Effect Mechanism Of Electric Current On The Bio-electrochemical Denitrification Processs

The excessive use of nitrogen fertilizer leads to the aggravation of nitrate pollution in groundwater,which directly affects the safety of residential drinking water.The combination of bio-denitrification and electrochemical technology could effectively treat polluted groundwater by nitrate removal.However,in previous study,the authors only focused on the operating conditions effect on denitrification performance,the mechanisms of how current affects microorganism under the different operating conditions remained unclear.This study used a bio-electrochemical reactor to conduct the denitrification experiments,investigate the effects of electric current stimulation on the denitrification performance and denitrifying bacteria under different conditions,elucidate the mechanisms of electro-stimulation on the denitrifying bacteria and provide a theoretical foundation for the application of the bio-electrochemical denitrification system.The efficiency of denitrification was studied using a stainless-steel rod,a carbon rod,and a Ti/IrO2 rod as the anode in the bio-electrochemical denitrification system,respectively.It was found that the optimal current density ranges of anodes using different types of materials were basically similar,which was in the range of 200-250mA/m~2.Within this current density range,nitrate removal efficiencies were high in all reactors coupled with highest ATP aggregate levels.In addition,the activity of microorganisms was inhibited at higher current density(350-400 mA/m~2)and the inhibition effect caused by the Ti/IrO2 rod anode was more significant than the other two anodes.By comparing the denitrificaition proformance,microbial activity and pH variation of the three materials as anodes,it was confirmed that the stainless steel rod was more suitable for constructing the bio-electrochemical system.The effects of initial pH on the denitrification performance and microbial activity under the condition of non-energized and suitable current density(200 mA/m~2)were compared,the results showed that the removal efficiency of NO3~--N was largest at the initial pH of 7.25 no matter current present or not.With the increase in the initial pH value,the removal efficiency of NO3~--N decreased accompanied by increased accumulation of NO2~--N.At the same time,under the same initial pH,a suitable current density not only enhanced the activity of denitrifying bacteria and denitrification,but also improved microbial resistance to pH conditions when beyond the optimal pH range.It was found that the similar optimum current density 250 mA/m~2 was obtained at different C/N ratios in the bio-electrochemical denitrification system.Under this current density,the removal efficiency was the largest and the accumulation of NO2~--N was the least.Furthermore,the highest ATP aggregate levels,maximum growth rates and shorter lag time were also obtained at this current density.Moreover,a secondary model of microbial growth was constructed using the microbial parameters from the modified Gompertz model and described the relationship between current and microbial growth status.The addition of trace elements solution to the reactors could effectively improve the ability of microorganisms to reduce nitrate under the same current density.Mo and Fe could remarkably increase the catalytic activity of nitrate reductase(Nar),while Cu and Fe could significantly increase the catalytic activity of nitrite reductase(Nir).In addition,combination of various trace elements could improve the bacterial resistance to high current.Furthermore,both optimal current stimulation(200 mA/m~2)and the addition of trace element had positive effects on microbial metabolic activity,growth,the largest specific growth rate and growth lag phase,and could effectively increase the activity of Nar and Nir,while the simulation effect of current was more significant.The microbial community structures under different current density(0,200 and400 mA/m~2)was studied,results showed that the evenness,abundance and diversity of the community significantly increased under 200 mA/m~2,this allowing the microorganisms to resist interference from external environment.Components of the microbial community in all samples were similar at different current densities.The most predominant phylum was Betaproteobacteria,followed by Bacteroidetes and Chloroflexi.However,the proportions of different phylum under different current density were varied.This study revealed the mechanisms of electric current stimulation on the effect of the bio-electrochemical denitrification performance and denitrifying bacteria,which provided a theoretical foundation and technical support for the treatment of nitrate-contaminatedgroundwaterusingbio-electrochemicaldenitrification technology.