Study On Denitrification And Methanogenesis/Bioelectrochemical For Nitrogen Heterocyclic Compounds Degradation And Carbon-Nitrogen Removal

Nitrogen Heterocyclic Compounds(NHCs),which have been reported significant toxicity,mutagenicity,and carcinogenicity,are frequently found in effluent from coal pyrolysis,cooking,and pharmaceutical processing units.The COD/NO₃^--N ratio is high in this type of industrial wastewater,which can lead to the problem of excessive carbon sources under denitrification condition.In addition to denitrifying degradation,the excessive carbon sources mainly degradation in the subsequent aerobic reaction pool,but the dynamic energy consumption is relatively large,and the effect of biological treatment is greatly limited,due to the presence of refractory NHCs,which result in the COD concentration is still high in effluent,and difficult to achieve the emission standards,during the current industrial wastewater treatment process.Traditional anaerobic biological treatment is an effective method to remove NHCs,but there are difficulties in slow degradation rate,low yield of bioenergy gas(CH₄),and accumulation of ammonia nitrogen during the degradation process.Simultaneous denitrification and methanogenesis(SDM)is an effective way to increase the degradation rate of organic compounds and improve the quality of biochemical effluent.Additionally,aiming at the bottleneck of application of traditional anaerobic technology,coupling microbial electrolysis cell with anaerobic digestion simultaneous(MEC-AD)has the potential to enhance anaerobic degradation and simultaneously remove carbon and nitrogen.Three typical NHCs compounds,quinoline,indole,and pyridine,were selected in this paper.The degradation performance of the typical NHCs in the SDM was analyzed,the mechanism of NO₃^--N promoting NHCs degradation and inhibiting methanogenesis was indicated,the degradation pathway and mechanism of NHCs in the SDM system were revealed,and the symbiotic relationship between denitrifying bacteria and methanogens under specific carbon sources was clarified;the method for NHCs degradation enhancement and carbon-nitrogen removal in MEC-AD system was proposed,the mechanism of simultaneous carbon and nitrogen removal in the bioelectrochemical system was explored,the degradation mechanism of NHCs in the MEC-AD system and the biological mechanism of functional bacteria was investigated;the degradation relationship of NHCs co-substrates in SDM and MEC-AD system was studied.The study can enrich the theory of anaerobic biological treatment of wastewater,and provide a theoretical basis and new ideas for effective removal of NHCs from industrial wastewater.Results showed that the optimal NO₃^--N value of 250 mg/L quinolone,indole,pyridine were 50,50,35 mg/L,respectively,that could obtain the fastest degradation rate of NHCs and the least inhibition of methanogenesis,relative to the AD,the degradation rate was increased by 0.54,1.15,1.41 mg/(L·h),respectively;the production of methane decreased by 58.58%?64.13%?30.36%,respectively;dehydrogenase and urease were increased by 6.50 and 3.70,6.80and 8.50,6.70 and 3.50 mg/MLVSS/h,respectively;the degradation efficiency of high concentration NHCs(550 mg/L)was increased by 39.86%,54.34%,22.33%,respectively;due to the accelerated degradation of NHCs,the accumulation of ammonia nitrogen was increased by 4.90%,6.25%,8.41%,respectively.Through the balanced of C and N analysis,the direction of carbon and nitrogen during NHCs degradation in the SDM system was determined.It was found that during quinoline,indole,pyridine degradation,the inorganic carbon was increased by 21.30%,20.73%,4.51%,respectively;the organic carbon was decreased by 19.63%,24.04%,17.80%,respectively;the CH₄-C was decreased by 12.76%,21.79%,14.45%,respectively;the content of biomass carbon and nitrogen in the solid state was increased.In the SDM,results showed that quinoline was hydroxylated at the C-2 position;indole was hydroxylated to oxindole at the C-2 position,then to form isatin,and the methylation product of indole,3-methyindole was also detected;pyridine was broken at N-C-2 without hydroxylation,to form glutarate dialdehyde.Moreover,it was found that the intermediates were formed faster and removed more thoroughly without any lag phase,compared with the AD system.In the SDM system,functional bacteria,such as degrading bacteria,denitrifying bacteria,nitrate-reducing bacteria,and intermediate product degrading bacteria were generated and enriched,which was an interactive symbiotic relationship.At the methanogenic level,the methanogenesis of quinoline and indole were dominated by hydrogenotrophic methanogens,while the pyridine group was dominated by acetoclastic methanogens.Due to the phenomenon of the increased ammonia accumulation and decreased methane production during NHCs degradation in the SDM,the MEC-AD was explored to study the enhancement of NHCs degradation and the mechanism of NHCs carbon-nitrogen removal.Results showed that the applied voltage promoted the degradation of quinoline,indole,and pyridine,and the optimal degradation voltage was 1.0,1.0,0.7 V with the NHCs concentration of250 mg/L,respectively,and the degradation rates increased by 2.65,3.74,8.32mg/(L·h),respectively;TOC removal efficiency increased by 29.41%,29.19%,24.69%,respectively;methane production increased by 1.5,1.4,1.4 fold,respectively;the degradation efficiency of the high concentration(550 mg/L)increased by 84.26%?84.22%?55.56%,respectively.In addition,the effect of simultaneous removal of ammonia in the MEC-AD system was analyzed,and it was found that NH₄⁺-N could be completely removed.The synergistic mechanism of electrochemical oxidation,anaerobic ammonia oxidation,and denitrification in the process of ammonia removal was analyzedIn the MEC-AD,quinoline was transformed to 2-hydroxyquinoline,and8-hydroxycoumarin,and following the coumarin pathway;indole was transformed to oxindole and isatin;pyridine was transformed to glutarate dialdehyde,also broken between C-2 and C-3 position to form formamide;the formation and degradation rates of intermediates are faster in the MEC-AD.Meanwhile,it was found that the current was played a selective role in functional bacteria in MEC-AD,and the enrichment of functional bacteria was higher than the AD system.During the degradation of quinoline,the quinolone degrading and electrogenic bacteria,anaerobic ammonia-oxidizing bacteria,autotrophic denitrifying bacteria,heterotrophic denitrifying bacteria were mutual symbiotic,the proportions are as high as 36.99%,4.84%,8.18%,1.28%,respectively;the hydrogenotrophic methanogens played the dominated role during the methanogenesis process,moreover,the ammonia-oxidizing archaea(Nitrosoarchaeum)which is related to the denitrification process is enriched.The NHCs exist in a mixed substrate state in the actual wastewater,the degradation characteristics of co-substrates of NHCs were investigated.It was showed that low concentration of indole(0-100 mg/L)promoted the degradation of quinoline and its intermediates,and higher concentration(150 mg/L)inhibited the degradation;the degradation of indole was inhibited by quinoline,no matter the concentration of quinoline was high or low.The indole and pyridine co-substrates had a similar degradation characteristic.Due to different degradation systems and substrates,the functional flora structure of NHCs co-substrates differs greatly.