Performance Of Nitrate Reduction Driven By Electricity Generated From Anammox Sludge In The Anode

As a new biological nitrogen removal technology,anaerobic ammonium oxidation?Anammox?can remove two kinds of nitrogen pollutants in wastewater simultaneously,ammonium and nitrite.This process does not need to add organic carbon source,only lower oxygen consumption and lower mud production which has a good development prospect.However,the reaction has by-product nitrate,so that the total nitrogen?TN?removal efficiency is only about 70%in actual operation,and the Anammox bacteria have strict requirements on the growth environment,which limits the development and application.Therefore,finding a simple and economical method for removing nitrate by-products in situ and improving the TN removal efficiency of Anammox process has become a central issue in this field.In this study,Anammox was coupled with microbial fuel cell?MFC?technology,Anammox sludge was used as a microbial catalyst in the anode region of MFC to drive electrocatalytic reduction of nitrate in the SnCu-Pd/CFC catalytic cathode.The nitrogen removal,electricity generation performance and community structure of microorganisms at different locations were mainly discussed.In addition,the temperature in the operating conditions was changed to room temperature,and the main substrate concentration ratio of the influent water was extended.Changes of the nitrogen removal and electricity generation performances were analyzed.Contents and conclusions of this research are as follows:?1?The pre-treated carbon fiber cloth?CFC,with good conductivity and sufficient sites?was used as the base material,and Pd,Cu and Sn were respectively deposited to prepare SnCu-Pd/CFC catalytic cathode.The SEM characterization and reduction activity of nitrate by CV test were investigated.The effects of pretreatment method,deposition method and addition of complexing agent on reducing activity were observed.The results showed that CFC soaked in concentrated sulfuric acid solution,electrodeposited and added a complexing agent?1mmol/L EDTA and 4 mmol/L glucose?in electrolyte solution will make the catalyst deposition on the surface of the substrate material more uniform and firmly.The optimal preparation method of the electrode is as follows:the concentrated acid solution in the pretreatment process was concentrated sulfuric acid:concentrated hydrochloric acid=3:1,the deposition mode of Pd was electrodeposition,and the complexing agent was added in the preparation of the entire deposition process solution?1 mmol/L EDTA and 4 mmol/L glucose?.The electrodes prepared under the optimal conditions were characterized by XRD,SEM,EDX and CV test.It was found that the catalyst existed in a cluster form and uniformly attached to the surface of the substrate material.The reduction peak was observed at the position of-0.60 V with a response current of approximately-0.019 A.?2?The SnCu-Pd/CFC prepared under the optimal conditions was used as the cathode,and the Anammox was filled by activated carbon granules in the anode.The Anammox sludge-MFC reactor was constructed and operated for 54 days.High nitrogen removal rate and power generation capability were observed.When the nitrogen loading rate of influent TN was1.74 kg·N/m³·d,the TN removal efficiency was 96.3%,and the nitrogen removal rate can reach1.69 kg·N/m³·d.Compared with the control group of this experiment?open circuit when other conditions are the same?,the TN removal efficiency and nitrogen removal rate rate were increased by 14.9%and 0.30 kg·N/m³·d,respectively.At the same time,due to the effective removal of nitrate in the cathode region,the ratio of produced NO₃^--N and consumed NH₄⁺-N?Rp?in the effluent was-0.27-0.02,which was significantly lower than that of theoretical value0.26.The content of heme c was 0.99?M/mg·pro,which is about double than that of control group.It can be speculated that heme c existed as an electron transporter of the anode microorganism,and its high concentration was more conducive to the transfer of extracellular electrons,thereby realizing electricity production.What's more,the maximum voltage was39.8 mV with the corresponding power density was 21.20±0.05 mW/m³.Microbial community analysis showed that the dominant of anammox bacteria closed to the surface of the anode electrode?carbon felt?was from Candidatus_Kuenenia to Candidatus_Brocadia.?3?Anammox sludge-MFC was operated at room temperature,the main substrate concentration ratio of influent NO₂^--N/NH₄⁺-N increased from 1 to 1.64 gradiently,produing the effect on nitrogen removal and electric generation.When the ratio was slightly lower?1-1.32?,the nitrogen removal effect was better,especially the NH₄⁺-N which was the main fuel in the anode region,and the maximum NH₄⁺-N,NO₂^--N,TN nitrogen removal efficiency and nitrogen removal rate were 94.0%,90.1%,88.7%and 0.54 kg·N/m³·d,respectively.Compared with the control group of this experiment?open circut when other conditions were the same?,TN removal efficiency and nitrogen removal rate were increased by 13.2%and 0.07 kg·N/m³·d,respectively.The maximum output voltage was 28.1 mV,which was higher than of the theoretical value 1.32.When the ratio was slightly higher,the nitrite will compete with the nitrate in the cathode,resulting in a higher nitrate content in the effluent.The maximum output voltage was 21.8 mV,which was close to the theoretical value of 1.32.