Long-Term Performance And Microbial Community Structure Of MFC Using Mustard Tuber Wastewater As Substrate

Microbial fuel cell(MFC)is a new energy technology.In the case of energy shortage and environmental contamination in the whole world,it has been gaining increasingly wide attention because of its great advantage in energy recovery and wastewater treatment.MFC can utilize exoelectrogenic bacteria as bio-catalyst to produce electric energy directly from simple organic matter or complex organic matter in the actual wastewater.Mustard production has developed in Three Gorges reservoir area for more than 100 years.With the rapid development of economy in Three Gorges reservoir area,as the pillar industry,the intensification and scale of mustard production are getting bigger.And a large number of mustard industrial parks are built in Chongqing City.Above those lead to lots of high salt and concentration wastewater discharging in Three Gorges reservoir area.Nowadays,anaerobic technology is usually used as pretreatment for mustard tuber wastewater treatment.And then using aerobic technology for further processing.To meet the discharge standards,the physical and chemical units are used for effluent polishing.However,energy consumption and operating costs are high when using conventional aerobic technology and sludge production is large,which will increase the cost of mustard tuber wastewater treatment.As it is suitable to use mustard tuber wastewater as fuel in MFC when the overall characteristics: optimum salinity,sufficient and readily biodegradable organic matters,are taken into account.Combining mustard wastewater treatment with MFC is worthy of exploration and discussion.This study systematically investigated long-term performance using mustard tuber wastewater as substrate in two-chambered MFC,including the characteristics of power generation and contaminant removal.Biofilm and microbial community structure were investigated by the method of scanning electron microscopy(SEM)and 16 S rRNA gene sequencing.In addition,the effect of aeration on production performance as well as simultaneous removal of organic matter and nitrogen was investigated.Some innovative findings have been made as follows.(1)The multi-cycle performance of MFC using high strength mustard tuber wastewater were in stable batch operation with a 1000? external resistor.The maximum power density of 7.44W/m3 were observed in the fifth cycle,and the according internal resistance,open circuit voltage,COD removal and columbic efficiency were 88?,746 mV,(65±2.5)% and(19.3±1)%,respectively.COD removal continuously increased to(73±3.3)%,the maximum rate,in the eighth cycle after start-up;meanwhile,the rapid increase of columbic efficiency till(19.3±1)% in the fifth cycle were followed by the slow declination.PH values of the anode effluent continuously decreased during the operation leading to the acidification.A sustainable power generation was able to be achieved with a 500? external resistor.An overshoot was also observed in power curves in the multi-cycle operation.(2)The tested MFC with a 500? external resistor can produce electricity over the entire testing period by single feed of mustard tuber wastewater,with stable power output in the period of day 20–100,lasting 80 d.In the stable and efficient production stage,the maximum power density of 12.43W/m3 were observed.Correspondingly,the minimum internal resistance,the maximum open circuit voltage,the peak COD removal rate and coulomb efficiency(CE)were 148?,756 mV,(89±1.5)% and(36.7±1.2)%,respectively.Voltages of MFC using mustard tuber wastewater as fuel rapidly increased and peaked on 32 d.COD removal continuously increased but columbic efficiency declined after the successful exoelectrogenic microbes acclimation.Moreover,the anode pH first descended and then rose and its minimum value was 4.36±0.05.The change trend of pH value demonstrated that anaerobic fermentation occurred firstly and then methane production process was carried out.In the later period of long-term running,we could improve the operating efficiency of MFC by inhibiting methane production process.(3)In the bio-cathode MFC,the system had the best performance when the aeration intensity was 150 mL/min.The maximum power density,internal resistance and open circuit voltage were 0.803W/m3,651? and 760 mV,respectively.Overpotential for oxygen reduction significantly lowered energy recovery.Moreover,COD along with ammonium present in catholyte declined voltage output.When involved Contaminant removal,COD removal rate was greater than 89% in both anode and cathode chamber under different aeration intensity.However,the aeration intensity had a great influence on the denitrification and good denitrification occured when the aeration intensity was between 100 and 150 mL/min.The maximum total nitrogen removal rate was up to 80% and those upon ammonia was greater than 92%.The accumulation of nitrate nitrogen was also observed in cathode chamber.(4)Biofilm and microbial community structure were investigated by the method of SEM and 16 S rRNA gene sequencing.The results showed that the biofilm did not totally cover the electrode surface,which might be due to the biofilm detachment after long-term running.Bacterial nanowires were used for electronic transmission when using mustard tuber wastewater as substrate.Compared to anodic biofilm,cathode biofilm distribution is more uniform.Considering the microbial community structure,bacterial diversities of electrode-respiring biofilms on anode and cathode were improved compared to the inoculum resources.In anodic biofilms,Bacteroidetes,Synergistetes,Firmicutes and Proteobacteria were the dominant bacteria at level of phylum.In cathodic biofilms,Proteobacteria,Bacteroidetes,and Planctomycetes were the dominant bacteria at level of phylum.