,the changes of AnAOB enzyme activity and the succession of the microbial community,and reveal the mechanism of UMER enhancing nitrogen and sulfur removal in the low NO₂^--N condition.With constructed UMER as the core technology,the influence of influent NH₄⁺-N/NO₂^--N ratio on the nitrogen removal efficiency of Anammox was investigated under constant Total nitrogen（TN）concentration.The results showed that the removal efficiency of NH₄⁺-N could be recovered to 78.8%when the ratio of NH₄⁺-N/NO₂^--N was 1:0.72.However,when the condition is 1:0.52,the potential difference between the two poles decreases,and the NH₄⁺-N removal rate continues to decline.In addition,high NH₄⁺-N concentration led to a decrease in the abundance of Ca.Kuenenia and inhibited the expression of Ca.Kuenenia and NB1-j nitrite reductase-related genes.Ca.Brocadia showed a strong niche preference for NH₄⁺-N on biofilms,and the NO₂^--N reaction with NH₄⁺-N was enhanced.In the low NO₂^--N condition,the decreased H⁺transfer efficiency led to the failure of heterotrophic SRB（HSRB）such as Thiobacillus and Desulfovibrio to adapt to environmental changes.At the same time,Acetobacterium gradually became dominant in the process of competing with SRB for electrons in the system.The SO₄^2-removal rate was reduced to 57.0%.With the increase of Acetobacterium abundance,Desulfobacterota,and Synergistota could maintain sulfur reduction by transferring electrons through carbon-felt electrodes.These results indicate that electrocatalysis can induce electrochemically active bacteria to enrich biofilm on carbon-felt electrodes and enhance the effect of EET under low NO₂^--N conditions,and achieve a better simultaneous nitrogen and sulfur removal level of UMER,which provides technical and engineering guidance for ammonia nitrogen sulfate wastewater treatment.