Denitrification And Sulfide Removal From Wastewater Based On In-situ Utilization Of Generated Electricity In Microbial Fuel Cells

Industrial wastewater often contained sulfide and nitrate,which should be treated to meet the wastewater discharge standards before being discharged.Microbial fuel cell（MFC）,a promising and emerging wastewater treatment technology,can produce electricity and remove both sulfide and nitrate.The denitrifying sulfide removal MFC was previously studied in our lab,but it was a problem to be urgently solved to enhance the removal efficiency of pollutants and the formation percent of sulfur and gaseous nitrogen.In this study,denitrifying sulfide removal MFCs basing on in-situ utilization of generated electricity,i.e.MFCs run in an alternate charging and discharging（ACD）mode,were started-up with two ways.MFC1 was run firstly in a constant resistance mode（R mode）and then in an ACD mode to start up while MFC2 was directly started up in an ACD mode.When the in-situ utilization of generated electricity in the denitrifying sulfide removal MFC was accomplished using an ACD mode,the effects of external circuit parameters such as the charging-discharging duration of capacitor and the distribution of external resistance,substrate concentration and temperature on the electrode potential and pollutant removal performance were studied.In addition,the extracellular electron transfer mode（EET）and rate-limiting steps of denitrifying biocathode were explored by cyclic voltammetry（CV）.Main conclusions are listed as follows:（1）MFC1 and MFC2 were successfully started up in 109 and 103 d,respectively.The cathodic total nitrogen removal loadings of MFC1 and MFC2 were 6.3±0.3 g N/（m³?d）and 5.5±0.5 g N/（m³?d）,respectively,achieving a gaseous nitrogen formation percent of 23.8±1.1%for MFC1 and 20.2±1.8%for MFC2.The sulfide removal loadings of MFC1 and MFC2 were 0.134±0.002 kg S/（m³?d）and 0.134±0.001 kg S/（m³?d）,respectively,achieving a sulfur formation percent of 20.7±0.7%for MFC1and 15.1±1.1%for MFC2.In addition,the average output power density of MFC1and MFC2 were 3.11 W/m³ and 4.53 W/m³,respectively.Compared with MFC2,MFC1owned less powerful electricity generation capacity but obtained higher sulfide removal loading,sulfur formation percent and gaseous nitrogen formation percent.The MFC that being operated in an ACD mode enhanced electricity generation and sulfide removal but promoted sulfate formation as compared to that being operated in an R mode.（2）Low range of the anode and cathode potentials favored the formation of sulfur and gaseous nitrogen.With the decrease of charging and discharging duration（TC=20,15,10,5 min）,the upper and lower limit value for both anode and cathode potentials decreased firstly and then increased,achieving the lowest potential range of-0.342^-0.242 V for the anode and-0.021⁰.063 V for the cathode at TC=10 min.At the same time,the sulfur and gaseous nitrogen formation percent respectively reached the maximum level of 21.5±0.4%and 6.4±0.3%,obtaining a sulfide removal loading of 0.212±0 kg S/（m³?d）and a total nitrogen removal loading of 2.6±0.2 g N/（m³?d）.Among the three methods of regulating the external resistance,the constant anode potential regulating method regulated the cathode potential range to the lowest range of-0.276⁰ V,accompanying with a cathode potential range of-0.31^-0.151 V.As a result,the gaseous nitrogen formation percent and cathodic nitrogen removal loading reached the maximum level of 32.2±1.5%and 16.5±0.8 g N/（m³?d）,accompanying with a preferable sulfur formation percent of 10.3±0.1%and a sulfide removal loading of 0.21±0.002 kg S/（m³?d）.（3）With the increase of feeding sulfide concentration from 64 mg S/L to 160 mg S/L,the upper limit value of anode potential showed an increase followed by a decrease and the lower limit value of anode potential fluctuated slightly,while both upper and lower limit values of cathode potential showed a decrease followed by an increase.With the increase of feeding nitrate concentration from 14 mg N/L to 56 mg N/L,both upper and lower limit values of anode potential firstly rose and then decreased,and the upper limit value of cathode potential had an opposite trend while the lower limit value of cathode potential gradually increased.The optimum substrate removal was obtained at a feeding sulfide concentration of 128 mg S/L and a feeding nitrate concentration of28 mg N/L,achieving a sulfide removal loading of 0.212±0 kg S/（m³?d）,a total nitrogen removal loading of 16.5±0.8 g N/（m³?d）,a sulfur formation percent of 10.3±0.1%and a gaseous formation percent of 32.2±1.5%.When the temperature increased from 10 to 30°C,both upper and lower limit values of anode potential increased firstly and then decreased while those of cathode potential showed an opposite trend.30°C was beneficial for nitrate removal and gaseous nitrogen formation,achieving a nitrogen removal loading of 16.5±0.8 g N/（m³?d）and a gaseous nitrogen formation percent of 32.2±1.5%.40°C was beneficial for sulfur formation,achieving a sulfur formation percent of 48.8±0.2%.（4）Three peak potentials presented in CV curves of the denitrifying biocathode,where E₁（-0.471^-0.465 V）and E₂（-0.412^-0.428 V）represented respectively nitrate reduction and nitrite oxidation while E₃（-0.822^-0.826 V）represented nitrite reduction.Both nitrate reduction and nitrite oxidation in denitrifying biocathodes involved the direct electron transfer（DET）mode while nitrite reduction involved the mediated electron transfer（MET）mode.Intracellular catalytic reaction was the rate-limiting step for nitrate reduction.The rate-limiting step for nitrite reduction was affected by both nitrate supply and electrochemical activity.The mediator transfer was the rate-limiting step for nitrite reduction in the absence of nitrate,irrespective of the electrochemical activity level of denitrifying biocathode.But both mediator transfer and intracellular catalytic reaction became the rate-limiting steps for nitrite reduction in the presence of sufficient nitrate.