Performance Of Denitrification And Organic Pollutants Removal And Bacterial Community Characteristics In Bioelectrochemical System

Recently,advanced nitrate removal and refractory organic matters degradation has been drawing a growing interest due to upgrading and reconstruction of wastewater treatment plants in China.However,the domestic wastewater of costal cities is characterized as low C/N,high saliniy and poor biodegradability.Based on incorporation of electrochemical and biological processes,two bioelectrochemical systems were set up:one is dicyclic-type electrode based biofilm electrode reactor（BER）;the other one is H-type two-chamber microbial electrolysis cell（MEC）.In this study,the key scientific problems of simultaneous removal of nitrogen and organic matter from wastewater using bioelectrochemical system were investigated.BER was used to explore the performance of denitrification and organic matters removal saline municipal wastewater with low C/N.MEC was used to explore the biotransformation of refractory organic matter under nitrate reducing condition.The molecular biotechnology was used to have a deep insight of the microbial structural characteristics in these two bioelectrochemical systems.The research results are meaningful to promote the development and application of bioelectrochemical system in the field of advanced sewage treatment.A dicyclic-type electrode based biofilm electrode reactor（BER）was developed for simultaneous removal nitrate and organic matters from saline municipal wastewater.The denitrification efficiency was evaluated under carbon to nitrogen ratios（C/N）,hydraulic retention times（HRTs）and electric currents.Compared with convertional biofilm reactor（BR）,BER was more competive for wastewater nitrate removal under low C/N（2.5）and high salinity（1.0%）conditions.The results showed that high nitrate efficiency（≥90.0%）were achieved when the HRT was 8 h and the current was 10 m A.It was found that autotrophic and heterotrophic denitrifying bacteria coexisted in the BER,which was responsible for the higher abundance of denitrifying functional genes.The cooperation between heterotrophic an d autotrophic denitrifiers in the BER system provides a more efficient and feasible solution for nitrate removal from saline municipal wastewater.A dicyclic-type electrode based BER was used to study the response of bacterial community structures and denitrification functional genes under high salinity stress.As the salinity was 2.5%,BER could overcome the impact of high salinity and maintained a relatively stable denitrification performance,the effluent nitrate was lower than 1.5mg/L.High salinity high salinity（>2.5%）impoverished microbial diversity and altered the microbial community in both BR and BER.However,two denitrifiers Methylophaga and Methyloexplanations were enriched in BER due to the under the electrochemical stimulation,and their relative abundance were 10 times compared to BR.These two bacteria Methylophaga and Methyloexplanations can tolerate high salinity（>3.0%）and join the denitrification process.The abundance and proportion of denitrifying functional genes of the q PCR results confirmed that main denitrifiers shift to salt-tolerant species（nir K-type denitrifiers）to reduce the toxic effects.The abundance of nap A and nos Z genes in BER was much higher than BR’s,which proved that BER had greater denitrification potential under high salinity stress.In order to further improve the denitrification efficiency of the BER,conductive polymer polypyrrole（PPy）and electronic shuttle anthraquinone-2,6-disulfonate disodium（AQDS）were used to modify graphite felt.The modified electrode was applied to enhance the denitrification process of BER.In the application of PPy/AQDS modified electrode,the BER gained a better denitrification performance.The HRT was shortened to 4 h,and the removal rate of NO₃^--N was high than 95.0%.It is proved that PPy/AQDS modified electrode had better biocompatibility.The attached biomass of PPy/AQDS cathode had an obvious increase,and mass ratio of biofilm/electrode was0.32.The immobilized AQDS promote the enrichment of specific microorganisms effectively,such as Thauera,Rhodobacter and Paracoccus.With the applicaation of AQDS,the relative abundance of Thauera was two times to the only PPy modified electrode.In order to further reduce electrical energy consumption,a H-type two-chamber MEC was developed to remove nitrate and phenolic compounds simultaneously.Two different culture were innoculated to the MEC,the electrochemical performance,H₂yield and 4-hydroxybenzoic acid（HBA）biotransformation were investigated under nitrate reducing condition.The results showed that:in the absence of nitrate,HBA converted to phenol;in the presence of nitrate,HBA achieved complete degradation of benzene ring through anaerobic metabolism and bezoy-coa pathway.Analysis of high throughput sequencing found that a denitrifying bacterium-Magnetospirillum（relative abundance>30%）was enriched in the MEC-2 bioanode under nitrate reducing condition.Magnetospirillum is able to biodegrade a variety of aromatic compounds through benzoyl-co A metabolic pathway.MEC-2 showed good electrochemical performance and H₂ yield,and analysis of electron balance showed that more than 5.8%electron equivalent of the HBA converted to current.