Research On Simultaneous Denitrification And Desulfurization And Enhanced Degradation Efficiency Of Halogenated Organic Compounds In Bioelectrochemical Reactor

In this study,bioelectrochemical reactor?BER?is the main research system and the aim of that is to enhance the microbial treatment of target pollutants by using micro-electric field stimulation.For sulphate-(SO₄^2-)and nitrate-?NO₃^-?contained wastewater,the effects of different external voltage on the enhanced treatment efficiency of heterotrophic or autotrophic desulfurization and denitrification,the analysis of intermediate products and the change of bacterial community structure were studied.In addition,in order to achieve simultaneous degradation of anodic nitrate and cathodic sulphate,the unique two-chamber structure of BER was also used to respectively inject sulphate wastewater and nitrate wastewater into both chambers.For toxic organic wastewater,the effects of different additives?glucose and acetate?on the anaerobic degradation efficiency of target pollutants,the analysis of intermediates and the change of bacterial community structure under the stimulation of micro-electric field were studied.Firstly,the BER was used to achieve simultaneous heterotrophic sulphate and nitrate reduction in a biocathode under different external voltages?0.1,0.2,0.3,0.4 and 0.5 V vs.Ag/AgCl?,indices such as the COD removal rate?CRR?,sulphate-reducing rate?SRR?,nitrate-reducing rate?NRR?and the possible formation of elemental sulfur were estimated to evaluate the effects of external voltages on treatment efficiency.Illumina high-throughput sequencing technology was utilized to characterize the bacterial community quantitatively.The results indicated that COD was highly affected and nitrate was the lowest affected by the variation in the external voltage.Furthermore,the removal rate of sulphate was attributed to the fact that sulphate was not affected entirely by voltage and it maybe affected by nitrate potentially.It can be found that there is a serious imbalance between the sulphate reduction rate and the hydrogen sulfide production rate after a analysis of the reduction products that may be formed in sulphate-reducing process,thus,in the presence of nitrate,the final product of sulphate reduction may be elemental sulfur.When the external voltage was 0.3V,the relative abundance of sulphate-reducing bacteria?SRB?Desulfococcus was the highest.This may be due to the promotion of nitrate reduction under this voltage,which slows down the inhibition of sulphate reduction,thus increasing the relative abundance of SRB and enhancing the sulphate reduction efficiency.Therefore,it can be concluded that the optimum external voltage is 0.3 V for this system.Secondly,the BER was used to achieve simultaneous autotrophic sulphate and nitrate reduction in a biocathode under different external voltages?0.2,0.4,0.6,0.8 and 1.0 V vs.Ag/AgCl?,indices such as the sulphate-reducing rate?SRR?,nitrate-reducing rate?NRR?,nitrite accumulation rate?NCR?and removal trend of nitrate and sulphate were estimated to evaluate the effects of external voltages on treatment efficiency.Illumina high-throughput sequencing technology was utilized to characterize the bacterial community quantitatively.The results indicated that nitrite removal efficiency and nitrite accumulation were affected significantly by the external voltage and the sulphate removal efficiency was only slightly affected.For the limited electrons supplied by the external electric field,there existed a competitive relationship between nitrated-reducing bacteria?NRB?and SRB.In five BER reactors,when the nitrate concentration was below 80 mg/L,the sulphate reduction process began.When the nitrate concentration was below 70 mg/L,SRR gradually increased.In addition,the increase of external voltage made the sulphate reduction start faster,that is to say,the inhibition time of the sulfate reduction process decreased.When the voltage increased from 0.4V to 0.8V,it can promote the autotrophic reduction of sulphate in this system.However,at a higher voltage of 1.0V,the autotrophic reduction of sulphate may be negatively affected.Moreover,although the dominant bacterial orders and genera were similar at voltages from 0.2 V to 0.8 V,their relative abundances were different due to different voltages.Significant differences in community compositions were observed when the voltage increases further to 1.0 V.Thirdly,in a two-chamber BER,the simultaneous degradation of anodic nitrate and cathodic sulphate was achieved by applying 0.2V vs.Ag/AgCl external voltage to the anode and injecting sulphate wastewater and nitrate wastewater into the cathode and cathode,respectively.Indexes such nitrate removal efficiency?NRE?,sulphate removal efficiency?SRE?,nitrite production efficiency?NPE?and ammonia production efficiency?APE?were estimated to evaluate the treatment performance.In addition,the Illumina high-throughput sequencing technology was used to characterize the bacterial community quantitatively.Although the removal efficiency of nitrate and sulphate in BER reactor was much lower than that in conventional biological treatment process,the simultaneous degradation of anodic nitrate and cathodic sulphate was achieved.Although denitrification and nitrate reduced to ammonia were both potential degradation processes by introducing nitrate into the anode,heterotrophic denitrification was the main degradation pathway of nitrate in this mixed bacteria system.In the anode,the existence of dominant species Pseudomonas and Azoarcus proved that nitrate was reduced by NRB using acetate as electron donor;in the cathode,Desulfomicrobium and Thauera are the main functional bacteria for sulphate reduction;the existence of major functional bacteria indicated the feasibility of the original idea of this study.Fourthly,the efficiency of bio-anaerobic degradation of 2,4,6-TCP and the diversity and composition of bacterial community with or without external carbon sources?acetate and glucose?under or no microelectric field stimulation were studied?0.2 V vs.Ag/AgCl?.In six BERs,the efficiency of 2,4,6-TCP was significantly different,and the degradation efficiency of 2,4,6-TCP was effectively improved due to the existence of micro-electric field and the addition of external carbon sources?acetate and glucose?as co-substrates.Compared with acetate,glucose has the higher promotion efficiency.Due to the existence of micro-electric field and the addition of external carbon sources,the bacterial community structure in six BERs had obvious differences,and the bacterial enrichment had obvious selectivity.Fifthly,the efficiency of bio-anaerobic degradation of p-CIA and the diversity and composition of bacterial community with or without external carbon sources?acetate and glucose?under or no microelectric field stimulation were studied?0.2 V vs.Ag/AgCl?.The results indicated that in six BERs,the p-CIA efficiency showed significant differences,and the degradation efficiency of p-CIA was effectively improved due to the presence of micro-electric field and the addition of external carbon sources?acetate and glucose?as co-substrates.In addition,compared with glucose,acetate is more suitable to be the co-substrates of p-CIA degradation in terms of cathodic current generation and its degradation efficiency.The analysis of intermediate products in two BER reactors without external carbon source showed that the final degradation products of p-CIA are harmless to the environment and achieve the purpose of this biodegradation.The bacterial community structure was analyzed and it showed that in each BER reactor,the relative abundance of functional bacteria related to p-CIA biodegradation was significantly different,which may be the main reason for the difference of degradation efficiency.Enhancing and strengthening the treatment efficiency of refractory organic matter,sulphate and nitrate in water is a subject of continuous progress and exploration in the process of water treatment.Therefore,it is of great significance for environmental protection and treatment to explore a fast,efficient,economical and controllable biological intensification treatment method for refractory organic pollutants or sulphate-and nitrogen-containing wastewater as the target pollutants,so as to achieve the effective removal and degradation of the target pollutants.