Study On Nitrate Removal From Groundwater By Electrode Biofilm

The nitrate pollution in groundwater is becoming more and more serious.In order to overcome the disadvantages of traditional biological denitrification,such as poor denitrification performance and secondary pollution,a new electrode biofilm process with simple operation,high denitrification performance and less secondary pollution was studied.Biofilm electrode reactor（BER）is a method of nitrogen removal using a combination of hydrogen autotrophic denitrification and electrolytic hydrogen production,which has the characteristics of no organic carbon source and less secondary pollution.In this study,an electrode biofilm denitrification reactor was constructed and the start-up was investigated.The effects of several factors on denitrification such as current strength,HRT,influent p H,influent NO₃^--N were studied.Further,heterotrophic denitrification using polycaprolactone（PCL）as an efficient solid phase carbon source and electrochemical hydrogen production autotrophic denitrification were combined to enhance the denitrification.The main conclusions obtained were as follows:（1）The NO₃^--N removal rate during the domestication of BER microorganisms constructed in this study was positively correlated with the current.When the current was adjusted to 70 m A,the NO₃^--N removal rate could be stabilized to more than 75%after running the reactor for 7 days at an HRT of 12 h.At this time,the domestication was considered to be completed,and the experiments for optimizing the BER operation conditions were subsequently conducted.With the increase of current,the NO₃^--N removal rate accelerated and reached the best removal rate of 96%at 80 m A.When the current intensity exceeded 86 m A,the NO₃^--N removal rate no longer increased with current.With the extension of HRT,the nitrate removal rate increased continuously.When the HRT exceeded 9 h,the nitrate removal rate was higher than 90%and the variation became smaller.The influent p H ranging from 6.7 to 8.1,the NO₃^--N removal rate was higher than 85%,and there was no obvious accumulation of NO₂^--N,which proved that the reactor had good p H buffering ability.The denitrification rate increased with the increase of the initial NO₃^--N concentration.When the influent NO₃^--N concentration reached 40 mg/L,the NO₃^--N removal reached the maximum value of22.3 mg/L.The NO₂^--N concentration in the effluent increased with the influent NO₃^--N concentration,and there was an obvious accumulation phenomenon.The optimum operating parameters of the reactor were determined as follows:current density（86～96m A）;HRT（9 h）;influent p H（6.7～8.1）;influent NO₃^--N concentration（40 mg/L）.The BER was operated under the optimum conditions for 7 days,and the NO₃^--N removal rate was maintained between 80%and 90%;the effluent NO₃^--N concentration was basically maintained at 4～6 mg/L.while the effluent NO₂^--N was in the range of 4～6mg/L.（2）The main genera in BER included norank＿f＿Blastocatellaceae、Thauera、Hydrogenophaga and Denitratisoma.The community diversity of BER was less than BR,probably due to microorganisms becoming more functionally specialized in BER.Appropriate electrical stimulation might lead to the evolution of specific communities and the development of specific microbial functions.（3）Solid-phase heterotrophic denitrification and electrochemical hydrogen autotrophic denitrification（SHD-EHD）with PCL was combined to enhance the denitrification.In this reactor,the slow-release carbon source PCL could not only provide both carbon and electron donor,but also could act as a growth carrier for microorganisms.The system overcome the disadvantage of heterotrophic denitrification in which the amount of carbon source dosing was difficult to control and therefore easily caused secondary pollution.Compared with the above-mentioned hydrogen autotrophic denitrification,the SHD-EHD reactor removed NO₃^--N more efficiently with a shorter HRT and a lower applied current.The NO₃^--N removal rate of the SHD-EHD reactor was maintained above 90%under optimal conditions for 7days.The effluent NO₃^--N concentration was ranged from 1.41 to 3.27 mg/L,while the effluent NO₂^--N concentration was ranged from 0.27 to 0.62 mg/L.Compared with the BER reactor,the SHD-EHD reactor obtained higher NO₃^--N removal rate within a shorter HRT.Besides,the SHE-EHD increased the efficiency of denitrification by increasing the denitrification rate and reducing the accumulation of NO₂^--N,indicating that the autotrophic/heterotrophic synergistically facilitated the denitrification.（4）Dechloromonas was the dominant genus with the highest relative abundance in SHD-EHD reactor,which could use PCL as the organic carbon source to reduce NO3--N.Thauera,Acidovorax,Hydrogenophaga were also the dominant genera in SHD-EHD,which could utilize H2 as the electron donor for hydrogen autotrophic denitrification.The electroactive bacteria Stenotrophomonas were also present in SHD-EHD reactor.By comparing the community structural characteristics in SHD-EHD reactors and conventional heterotrophic denitrification（SHD）reactors,it was obtained that electrical stimulation increased the microbial nitrate reductase and nitrite reductase activities by N₂O accumulation and low expression of the nos Z gene led to the conclusion that NO to N₂O conversion was the rate limiting step in the overall denitrification pathway.The PCL-enhanced electrode biofilm process developed in this study effectively solved the problems of microbial activity inhibition,high cost and secondary pollution in the traditional electrode biofilm method,laid the foundation for the application and development of bioelectrochemical technology in groundwater nitrate treatment,and provided a new idea for in situ remediation of nitrate contaminated groundwater.