Study On Diet In Anaerobic Treatment Of Organic Wastewater By Conducting Iron Materials

Anaerobic digestion process is considered to be one of the effective ways to turn pollutants into energy.The key process in traditional anaerobic digestion is that hydrogen or formate are electron carriers,acting as interspecies electron transfer shuttles between volatile fatty acids（VFAs）-oxidizing bacteria and hydrotrophic methanogens.In recent years,direct interspecies electron transfer（DIET）has been found as a faster and more specific alternative to IHT/IFT,since the electrons are transferred directly on microbial surfaces.In the studies of enhancing DIET methanogenic processes,conductive material（CM）has been widely studied because of its material availability,which can promote the enrichment of functional microorganisms and accelerate the metabolism of VFAs.Besides,conductive iron materials having not only the characteristic of common CM,but also the variable value of Fe element,may have more possibilities in enhancing DIET methanogenic processes during anaerobic digestion.Based on the above considerations,this study aimed to explore the effect of different existence forms of the conductive iron materials on anaerobic decomposition of organic pollutants in wastewaters and the DIET processes.The detailed research contents are as follow:（1）The effects of different doses of magnetite on DIET pathway in anaerobic digestion were studied using tetrachlorophenol（4-CP）as substrate.The study found that compared with the biological group without magnetite,the removal efficiency of 4-CP in the biological group supplemented with magnetite increased by 156-203%.On day 60 of the experiment,no hexanoic and pentanoic acids were detected in the biological group with magnetite,while the acetic acid was 5.6-6.2 times higher than that in the biological group without magnetite,suggesting that biodegradation of the complex intermediate products of 4-CP was accelerated.In the analysis of the final products of 4-CP degradation,no CH₄was detected in the biological group with magnetite,but the content of CO₂accounted for 39-83%,whichimplied that the addition of magnetite completely mineralized 4-CP into CO₂.High-throughput sequencing analysis showed that the abundance of functional bacteria,Desulphuromonas,Pseudomonas and Bacillus species in the biological group supplemented with magnetite increased by 1.38-1.97,1.50-2.04 and 11.60-17.18 times,respectively,compared with the group without magnetite.This functional microorganisms helped the biodegradation of 4-CP and its intermediates（SCFAs）with the existence of magnetite.Fe²⁺concentration and cyclic voltammetry（CV）analysis showed that the underlying Fe（III）/Fe（II）transition occurred and accelerated the 4-CP degradation.After the experiment,XRD analysis was conducted to analyze the potential crystalline structure of magnetite in different groups.The results showed that magnetite had been completely consumed in the biological group,indicating that the dissimilated Fe（III）reduction destroyed the crystal structure of magnetite.In addition,magnetite can also be used as an electron conduit to establish DIET between syntrophs,accelerating the biodegradation of 4-CP and its intermediates.（2）In the case of cellulose wastewater,the effect of Fe（0）acting as electrode materials on organics biodegradation and methane production via DIET was explored in anaerobic digestion.Minor voltage（0.6-1.2 V）was applied to drive the occurrence of anodic oxidizing reactions and cathodic methane production to construct a Microbial Electrochemical system（MES）.The results showed that a higher CH₄yield（1205-1508 m L/d）and COD removal rate（79.0%-93.8%）were detected in Fe（0）-MES digester（R1）,compared with those in graphite-MES digester（R2,720-1090 m L/d and 63.6%-85.6%）and the conventional anaerobic digester（R3,384-428 m L/d and 35.2%-41.0%）.In addition,the COD removal efficiency of cellulose at 0.9 V is better than that at a higher voltage（1.2 V）or a lower voltage（0.6 V）.The activity analysis of endoglucanase,endoglucanase andβ-glucosidase showed thatβ-glucosidase in Fe（0）-MES digester had the highest activity of cellulose hydrolysis.β-glucosidase in R1 was 8.62%higher than that in R2 and 43.18%higher than that in R3,indicating that microbial electrochemical process was beneficial to cellulose hydrolysis,especially when Fe（0）was used as the electrode materials.The calculated energy efficiency showed that the output energy produced by CH₄was 8.16 times of the electric power input in the Fe（0）-MES digester.According to the results of high-throughput sequencing and FISH:Methanothrix,Methanosarcina and Methanobacterium were particularly enriched on the Fe（0）-cathode surface of R1.Besides,Clostridium was also enriched on the Fe（0）-cathode surface.It implied that the dual direct electron transfer including both the electrode-microbe and the microbe-microbe were occurred on Fe（0）-cathode to accelerate the degradation of cellulose and the production of methane.