Regulation Mechanism Of Magnetite On Direct Interspecies Electron Transfer In Wastewater Anaerobic Biological Treatment Process

Anaerobic biological treatment technology could convert organic pollutants ofsewage into resources such as methane and hydrogen,and is the core technology for recovery of sewage resources.Developing an efficient and stable anaerobic wastewater treatment technology has become one of the hot spots in the world.In the process of anaerobic methane production,the degradation of volatile fatty acids（VFAs）requires a low hydrogen partial pressure based on the theory of interspecies hydrogen transfer（IHT）,and this step is considered as the rate-limiting step.Recent studies have shown that anaerobic syntrophic microorganisms in co-culture can perform direct interspecies electron transfer（DIET）through extracellular cytochromes or conductive pili,and the addition of conductive materials such as biochar and magnetite could improve methanogenesis performance via enhancing DIET.However,the potential DIET functional microbes and enhanced mechanism of conductive materials in anaerobic biological reactor are still unknown.In view of magnetite enhancing DIET among anaerobic mix-culture,theperformance and stability of anaerobic bioreactor with magnetite addition were studied in this thesis.At the same time,response process of magnetite enhanced anaerobic bioreactor was analyzed under typical adverse conditions such as starvation,organic loading shock and high ammonia inhibition.Furthermore,the long-term reaction migration behavior of magnetite,characteristics of interspecies electron transfer characteristics and succession of dominant microbial community were analyzed during anaerobic sludge granulation.Finally,the mechanism of magnetite enhanced anaerobic process via DIET in methanogenesis pathway was revealed.The main results are as follows:1. Dosage and size effects of magnetite on the performance of anaerobic wastewater treatment process were studied,and results showed that the reactor with10 g Fe·L^-1 micro-sized magnetite（R3）had a better performance of pollutants removal.Especially after a short-term ethanol stimulation(500 mg·L^-1),COD removal efficiency of R3 increased obviously,which was 34.8%-38.7%higher than those of reactors without magnetite（R1）,with 2 g Fe·L^-1 micro-sized magnetite（R2）,and with2 g Fe·L^-1 nano-sized magnetite（R4）under the same condition.Methanosarcina capable of DIET was enriched in R3 with abundance of 63.64%.Meanwhile,the abundance of Chloroflexi in R3 reached 16%,which was 3.61-7.57 times of the others,and the electrochemical active Longilinea became the dominant bacteria.2. Effect of magnetite addition during the starvation-recovery period in anaerobic reactor was studied.Results showed that the COD removal efficiency and gas production rate of RM reactor with addition of magnetite were 87.73%±0.66%and0.30 m³·kg^-1COD·d^-1 after 60 days’starvation,which were higher than the control starvation-recovery time of RM was 5 days,while that of RB was 10 days,this might be relative to lower oxidation-reduction potential（ORP）of RM with-351±5.64 m V.Although the presence of magnetite reduced the heme c concentration of anaerobic sludge in RM to one fifth of that in RM,the sludge conductivity of RM(25.59±2.79Syntrophaceae（7.93%）enriched in RM,and probably were used by Methanothrix（90.11%）in RM.The potential DIET between Syntrophaceae and Methanothrix was enhanced by magnetite,which increased the sludge conductivity and reduced 50%of the recovery time after starvation.3. The migration behavior of magnetite during long-term operation of anaerobic bioreactor were analyzed.Results showed that the total iron concentration in the effluent of RM reactor with magnetite addition was up to 32.49±0.30 mg·L^-1,and the loss of iron element through the effluent accounted for 94.51%of the initial total iron in RM after 130 days’operation,because of the neutral-acid niche.DIET partners such as Geobacter,Desulfovibrio,Methanothrix and Methanosarcina,enriched in RM with organic loading rate（OLR）increased to 20 kg COD·m^-3·d^-1,and a higher EPS redox activity and sludge conductivity were achieved.The electronic transfer system than that in RB.Enrichment of DIET-based syntrophic microorganisms and establishment of metabolic network were facilitated by the gradual loss of magnetite,and dense anaerobic granular sludge（An GS）with high methanogenic performance formed.4. Effect of magnetite on the key methanogenic metabolic pathway was further studied.Results showed that under the low ammonia concentration of 147.29±2.57mg NH₄⁺-N·L^-1,acetoclastic methanogenesis was dominated,and effect of magnetite was not obvious.Under the high ammonia concentration of over 1600 mg NH₄⁺-N·L^-1,the acetoclastic methanogenesis was inhibited,but the methane production rate constent was still at 0.023（R²=0.85）with magnetite addition（group M）.Meanwhile,DIET partners like Desulfovibrionales and Methanosarcina were enriched in group M,and biological electric network was possibly formed with more extracellular proteins.The activities of four key enzymes/coenzymes like cytochrome c,coenzyme A,coenzyme M and coenzyme F420 in RM,were increased by 42.39%,23.74%,18.14%and 41.56%respectively,accompanied by 1.49 times higher genes expression related to acetoclastic methanogenesis than RB.Results indicated that the addition of magnetite enriched methanogens capable of DIET,and improved the tolerance of acetoclastic methanogenesis in the adverse niche to enhance the performance and stability of anaerobic system.In summary,the addition of appropriate magnetite not only favors the enrichment of potential DIET partners and formation of An GS with high methanogenic performance,but also improves the tolerance of acetoclastic methanogenesis under adverse conditions such as starvation-recovery,organic loading shock and high ammonia inhibition to achieve a stable performance of anaerobic bioreactor.This study has prominent academic achievement and potential application value for innovative development of efficiency and stable anaerobic wastewater treatment process for energy and resources recovery from wastewater.