Research On Microbial Nitrate Removal In Coal Pyrolysis Wastewater Enhanced By Electrochemistry

Coal is the main fuel in China,while the development of society and the transformation of economic structure urge the coal industry to produce innovatively for pollution-reducing,as a new coal chemical industry technology,low-rank coal pyrolysis and qualitative utilization facilitated the clean and low-carbon utilization of coal.However,the contaminants in coal pyrolysis wastewater(CPW)were complex and harmful to the environment,and the strict standard of near-zero liquid discharge for the coal chemical industry made the CPW require to be treated properly and reuse economically.This dissertation focuses on nitrate removal in the effluent from the biological pretreated CPW with low COD to nitrate nitrogen ratio,and the nitrate removal efficiency was unsatisfying in this wastewater.Based on the treatment status,this dissertation carried out the basic research of nitrate removal in biological pretreated CPW by mixotrophic denitrification enhanced by electrochemistry.The external voltage coupled with microorganism was used to remove nitrate in CPW,then an iron electrode was introduced in microbial electrolysis cell(MEC)to explore the effect of the iron anode on microbial denitrification process in CPW.Finally,the biological denitrification coupled with iron-carbon micro-electrolysis(IC-ME)was established to reveal that nitrate removal in CPW was enhanced by mixotrophic denitrification.A study coupled external voltage with microorganisms for nitrate removal from CPW was carried out.The experiment results confirmed that when the influent conductivity was 3.0 m S/cm,p H was 7.5,and the applied voltage was 0.9 V,the COD and TN removal efficiency were 95.30±2.25%and 94.93±2.00%,respectively,which were 6.7%and 15.15%higher than those in the control experiment.With the applied voltage increased continuously,the TN removal efficiency decreased to 88.29±3.28%because of the reoxidation of nitrite by the anode.The existence of the proper voltage constructed a higher anode potential and a lower cathode potential,which facilitated the exoelectrogenic bacteria degraded organics with the anode being electron acceptor and the autotrophic denitrifying bacteria reduced nitrate with the cathode being electron donor.The analysis of nitrogen species in wastewater demonstrated that the concentration of nitrous oxide was 758.37 ppm higher than the control experiment,which indicated that part of nitrate was reduced to nitrous oxide rather than nitrogen gas.Under the stimulation of the external voltage,the protein content in extracellular polymeric substances(EPS)of the activated sludge increased,and a coating film was formed on the electrode,which increased the contact area between the bacteria and the electrode.The abundance of key enzymes in the denitrification process implied that the external voltage promoted the abundance of periplasmic dissimilatory nitrate reductases(Nap A),which accelerated the reduction of nitrate.The influence of voltage and the lack of carbon source declined the activity of nitrous oxide reductase(Nos Z),therefore inhibited the reduction of N2O.Microbial community structure analysis revealed that the phenol-degrading bacteria Zoogloea,the exoelectrogenic bacteria Pseudomonas and the autotrophic denitrifier Arenimonas play an important role in the nitrate removal process in CPW.For the complete removal of nitrate,an iron anode that providing extra electron donors was introduced into the system to enhance the biological denitrification process.The results demonstrated that under the low voltage of 0.5 V applied,the Fe²⁺generated from the iron anode could enhance nitrate removal in CPW by enriching nitrate-reducing Fe(?)-oxidizing bacteria(NRFOB).The nitrate was mainly reduced by membrane-bound dissimilatory nitrate reductases(Nar)which indicated by analysis of key enzymes in the denitrification process,Nar and Nap A reduced nitrate by accepting electrons from quinone and cytochrome,respectively.Therefore,the nitrate was reduced by Nar rather than Nap A,which revealed that under the influence of iron anode and external voltage of 0.5 V,the contribution of autotrophic denitrifying bacteria to nitrate reduction might be lower than NRFOB.Moreover,with the extra electrons provided by Fe²⁺,the NO and N2O were reduced by nitric oxide reductase(Nor B)and Nos Z,respectively.NRFOB concluded Acidovorax,Bradyrhizobium and Azospira,exhibited the relative abundance of 1.30%,2.74%and 10.64%,respectively.While the relative abundance of autotrophic denitrifying bacteria were all lower than 0.1%.As the Fe²⁺oxidation product,?-Fe OOH enriched Geobacter,this kind of Fe(?)-reducing bacteria(FRB)reduced Fe³⁺to Fe²⁺with electrons accepted by oxidizing organics,and the cycle of iron was beneficial to the synergistic removal of organics and nitrate in wastewater.In order to reduce electricity consumption and promote the goal of carbon neutrality,the biological denitrification coupled with IC-ME for nitrate removal in CPW was investigated,and the nitrate removal strategy in CPW was proposed.When the IC-ME carrier dosage was 100 g/L(the filling ratio was 7%),the p H of the influent was 6.5?7.0,the temperature was 25?,the COD and TN removal efficiency reached93.08±1.74%and 89.54±2.20%.The relative abundance of denitrifying bacteria including Thauera,Thermomonas and Hyphomicrobium was 38.67%,4.52%and3.36%,respectively.Fe²⁺produced by iron corrosion was utilized by NRFOB including Acidovorax(6.02%)and Thiobacillus(4.43%)to reduce nitrate,therefore the TN removal in CPW was enhanced by the mixotrophic denitrification process.Furthermore,the combination of IC-ME with bacteria declined the ammonium concentration which produced during the chemical reaction between iron and nitrate.The principal pollutants in CPW had inhibitory effects on denitrifying bacteria and NRFOB with the following order:phenol<resorcinol<4-methylphenol<catechol<p-nitrophenol.And under the influence of IC-ME,the degradation rate of phenol,resorcinol,4-methylphenol,catechol and p-nitrophenol was 0.1833 h^-1,0.1318 h^-1,0.1918 h^-1,0.1694 h^-1and 0.0583 h^-1,respectively,the degradation rates were all accelerated compared with the rate in biological denitrification,especially for p-nitrophenol.Fe²⁺improved the establishment of tryptophan in EPS of the sludge,therefore enhanced the adsorption and bridging function of EPS and promoted the formation of biofloc,which built an anoxic micro-environment for denitrifying bacteria and NRFOB to reduce nitrate.Microbial network analysis revealed that the“small world”established among bacteria in the sludge,such as the interaction between the phenol-degrading bacteria?-Proteobacteria class and the nitrogen removing bacteria Phycisphaerae class,as well as the FRB Geothrix and the denitrifying bacteria Flavobacterium,the interaction between bacteria showed a potential effect on organics and TN removal in CPW.