Performance And Mechanism Of Palm Fiber/Woodchip Enhanced Electrode Biofilm For Nitrate-contaminated Groundwater Treatment

The increasingly serious groundwater nitrate pollution has become an environmental issue of global concern.Biofilm-electrode denitrification process has been widely concerned because of its high nitrogen removal efficiency and easy operation.However,the traditional biofilm electrode process is costly and easily causes secondary pollution.At the same time,the negative effect of anodizing agent on microorganisms also restricts the further improvement of denitrification performance.In this study,palm fiber and woodchip were used as carriers to develop the biofilm electrode reactor,respectively,and the denitrification performance and mechanism were explored.After that,an in-situ bioelectrochemical remediation reactor was constructed to provide technical support for the bioelectrochemical system to treat groundwater nitrate in situ.A palm fiber biofilm electrode reactor（PBER）was developed with CH₃COONa as carbon source and palm fiber as carriers.The sequencing batch experiments showed that the highest activity of nitrate reductase and electron transport system were obtained at 250 m A/m²,and the optimal electrical stimulation promoted the formation of extracellular polymers and the enrichment of hydrogen autotrophic,heterotrophic denitrifying bacteria and electroactive bacteria.The nitrate removal efficiency was 100%at 250 m A/m²,among which the contributions of heterotrophic denitrification,hydrogen autotrophic denitrification,extracellular electron transfer and electrochemical reduction were 60.1%,21.3%,11.2%and 7.4%,respectively.The upflow PBER continuous operation performance demonstrated that when the current density was 250m A/m²,the denitrification performance was the highest at vertical water inlet,C/N 1.5 and HRT8 h,and the nitrate removal efficiency and rate were 97.9%±0.6%and 6.15±0.08 mg-N/（L·h）,respectively,with lower denitrification cost（19.68 yuan/kg NO₃^--N）.An upflow woodchip biofilm electrode reactor（WBER）was constructed with woodchip as carrier embedding bipolar electrodes.When the current density was 350 m A/m2,the nitrate removal rate and cost of the reactor with carbon rod as anode were 2.58 mg NO₃^--N/（L·h）and 14.67yuan/kg NO₃^--N,respectively.The following three aspects contribute to the denitrification performance improvement:i)the electrochemical oxidation effectively destroyed the lignin structure and released more organic matter;ii)the CO₂ generated by carbon rod oxidation could buffer the solution p H;iii)the anode embedding technology alleviated the inhibition of dissolved oxygen and free radicals on microorganisms.The lignin degrading,nitrifying and aerobic denitrifying bacteria enriched at anode and hydrogen autotrophic and heterotrophic denitrifying bacteria enriched at cathode together realized the efficient nitrate removal in WBER.The contributions of heterotrophic denitrification,autotrophic denitrification and electrochemical reduction were 32.1%,62.2%and 5.8%,respectively.In addition,a double cathode in-situ bioelectrochemical remediation reaction process was constructed.And the nitrate removal efficiency and rate for actual groundwater can reach 97.6%±0.4%and 2.09±0.14 mg-N/（L·h）at a temperature of 18.5±0.8℃,a current density of 350m A/m²,and a flow rate of 10 cm/d.Moreover,there was no blockage during the operation.Based on the anode loss estimation and carbon balance,the life span of carbon rod anode was about 3.81years,and the life span of woodchips in front-end cathode components,back-end cathode components and anode components were about 7.14～9.08,18.26～25.42 and 0.92 years,respectively,indicating that woodchip as a solid carbon source could continuously provide organic matter for denitrifying microorganisms.The development of palm fiber/woodchip intensified electrode biofilm technology effectively solves the problems of microbial activity inhibition,high cost and secondary pollution in the traditional electrode biofilm reactors.The results lay the foundation for the application and development of bioelectrochemical technology in groundwater nitrate treatment,and provides new ideas for in-situ remediation of nitrate-contaminated groundwater and effective utilization of waste.