Nitrogen And Phosphorus Removal Efficiency And Mechanism In Biochar And Electrochemical-Assisted Constructed Wetland

Constructed wetland（CW）is an ecological treatment technology with low investment cost,easy maintenance and management,which is used various wastewaters treatment in the world.However,the wetland has poor nitrogen and phosphorus removal efficiency when treating low C/N wastewater.In this study,intensified constructed wetland with biochar combined with electrochemistry was established.Firstly,the efficiency of biochar and electrochemistry on nitrogen and phosphorus removal were investigated.Then the effects of different HRT and current density on biochar and electrochemical-assisted vertical subsurface flow constructed wetland system（E-BFCW）were investigated.At the same time,the growth and physiological indexes of the plants,the activity of the substrate microorganisms and the denitrification performance were analyzed.The mechanism of nitrogen and phosphorus removal was deeply analyzed in E-BFCW by high-throughput sequencing,FITR and XPS.The four CWs were constructed with biochar and electrochemistry.Firstly,the performance of biochar and electrochemical enhanced CW for nitrogen and phosphorus removal was investigated.The results showed that biochar significantly increased the removal rate of ammonia nitrogen,nitrate nitrogen and total nitrogen in CWs,but the removal of total phosphorus was reduced,the contribution of biochar to TN removal was 25.07-33.33%.Electrochemistry was not conducive to the removal of ammonia nitrogen,but significantly improved the removal efficiency of the nitrate nitrogen and total nitrogen,the contribution of electrochemistry to TN removal was 27.64-30.16%.The biochar and electrochemical-assisted constructed wetland system（E-BFCW）effluent total nitrogen meet the water quality requirements of surface class IV water（HRT=12 h）.The total phosphorus concentration of the effluent meet the water quality requirements of surface class III water.Both current density and HRT affected the denitrification efficiency of E-BFCW.The current density was 0.0531 mA/cm²,and the removal efficiency of TN was the highest.The removal rate of TN was significantly positively correlated with HRT,and HRT had no significant effect on TP removal.C/N directly affected the removal efficiency of nitrate nitrogen,and low C/N led to a large accumulation of nitrous nitrogen.There are differences in the growth and physiological indexes of the four wetland systems,and both biochar and electrochemistry affect the growth of plants.The enzyme activity in plants was slightly increased by electrochemical enhancement,and the addition of biochar could alleviate the adverse effects of electrochemistry.The MDA content indicated that the electrochemistry does not damage the cell membrane of plant cells.Both biochar and electrochemistry enhanced the activity of substrate microorganisms.The nitrification intensity of the upper part of the four wetland systems was relatively low.The microbial community structure was mainly affected by electrochemistry,and the depth of the substrate layer also affected the microbial community structure.Based on microbial community structure analysis and electrochemical characterization,the mechanism of nitrogen and phosphorus removal was analyzed.The results showed that microbial is the main way of nitrogen removal.Both biochar and electrochemistry increased the abundance of denitrifying bacteria,The cathode region in E-BFCW was enriched with several denitrifying microbial genus,including Hydrogenophag,Simplicispira,Rubrivivax,Proteiniclasticum and Thauera.Among them,Hydrogenophag was a dominant genus and a hydrogen-based autotrophic denitrifying bacterium.There may be multiple denitrification pathways:hydrogenotrophic denitrification,heterotrophic denitrification,Fe（II）-mediated autotrophic denitrification,and ANAMMOX.FTIR analysis showed that the content of P=O bond in biochar increases obviously,and iron ion formed a complex structure with biochar,which was beneficial to phosphate adsorption.The results of XPS showed that the precipitation of Fe₃（PO₄）₂and the adsorption of FeOOH were the main reasons for phosphorus removal.