Study On The Denitrification Characteristics Of Wastewater Enhanced By Microbial Electrochemical Coupling System

In recent years,eutrophication of lake water bodies has been a focus on our attentions,and nitrate is an important cause of eutrophication in water bodies.In order to discharge treated wastewater directly into sensitive lakes,the discharged water should meet the Class III standard of the"Surface Water Environmental Quality Standards"（GB3838-2002）,withρ（NO_x^--N）≤1.0 mg·L^-1.The bioelectrochemical system（BES）has been widely studied due to its advantages in reducing carbon source dosage and inducing the generation of autotrophic denitrifying bacteria,and an applied voltage can also influence denitrification in the BES system.Currently,research on nitrate removal is mostly concentrated on microbial fuel cells（MFCs）,and there is little research on microbial electrolysis cells（MECs）,and the denitrification mechanism of MECs is not yet clear.Therefore,this paper constructed a denitrification MEC system（DN-MEC）by installing electrodes in an anaerobic denitrification system（DN-A）and studied the effects of carbon sources sourced type,COD/N,applied voltage,and other factors of the denitrification efficiency of the DN-MEC system.Scanning electron microscopy and high-throughput sequence technology were used to analyze the sludge in each DN-MEC system,and the enhanced denitrification mechanism of the system under different voltages was further explored,in order to provide technical references and inspiration for the deep denitrification of wastewater treatment plant effluent.The effects of three different external carbon sources（glucose,methanol,and sodium acetate）on the degradation efficiency of a DN-MEC system were investigated by constructing a DN-MEC system with an additional voltage of 0.5 V and a control group（DN-A system）without an external voltage.The systems were run for 66 cycles with six replicating,and the effluent NO_x^--N concentrations were maintained below 1 mg·L^-1.For the control group,the COD/N ratios required to achieve the effluent NO_x^--N concentrations were 11,9,and 7 for glucose,methanol,and sodium acetate carbon sources,respectively,while for the DN-MEC system,the COD/N ratios were 9,7,and 4.5.The results showed that the COD/N ratio required for the lowest nitrogen removal was achieved with sodium acetate as the carbon source.The DN-MEC system required less COD/N and carbon source input than the control group under the same carbon source.Under the optimal carbon source conditions,the effects of different COD/N ratios and external voltages on the deep denitrification of the DN-MEC system were investigated by setting different voltage systems（0.2 V,0.5 V,0.8 V,and 1.1 V）with the no-voltage system as the control group（OCV）.When COD/N was 4.5,the effluentρ（NO_x^--N）of the control group was 4.06 mg·L^-1,while the DN-MEC system effluent met the requirement ofρ（NO_x^--N）≤1.0 mg·L^-1.In the typical cycle with the optimal COD/N ratio,the 0.5 V voltage system had the fastest and the most effective removal rates for NO_x^--N and COD,with an increase of 16.77%and 10.97%compared to the control group,respectively.The sludge in the typical cycle was studied to investigate the denitrification mechanism of the DN-MEC system under different external voltages.Compared with the control group,the microbial diversity of the0 V,0.2 V,0.5 V,0.8 V,and 1.1 V systems all showed varying degrees of increase,with the 0.2V system exhibiting the highest species richness and diversity.At the phylum level,the external voltage systems were similar,with Proteobacteria,Bacteroidetes,and Chloroflexi as the dominant phyla.In the 0.5 V system,the proportion of dominant denitrifying bacterial communities,such as the Proteobacteria phylum,Gammaproteobacteria class,and Thauera genus was high.At the genus level,the electrically active bacteria Flavobacterium,Pseudomonas,andthedenitrifyingbacteria Bacteroidetes＿vadin HA17 and Sulfuritalea showed significant increases in the 0.5 V system,which had strong capabilities for organic pollutant degradation and electron transfer.