Bioelectrocatalysis Coupled With UASB To Enhance Anaerobic Treatment Of Sulfate-containing Methanolic Wastewater And Regulatory Mechanism

The wastewater from chemical,pupling,textile,and coal industries,as the major dischargers of industrial wastewater,account for almost half of the industrial wastewater emissions in China.Methanol is the main pollutant of these effluents,and varying concentrations of sulfate are also contained.They possess an abundance of energy,and in order to achieve simultaneous industrial wastewater treatment and bioenergy recovery,anaerobic digestion（AD）process with up-flow anaerobic sludge bed（UASB）as the technological core is widely used.However,the long-term treatment of methanolic wastewater in UASB often results in anaerobic granular sludge（AnGS）refinement and washing-out due to single microbial community,low extracellular polymers substances（EPS）secretion,and lack of carrier material for microorganism attachment,which ultimately lead to poor operation.Besides,sulfate reduction has an inhibiting effect on the AD process,which is not conducive to methanogenesis and sulfate reduction taking place simultaneously.To address these issues,this thesis introduces bioelectrocatalysis（BE）technology based on a lab-scale UASB reactor（BE-UASB）with sulfate-containing methanolic wastewater to enable efficient operation,with a view to providing basic data and technical support for the wide application of UASB for the industrial wastewater.The main research conclusions are as follows:（1）Bioelectrocatalysis coupled with UASB（BE-UASB）was constructed to investigate the long-term performance in treating methanolic wastewater and the feasibility of promoting the sludge regranulation.BE-UASB exhibited a better COD removal of 90%and a methane production rate of 388.0 ml/L_reactor/d.The disintegration trend of AnGS was effectively mitigated and the percentage of AnGS exceeding 300μm increased from 1.3%to 22.4%.Analysis of EPS revealed that bioelectrocatalysis stimulated the EPS secretion,which bonded to cells to form a three-dimensional[-EPS-cell-EPS-]structure.16S r RNA disclosed that bioelectrocatalysis enriched the specific functional microorganisms such as Acetobacterium,Methanobacterium,and Methanomethylovorans,increased microbial community richness,and diversified metabolic pathways.Particularly,Methanobacterium,which accepts electrons at the electrode and catalyzes the conversion of CO₂ to CH₄,accounted for 10.8%on electrodes,compared to only 0.3%in UASB,which induced a upto 52.8%reduction in the CO₂ emission from BE-UASB reactor than that of control.（2）To overcome the limitations of BE-UASB in treating methanolic wastewater with a single substrate,sulfate was added to methanolic wastewater to adjust COD/SO₄^2-,and a study on the treatment effect of BE-UASB on sulfate-containing methanolic wastewater was carried out,the characteristics of AnGS were investigated,and the microbial community evolution of AnGS and electrodes was elucidated.The results showed that BE-UASB maintained higher CH₄ production,COD and sulfate removal than those of the UASB at all COD/SO₄^2-from 10∶1 to 3∶1.At COD/SO₄^2-of 10∶1-5∶1,BE-UASB achieved a COD and sulfate removal of 93%and 83%,the CH₄ production rate was 436.8 ml/L_reactor/d,realizing efficient energy recovery.The particle size of the AnGS showed a trend of increasing and then decreasing.At COD/SO₄^2-of 10∶1,the AnGS in BE-UASB was optimal with an average particle size of 569.6μm（224.9μm in UASB）,with 31.8%of the AnGS exceeding 1000μm（only1%in UASB）.As judged by scanning electron microscopy（SEM）,there was a gradual evolution from cocci-microorganisms to interwoven filaments,rods,and spherical bacteria,and bioelectrocatalysis in turn contributed to the enrichment of these microorganisms growing on the electrodes.Methanomethylovorans,Methanosaeta,Candidatus＿Methanofastidiosum,and other methane-producing archaea（MPA）were present in BE-UASB reactor.G＿norank＿f＿Syntrophobacteraceae,Desulfomonile,Syntrophobacter,and other sulfate-reducing bacteria（SRB）were in small abundance and tended to enrich on bioelectrodes,easing the competition with MPA.The simultaneous removal of COD and sulfate was realized under the synergistic action.（3）Further increase in sulfate concentration to investigate the effect of bioelectrocatalysis on wastewater treatment and sludge characteristics.The MPA with high sulfate stress were domesticated,and the succession process of MPA and SRB in BE-UASB was elucidated.The microorganisms mainly attached to the bioelectrodes and with high metabolic activity,the CH₄ production rate of 243.7 m L/L_reactor/d was consistently higher than that of the UASB reactor,by a factor of approximately 1.4,corresponding to a COD removal rate of 93%.But the low sulfate removal rate of 20%further suggests that the system has successfully regulated methanogenic metabolic pathways suitable for high sulfate concentrations and inhibited the sulfate-reducing process,particularly the Methanomethylovorans of MPA,while Syntrophobacteraceae and Desulfomicrobium of SRB were also adapting to this environment,but the abundance（4%）was much lower than the MPA（35%）.The results of the PSD analysis show that the particle size of AnGS decreased significantly with increasing sulfate concentration.The effect of the bioelectrodes in trapping biomass somewhat alleviated the problem of sludge scouring loss,but new techniques are still needed to improve sulfate removal and sludge regranulation.