Nitrogen Removal Based On Hydrogenotrophic Denitrification At Biocathode In Bioelectrochemical System(BES)

With the developing of industry,agriculture and animal husbandry,a large amount of nitrogen was discharged into the water and keeps accumulating,which threatens the survival of humans and other creatures.Efficient treatment of wastewater containing nitrogen has caused much attention.Compared with the traditional chemical,physico-chemical and biological treatment methods proposed for the nitrogen removal from wastewater,bioelectrochemical system(BES)based denitrification at biocathode has drawn extensive public attention featured for its high treatment efficiency,low energy consumption,and low sludge yields.Based on hydrogenotrophic denitrification,biohydrogen production coupled BES was investigated to provide more electrons for nitrogen removal.Three groups of bioelectrochemical system(BES),including biocathode(BC),hydrogen facilitated biocathode(HBC)and coupled hydrogen and biocathode(CHBC),were investigated for denitrification at biohydrogen facilitated biocathode.In addition to kinetic of nitrogen reduction and hydrogen production,electrochemical performances such as polarization curve and electrochemical impedance spectroscopy(EIS)of the three groups were all investigated.Most of all,molecular biology method like Illumina MiSeq high-throughput sequencing was also applied to further explore the microbial denitrification communities at the three different biocathodes.It was found that chree-chambered CHBC was the most efficient for nitrogen removal.(1)Anode exoelectrogens,biocathode denitrifiers and hydrogen producers were enriched by different ways.When the exoelectrogens and denitrifiers were successfully enriched,anode potential across electrodes could poise at-0.55 V and the biocathode nitrate removal rate showed to be stable.Prior to inoculating for biohydrogen production,anaerobic granular sludge was pretreated to eliminate non-spore-harboring methanogens.H2 could accounted for 40-50% of the fermentation gas with hydrogen producer successfully enriched,and no CH4 could be detected.(2)It was found nitrate removal efficiency reached 95% during 6 days for the three-chambered CHBC reactor,while 8 and 12 days were required for HBC and BC,respectively,with an initial nitrate concentration of 130±10 mg/L at biocathode.As the direct reduction product of nitrate,the highest nitrite concentration of 4.20 mg/L,15.92 mg/L and 32.56 mg/L was observed in CHBC,HBC and BC,respectively.Thus,CHBC was the most efficient for nitrogen removal of the three groups and complete denitrification might be accessed through CHBC as the lowest nitrite concentration was observed.Dissimilation in periplasm pathway might take place in BC for nitrogen removal as nitrite was accumulated in BC greatly.(3)Butyrate type fermentation occurred in all three BES as acetate and butyrate acid were the main organic acid accumulated in each group along with the biohydrogen process.Total acid in HBC was minimum,where biohydrogen yield(32.70 mL/gVS)was the lowest as well.Although the total acid concentration was the highest in CHBC,biohydrogen yield only reached 75.91% of BC(50.23 m L/gVS)for the inhibition of much more acetate.In addition,alcohol type fermentation also took place in CHBC and HBC,which led to the biohydrogen content of CHBC(52.49%)and HBC(45.59%)higher than BC(44.41%).It seemed that the in-situ biohydrogen production through BES would be competitive with the control,namely the external biohydrogen fermenter,for the purpose of electron supplying for nitrate reduction.(4)Both current(3.60 mA)and open circuit voltage(336.7 mV)were the highest in CHBC and maximum power density in CHBC(118.43 mW/m3)increased by 46.86% and 20.10% compared with BC and HBC,respectively.Coulombic efficiency of CHBC(79.1±0.2%)was also increased by 35.21% and 9.56% compared with BC and HBC,respectively.Electrochemical impedance spectroscopy showed total resistance in CHBC(35.45 ?)was a little lower than that of the HBC(39.83 ?),while was less than half of the BC(81.52 ?).In addition,electrode membrane of CHBC had a better ability for electron transfer and the most total cells were observed in denitrification biocathode biofilm of CHBC.In conclusion,the three-chambered CHBC could reach better electrochemical performance.(5)Through Illumina MiSeq high-throughput sequencing,it was found microbial diversity was the most infertile in CHBC,but 5 core genus promoting denitrification got enriched in CHBC,including Simplicispira,Thauera,Thermomonas,Azoarcus and Ottowia,while only 1 core genus was enriched in BC(Denitratisoma)and HBC(Thiobacillus),respectively.Relative abundance of these 7 genus in CHBC(45.44%)was greatly higher than BC(15.13%)and HBC(15.96%),which suggested that the three-chambered BES was better for denitrifies growth,and then improved nitrogen removal efficiency.Moreover,hydrogen producing bacterium and hydrogen consumers were both found in three denitrification biocathodes.As the most hydrogen consumers for nitrogen removal were identified in CHBC,hydrogenotrophic denitrification at biocathode could be enhanced in the three-chambered BES reactor and nitrate respiration pathway might be the most preferred for nitrogen removal in it.