Effect Of The Carbon-nitrogen Ratio And Salinity On Performance Of The Single Chambered MFCs

Microbial fuel cell(MFC)has attracted much attention under the environment of the energy shortage and serious water pollution,because they could use a variety of organic-rich wastewater as fuel and convert the chemical energy to electrical energy directly.With the rapid development of the industry and agriculture,the discharge of the high-salt nitrogen wastewater has increased heavily.The high-salt nitrogen wastewater contains not only nitrogen,phosphorus and organic compounds with low or medium carbon chains,it also contains a large amount of inorganic ions.If it is discharged directly without effective treatment,it will cause serious water environmental problems.However,the current conventional biochemical methods which result in large sludge production,high energy consumption and unstable operation have been used to treat the wasterwater.Therefore,microbial fuel cell technology was introduced to treat high-salt nitrogen wastewater.In order to save energy consumption,the single-chambered MFC was the main research object.Six sets of air-cathode single-chambered MFCs were constructed to investigate the effects of carbon-nitrogen ratio and salinity on the power generation performance and pollutant removal efficiency when using the high-salt nitrogen wastewater as the substrate.And 16S rRNA gene sequencing technology was applied to investigate the electrode microbial membrane and analysed the dominant species.The main findings have been made as follows:(1)When the salinity of the substrate was 15g/L,five sets of single-chambered microbial fuel cells were started and operated by simulated wastewater with carbon nitrogen ratio of 3:1,4:1,5:1,6:1 and 7:1,respectively In the aspect of the power generation:the improvement of carbon nitrogen ratio could optimize the power generation performance of single-chambered MFCs.The battery’s power generation performance was the best when the carbon nitrogen ratio was 7:1.The open circuit voltage,internal resistance and maximum power density were 765mv,78.4Ωand7.33w/m~3 respectively.In the aspect of the pollutant removal,the single-chambered MFCs could remove nitrogen by simultaneous nitrification and denitrification.When the carbon-nitrogen ratio was 4:1,the removal efficiency of the MFC was the best.And the COD removal rate reached(86.17±2.4)%,the removal rates of ammonia nitrogen and total nitrogen were(96.98±1.8)%and(96.64±1.8)%,respectively.When the carbon-nitrogen ratio fluctuated from 4:1 to 6:1,COD and total nitrogen removal rates still remained above 80%and 75%,respectively When the carbon-nitrogen ratio was 7:1,the removal rate of COD was still more than 80%,but the removal rate of total nitrogen was only(22.21±1.2)%.(2)A set of single-chambered MFC was started and operated by the simulated wastewater with the carbon-nitrogen ratio of 4:1.When the salinity of the substrate changed in the range of 15g/L-35g/L according to the three stages of risinng,falling and irregular changing,the MFC’s electricity production performance and pollutant removal effect would be affected.First of all,in the salinity rising stage,the MFC’s power generation performance increased first and then decreased.It was optimal when the salinity was 20g/L.The maximum power density and internal resistance were 6.06W/m~3 and 84.6Ω,respectively;The removal rate of pollutants decreased gradually with the increasing of salinity.The removal rate of COD and total nitrogen decreased by40.7%and 62.7%,respectively,compared with the salinity load applied before.Secondly,as the salinity of the substrate decreased,the MFC’s power generation performance and the ability to remove COD gradually increased.When the salinity dropped to 15g/L,the maximum output voltage and COD removal rate of the MFC were 515m V and(78.82±1.8)%,which could not reach the level before the impact load was applied.And the total nitrogen removal rate was only 25%.Finally,the single-chambered MFC was subjected to a salinity impact load irregularly.The output voltage of the MFC and the ability of removing COD continued to decrease.The removal rate of COD was reduced by 37%compared with the impact load which was applied before,and the total nitrogen removal rate remained only 20%-26%.(3)16S rRNA sequencing results showed that changes in the carbon-nitrogen ratio and salinity had a effect on the community structures of the electrode microorganisms at the phylum level.In the single-chambered MFCs started by the high-salt simulated wastewater with the different carbon-nitrogen ratios,the dominant species in the cathode biofilm were Proteobacteria and Bacteroidetes,and the dominant species in the anodic biofilm were Proteobacteria,Bacteroidetes,and Chloroflexi.Compared with the biofilm which was not exerted the additional salinity load,in the cathodic biofilm,besides Proteobacteria and Bacteroidetes,Epsilonbacteraeota and Spirochaetes were also evolved into dominant species.And Epsilonbacteraeota was abundant in the anode biofilm.At the genus level,the cathodic and anodic bacteria had large differences under different carbon-nitrogen ratio conditions,but were both mainly based on Thauera.The bacteria related to nitrogen removal were heterotrophic nitrifying and aerobic denitrification bacteria.With the increasing of the carbon-nitrogen ratio of the substrate,sulfate-reducing bacteria gradually enriched in the anode membrane and became the dominant species.The changes in salinity also affected the microbial community structures at the genus level.The abundance of Thauera decreased sharply,and the abundance of the other heterotrophic nitrifying bacteria and aerobic denitrifying bacteria also decreased sharply.Sulfate-reducing bacteria were abundantly integrated into dominant bacteria.